a study of corrosion in hydroprocess reactor efﬂuent air

A Study of Corrosion in Hydroprocess Reactor EffluentAir Cooler Systems

API PUBLICATION 932-ASEPTEMBER 2002

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A Study of Corrosion inHydroprocess Reactor Effluent Air Cooler Systems

Downstream Segment

API PUBLICATION 932-ASEPTEMBER 2002



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SPECIAL NOTES

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Copyright © 2002 American Petroleum Institute



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FOREWORD

API publications may be used by anyone desiring to do so. Every effort has been made bythe Institute to assure the accuracy and reliability of the data contained in them; however, theInstitute makes no representation, warranty, or guarantee in connection with this publicationand hereby expressly disclaims any liability or responsibility for loss or damage resultingfrom its use or for the violation of any federal, state, or municipal regulation with which thispublication may conflict.

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iii



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CONTENTS

Page

1 INTRODUCTION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 PREAMBLE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3 1975 NACE SURVEY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

4 1996 UOP SURVEY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

5 1998 API SURVEY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.1 Preliminary Survey—Broad Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75.2 Interview Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105.3 Reactor Effluent Air Coolers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125.4 REAC Inlet and Outlet Piping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135.5 Water Wash Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225.6 Water Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.7 Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235.8 Corrosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.9 Alloy Substitution to Prevent Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255.10 Inhibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6 DISCUSSION AND ANALYSIS OF SURVEY RESPONSES . . . . . . . . . . . . . . . . . 276.1 General Equipment Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276.2 Nitrogen Content of the Feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276.3 Salt Deposition and Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286.4 Management of Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286.5 Factors Influencing Corrosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286.6 Corrosion Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.7 Water Washing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296.8 Corrosion Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306.9 Flow Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

7 CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

8 FUTURE RESEARCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

APPENDIX A PLOTS OF CORROSION SEVERITY VERSUS VARIOUS PARAMETERS FROM THE UOP SURVEY DATA FOR ALL COOLER TUBES (REFERENCE 7) . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

APPENDIX B PLOTS OF CORROSION SEVERITY VERSUS VARIOUS PARAMETERS FROM THE UOP SURVEY DATA FORREAC PIPING (REFERENCE 7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

APPENDIX C QUESTIONNAIRE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Figures1 Effect of

K

p

on REAC Tube Corrosion (from Ref. 7) . . . . . . . . . . . . . . . . . . . . . . . 42 Effect of NH

4

HS Concentration in the Downstream Separator on REACTube Corrosion (from Ref. 7) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Effect of Velocity on REAC Tube Corrosion (from Ref. 7) . . . . . . . . . . . . . . . . . . . 6

v



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Preliminary Survey of Subcommittee Participants for Levels of Experience . . . . . 8A-1 Corrosion of Air Cooler Tubes—The Combined Effect of Calculated

Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Tube Velocity (ft/s) on Corrosion Severity. . . . . . . . . . . . . 34

A-2 Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separatorand Calculated Maximum Tube Velocity (ft/s) on Corrosion Severityfor Balanced and Unbalanced Headers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

A-3 Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator andCalculated Maximum Tube Velocity (ft/s) on Corrosion Severity for Balanced Header Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

A-4 Corrosion of Air Cooler Tubes—The Combined Effect of CalculatedAmmonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Tube Velocity (ft/s) on Corrosion Severity for Balanced Inlet and Unbalanced Outlet Header Systems . . . . . . . . . . . . . . . . . . . . 35

A-5 Corrosion of Air Cooler Tubes—The Combined Effect of CalculatedAmmonium Bisulfide Concentration in the Downstream Separator andCalculated Maximum Tube Velocity (ft/s) on Corrosion Severity for Unbalanced Header Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

B-1 Corrosion of REAC Outlet Header—The Combined Effect of CalculatedAmmonium Bisulfide Concentration in the Downstream Separator andCalculated Maximum Outlet Header Velocity (ft/s)on Corrosion Severity . . . . . . 38

B-2 Corrosion of REAC Outlet Header or Piping—The Combined Effect ofCalculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Velocity (ft/s) in the Outlet Headeror Piping on Corrosion Severity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

B-3 Corrosion of REAC Outlet Piping—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator andCalculated Maximum Outlet Piping Velocity (ft/s) on Corrosion Severity . . . . . . 39

B-4 Corrosion of REAC Outlet Header—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator andCalculated Maximum Outlet Header Velocity (ft/s) on Corrosion Severityfor Balanced and Unbalanced Header Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

B-5 Corrosion of REAC Outlet Header—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Outlet Header Velocity (ft/s) on Corrosion Severityfor Balanced Header Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

B-6 Corrosion of REAC Outlet Header—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator andCalculated Maximum Outlet Header Velocity (ft/s) on Corrosion Severity for Balanced Inlet and Unbalanced Outlet Header Systems . . . . . . . . . . . . . . . . . 40

B-7 Corrosion of REAC Outlet Header—The Combined Effect of CalculatedAmmonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Outlet Header Velocity (ft/s) on Corrosion Severity for Unbalanced Header Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Tables 1 Summary of REAC Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Preliminary Survey Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Summary of Preliminary Survey Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 Preliminary Survey Results (Showing Only Units Selected for Site Visits) . . . . . 11

vi



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Summary of Preliminary Survey Results (Showing Only Units Selected for Site Visits) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

6 Compilation of Information from Plant Interviews . . . . . . . . . . . . . . . . . . . . . . . . 147 Compilation of Information from Plant Interviews—Air Cooler Tubes . . . . . . . . 178 Air Cooler Tube Experience from Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Compilation of Information from Plant Interviews—Piping Information. . . . . . . 1910 Piping Experience from Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2011 Wash Water Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2112 Inspection Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2413 Corrosion Experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

vii



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1

A Study of Corrosion in Hydroprocess Reactor Effluent Air Cooler Systems

This study was sponsored by the American PetroleumInstitute (API), Committee on Refinery Equipment, Subcom-mittee on Corrosion and Materials Research. The purpose ofthe study was to provide technical background, based onindustry experience and consensus practice, for controllingcorrosion in hydroprocess reactor effluent systems. The find-ings reported herein will be used as an interim resource by theindustry and as a basis for a future API recommended prac-tice document.

1 Introduction

The treatment of crude distillation products with hydrogento produce higher yields of gasoline and jet fuel became amajor part of refinery technology with the introduction ofhydrocracking in the 1950s. Later, other hydrofining pro-cesses for removal of impurities from products were intro-duced. These technologies all had similar corrosionexperiences that eventually were identified as being associ-ated with the presence of hydrogen sulfide (H

2

S) and ammo-nia (NH

3

), and their reaction product ammonium bisulfide(NH

4

HS), in the reactor effluents. Minor contaminants suchas chlorides and oxygen were also believed to have an influ-ence on corrosion. But in general, corrosion was associatedwith salt deposition, concentrated solutions of ammoniumbisulfide and the flow velocity.

In 1976, R. L. Piehl (Standard Oil of California) conducteda survey for the National Association of Corrosion Engineers(NACE) on corrosion in Reactor Effluent Air Cooler (REAC)systems. Industry-wide experience was gathered and ana-lyzed to establish guidelines to minimize REAC corrosion. In1996, Unocal/UOP conducted an extensive survey of theirlicensees world wide. Their conclusions confirmed the valid-ity of the original parametric guidelines and contributed to theimportance of certain design features in avoiding corrosionproblems. Since the earlier survey, failures have continued tooccur and while it is believed that most have resulted fromoperation outside of the guidelines, there has been no system-atic study of the experience or open documentation of indi-vidual events.

2 Preamble

Information for this report has been gathered from open lit-erature, private company reports and interviews with represen-tatives of major refining companies. The terminology used byeach of these sources to describe some of the corrosion eventshas varied slightly and may have introduced some doublemeanings. Every effort has been made in this document tomaintain consistency and avoid confusion. Specifically, wehave tried to differentiate between “localized corrosion” as acategory of corrosion phenomena (e.g., pitting, crevice corro-

sion or stress corrosion cracking) and corrosion that has beenconfined to a small area of the corroding surface at a specificlocation in the process equipment. For the most part, localizedcorrosion will refer to the latter, i.e., corrosion occurring on asmall area, or at a specific location in the process. Where aspecific type of corrosion has occurred the proper description,for example, pitting, or stress corrosion cracking, will be used.

Throughout the report, reference is made to the effects ofvelocity on corrosion. It is commonly acknowledged thatvelocity induces accelerated attack by a mechanism involvingthe erosive effect of the high velocity liquid. The many con-tributors have frequently referred to this phenomenon as ero-sion. The consensus however appears to be that themechanism is one of accelerated corrosion. This should moreproperly be categorized as erosion-corrosion. Thus, unlessotherwise stated, the use of the term erosion-corrosion ismeant to describe the severe corrosion experienced by highvelocity fluid flow.

3 1975 NACE Survey

Prior to this survey, R. L. Piehl had presented the results ofstudies conducted by his company at an API Division ofRefining meeting in Philadelphia, May 15 – 17, 1968

1

. Thesestudies provided the earliest insights into the corrosion prob-lems associated with this new technology and attempted toidentify critical parameters contributing to the corrosion ofcarbon steel equipment.

Among the important conclusions reached at that time wasthe recognition that sulfide corrosion in an alkaline environ-ment was the primary cause of corrosion. The quantities andrelative ratios of ammonia and hydrogen sulfide were per-ceived as important and the possible influence of minor con-taminants such as chlorides and oxygen were acknowledged.The study concluded that the dominant mode of attack waserosion-corrosion of air cooler tube ends and some flowvelocity limitations were suggested to avoid the problem.

It was subsequently recognized that the subject was muchmore complex than originally thought and that other equip-ment, especially piping, was also affected. To broaden the database and capture as much experience as possible, NACE GroupCommittee T-8 (Refining Industry Corrosion) set up TaskGroup T-8-1 to conduct a survey of the T-8 membership on thesubject of corrosion of hydroprocess effluent air coolers

2

.Information was gathered on forty-two plants from fifteenrefining companies, mostly in the United States. In general, theresults were supportive of the original definition of the prob-

1

R. L. Piehl, “How to Cope with Corrosion in Hydrocracker EffluentCoolers,”

Oil & Gas Journal

, July 8, 1968.

2

R. L. Piehl, “Survey of Corrosion in Hydrocracker Effluent AirCoolers,”

Materials Performance

, January 1976.



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2 API P

UBLICATION

932-A

lem, but tended to underscore the complexity of the problemrather than provide clearer guidance. The ability of bothammonium chloride and ammonium bisulfide to condense assolids from the vapor phase and thereby cause blockage of theflow path was the motivation to introduce a water wash to solu-bilize the deposited material. Unfortunately, the resulting aque-ous solutions are extremely corrosive unless substantiallydiluted, and are in fact the cause of the corrosion problems inthese systems.

The survey gathered data on the chemical composition ofthe effluent stream including contaminants, and attempted todefine the corrosivity of the aqueous phase by factoring in theamount of water added to the system and its velocity. Theconcentration of bisulfide solution was measured in mostcases at the downstream water separator and this value wasused as a measure of the aggressiveness of the processstream. It was observed that at a concentration of 2% bisul-fide or below corrosion was mild but at 3% – 4% or more,significant corrosion began to occur.

The results helped to support a previously suggested rela-tionship between the bisulfide concentration and velocity,wherein the bisulfide level was represented by the product ofmole% NH

3

×

mole% H

2

S, designated as the

K

p

(Piehl) fac-tor. It was proposed that for

K

p

values of 0.1 – 0.5, velocitiesin the range 15 ft/s – 20 ft/s would be appropriate but for

K

p

values above 0.5 there was no suitable velocity. The higherthe

K

p

value the tighter the tolerance on velocity. It was alsofound that velocities of 10 ft/s – 12 ft/s could result in stag-nant deposits underneath which severe corrosion could occur,hence a lower limit of 10 ft/s was suggested, with an upperlimit of 20 ft/s and an optimum of 15 ft/s.

A major conclusion drawn from this survey was that aircooler corrosion is a complex phenomenon having numerousinterdependent variables. This reduces the prospects of suc-cessfully eliminating corrosion by control of one or even sev-eral of the variables.

4 1996 UOP Survey

In the 20 years following the NACE survey the problemswith corrosion continued, giving rise to a number of publica-tions discussing various aspects of the phenomenon

3

,

4

. Aircooler tubes continued to be the principal focus of the discus-sions although piping problems were also recognized. Labo-ratory studies of corrosion were unable to clarify the use ofparametric variables in controlling corrosion

5,6

. Thus, in1996, Unocal/UOP initiated a survey of its licensees to

expand the experience data base and possibly identify anynew factors in the corrosion of REACs and related piping.

The survey consisted of a comprehensive 10-page ques-tionnaire covering a variety of process and mechanical designinformation and corrosion experience. Topics included gen-eral operating conditions such as process mode, feedstocks,gas flow rates, contaminants (H

2

S, NH

3

, Cl, CN), water washdetails, and flow velocities. Air cooler and piping design andlayout, materials and corrosion experience were also covered.

Forty-six responses were received from operators of five dif-ferent types of hydroprocess unit. The information in theresponses was compiled into tabulated formats and analysesmade of the effects of certain variables on the corrosion experi-enced. Not all the respondents were able to provide values foreach of the key parameters requested so that UOP had to pro-vide estimated values by calculation from key operating datasuch as feed quality, charge rates, reactor efficiencies, flowrates, temperatures, pressures, and tube and piping flow areas.Note that these calculations, in particular velocities, were notbased on rigorous process simulation but rather on factoredestimates based on representative models for each unit configu-ration. To ensure consistency, UOP calculated values for

K

p

,NH

4

HS concentration and velocities for all of the units.

The results were presented in Paper 490 at Corrosion 97

7

.The diversity in the responses is illustrated in Table 1, whichsummarizes the range of values received for the key vari-ables; however, it cannot be construed that such a range ofvalues in the key parameters will necessarily result in widepattern of corrosion behaviors because of the interdepen-dency of corrosion on several parameters at the same time.Only if all the parameters are at one end of the range or theother can extreme behavior be anticipated. In addressing cor-rosion of carbon steel air cooler tubes, the effect of

K

p

factoron the severity of corrosion was evaluated by plotting

K

p

fac-tor versus level of corrosion experienced. The level of corro-sion was associated with tube life and the following rankingsused.

3

A. M. Alvarez and C. A. Robertson, “Materials and Design Consid-erations in High Pressure HDS Effluent Coolers,”

MaterialsProtection and Performance

, May 1973.

4

E. F. Ehmke, “Corrosion Correlations with Ammonia and Hydro-gen Sulfide in Air Coolers,”

Materials Performance,

July 1975.

5

D. G. Damin and J. D. McCoy, “Prevention of Corrosion inHydrodesulfurizer Air Coolers,”


, December1978.

6

C. Scherrer, M. Durrieu and G. Jarno, “Distillate and Resid Hydro-processing: Coping with Corrosion with High Concentrations ofAmmonium Bisulfide in the Process Water,”


,November 1980.

7

A. Singh, C. Harvey and R. L. Piehl, “Corrosion of Reactor Efflu-ent Air Coolers” Paper 490, Corrosion 97.

Severe (S) Tube life of 5 years or lessModerate (M) Tube life of 6 – 10 yearsLow (L) Tube life greater than

10 years with reported corrosion occurring

None (N) No corrosion reported



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A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

3

A plot of the data is shown in Figure 1. The four horizontalbands represent the four levels of corrosion but it can be seenthat there is considerable overlap in the range of

K

p

factors ateach level. The best conclusion that can be drawn is that thereis a trend showing that the severity of corrosion increaseswith increase of

K

p

, confirming Piehl’s observation. Theimprecise nature of the correlation however does not permit auseful guideline to be developed. Similar plots were devel-oped for corrosion severity versus:

1. Downstream separator bisulfide concentration (seeFigure 2),2. Maximum air cooler tube velocity (see Figure 3).

Note: Where values were not reported by the plants, values calculatedby UOP were used. These calculated values are valid only for bal-anced systems with assumed uniform flow distribution. They may beinaccurate where less than full condensation has occurred. As pre-sented, the data were so scattered that no correlation or inferencecould be drawn. It was evident that occurrences of corrosion do notcorrelate well with individual parameters partly because of the inac-curacies just discussed and in addition, because of the interdepen-dency of some of the parameters. By simultaneously plotting thelevel of corrosion against both bisulfide concentration and velocity,only a very slight improvement was obtained (see Appendix A, Fig-ure A-1). However, when the influence of piping symmetry on thedistribution of flow through the air coolers was introduced as a fourthparameter, more striking relationships were apparent (see AppendixA, Figures A-2 through A-5).

The air cooler piping system consists of a single inletpipe connected to a branched manifold system called theinlet header, which distributes the flow equally to each cellof the air cooler. The outlet flow from each cell is gatheredby a similar manifold arrangement called the outlet header,which reduces to a single outlet pipe leading to the separa-tor. With the piping headers (inlet and outlet) hydraulicallybalanced (see Figure A-3), no corrosion or low corrosionof the air cooler tubes is apparent. Several moderate corro-

sion points appeared, but two of these were associatedwith very high bisulfide concentrations. Similarly, wherethe inlet piping header is balanced, corrosion tends to below (see Figure A-4). On the other hand, when both head-ers were unbalanced (see Figure A-5), corrosion of thetubes was predominantly severe or moderate. It is clearthat uneven distribution of flow through the air cooler cre-ates conditions of either low or high velocities where cor-rosion can occur. Unfortunately, in most cases reported,neither the bisulfide concentration nor the magnitude ofthe velocity at the location where corrosion occurs isknown, so that a more refined correlation is not possible.The data were screened to exclude air coolers with returnbend tubes and those with ferrules. All of the plots areincluded in Appendix A (see Figures A-1 to A-5).

The above analytical approach was also applied to theassociated REAC piping. Piping failures have been the causeof some of the most serious incidents in these units. Discus-sions of experiences with REAC piping corrosion can befound in the NACE T-8 committee minutes and have beendocumented by Piehl

8

as well as the UOP study. Appendix B (see Figures B-1 to B-7) are data plots for out-

let headers and piping corresponding to the air cooler tubeseries. Two sets of data have been used, one for the maximumvelocity in the air cooler outlet header and the other for maxi-mum velocity in the piping from the header system to the sep-arator, if greater. Since the piping has a greater thickness thanair cooler tubing, the classification of corrosion severity hasbeen changed. Severe corrosion is considered as less than 10years life, moderate corrosion between 10 and 20 years andlow corrosion as measurable but with a greater than 20 yearpredicted life.

The conclusions are similar to those for the air coolersshowing an unclear relationship when just two or threeparameters are plotted and an improvement when the fourthparameter is added. Figure B-1 shows the combined effect ofbisulfide concentration and header velocity on corrosion. Theexceptions to good performance generally lie in the regime ofhigh bisulfide and high velocity. There are however severalsevere corrosion data points that fall below 4% bisulfide andless than 20 ft/s velocity, a regime that is normally regardedas acceptable. If the maximum piping velocity is included inthe data (see Figure B-2), severe corrosion shifts to the rightindicating that corrosion is associated with higher velocities.The correlation is stronger when only the maximum outletpiping velocity is plotted (see Figure B-3), and severe corro-sion is shown only when velocities are greater than 25 ft/s.

Figure B-4 includes the effect of the header configurationand indicates that serious corrosion has been experienced onlywhen the system is unbalanced whereas with balanced headersexperience has been favorable even at surprisingly high veloc-

Table 1—Summary of REAC Environments

Minimum Value

Median Value

Maximum Value

K

p

0.00 0.17 8.1

Wash water rate, vol-% feed

0.5 5.2 15.7

Foul water NH

4

HS content, wt-%

a

0.6 4.8 17.2

REAC tube velocity, ft/s

3.0 ~ 13.0 39.0

REAC outlet piping velocity, ft/s

4.3 15.1 44.9

Note:

a

Interpreted to mean NH

4

HS content of sour water in downstreamseparator.

8

R. L. Piehl, “Survey of Corrosion in Hydrocracker Effluent AirCoolers,”


, January 1976.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

4 API P

UBLICATION

932-A

Fig

ure

1—E

ffect

of K

p on

RE

AC

Tub

e C

orro

sion

(fr

om R

ef. 7

)

0.00

10.

010.

11

10

Kp,

mol

-% H

2S ×

mol

-% N

H3

Corrosion severity

NLMS

Not

e:F

= Fe

rrul

es u

sed

at tu

be e

nds

FF

F

FF

S =

Tub

e lif

e ≤

5 ye

ars

M =

Tub

e lif

e 6

– 10

yea

rsL

= Tu

be li

fe >

10

year

sN

= N

o co

rros

ion

F



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

5

Fig

ure

2—E

ffect

of N

H4H

S C

once

ntra

tion

in th

e D

owns

trea

m S

epar

ator

on

RE

AC

Tub

e C

orro

sion

(fr

om R

ef. 7

)

04

812

16

Am

mon

ium

bis

ulfid

e co

ncen

trat

ion,

wt-

%

Corrosion severity NLMS

Not

e:F

= F

erru

les

used

at t

ube

ends

F

F

FF

F

S =

Tub

e lif

e ≤

5 ye

ars

M =

Tub

e lif

e 6

– 10

yea

rsL

= T

ube

life

> 1

0 ye

ars

N =

No

corr

osio

n



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

6 API P

UBLICATION

932-A

Fig

ure

3—E

ffect

of V

eloc

ity o

n R

EA

C T

ube

Cor

rosi

on (

from

Ref

. 7)

0

3

6

9

12

Nom

inal

pea

k tu

be v

eloc

ity, m

/sec

Corrosion severity NLMS

FF

FF

FF

Not

e:F

= F

erru

les

used

at t

ube

ends

S =

Tub

e lif

e ≤

5 ye

ars

M =

Tub

e lif

e 6

– 10

yea

rsL

= T

ube

life

> 1

0 ye

ars

N =

No

corr

osio

n



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

7

ities and bisulfide concentrations. The data in Figure B-4 isclarified by showing the effect of each configuration sepa-rately as in Figures B-5, B-6, and B-7. Unfortunately, theseobservations do not provide any precise guidance as to corro-sion behavior with velocity but on the other hand are not inconflict with the suggested ranges proposed by Piehl. A seri-ous shortcoming is the lack of predictability of erosion-corro-sion in terms of both the velocity conditions that initiate it andthe locations where it might occur.

The UOP survey also reported on the performance of vari-ous alloys for REAC tubes and piping, corrosion of othercomponents and in addition discussed air cooler tube fouling.The report concluded with a list of design and operating rec-ommendations which support the 1975 NACE study but didnot add any further enlightenment or provide new guidelinesfor dealing with the problem.

5 1998 API Survey

The present study was initiated in 1997 and consisted oftwo parts:

1. A preliminary survey to obtain a broad overview of theproblem within the task group members’ experience. 2. An interview process of selected companies in whichdetails of their corrosion experiences were explored.

5.1 PRELIMINARY SURVEY—BROAD OVERVIEW

A brief questionnaire was sent to all task group membersinviting them to provide general information about two unitswithin their company’s operations where the corrosion expe-rience in one unit was significantly different from the other.Any unit that had experienced a catastrophic event such as anexplosion or fire was to be included. Where possible the sec-ond unit would be one that had a predictable and essentiallytrouble free corrosion record.

The preliminary questionnaire is shown in Figure 4. Thesurvey was divided into three major categories.

1.

Level of distress.

This was intended to give a measureof the severity of the corrosion problems experienced foreach unit and to identify units with poor experience fromthose with good experience.2.

Economic levels.

Another measure of the seriousnessof the corrosion problem is the frequency with whichequipment replacements have to be made or if a large cap-ital investment in alloy replacements is believed to benecessary. This category provided some insight into thoseaspects.3.

Corrosion control.

The level of effort needed to keepcorrosion under control is an indication of the seriousnessof the problem to the owner. Included in this categorywere some corrosion control measures and the quality ofinspection.

Unfortunately, in attempting to keep the survey brief toelicit maximum and timely response, some line items con-

tained more than one subject. This ambiguity has been takeninto account in drawing conclusions from the responses anddoes not appear to be a major deterrent to the usefulness ofthe results.

Table 2 is a complete compilation of the responsesreceived. The categories discussed above are listed at the leftside of the table. Each column represents an individual unitidentified by a code letter which has been used consistentlythrough the remainder of this report. The type of unit is iden-tified by a code letter as follows:

HCU

hydrocracking

HTU

hydrotreating—this includes hydrodesulfuriza-tion and hydrodenitrification units.

Table 3 summarizes the data from the responses and thefollowing conclusions have been drawn.

a. Out of 24 units included in the survey:• Five units reported fires and explosions.• Four units reported unscheduled outages.• Ten units reported experiencing corrosion but manage it

by regular replacement of carbon steel or have substi-tuted alloy for carbon steel.

• Five units reported no significant corrosion.b. A total of 12 units have substituted alloy for carbon steel.c. Ten units use alloy extensively.d. Two units report low corrosion rates but have experienced

fires. This indicates localized corrosion resulting in a seri-ous leak.

e. Only two units report using inhibition. One of these had anunscheduled outage.

f. Fifteen plants use regular inspection, that is, inspectionsconducted at planned intervals, usually at unitturnarounds.

g. Eleven plants use extensive inspection. The thickness mea-suring locations have been increased or 100% UTinspection is used to detect localized corrosion.

h. Seventeen plants use special frequent inspection. Thisincludes on-line UT and radiography.

There is no distinction of behavior by type of unit. Bothhydrocrackers and hydrotreaters have had comparable corro-sion experiences. The experience is clearly diverse although itis apparent that those units that have suffered a catastrophicevent have upgraded to alloy even when corrosion is gener-ally mild, whereas some units have never had a problem andcarbon steel has been quite satisfactory for all equipmentitems. Inhibition is not a widely used method of corrosioncontrol but those who use it clearly perceive an economicadvantage in doing so. It is evident that many reactor effluentsystems are subjected to more rigorous inspection than otherrefinery units as indicated by the number of units receivingfrequent or special inspections. The latter include techniquesthat are detailed and time consuming but are more thoroughthan alternative methods.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

8 API P

UBLICATION

932-A

Figure 4—Preliminary Survey of Subcommittee Participants for Levels of Experience

Levels of distress

a) No corrosion or corrosion requiring replacement of equipment on a scheduled basis.

b) Corrosion requiring replacement of equipment on an unscheduled basis.

c) Corrosion causing an outage (unscheduled and sudden).

d) Corrosion causing a fire, explosion, and property damage.

Economic levels

a) Replacement of carbon steel items i) exchanger bundles, ii) piping, iii) weld repair.

b) Frequent extensive in-kind replacement of carbon steel items.

c) Replacement of carbon steel with alloy.

d) Replacement of alloy with alloy.

e) Prolonged outage for replacement.

f) Unit reconstruction.

Corrosion Monitoring Control (Mark as many as are applicable.)

a) Mild corrosion monitored by regular maintenance.

b) Extensive use of alloy in critical locations.

c) Successful use of inhibition.

d) Extensive inspection—all equipment frequently.

e) Special inspection techniques/frequent inspection. (Define frequent as less than 1 year interval.)

f) Use of chemical analysis indicators—e.g., Kp factors, NH 4 HS concentration.

g) Wash water quality.

h) Wash water quantity.

Comments (Please add additional pages as needed.)



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

9

Tabl

e2—

Pre

limin

ary

Sur

vey

Res

ults

AB

CD

EF

GW

HJ

KL

MN

YO

RU

DD

PQ

ST

XH

CU

HT

UH

CU

HT

UH

TU

HC

UH

CU

HT

UH

CU

HC

UH

CU

HT

UH

CU

HT

UH

CU

HT

UH

TU

HT

U

Lev

el o

f D

istr

ess

a

) N

o c

orr

osi

on

XX

XX

X X

Xb

) S

che

du

led

re

pla

cem

en

tX

XX

XX

XX

XX

XX

c)

Un

sch

ed

ule

d o

uta

ge

XX

XX

d)

Exp

losi

on

, fir

eX

XX

XX

Eco

no

mic

Lev

els

e)

Re

pla

cem

en

t ca

rbo

n s

tee

lX

XX

XX

XX

XX

XX

XX

XX

Xf)

F

req

ue

nt,

ext

en

sive

CS

re

pla

cem

en

tX

Xg

) A

lloy

for

CS

XX

XX

XX

XX

XX

XX

h)

Allo

y re

pla

cem

en

tX

XX

i)

Pro

long

ed o

utag

eX

XX

j)

Un

it re

con

stru

ctio

nX

Co

rro

sio

n C

on

tro

lk)

M

ild c

orr

osi

on

, re

gu

lar

insp

ect

ion

XX

XX

XX

XX

XX

XX

XX

Xl)

E

xte

nsi

ve u

se o

f a

lloy

XX

XX

XX

XX

XX

m

) In

hib

itio

nX

Xn

) E

xte

nsi

ve,

fre

qu

en

t in

spe

ctio

nX

XX

XX

XX

XX

XX

o)

Sp

eci

al,

fre

qu

en

t in

spe

ctio

nX

XX

XX

XX

XX

XX

XX

XX

XX

p)

Ch

em

ica

l a

na

lyse

s: K

p f

act

ors

XX

XX

XX

XX

XX

XX

XX

XX

XX

X

q)

Wa

sh w

ate

rÑq

ua

lity

XX

XX

XX

XX

XX

XX

XX

XX

XX

XX

XX

X

r)

W

ash

wa

terÑ

qu

an

tity

XX

XX

XX

XX

XX

XX

XX

XX

XX

XX

XX

X

C

om

me

nts

:A

: C

.S.

pip

eÑ

> 8

25

; 4

44

exc

ha

ng

er

tub

es;

> 0

.5 m

m/y

r; v

elo

city

= 6

m/s

; b

isu

lfid

e =

4 w

t%

B:

Re

pla

cem

en

t o

f ca

rbo

n s

tee

l in

exc

ha

ng

ers

; (s

ee

co

lum

n E

ab

ove

)C

: F

ou

r la

rge

le

aks

; th

ree

fir

es,

all

rela

ted

to

bis

ulfi

de

/Cl

corr

osi

on

H:

Wa

shÑ

8 k

g/h

r a

t 1

9.2

mb

sdM

: W

ash

Ñ6

.8 k

g/h

r a

t 1

4.0

mb

sd

O:

Cl

corr

osi

on

at

wa

ter

inje

ctio

n p

oin

tR

: C

l, F

co

rro

sio

n a

t w

ate

r in

ject

ion

po

int

X:

No

sig

nifi

can

t re

pla

cem

en

t o

f ca

rbo

n s

tee

l ite

ms

for

ma

ny

yea

rs (

see

co

lum

n E

ab

ove

)



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

10 API P

UBLICATION

932-A

The results of this brief survey clearly invite a moredetailed investigation into the differences between those unitsthat have experienced no significant corrosion and those thathave had explosions or fires. It is also evident that some oper-ators are required to spend more time and money in maintain-ing their units than others. Previous attempts to address theseissues by studying the underlying factors known to affect cor-rosion behavior have failed to produce a clear picture of thedifferences. This is believed to be due to the complexity offactors and the lack of precise control of those factorsthroughout the system.

The next step in this present survey was designed, there-fore, to take a different approach to test the above premise. Itwas decided that instead of gathering broad, detailed informa-tion on each of the units selected for study, the informationsought would be relevant to a specific corrosion experienceonly. In this way, it was hoped to separate local aberrationsfrom general unit performance and possibly improve theparametric relationships between causes and effects.

To accomplish this, a selection was made of a number ofoperating companies that met the following criteria.

1. They operate units with experience at both ends of thespectrum.2. They were known to have reliable engineeringresource.

The number of locations visited was influenced by budge-try considerations.

To test the validity of the selection of companies for inter-view, their responses to the preliminary survey were sepa-

rated from Table 2 and are presented in Table 4. A summaryof the information is given in Table 5. Comparing Tables 3and 5 it can be seen that the percentage of units falling intoeach category is approximately the same in both cases indi-cating that the selected units are representative of the totalpopulation.

5.2 INTERVIEW PROCESS

Following review of the responses, a number of companieswere selected and arrangements made to interview key per-sonnel with respect to reactor effluent system corrosion.These interview arrangements were made through the taskgroup members but a request was made to have presentappropriate staff engineers with an intimate knowledge ofcorrosion, process and inspection aspects involved in each ofthe units selected for study. All of the participants were veryco-operative, but it was evident in some companies that staffreductions and re-assignments had an effect on the time avail-able for such studies. Few engineers remain that have a life-time of experience with these units.

Prior to the visits, a detailed questionnaire (shown inAppenedix C) was sent to the interviewees to provide a basisfor the discussions and indicate the quality of informationbeing sought. The UOP survey demanded a substantialamount of detailed information, which was needed to com-pute values for the process parameters of interest. The presentsurvey was designed to avoid imposing an unnecessary bur-den on the respondents in requesting extraneous process datathat would not have a foreseeable use. However, a certain

Table 3—Summary of Preliminary Survey Results

a) No corrosion 5b) Scheduled replacement 10c) Unscheduled outage 4d) Explosion, fire 5

e) Replacement carbon steel 1 2 8 5f) Frequent, extensive CS replacement 0 2 0 0g) Alloy for CS 5 1 5 1h) Alloy replacement 1 0 2 0i) Prolonged outage 3 0 0 0j) Unit reconstruction 1 0 0 0

k) Mild corrosion, regular inspection 1 4 6 4l) Extensive use of alloy 5 1 3 1

m) Inhibition 0 1 0 1n) Extensive, frequent inspection 4 2 3 2o) Special, frequent inspection 4 2 8 3p) Chemical analyses: Kp factors 5 2 9 3q) Wash waterÑquality 4 4 10 5r) Wash waterÑquantity 4 4 10 5

SUBTOTALS

Economic Levels

Level of Distress

Corrosion Control



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

11

Tabl

e4—

Pre

limin

ary

Sur

vey

Res

ults

(S

how

ing

Onl

y U

nits

Sel

ecte

d fo

r S

ite V

isits

)

BC

EW

IY

OR

UQ

XS

UB

TO

TA

LS

HC

UH

TU

HT

UH

CU

HC

UH

CU

HT

UH

CU

HT

UH

CU

HT

U

Lev

el o

f D

istr

ess

a)

No

co

rro

sio

nX

X X

2

b)

Sch

ed

ule

d r

ep

lace

me

nt

XX

XX

X6

c)

Un

sch

ed

ule

d o

uta

ge

X

d)

Exp

losi

on

, fir

eX

XX

3

Eco

no

mic

Lev

els

e)

Re

pla

cem

en

t ca

rbo

n s

tee

lX

XX

XX

XX

14

2

f)

Fre

qu

en

t, e

xte

nsi

ve C

S r

ep

lace

me

nt

XX

02

0

g)

Allo

y fo

r C

SX

XX

XX

XX

33

1

h)

Allo

y re

pla

cem

en

tX

X1

10

i)

Pro

long

ed o

utag

eX

X2

00

j)

Un

it re

con

stru

ctio

nX

10

0

Co

rro

sio

n C

on

tro

l

k)

Mild

co

rro

sio

n,

reg

ula

r in

spe

ctio

nX

XX

XX

XX

14

2

l)

Ext

en

sive

use

of

allo

y X

XX

XX

31

1

m)

Inh

ibiti

on

XX

01

1

n)

Ext

en

sive

, fr

eq

ue

nt

insp

ect

ion

XX

XX

21

1

o)

Sp

eci

al,

fre

qu

en

t in

spe

ctio

nX

XX

XX

XX

X3

41

p)

Ch

em

ica

l a

na

lyse

s: K

p f

act

ors

XX

XX

XX

XX

XX

X3

62

q)

Was

h w

ater

Ñqu

ality

XX

XX

XX

XX

XX

X3

62

r)

Wa

sh w

ate

rÑq

ua

ntit

yX

XX

XX

XX

XX

XX

36

2

K p V

alu

es0

.12

0.0

30

.33

0.4

50

.23

0.3

40

.3 Ð

0.4

50.

4 Ð

10

.04

0.0

5

Co

mm

ents

:

X:

No

sig

nifi

can

t re

pla

cem

en

t o

f ca

rbo

n s

tee

l ite

ms

for

ma

ny

yea

rs (

see

co

lum

n E

ab

ove

)

B:

Re

pla

cem

en

t o

f ca

rbo

n s

tee

l in

exc

ha

ng

ers

(se

e c

olu

mn

E a

bo

ve)

C:

Fo

ur

lar g

e l

ea

ks;

thre

e f

ire

s, a

ll re

late

d t

o b

isu

lfid

e/C

l co

rro

sio

n

O:

Cl

corr

osio

n at

wat

er i

njec

tion

poin

t

R:

Cl,

F c

orr

osi

on

at

wa

ter

inje

ctio

n p

oin

t



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

12 API P

UBLICATION

932-A

amount of detail to evaluate each experience was requested. Itwas decided that the data collected would be specific to eachcorrosion incident discussed. In this way, the parametersaffecting each incidence of corrosion would be available, butthe laborious task of gathering all the process, operating andmaintenance records could be eliminated. This made theinformation base more manageable. The questionnaire cov-ered all of the subjects pertinent to corrosion in REAC sys-tems and served as an agenda for the discussions.

The contents of each interview were recorded as hand-writ-ten notes backed up by copies of relevant data sheets, flowdiagrams and reports. Each significant corrosion experiencewas discussed in chronological order until the entire historyof the unit was completed. The results of these discussionswere compiled into a spreadsheet presented in Table 6.

5.3 REACTOR EFFLUENT AIR COOLERS

All of the data relevant to the reactor effluent air coolershave been sorted from Table 6 and is presented separately inTable 7. The table identifies the plant and type of unit and givesa brief description of the corrosion problem, including theapparent cause. Where additional background information orobservations are available, they have been added as comments.Numerical data has been compiled separately in Table 8.

There has been a wide variety of experience with air cool-ers in terms of useful life. Tube failures have occurred in aslittle as a few months in the worst case (Plant C), but inanother unit (Plant U), carbon steel tubes have lasted 25 years

with no failures. In the first case, corrosion was due to deposi-tion of ammonium salts in the air cooler tubes with probablyjust enough water present to create an aggressive concen-trated salt solution. Subsequent efforts to solve the problemby substituting chrome steels and later stainless steels wereunsuccessful until Alloy 800 was installed some 14 yearslater. The maximum tube velocities in this air cooler werehigh with respect to Piehl’s guidelines and the ammoniumbisulfide concentration in the downstream separator was 13%suggesting that this too might have been a contributing factor.In the second case, the operating conditions are very mild.There has never been any plugging or corrosion because theair cooler operates at an outlet temperature above the salt con-densation point (150°F). It is interesting that this unit also hasunbalanced outlet piping, which apparently is not a factorbecause the deposition of salts does not come into play.

One of the earliest problems with carbon steel air coolertubes was severe localized corrosion of the tube ends (PlantsW, EE, and Y). This was caused by high velocity and turbu-lence at the entrance to the tube and in some cases at the out-let end of the tube. This type of attack is thought by many tobe an example of erosion-corrosion. The problem has beensolved by the use of alloy ferrules with carbon steel tubes.Both stainless steel and Alloy 800 ferrules are used. Whereferrules are used, they must be designed with a tapered end sothat there is no abrupt transition from the ferrule to the tubecausing downstream eddies. Loose fitting ferrules can admitcorrosive solutions to the annulus with the tube wall and ini-

Table 5—Summary of Preliminary Survey Results (Showing Only Units Selected for Site Visits)

Level of Distressa) No corrosion 2b) Scheduled replacement 6

c) Unscheduled outaged) Explosion, fire 3

Economic Levelse) Replacement carbon steel 1 4 2f) Frequent, extensive CS replacement 0 2 0g) Alloy for CS 3 3 1h) Alloy replacement 1 1 0i) Prolonged outage 2 0 0j) Unit reconstruction 1 0 0

Corrosion Controlk) Mild corrosion, regular inspection 1 4 2l) Extensive use of alloy 3 1 1

m) Inhibition 0 1 1n) Extensive, frequent inspection 2 1 1o) Special, frequent inspection 3 4 1p) Chemical analyses: Kp factors 3 6 2q) Wash waterÑquality 3 6 2r) Wash waterÑquantity 3 6 2

SUBTOTALS



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

13

tiate crevice corrosion (Plant W). Alloy 800 tubes have notbeen reported to have tube end corrosion problems.

The results of discussions with respondents indicated thatvelocity must be controlled to avoid corrosion problems.Although the data in Table 8 is incomplete, it can be seen thatserious corrosion is associated with either high bisulfide con-centrations or high velocity, or both (Plants C, W, AA, andEE). Conditions of high bulk fluid velocity also lead to turbu-lence and localized corrosion or erosion-corrosion. The datain Table 8 is not in conflict with 20 ft/s as a reasonable upperlimit on tube velocity when associated with 4% bisulfide.Plant Y appears to be in conflict with this conclusion, how-ever the lower velocities could have resulted in underdepositcorrosion from salt build-up when the bisulfide concentrationwas 7% – 8%. The data for Plant O in Table 8 indicates thathigher velocities can be tolerated if the bisulfide concentra-tions are low.

One of the factors emphasized by this review is the criticalrole of wash water. Plants that experienced severe corrosionearly in their history have attributed the problem to lack ofwash water (Plants B, R, and Y) and have often been able tocorrect the problem by increasing the water rate. There is awide variation from process to process in the amount ofinjected water that vaporizes when introduced to the processstream. None of the respondents reported more than 75%vaporization, indicating a minimum of 25% of the injectedwater remains as liquid after introduction. When a singleinjection point is used, the aqueous phase has to distributeitself through the inlet piping header system and the air coolertube bundles in proportion to the effluent flows in that equip-ment. To eliminate the influence of an unbalanced pipingheader system, some operators use multiple injection points(see Table 11). These are usually located on the air coolerinlet nozzle close to the tube bundle inlet. The purpose is toensure that each bundle receives the same amount of washwater. However, individual monitoring of each injection lineis required, and frequent manual adjustment of the flow con-trol valves is often needed. The small diameter water linesand injectors are also prone to plugging depending on thequality of water used. Wash water preferences will be dis-cussed below when that specific topic is addressed.

Where corrosion has been serious and persistent, unit oper-ators have sometimes invested in Alloy 800 or Alloy 825 as apermanent solution. Alloy 800 has been used for at least 17years with no major failures (Plant B); however, pitting corro-sion has been reported (Plants Q and E) so that the long-termreliability has been put in question. One of these casesreported a chloride content of 50 ppm in the separator water.The level at which chlorides are significant with respect topitting of Alloy 800 has not been established although labora-tory tests

9

identify 100 ppm as harmful. One operator alsoreceived laminated Alloy 800 tubes that were not discovereduntil they had been in service some time.

5.4 REAC INLET AND OUTLET PIPING

All comments relating to inlet and outlet piping associatedwith the REACs have been sorted from the general commentsin Table 6 and presented as a separate compilation in Table 9.The numerical data have been reformatted and are presentedin Table 10.

The piping system discussed in this section covers the pip-ing from the water injection point, the inlet headers to the aircooled exchangers, and the outlet headers and piping to theseparator drums.

Out of the 12 units included in the detailed survey, one halfexperienced piping corrosion problems. Of the better per-forming group two were inhibited, two use alloy and two didnot experience any significant corrosion. Corrosion has beenexperienced on both the inlet and outlet sides of the REACs.The corrosion has often been localized at tees, elbows or asgrooving of straight run pipe. Depending on upstream factors,such as type of catalyst and feedstock quality, condensed saltscan be ammonium fluoride, chloride or bisulfide, or combina-tions of the three. Some operators employ fluoride catalystactivators giving rise to residual fluorides in the system (PlantR). The deposition temperature for the chloride salt is higherthan the fluoride salt deposition temperature which in turn ishigher than the bisulfide condensation temperature. Appar-ently, the aggressiveness of the salts is in the same order.Severe corrosion by deposition of ammonium chloride is themost prevalent type of attack.

5.4.1 Inlet Piping

Corrosion of the inlet piping was reported for two areas,immediately around the water injection point and in the head-ers before the REACs. Around the injection point, direct waterimpingement has been a problem. When excessive vaporiza-tion occurs, the amount of liquid water remaining could be toolow and this results in concentrated aqueous salt corrosion.Salt deposits in the feed/effluent exchangers upstream of thewater injection point have resulted in severe corrosion whenwater has come in contact with the deposits, from eithersplashing or saturation above the dew point. One solution hasbeen to use an alloy lining in the immediate area. One operatorhas used Alloy 625 successfully for this purpose. Localizedcorrosion can also occur as a concentrated aqueous phaseresulting from insufficient wash water flows through the pip-ing system. At elbows where surface velocities can increasedue to turbulence, the attack may be particularly severe. Bothstraight-pipe corrosion and erosion-corrosion of elbows havebeen reported by respondents. Again, the solution has been touse alloy for protection. In one case, Plant Q, installing Alloy

9

C. Scherrer, M. Durrieu and G. Jarno, “Distillate and Resid Hydro-processing: Coping with Corrosion with High Concentrations ofAmmonium Bisulfide in the Process Water,”


,November 1980.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

14 API P

UBLICATION

932-A

Tabl

e6—

Com

pila

tion

of In

form

atio

n fr

om P

lant

Inte

rvie

ws

Pla

nt

Un

itL

oca

tio

n o

f C

orr

osi

on

Typ

e o

f C

orr

osi

on

Cau

sati

ve F

acto

rsC

om

men

ts

BH

CU

Air

co

ole

r tu

be

sB

isu

lfid

e c

orr

osi

on

La

ck o

f w

ash

wa

ter.

Inst

alle

d 1

97

1;

faile

d i

n 6

ye

ars

(fir

e);

re

pla

ced

with

Allo

y 8

00

.

A/C

ou

tlet

pip

ing

Gro

ovi

ng

an

d c

ha

nn

elin

g2

5 f

t/s

att

ack

in

tu

rbu

len

t a

rea

do

wn

stre

am

of

elb

ow

. N

ow

co

rro

din

g a

t 5

0 m

py.

S

usp

ect

oxy

ge

n a

nd

ch

lori

de

s. N

o

corr

osi

on

wh

en

oxy

ge

n a

nd

ch

lori

de

s u

nd

er

con

tro

l.

Ori

gina

l 18"

rep

lace

d w

ith 2

2" in

199

4.

Tri

m c

oo

ler

U-b

en

d c

orr

osi

on

Lo

we

r ve

loci

ty a

nd

low

er

tem

pe

ratu

re t

ha

n A

/C.

Lo

we

r ra

te o

f co

rro

sio

n.

Re

pla

ced

ca

rbo

n s

tee

l w

ith A

lloy

80

0 i

n 1

98

1 (

10

ye

ars

).

Eff

lue

nt/

fra

ctio

na

tor

fee

d e

xch

an

ge

rT

ub

e t

hin

nin

g (

24

ye

ars

);

bis

ulfi

de

co

rro

sio

nS

usp

ect

oxy

ge

n (

50

pp

b)

invo

lve

d;

inle

t p

ipin

g

@

29

ft/

sÑst

ill i

n s

erv

ice

.F

aile

d i

n 1

99

5 a

fte

r 2

4 y

ea

rs a

t <

5 m

py.

QH

CU

A/C

in

let

pip

ing

(c.

s.)

Bis

ulfi

de

co

rro

sio

nH

igh

ve

loci

tyÑ

43

ft/

s: l

ow

wa

ter

inje

ctio

n.

No

n-s

ymm

etr

ica

l p

ipin

g l

ayo

ut;

14

" p

ipe

. In

sta

lled

Allo

y 8

00

el

bow

s in

inle

t un

til p

ipe

repl

aced

with

825

. B

oth

inle

t an

d ou

tlet

pipi

ng n

ow 8

25.

A/C

ou

tlet

pip

ing

(c.

s.)

Bis

ulfi

de

co

rro

sio

nH

igh

ve

loci

tyÑ

34

ft/

s: l

ow

wa

ter

inje

ctio

n.

A/C

tu

be

s (c

.s.)

Tu

be

th

inn

ing

Ñb

isu

lfid

e

corr

osi

on

Allo

y 8

00

re

pla

cem

en

t; h

ave

fo

un

d p

ittin

g.

Als

o

exp

eri

en

ced

le

aks

fro

m e

xte

rna

l co

rro

sio

n.

Th

irty

-six

tu

be

le

aks

fro

m 1

96

7 Ð

19

83

. In

sta

lled

fe

rru

les

in

19

83

. A

fte

r 1

mo

nth

, o

ne

tu

be

le

ak.

R

etu

be

d w

ith A

lloy

80

0

tub

es

in 1

98

4 Ð

19

85

.

Fra

ctio

na

tor

fee

d/e

fflu

en

t ex

chan

ger

Tu

be

co

rro

sio

nB

isu

lfid

e c

orr

osi

on

fro

m w

ate

r in

fra

ctio

na

tor

fee

d;

retu

be

d

in 1

981

with

Allo

y 80

0.S

tart

up

19

67

with

ca

rbo

n s

tee

l tu

be

s. E

igh

ty-s

ix U

-be

nd

tu

be

s p

lug

ge

d b

y 1

97

6 l

ea

k. L

ea

k in

19

78

ca

use

d o

uta

ge

. R

etu

be

d

with

ca

rbo

n s

tee

l in

19

81

. R

etu

be

d w

ith A

lloy

80

0 in

19

83

.

Tri

m c

oo

ler

Bis

ulfi

de

co

rro

sio

n;

als

o

wa

ter

sid

e a

tta

ckS

usp

ect

oxy

ge

n i

nvo

lve

d.

Film

ing

am

ine

ad

de

d i

n 1

99

4.

Wa

ter

sid

e c

orr

osi

on

by

MIC

; p

oo

r ch

lori

na

tion

;

10

mp

y Ð

15

mp

y o

n c

ha

nn

els

. R

etu

be

d i

n 1

98

3 w

ith c

arb

on

ste

el.

CH

TUA

/C t

ub

es

(c.s

.)B

isu

lfid

e c

orr

osi

on

13

% b

isu

lfid

e;

25

ft/

s.F

ailu

re 7

7 d

ays

(1

96

7).

Tri

ed

12

Cr,

17

Cr,

an

d

18

-8

up

gra

de

s. A

lloy

80

0 i

nst

alle

d i

n 1

98

1.

17

% m

ax

(13

%

ave

rag

e)

am

mo

niu

m b

isu

lfid

e;

25

ft/

s.

A/C

o

utle

t p

ipin

gB

isu

lfid

e c

orr

osi

on

Sw

irlin

g g

roo

ved

pa

tte

rn.

Up

gra

de

d t

o A

lloy

80

0.

No

w 8

25

be

cau

se o

f P

TA

.

Co

mp

ress

or

suct

ion

lin

eB

isu

lfid

e c

orr

osi

on

10

% Ð

15

% b

isu

lfid

e.

Ra

tes

of

10

0 m

py

Ð 5

00

mp

y in

tu

rbu

len

t flo

w b

eh

ind

we

ld p

rotr

usi

on

. H

igh

NH

3 i

n g

as

phas

e re

duce

d by

low

erin

g w

ash

wat

er t

empe

ratu

re.

Pro

ble

m c

au

sed

by

hig

h a

mm

on

ia i

n v

ap

or

cau

sin

g b

isu

lfid

e

salt

de

po

sit.

P

inh

ole

ju

st b

eh

ind

we

ld j

ett

ed

on

to o

pe

rato

r re

sulti

ng

in

sh

ut

do

wn

. N

ea

r m

iss.

Co

rro

sio

n i

n c

olu

mn

to

lera

ble

at

10

% b

ut

no

t 1

5%

bis

ulfi

de

.

Hyd

rog

en

re

cycl

e

dead

leg

Am

mo

niu

m c

hlo

rid

e

corr

osi

on

De

ad

le

g a

llow

ed

co

nd

en

sate

to

fo

rm.

Ga

s o

il H

T h

ydro

ge

n.

12

Cr

de

ad

le

g 6

" p

ipe

la

rge

en

ou

gh

to

a

llow

cir

cula

tion

an

d s

alt

acc

um

ula

tion

.

OH

TUS

ep

ara

tor

liqu

id/r

ea

cto

r e

fflu

en

tA

mm

on

ium

ch

lori

de

co

rro

sio

nN

ot

en

ou

gh

in

term

itte

nt

wa

sh w

ate

r. W

ett

ed

bu

t n

on

-flo

win

g d

ep

osi

ts.

Op

era

tion

al

de

fect

. S

olid

de

po

sits

pu

she

d d

ow

nst

rea

m i

nto

e

xch

an

ge

r.

30

0 m

py

Ð 4

00

mp

y.

No

w 6

25

ove

rla

id.

A/C

tu

be

sN

o c

orr

osi

on

No

co

rro

sio

n b

eca

use

of

3-p

ha

se f

low

.N

o s

olid

s d

ep

osi

ts;

1%

bis

ulfi

de

in

se

pa

rato

r; 2

6 f

t/s.

RH

CU

A/C

tu

be

sA

mm

on

ium

flu

ori

de

/ch

lori

de

co

rro

sio

nU

nd

erd

ep

osi

t co

rro

sio

n.

Ina

de

qu

ate

wa

sh w

ate

r (d

istr

ibu

tion

).F

luo

rid

es

fro

m h

ydro

cra

ckin

g c

ata

lyst

. N

ee

ds

re-f

luo

rin

atio

n.

U

pg

rad

ed

to

typ

e 4

10

sta

inle

ss s

tee

l (1

98

4).

D

esi

gn

ed

fo

r a

nn

ula

r flo

w.

We

irs

in i

nle

t h

ea

de

r.

Co

rro

sio

n i

n t

ub

es

< 1

m

py

sin

ce 1

98

4.

Wa

ter

inje

ctio

n p

oin

tC

orr

osi

on

-ero

sio

n

ad

jace

nt

to i

nle

tIn

suff

icie

nt

wa

sh w

ate

r. 1

in

. m

eta

l lo

ss i

n 1

0 y

ea

rs.

Se

vere

att

ack

(6

5 m

py)

. R

ed

esi

gn

ed

with

sp

ray

no

zzle

an

d

we

ld o

verl

aid

with

In

con

el

62

5.

Inst

alle

d s

tatic

mix

er.

A/C

in

let

pip

ing

A

mm

on

ium

flu

ori

de

co

rro

sio

n4

00

mils

in

20

ye

ars

. N

H4F

or

NH

4Cl

; to

o h

ot

for

bis

ulfi

de

.R

ep

lace

d i

n 1

97

8.

La

ste

d t

o 1

99

7.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

15

Tabl

e6—

Com

pila

tion

of In

form

atio

n fr

om P

lant

Inte

rvie

ws

(Con

tinue

d)

Pla

nt

Un

itL

oca

tio

n o

f C

orr

osi

on

Typ

e o

f C

orr

osi

on

Cau

sati

ve F

acto

rsC

om

men

ts

WH

CU

A/C

inl

et p

ipin

gT

hinn

ing

at e

xcha

nger

in

let.

Ba

lan

ced

pip

ing

Hig

h n

itro

ge

n f

ee

d;

Kp

> 0

.45

; 8

% b

isu

lfid

e.

Aft

er

first

exp

eri

en

ce w

ith c

arb

on

ste

el,

use

d 1

2 C

r.

Inte

rmitt

en

t w

ash

to

pre

ven

t sa

lt fo

ulin

g.

Use

in

hib

itor.

A/C

tu

be

sR

ecu

rrin

g p

rob

lem

s w

ith

carb

on

ste

el

tub

es;

b

isu

lfid

e c

orr

osi

on

8%

bis

ulfi

de

; <

20

ft/

s ve

loci

ty i

n t

ub

es;

ca

rbo

n s

tee

l tu

be

s.19

68 Ð

198

0 un

sche

dule

d re

plac

emen

t; a

fter

198

0 w

ent

to

5-y

ea

r sc

he

du

le.

Fo

ur

lea

ks s

ince

. F

ou

r ye

ars

min

imu

m,

ave

rag

e >

5 y

ea

rs.

Ero

sio

n a

t o

utle

t o

f la

st p

ass

. U

se t

ype

3

04

fe

rru

lesÑ

pitt

ed

in

an

nu

lus

du

e t

o c

hlo

rid

es.

A/C

ou

tlet

he

ad

er

Bis

ulfi

de

co

rro

sio

nR

eq

uir

ed

re

pla

cem

en

t o

f h

ea

de

rs.

Un

ba

lan

ced

pip

ing

.W

ash

wa

ter

ad

just

ed

acc

ord

ing

to

bis

ulfi

de

co

nte

nt,

nitr

og

en

co

nte

nt,

an

d f

ee

d r

ate

. 7

5%

no

n-v

ap

ori

zin

g.

A/C

ou

tlet

pip

ing

No

se

rio

us

corr

osi

on

Ñn

o

rep

lace

me

nt

So

me

Ls

ove

rla

id w

ith 3

09

/18

-8.

In

dic

ate

s e

rosi

on

-co

rro

sio

n a

t >

20

ft/

s.S

tric

t co

ntr

ol

of

bis

ulfi

de

in

acc

um

ula

tor

(8%

).

W(2

)H

CU

A/C

ou

tlet

pip

ing

Se

vere

bis

ulfi

de

e

rosi

on

Ñfir

eIn

cre

ase

d v

elo

city

be

cau

se o

f 1

0%

in

cre

ase

in

liq

uid

fe

ed

.U

nsy

mm

etr

ica

l p

ipin

g w

ith i

mp

ing

em

en

t d

ue

to

sh

arp

tu

rns.

V

elo

city

ch

an

ge

fro

m 2

0 f

t/s

Ð 2

7 f

t/s.

XH

TUA

ll ca

rbo

n s

tee

l e

qu

ipm

en

tN

o s

eri

ou

s co

rro

sio

n

pro

ble

ms

Kp =

0.0

5; 2

% Ð

3%

bis

ulfid

e; 1

5 ft

/s t

ube

velo

city

.B

alan

ced

pipi

ng;

annu

lar

flow

; st

ripp

ed s

our

wat

er w

ash,

60%

va

pori

zed;

low

chl

orid

es.

EE

HC

UA

/C t

ub

es

Bis

ulfi

de

co

rro

sio

nT

ub

e t

hin

nin

g.

Fe

rru

les

inst

alle

d o

n i

nle

t o

f ro

w 2

.

Thi

nnin

g of

inl

ets

to 3

rd a

nd 4

th r

ows.

N

ever

had

tub

e p

lug

gin

g.

Ca

rbo

n s

tee

l tu

be

s, A

lloy

80

0 f

err

ule

s.

Ori

gin

al

top

ro

w t

ype

3

04

sta

inle

ss s

tee

lÑch

an

ge

d t

o A

lloy

80

0.

Tu

be

life

ha

s va

rie

d f

rom

5 Ð

16

ye

ars

. A

ll tu

be

s ch

an

ge

d t

o A

lloy

80

0 i

n

19

97

. Kp =

0.1

2 Ð

0.2

; b

isu

lfid

e =

4%

at

sep

ara

tor.

A/C

pip

ing

No

sig

nifi

can

t co

rro

sio

nL

ow

bis

ulfi

de

; V

elo

citie

s ~

30

ft/

s.

IH

CU

All

carb

on

ste

el

eq

uip

me

nt

No

se

rio

us

corr

osi

on

p

rob

lem

sU

nb

ala

nce

d p

ipin

g;

bis

ulfi

de

~ 4

%.

Ve

loci

ties

no

t p

rovi

de

d.

Kp =

0.2

3.

YH

CU

A/C

tu

be

sB

isu

lfid

e c

orr

osi

on

Kps

incr

ea

sed

fro

m 0

.09

to

0.3

2 o

ver

time

. V

elo

citie

s in

1

0 f

t/s

Ð 1

5 f

t/s

ran

ge

. 4

% b

isu

lfid

e.

Ca

rbo

n s

tee

lÑin

itia

l p

rob

lem

s tu

be

en

d c

orr

osi

on

. P

erf

ora

tion

ca

me

la

ter.

R

etu

be

d w

ith c

arb

on

ste

el

ap

pro

xim

ate

ly e

very

5

yea

rs.

Pla

n t

o u

pg

rad

e t

o A

lloy

82

5 o

n n

ext

re

tub

e.

Fo

rme

rly

sing

le w

ash

wat

er p

oint

Ñno

w m

ultip

le.

Poo

r w

ater

dis

trib

utio

n.

A/C

pip

ing

B

isu

lfid

e c

orr

osi

on

in

let

he

ad

ers

Ve

loci

ties

in i

nle

t h

ea

de

r sy

ste

m o

f 1

7 f

t/s

Ð 2

7 f

t/s.

Un

ba

lan

ced

in

let

an

d o

utle

t p

ipin

g.

> 2

0 m

py.

P

oss

ible

ch

lori

de p

robl

em d

ue t

o in

adeq

uate

wat

er in

inle

t.

Pip

ing

upgr

aded

to

Allo

y 82

5 on

sm

all

size

s.

Allo

y 62

5 ov

erla

y on

la

rge

ca

rbo

n s

tee

l p

ipin

g.

Pos

sibl

e ch

lori

de p

robl

em d

ue t

o in

adeq

uate

wat

er.

HP

Se

pa

rato

rB

isu

lfid

e c

orr

osi

on

-e

rosi

on

Imp

ing

em

en

t a

tta

ck.

Sta

inle

ss s

tee

l im

pin

ge

me

nt

pla

te o

f a

de

qu

ate

siz

e.

ZH

TUA

/C t

ub

es

Bis

ulfi

de

co

rro

sio

nÑ

thin

nin

g

lead

ing

to f

ire

Typ

e 4

10

sta

inle

ss s

tee

l tu

be

s. F

ailu

re a

fte

r 7

ye

ars

.

2

0 m

py

Ð 2

5 m

py.

B

ott

om

tu

be

gro

ovi

ng

. B

isu

lfid

es

conc

entr

ated

to

19%

in

oute

r ce

lls.

Poo

r w

ater

d

istr

ibu

tion

.

No

n-s

ymm

etr

ica

l p

ipin

g a

nd

sin

gle

po

int

wa

ter

inje

ctio

n.

T

ubes

upg

rade

d to

Allo

y 82

5.

Thr

ough

put

incr

ease

d be

fore

fa

ilure

.

A/C

pip

ing

No

co

rro

sio

nA

lloy

82

5.

He

ad

er

bo

xes

an

d o

utle

t p

ipin

g v

eri

fied

82

5.

No

ve

loci

ties

ava

ilab

le.

AA

HTU

A/C

ou

tlet

pip

ing

Co

rro

sio

n-e

rosi

on

of

carb

on

ste

el

pip

ing

Bis

ulfi

de

co

rro

sio

n.

Re

pla

ced

elb

ow

s w

ith A

lloy

80

0.

Ve

loci

ties

18

ft/

s Ð

32

ft/

s.

With

in 2

yea

rs u

pgra

ded

all

pipe

to

Allo

y 80

0.

Cha

nged

to

Allo

y 8

25

be

cau

se o

f p

oly

thio

nic

aci

d c

rack

ing

Ñin

sta

llatio

n

err

or.

B

ala

nce

d i

nle

t a

nd

ou

tlet.

1

4%

Ð 1

5%

bis

ulfi

de

.

A/C

tu

be

sN

o c

orr

osi

on

Allo

y 8

00

tu

be

s a

nd

Allo

y 8

00

bo

xes.

Ori

gin

al

tu

be

s A

lloy

80

0.

Ve

loci

ties

18

ft/

s Ð

20

ft/

s.

Eff

lue

nt/

fra

ctio

na

tor

feed

exc

hang

erA

mm

on

ium

ch

lori

de

d

ep

osi

tion

Co

rro

sio

n u

pst

rea

m o

f w

ate

r in

ject

ion

. A

dd

ed

in

term

itte

nt

inje

ctio

n p

oin

ts.

Me

tallu

rgy

up

gra

de

d f

rom

17

Cr

to 2

205

dupl

ex S

S.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

16 API P

UBLICATION

932-A

Tabl

e6—

Com

pila

tion

of In

form

atio

n fr

om P

lant

Inte

rvie

ws

(Con

tinue

d)

Pla

nt

Un

itL

oca

tio

n o

f C

orr

osi

on

Typ

e o

f C

orr

osi

on

Cau

sati

ve F

acto

rsC

om

men

ts

HP

LT s

epar

ator

wat

er

ou

tlet

pip

eB

isu

lfid

e c

orr

osi

on

Le

vel

con

tro

l p

rob

lem

. S

tra

tifie

d f

low

with

co

nce

ntr

ate

d

bis

ufid

e a

t st

ee

l su

rfa

ce 8

% Ð

28

% b

isu

lfid

e i

n a

qu

eo

us

ph

ase

, n

orm

ally

15

% Ð

16

%.

Sw

irl p

atte

rn o

f at

tack

ter

min

ated

at

wel

d pr

otru

sion

whi

ch

act

ed

as

a f

low

str

aig

hte

ne

r. L

oca

l a

tta

ck i

mm

ed

iate

ly a

fte

r w

eld.

CS

rep

lace

d w

ith A

lloy

800.

HP

LT

se

pa

rato

r o

utle

t p

ipe

Ñh

ydro

carb

on

Bis

ulfi

de

co

rro

sio

n d

ue

to

w

ater

ent

rain

men

t W

ate

r ca

rryo

ver

fro

m h

igh

le

vel

in s

ep

ara

tor.

Ext

rem

e

corr

osi

on

up

to

1-i

n.

loss

in

on

e w

ee

k.S

eve

re c

orr

osi

on

of

we

ldo

let.

De

tect

ed

by

rad

iog

rap

hy

be

fore

fa

ilure

occ

urr

ed

. N

ea

r m

iss.

LP

LT

se

pa

rato

r o

ff

ga

s p

ipin

gIm

pin

ge

me

nt

att

ack

S

ulfu

r d

ep

osi

t b

uild

up

ca

use

d r

est

rict

ed

flo

w a

nd

ero

sio

n-

corr

osi

on

. R

eq

uir

es

liqu

id c

arr

yove

r.P

rob

ab

ly e

rosi

on

-co

rro

sio

n f

rom

dro

ple

t im

pin

ge

me

nt.

By-

pa

ss d

ea

d l

eg

sB

isu

lfid

e c

orr

osi

on

(Se

e P

lan

t C

fo

r sa

me

pro

ble

m.)

C

orr

osi

ve d

ep

osi

ts

form

ed

by

circ

ula

tion

in

op

en

de

ad

le

g.

Lo

caliz

ed

att

ack

ove

r 1

ft2

are

a.

Eq

uip

me

nt

do

wn

stre

am

of

sep

ara

tors

Pro

ble

ms

cau

sed

by

bis

ulfi

de

s a

nd

cya

nid

es

Occ

urs

w

he

re s

alt

con

cen

tra

tion

ta

kes

pla

ce.

S

trip

pe

r o

verh

ea

d r

ece

ive

r va

po

r lin

e h

ad

to

be

re

pla

ced

at

6

mo

nth

in

terv

als

.

BB

HC

UF

ee

d/e

fflu

en

t ex

chan

ger

Ch

lori

de

pitt

ing

of

tub

es

on

fe

ed

sid

eT

ype

32

1 s

tain

less

ste

el

tub

es

pitt

ed

du

e t

o C

l in

fe

ed

.

Fa

il e

very

2 y

ea

rs.

Up

gra

de

d t

o 2

54

SM

O.

No

de

salte

r.

Eff

lue

nt

pip

ing

u

pst

rea

m A

/CU

nd

er

de

po

sit

att

ack

Am

mo

niu

m c

hlo

rid

e.

Co

rro

sio

n a

rou

nd

wa

ter

inje

ctio

n p

oin

t a

nd

do

wn

stre

am

pip

ing

.

A/C

tu

be

sC

arb

on

ste

el

tub

es

Hig

h v

elo

citie

s u

p t

o 6

0 f

t/s

in i

nle

ts.

Ve

loci

ties

red

uce

d

to 1

5 f

t/s

Ð 1

7 f

t/s.

Un

ba

lan

ced

pip

ing

.9

% b

isu

lfid

e m

ax.

; 3

.5%

Ð 4

% a

vg.

Ch

an

ge

d f

rom

4-p

ass

to

2-p

ass

to

re

du

ce v

elo

citie

sÑso

lve

d p

rob

lem

. W

ate

r a

ccu

mu

latio

n i

n l

ow

er

pre

ssu

re d

rop

tu

be

ba

nks

ca

use

d

seal

ing

and

incr

ease

d flo

w in

ope

n ba

nks.

CC

HC

UE

fflu

en

t p

ipin

g f

rom

e

xch

an

ge

rs t

o

sep

ara

tor

Bis

ulfi

de

co

rro

sio

n6

% Ð

6.8

% b

isu

lfid

e.

Ve

loci

ties

up

to

32

ft/

s.E

xtra

he

avy

CS

re

pla

ced

eve

ry 8

ye

ars

. U

pg

rad

ed

to

Allo

y 8

25

be

cau

se o

f in

cre

asi

ng

N i

n f

ee

d.

1st

sta

ge

eff

lue

nt

trim

co

ole

rB

isu

lfid

e c

orr

osi

on

~ 6

.5%

bis

ulfi

de

.U

-tu

be

exc

ha

ng

er.

Up

gra

de

d f

rom

CS

to

mo

ne

l a

fte

r 2

ye

ars

.

1st

sta

ge

eff

lue

nt/

fee

d

exch

ange

rB

isu

lfid

e c

orr

osi

on

~ 6

.5%

b

isu

lfid

e V

elo

city

38

ft/

s Ð

40

ft/

s.O

rig

ina

l C

S >

10

ye

ars

life

. U

pg

rad

ed

to

3R

E6

0 d

up

lex

allo

y.

2n

d s

tag

e e

fflu

en

t fe

ed

exc

ha

ng

er

Bis

ulfi

de

co

rosi

on

Ch

an

ge

d t

ub

es

fro

m 1

7 C

r to

3R

E6

0 a

fte

r 1

5 y

ea

rs.

EH

TU A

/C t

ub

es

Allo

y 8

00

Ñfo

un

d p

ittin

g

aft

er

7 y

ea

rsO

rig

ina

l 8

00

tu

be

s a

nd

he

ad

er

bo

xes.

Fo

un

d p

ittin

g a

nd

d

ela

min

atio

ns.

U

pg

rad

ed

to

Allo

y 8

25

. S

usp

ect

ch

lori

de

s ca

use

d p

ittin

g.

Tu

be

ve

loci

ties

12

ft/

s Ð

17

ft/

s.

A/C

pip

ing

Co

rro

sio

n-e

rosi

on

of

outle

t el

bow

res

ultin

g in

fir

e

Ca

rbo

n s

tee

l p

ipin

g.

In

let

pip

ing

ba

lan

ced

; o

utle

t p

ipin

g

un

ba

lan

ced

. B

isu

lfid

e <

8%

.

Ve

loci

ties

9 f

t/s

Ð 2

6 f

t/s.

Un

it m

od

ifie

d t

o i

ncl

ud

e H

PH

T s

ep

ara

tor

for

pro

cess

re

aso

ns.

F

ailu

re o

ccu

rre

d a

fte

r 9

mo

nth

s.

Fa

ilure

occ

urr

ed

in

ou

tlet

elb

ow

aft

er

inst

alla

tion

of

HP

HT

se

pa

rato

r.

Re

du

ced

wa

ter

wa

sh,

incr

ea

sed

bis

ulfi

de

, a

nd

un

ba

lan

ced

fa

n o

pe

ratio

n.

In

cre

ase

d H

2.

Eff

lue

nt/

fee

d

exch

ange

rN

o c

orr

osi

on

Op

era

tes

at

40

0+

with

no

sa

lt d

ep

osi

ts.

No

wa

ter

wa

sh o

n th

is i

tem

.P

lan

to

in

cre

ase

op

era

tion

to

50

0 d

eg

ree

s.

Co

nce

rn w

ith

na

ph

the

nic

aci

ds.

E

fflu

en

t h

as

5 p

pm

Ð 1

0 p

pm

ch

lori

de

s.

UH

TUA

/C t

ub

es

No

co

rro

sio

nH

2S

in

exc

ess

of

am

mo

nia

. 6

% b

isu

lfid

e.

V

elo

city

10

ft/

s.N

o p

lug

gin

g.

Dis

con

tinu

ed

up

stre

am

wa

ter

wa

sh.

Am

mo

nia

lo

w;

tem

pe

ratu

res

hig

h.

Sin

gle

po

int

wa

ter

inje

ctio

nÑ

tota

lly

vap

ori

zed

. C

arb

on

ste

el

tub

es.

5

Cr

cha

nn

els

. F

ou

r b

ays

to

tw

o a

fte

r a

dd

itio

n o

f H

PH

T s

ep

ara

tor

du

e t

o r

ed

uce

d

thro

ug

hp

ut.

A/C

pip

ing

No

co

rro

sio

nC

urr

en

tly c

arb

on

ste

elÑ

up

gra

din

g t

o A

lloy

82

5.

U

nb

ala

nce

d o

utle

t p

ipin

g.

RE

AC

in

let

velo

city

21

ft/

s.P

lan

nin

g a

dd

itio

n o

f H

PH

T s

ep

ara

tor.

K p

= 0

.8.

Allo

y 8

25

p

ipin

g f

rom

in

ject

ion

po

int

to s

ep

ara

tor.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

17

Tabl

e7—

Com

pila

tion

of In

form

atio

n fr

om P

lant

Inte

rvie

ws—

Air

Coo

ler T

ubes

Pla

nt

Un

itL

oca

tio

n o

f C

orr

osi

on

Typ

e o

f C

orr

osi

on

Cau

sati

ve F

acto

rsC

om

men

ts

BH

CU

Air

co

ole

r tu

be

sB

isu

lfid

e c

orr

osi

on

La

ck o

f w

ash

wa

ter.

Inst

alle

d 1

97

1;

faile

d i

n 6

ye

ars

(fir

e);

re

pla

ced

with

Allo

y 8

00

.

QH

CU

A/C

tu

be

s (c

.s.)

Tub

e th

inn

ing

Ñb

isu

lfid

e

corr

osi

on

Allo

y 8

00

re

pla

cem

en

t; h

ave

fo

un

d p

ittin

g.

A

lso

expe

rien

ced

leak

s fr

om e

xter

nal

corr

osi

on

.

36

tu

be

le

aks

fro

m 1

96

7 Ð

19

83

. In

sta

lled

fe

rru

les

in 1

98

3.

Aft

er

1

mo

nth

, 1

tu

be

le

ak.

R

etu

be

d w

ith A

lloy

80

0 t

ub

es

in 1

98

4 Ð

19

85

.

CH

TUA

/C t

ub

es

(c.s

.)B

isu

lfid

e c

orr

osi

on

13

% b

isu

lfid

e;

25

ft/

s.F

ailu

re 7

7 d

ays

(1

96

7).

Tri

ed

12

Cr,

17

Cr,

an

d 1

8-8

up

gra

de

s. A

lloy

80

0 i

nst

alle

d i

n 1

98

1.

17

% m

ax

(13

% a

vera

ge

) a

mm

on

ium

bis

ulfi

de

; 2

5 f

t/s.

OH

TUA

/C t

ub

es

No

co

rro

sio

nN

o c

orr

osi

on

be

cau

se o

f 3

-ph

ase

flo

w.

No

so

lids

de

po

sits

; 1

% b

isu

lfid

e i

n s

ep

ara

tor;

26

ft/

s.

RH

CU

A/C

tu

be

sA

mm

on

ium

flu

ori

de

/ch

lori

de

co

rro

sio

n

F c

on

de

nsa

tion

te

mp

era

ture

> c

hlo

rid

es.

Flu

ori

de

s fr

om

hyd

rocr

ack

ing

ca

taly

st.

Ne

ed

s re

-flu

ori

na

tion

.

Up

gra

de

d t

o t

ype

41

0 s

tain

less

ste

el

(19

84

).

De

sig

ne

d f

or

an

nu

lar

flow

. W

eir

s in

in

let

he

ad

er.

C

orr

osi

on

in

tu

be

s <

1 m

py

sin

ce 1

98

4.

WH

CU

A/C

tu

be

sR

ecu

rrin

g p

rob

lem

s w

ith c

arbo

n st

eel

tub

es;

bis

ulfi

de

co

rro

sio

n

8%

bis

ulfi

de

; <

20

ft/

s ve

loci

ty i

n t

ub

es;

ca

rbo

n s

tee

l tu

be

s.1

96

8 Ð

19

80

un

sch

ed

ule

d r

ep

lace

me

nt;

aft

er

19

80

we

nt

to 5

ye

ar

sch

ed

ule

. F

ou

r le

aks

sin

ce.

Fo

ur

yea

rs m

inim

um

, a

vera

ge

> 5

ye

ars

. E

rosi

on

-co

rro

sio

n a

t o

utle

t o

f la

st p

ass

. U

se t

ype

30

4 f

err

ule

sÑp

itte

d

in a

nn

ulu

s d

ue

to

ch

lori

de

s.

XH

TUA

ll ca

rbo

n s

tee

l e

qu

ipm

en

tN

o s

eri

ou

s co

rro

sio

n

pro

ble

ms

Kp =

0.0

5;

2%

Ð 3

% b

isu

lfid

e;

15

ft/

s tu

be

ve

loci

ty.

Bal

ance

d pi

ping

; an

nula

r flo

w;

stri

pped

sou

r w

ater

was

h, 6

0%

vapo

rize

d; lo

w c

hlor

ides

.

EE

HC

UA

/C t

ub

es

Bis

ulfi

de

co

rro

sio

nT

ub

e t

hin

nin

g.

Fe

rru

les

inst

alle

d o

n i

nle

t o

f ro

w 2

. T

hinn

ing

of in

lets

to

3rd

and

4th

row

s.

Ne

ver

ha

d t

ub

e p

lug

gin

g.

Ca

rbo

n s

tee

l tu

be

s, A

lloy

80

0 f

err

ule

s.

Ori

gin

al

top

ro

w t

ype

30

4

sta

inle

ss s

tee

lÑch

an

ge

d t

o A

lloy

80

0.

Tu

be

life

ha

s va

rie

d f

rom

5 Ð

16 y

ears

. A

ll tu

bes

chan

ged

to A

lloy

800

in 1

997.

Kp =

0.1

2 Ð

0.2

; b

isu

lfid

e =

4%

at

sep

ara

tor.

IH

CU

All

carb

on

ste

el

eq

uip

me

nt

No

se

rio

us

corr

osi

on

p

rob

lem

sU

nb

ala

nce

d p

ipin

g;

bis

ulfi

de

~ 4

%.

Ve

loci

ties

no

t p

rovi

de

d.

Kp

= 0

.23.

YH

CU

A/C

tu

be

sB

isu

lfid

e c

orr

osi

on

Kps

incr

ea

sed

fro

m 0

.09

to

0.3

2 o

ver

time

. V

elo

citie

s in

10

ft/

s Ð

15

ft/

s ra

ng

e.

7

% Ð

8%

bis

ulfi

de

.

Ca

rbo

n s

tee

lÑin

itia

l p

rob

lem

s tu

be

en

d c

orr

osi

on

. P

erf

ora

tion

ca

me

la

ter.

R

etu

be

d w

ith c

arb

on

ste

el

ap

pro

xim

ate

ly e

very

5 y

ea

rs.

Pla

n

to u

pgra

de t

o A

lloy

825

on n

ext

retu

be.

For

mer

ly s

ingl

e w

ash

wat

er

poin

tÑno

w m

ultip

le.

Poo

r w

ater

dis

trib

utio

n.

ZH

TUA

/C t

ub

es

Bis

ulfi

de

co

rro

sio

nÑ

thin

nin

g

lea

din

g t

o f

ire

Typ

e 4

10

sta

inle

ss s

tee

l tu

be

s. F

ailu

re a

fte

r 7

ye

ars

. 2

0 m

py

Ð 2

5 m

py.

B

ott

om

tu

be

g

roo

vin

g.

Bis

ulfi

de

s co

nce

ntr

ate

d t

o 1

9%

in

o

ute

r ce

lls.

Po

or

wa

ter

dis

trib

utio

n.

No

n-s

ymm

etr

ica

l p

ipin

g a

nd

sin

gle

po

int

wa

ter

inje

ctio

n.

Tu

be

s u

pg

rad

ed

to

Allo

y 8

25

. T

hro

ug

hp

ut

incr

ea

sed

be

fore

fa

ilure

.

AA

HTU

A/C

tu

be

sN

o c

orr

osi

on

Allo

y 8

00

tu

be

s a

nd

Allo

y 8

00

bo

xes.

Ori

gin

al

tu

be

s A

lloy

80

0.

Ve

loci

ties

18

ft/

s Ð

20

ft/

s.

BB

HC

UA

/C t

ub

es

Ca

rbo

n s

tee

l tu

be

sH

igh

ve

loci

ties

up

to

60

ft/

s in

in

lets

.

Ve

loci

ties

red

uce

d t

o 1

5 f

t/s

Ð 1

7 f

t/s.

U

nb

ala

nce

d p

ipin

g.

9%

bis

ulfi

de

ma

x.;

3.5

% Ð

4%

avg

. C

ha

ng

ed

fro

m

4-p

ass

to

2-p

ass

to

re

du

ce v

elo

citie

sÑso

lve

d p

rob

lem

. W

ate

r a

ccu

mu

latio

n i

n l

ow

er

pres

sure

dro

p tu

be b

anks

cau

sed

seal

ing

and

incr

ease

d flo

w i

n op

en

ba

nks

.

EH

TU A

/C t

ub

es

Allo

y 8

00

Ñfo

un

d

pitt

ing

aft

er

7 y

ea

rsO

rigi

nal

800

tube

s an

d he

ader

box

es.

Fou

nd

pitt

ing

an

d d

ela

min

atio

ns.

U

pg

rad

ed

to

Allo

y 8

25

. S

usp

ect

ch

lori

de

s ca

use

d p

ittin

g.

T

ub

e v

elo

citie

s 1

2 f

t/s

Ð 1

7 f

t/s.

UH

TUA

/C t

ub

es

No

co

rro

sio

nH

2S

in

exc

ess

of

am

mo

nia

. 6

% b

isu

lfid

e.

V

elo

city

10

ft/

s.N

o pl

uggi

ng.

Dis

cont

inue

d up

stre

am w

ater

was

h.

Am

mon

ia l

ow;

tem

pe

ratu

res

hig

h.

Sin

gle

po

int

wa

ter

inje

ctio

nÑ

tota

lly v

ap

ori

zed

.

Ca

rbo

n s

tee

l tu

be

s.

5 C

r ch

an

ne

ls.

Fo

ur

ba

ys t

o t

wo

aft

er

ad

diti

on

o

f H

TH

P s

ep

ara

tor

du

e t

o r

ed

uce

d t

hro

ug

hp

ut.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

18 API P

UBLICATION

932-A

Tabl

e8—

Air

Coo

ler T

ube

Exp

erie

nce

from

Sur

vey

Pla

nt

Un

itO

rig

ina

l M

ate

ria

lC

orr

osi

on

rat

e (l

ife

)

Ma

teri

al

Up

gra

de

(lif

e to

dat

e)E

rosi

on

-C

orr

osi

on

Sa

lt

dep

osi

tT

hin

nin

gO

ther

Cau

se o

f C

orr

osi

on

HS

co

nce

ntr

atio

n

(%)

Ve

loc

ity

(ft

/s)

K pB

HC

UC

S

18

mp

y

(6 y

ea

rs)

Allo

y 80

0

(1

7 y

ea

rs)

XX

Insu

ffic

ient

was

h w

ater

5% Ð

22

%

(ca

lcu

late

d)

17 Ð

19

0.2

QH

CU

CS

Allo

y 8

00

(1

4

yea

rsÑ

pitt

ing

)X

Bis

ulfi

de

co

rro

sio

n19

0.0

14

Ð 0

.04

0

CH

TUC

S5

00

mp

y

(77

da

ys)

Va

rio

us

to

Allo

y 82

5a

(9 y

ea

rs)

XH

igh

bis

ulfi

de

s; h

igh

ve

loci

ty1

3 a

vera

ge

;

1

7 m

ax.

250

.2 Ð

0.5

OH

TUC

SN

on

eN

oLo

w b

isul

fide

1%26

0.2

2

RH

CU

CS

200

mpy

(6 m

on

ths)

12

Crb

XX

Flu

ori

de

s/ch

lori

de

sÑh

igh

d

ep

osi

tion

te

mp

era

ture

.

Insu

ffic

ient

was

h w

ater

WH

CU

CS

Use

30

4 S

S

ferr

ule

sc

XH

igh

exi

t ve

loci

ty8%

Cu

rre

ntly

< 2

0>

0.4

5

XH

TUC

SS

ince

19

72

No

ne

No

Fa

vora

ble

op

era

ting

p

ara

me

ters

2% Ð

3%

150

.05

EE

HC

UC

S w

/ 80

0 fe

rru

les;

3

04

to

p

row

20

mp

yA

lloy

800

(new

)X

No

XH

igh

velo

city

and

low

w

ater

was

h4%

30

in

; 2

0 o

ut

0.1

2 Ð

0.2

IH

CU

CS

N

on

e4%

0.2

3

YH

CU

CS

25

mp

y (e

st.)

CS

, A

lloy

82

5

next

upg

rade

X?

Pitt

ing

Low

wat

er r

ate;

poo

r d

istr

ibu

tion

7% Ð

8%

(r

ed

uce

d t

o 4

%)

11 Ð

15

in;

8

Ð 1

1 o

ut

0.3

4

ZH

TU1

7 C

r2

0 m

py

Ð 2

5 m

py

Allo

y 8

25

No

XH

igh

bis

ulfi

de

Up

to

19

% i

n

lo

w fl

ow c

ells

AA

HTU

Allo

y 8

00

13

ye

ars

No

co

rro

sio

n1

2 Ð

15

%1

8 Ð

20

BB

HC

UC

SN

on

eX

Hig

h t

ub

e e

nd

ve

loci

ties

caus

ed b

y un

even

flo

w3

.5%

Ð 4

%

avg

.; 9

% m

ax.

60

ft/

s

re

du

ced

to

1

5 f

t/s

Ð 1

7 f

t/s

EH

TUA

lloy

80

0A

lloy

82

5P

ittin

gd

Su

spe

ct c

hlo

rid

es

< 8

%12

Ð 1

76

.1

UH

TU C

SN

on

eN

oLo

w N

H3

3.9

ave

rag

e;

6.0

%10

~0

.8

No

tes:

d M

an

ufa

ctu

rin

g d

efe

cts.

Typ

e o

f C

orr

osi

on

a C

arb

on

ste

el

faile

d i

n 7

7 d

ays

.b R

ep

lace

d f

ailu

re w

ith C

S.

Ch

an

ge

d t

o 1

2C

r fo

r p

roce

ss r

ea

son

s.

c C

hlo

rid

e c

orr

osi

on

be

hin

d t

he

ou

tlet

ferr

ule

.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

19

Table 9—Compilation of Information from Plant Interviews—Piping Information

Plant Unit Location of Corrosion Type of Corrosion Causative Factors Comments

B HCU A/C outlet piping Grooving and channeling.

25 ft/s attack in turbulent area downstream of elbow. Now corroding at 50 mpy. Suspect oxygen and chlorides. No corrosion when oxygen and chlorides under control.

Original 18" replaced with 22" in 1994.

Q HCU A/C inlet piping (c.s.) Bisulfide corrosion. High velocityÑ43 ft/s: low water injection. Non-symmetrical piping layout; 14" pipe. Installed Alloy 800 elbows in inlet until pipe replaced with 825. Both inlet and outlet piping now 825.

A/C outlet piping (c.s.) Bisulfide corrosion. High velocityÑ34 ft/s: low water injection.

C HTU A/C outlet piping Bisulfide corrosion. Sequential addition of Alloy 600 to downstream piping. Failure occurred downstream of last upgrade in remaining carbon steel section.

Occurred in 1982. No info on parameters.

R HCU A/C piping NH4F corrosion. 100 mpy.

1 mil in 10 years. Ammonium fluoride. Replaced in 1978 and again in 1996.

W HCU A/C inlet piping Thinning at exchanger inlet. Balanced piping.

High nitrogen feed; Kp > 0.45; 8% bisulfide.

After first experience with carbon steel used 12 Cr. Intermittent wash to prevent salt fouling. Use inhibitor.

A/C outlet piping No serious corrosionÑno replacement.

Some ells overlaid with 309 / 18-8. Indicates erosion-corrosion at > 20 ft/s.

Strict control of bisulfide in accumulator (8%).

W(2) HCU A/C outlet piping Severe erosion-corrosionÑfire.

Increased velocity because of 10% increase in liquid feed.

Unsymmetrical piping with impingement due to sharp turns. Velocity change from 20 ft/s Ð 27 ft/s.

X HTU All carbon steel equipment

No serious corrosion problems.

Kp = 0.05; 2% Ð 3% bisulfide; 15 ft/s tube velocity.

Balanced piping; annular flow; stripped sour water wash, 60% vaporized; low chlorides.

EE HCU A/C piping No significant corrosion.

Low bisulfide; Velocities ~ 30 ft/s.

I HCU All carbon steel equipment

No serious corrosion problems.

Unbalanced piping; bisulfide ~ 4%. Velocities not provided. Kp = 0.23.

Y HCU A/C piping Bisulfide corrosion inlet headers.

Velocities in inlet header system of 17ft/s Ð 27 ft/s.

Unbalanced inlet and outlet piping. > 20 mpy. Possible chloride problem due to inadequate water in inlet. Piping upgraded to Alloy 825 on small sizes. Alloy 625 overlay on large carbon steel piping.

Z HTU A/C piping No corrosion. Alloy 825. Header boxes and outlet piping verified 825. No velocities available.

AA HTU A/C outlet piping Erosion-corrosion of carbon steel piping.

Bisulfide corrosion. Replaced elbows with Alloy 800. Velocities 18 ft/s Ð 32 ft/s. Within 2 years upgraded all pipe to Alloy 800. Changed to Alloy 825 because of polythionic acid crackingÑ installation error. Balanced inlet and outlet. 14% Ð 15% bisulfide.

HPLT separator outlet pipe

Bisulfide corrosion. Carbon steel pipe replaced with Alloy 800. 12% Ð 15% bisulfide in aqueous phase.

HPLT separator outlet pipeÑ hydrocarbon

Bisulfide corrosion due to water entrainment; near miss.

LPLT separator off gas piping

Impingement attack downstream of sulfur deposits.

Deposit buildup restricted flow and caused erosion-corrosion. Requires water carryover.

BB HCU Effluent piping upstream A/C

Under deposit attack.

Ammonium chloride. Corrosion around water injection point and downstream piping.

CC HCU Effluent piping from exchangers to separator

Bisulfide corrosion. 6% Ð 6.8 % bisulfide. Velocities up to 32 ft/s.

Extra heavy CS replaced every 8 years. Upgraded to Alloy 825 because of increasing N 2 in feed.

E HTU A/C piping Erosion-corrosion of outlet elbow resulting in fire.

Carbon steel piping. Inlet piping balanced; outlet piping unbalanced. Bisulfide < 8%. Velocities 9 ft/s Ð 26 ft/s.

Unit modified to include HTHP separator for process reasons. Failure occurred after 9 months. Failure occurred in outlet elbow after installation of HTHP separator. Reduced water wash, increased bisulfide, and unbalanced fan operation. Increased H2 .

U HTU A/C piping No corrosion. Currently carbon steelÑupgrading to Alloy 825. Unbalanced outlet piping. REAC inlet velocity 21 ft/s.

Planning addition of HPHT separator. Kp = 0.8. Alloy 825 piping from injection point to separator.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

20 API P

UBLICATION

932-A

Tabl

e10

—P

ipin

g E

xper

ienc

e fr

om S

urve

y

Pla

nt

Un

itIn

letÑ

(I)

Ou

tlet

Ñ(O

)O

rig

inal

M

ate

ria

lC

orr

osi

on

Rat

e (l

ife

)

Mat

eria

l U

pg

rad

e

(lif

e to

dat

e)B

alan

ced

Un

bal

ance

dS

ing

leM

ult

iple

Wat

er w

ash

rat

e (g

pm

)

HS

co

nce

ntr

atio

n

(%)

Vel

oci

ty

(ft/

s)

KpB

HC

UO

CS

50

mp

ya

groo

ving

none

XX

120

n.a

.25

0.1

2

QH

CU

IC

S;

adde

d 80

0 el

bow

sIn

hibi

ted

Allo

y 82

5X

low

43

OC

SIn

hibi

ted

Allo

y 82

5X

340

.04

CH

TUO

CS

; A

lloy

600

liner

No

corr

osio

nA

lloy

825

XN

one

(ori

gina

lly

12

gpm

)7%

Ð 1

0%

RH

CU

I C

S6

5 m

py

NH

4F

co

rro

sio

nW

eld

over

lay

Allo

y 62

5 ar

ound

inj

ectio

n p

oin

t

X15

6 gp

m4%

n.a

.

WH

CU

IC

ST

hinn

ingb

Line

d w

ith 3

21

SS

X

XA

djus

ted

12 g

pm Ð

32

gpm

8%<

20

> 0

.45

OC

ST

hinn

ingb

Wel

d ov

erla

y 30

9/18

-8

elbo

ws

X8%

< 2

0c

> 0

.45

W2

HC

UO

CS

Imp

ing

em

en

t co

rro

sio

nD

uple

x 22

05X

n.a

27

IH

CU

I &

OC

SN

o se

riou

s co

rro

sio

nN

on

eX

X60

gpm

4%0

.23

ZH

CU

I &

OA

lloy

825

No

ne

XX

n.a

n.a

n.a

AA

HTU

OC

SE

rosi

on-

corr

osio

n of

pi

ping

Allo

y 80

0 to

82

5 la

ter

due

to

PT

A c

rack

ing

XX

n.a

.14

% Ð

15%

18 Ð

34

EH

TUO

CS

Ero

sion

-co

rros

ion

of

elbo

ws

(fire

)

Allo

y 82

5X

170

gpm

(ta

rget

)6%

Ð 7

%9

Ð 26

n.a

.

UH

TUI

CS

No

corr

osio

nN

one

XX

16 g

pm4.

6% a

vera

ge21

0.4

Ð 1

OX

IH

CU

I &

OC

SN

o se

riou

s co

rro

sio

nN

on

eX

X60

gpm

4%0

.23

ZH

CU

I &

OA

lloy

825

No

ne

XX

n.a

n.a

n.a

AA

HTU

OC

SE

rosi

on-

corr

osio

n of

pi

ping

Allo

y 80

0 to

82

5 la

ter

due

to

PT

A c

rack

ing

XX

n.a

.14

% Ð

15%

18 Ð

34

EH

TUO

CS

Ero

sion

-co

rros

ion

of

elbo

ws

(fire

)

Allo

y 82

5X

170

gpm

(ta

rget

)6%

Ð 7

%9

Ð 26

n.a

.

UH

TUI

CS

No

corr

osio

nN

one

XX

16 g

pm4.

6% a

vera

ge21

0.4

Ð 1

OX

No

tes:

Wat

er in

ject

ion

po

int

c V

eloc

ities

> 2

0 ft

/s c

ause

d er

osio

n-co

rros

ion

requ

irin

g w

eld

over

lay

b A

dded

inhi

bitio

n af

ter

early

exp

erie

nce

a C

urre

nt m

easu

red

rate

Ñsu

spec

t ox

ygen

and

chl

orid

es



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OOLER

S

YSTEMS

21

Tabl

e11

—W

ash

Wat

er D

etai

ls

So

urc

e o

f W

ash

Wat

er

Pla

nt

Ori

gin

al

Cap

acit

y (b

bls

/d)

Pre

sen

t C

apac

ity

(bb

ls/d

)S

team

C

on

den

sate

Str

ipp

ed

So

ur

Wat

er

Bo

ile

r F

eed

W

ater

Oxy

gen

(p

pb

)O

ther

Rat

e g

pm

(o

rig

ina

l)%

V

apo

riza

tio

n

gp

m P

er

Co

ole

r B

un

dle

Sin

gle

In

ject

ion

P

t.

Mu

ltip

le

Inje

ctio

n

Pts

.In

let

Pip

ing

Ou

tlet

P

ipin

g

B35

,000

5200

0X

X<

20

a,b

,c12

07

.5X

UU

Q2

2,0

00

40

,00

0X

Inh

ibite

d21

XU

U

OX

< 1

001

0 p

pm

Cl

6851

X

X<

100

< 5

0 p

pm

Cld

RX

< 1

005

0 p

pm

Cl

156

< 5

01

9.5

XU

U

X<

100

< 3

0 p

pm

Cld

CX

100

5 p

pm

Cl

3851

6 (

min

.)X

X0

80

pp

m C

l

W4

0,0

00

50

,00

0X

< 1

55

0 p

pm

Cl

12

Ð 3

225

XB

U

Xp

rese

nt

Fe

ad

just

ab

le

Inh

ibite

d

X10

%90

%33

0<

60

XB

B

I2

5,0

00

X60

10X

U

EE

45 T

DB

55 T

DB

Xde

-aer

ated

2766

XU

U

ZX

< 7

5X

UU

Y1

1,5

00

20%

24%

56%

10

0b

6 p

pm

Ð 1

0 p

pm

Cld

35<

75

XU

U

AA

45

,00

06

5,0

00

XX

50 p

pba,

bS

ign

ifica

nt

100

8.3

Xe

BB

CC

36

,00

0X

< 5

0 p

pm

Cl

105

n.a

.n

.an

.a.

E3

0,0

00

XX

low

20

pp

m Ð

50

pp

m C

l17

020

21X

BU

U1

4,0

00

XX

> 4

01

0 p

pm

Cld

16>

50

4X

BU

No

tes:

a R

ed

uce

d f

rom

1 p

pm

Ð 4

pp

mb

Use

an

oxy

ge

n s

cave

ng

er

an

d n

itro

ge

n b

lan

ket

c

Elim

ina

ted

pu

mp

se

al

lea

kag

ed T

race

NH

4H

Se

No

in

div

idu

al

flow

in

dic

ato

rs o

r m

on

itors

Ñu

nb

ala

nce

d w

ate

r d

istr

ibu

tion

W

ate

r lin

e p

lug

s fr

om

co

nta

min

ate

d p

roce

ss w

ate

r

B =

bal

ance

d

U =

unb

alan

ced



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

22 API P

UBLICATION

932-A

800 elbows allowed the continued use of carbon steel pipinguntil the entire piping was replaced with Alloy 825. In anotherplant (Plant W), the piping was upgraded to 12 Cr.

Plant Q also reported that the inlet piping system wasunbalanced, the velocities were high (43 ft/s) and that thewater injection rate was too low. However, this was not thecase with Plant W where the piping was balanced and veloci-ties were under 20 ft/s. The amount of wash water appearedadequate and yet corrosion was severe.

5.4.2 Outlet Piping

Similar problems to those reported in the inlet piping werealso reported in the outlet piping and in some cases appearedin both locations. In the two systems discussed above, Q andW, the REAC tube bundles also corroded. This indicates thatthe corrosivity of the process fluid is sustained throughout thesystem and the corrosive is not necessarily depleted. It hasbeen reported that pipe thinning is localized and usually takesthe form of grooving. This is descriptive of a continuouschannel cut into the pipe wall following the liquid flow pathof the corrosive water phase. The groove may be at the bot-tom of the pipe but often follows a spiral, or swirl pattern.These patterns are seen going into vertical nozzles or emerg-ing from elbows on the horizontal pipe run. Grooving inelbows tends to follow the outer radius of the bend asexpected but can be offset and hence escape detection by spotultrasonic inspection.

The most prevalent problem in the outlet piping was ero-sion-corrosion in turbulent areas. This commonly occurs inelbows and in some cases protecting the elbows is sufficientto extend the life of the piping system. For example, Plant Qused three 800 elbows with carbon steel pipe runs until thecarbon steel life was exhausted and Alloy 825 was substitutedfor both components. Plant W utilized stainless steel weldoverlay on the elbows only, but their experience with respectto the carbon steel pipe is tempered by the use of inhibitor.Note also the experience of Plant C, where the lining wasextended downstream during successive turnarounds but fail-ure occurred in the last remaining segment of carbon steel,where the swirl pattern of attack was missed by pulse-echoultrasonic inspection. In another plant (Plant W2), impinge-ment attack of the air cooler outlet header piping resulted inperforation of the pipe and a fire ensued. In this case, the subheaders were connected to the headers by vertical nozzles,with no elbows such that the flow impinged directly on thehorizontal pipe under the nozzle. Severe metal loss occurreddirectly under the nozzle.

Unbalanced piping configurations have been implicatedwith piping system corrosion, as illustrated in the data plotsfrom the UOP survey. While it is logical to deduce that anunbalanced system will produce velocity differences in theheader system, and, therefore, some parts of the system maybe subjected to higher risk of erosion-corrosion, not all cases

clearly follow this pattern. For example, Plants I and U havenot experienced any significant corrosion. This is evidencethat velocity must be linked to other factors, such as the corro-sivity of the process, and is not a stand alone parameter. It isalso evident that compliance with the guidelines for linearvelocity does not ensure freedom from high localized turbu-lence or impingement and consequential damage (Plant W2).

It should also be noted that imbalance in the flow patternsthrough the headers may also be caused by differences in therate of cooling through the various cells of the REAC. Oneoperator reported an air cooler design where reduced coolingwas obtained by shutting down or reducing speed of the fanson the middle group of cells while maintaining higher fanspeeds on the outer cells. This clearly created a thermalimbalance over the REAC.

A number of different alloys have been used in pipingreplacements with varying degrees of success. Austeniticstainless steels apparently have satisfactory general corrosionresistance but are prone to pitting attack and stress corrosioncracking where significant chlorides are present. There is noclear definition of a limiting concentration of chlorides so thatmany operators are unwilling to take the risk and opt for thestress cracking resistance of other alloys.

Among those that have been used are Alloy 800, Alloy825, Alloy 625, Alloy 400 and duplex Alloy 2205. Untilrecently, Alloy 800 has been very popular, but several opera-tors have encountered polythionic acid stress corrosion crack-ing as a result of sensitization of the alloy during fieldwelding. Inadvertent use of Alloy 800H, a higher carbon ver-sion, has exacerbated the problem. Reports of pitting in Alloy800 REAC tubing have also dampened enthusiasm for thealloy in piping replacements. Current upgrades favor Alloy825, which so far has not experienced any of the problemsassociated with Alloy 800.

5.5 WATER WASH TECHNOLOGY

Table 11 is a compilation of all the data made availablethrough this survey on factors influencing the role of washwater in corrosion control. This does not address the eco-nomic issues nor does it discuss the various operational diffi-culties experienced with the injection systems including thespecific hardware requirements. The purpose is to provide abasis of comparison of the quantities and qualities of thewash water used in each of the various units.

There are three aspects to be considered:

1. A quantity of water sufficient to solubilize the salts anddilute the aqueous phase sufficiently that it is not overlyaggressive.

2. A quality of water that does not introduce contami-nants into the process stream that aggravate corrosion.

3. Adequate water/vapor contact at the point of injectionto ensure removal of acidic components from the vapor.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

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Discussion of the problem with the various respondentsindicated that process restraints and economics often dictatethe availability and quality of water and the freedom ofchoice is limited. Some uncertainty exists as to how to estab-lish the required amount and what level of contaminants areacceptable.

An early rule of thumb for determining the amount of washwater to be injected was 1 gallon per minute of water for every1000 barrels of feed processed. This survey did not find anyconsistency with this guideline. Injection rates from 1 gpm/1000 bbls – 6 gpm/1000 bbls were reported and it is possiblethat other rules have been used. (See Turner [Reference 8].)The amount provided to each air cooler bundle varied from 4gpm – 21 gpm. It is likely that operators adjust the rates untileach finds a rate that is economical and provides satisfactoryresults. This trial and error approach has been costly in somecases where severe corrosion has been attributed to insuffi-cient wash water. Several respondents claimed improved cor-rosion performance after the wash water had been increased.Examination of the data in Table 11 shows a wide variation inthe amount of wash water relative to the capacity of the unit.For example, for Plant Q, based on a unit capacity of 22,000bbls/d, a water injection rate of 22 gpm would be required bythe above rule. The reported amount of 21 gpm is, therefore,consistent with the rule. However, for Plant B the amount ofwater per 1000 bbls/d is almost four times higher than the rulerequires and yet this unit experienced severe corrosion of car-bon steel REAC tubes and Alloy 800 tubes were substituted.

According to this survey, there is no consensus withrespect to wash water requirements, either as total quantityinjected or the amount distributed to each air cooler bundle.This may in part be due to the variations in nitrogen contentof the feed from unit to unit as discussed later. The surveyshows, however, that all operators introduce enough waterthat at least 25% remains unvaporized after flashing.

Another issue with respect to water injection is single-ver-sus-multiple injection points. With single injection points, anumber of operators have experienced localized corrosion atthe injection point. This has been successfully countered bythe use of alloy protection of the pipe in the vicinity of thewater entry and atomization of the spray to prevent impinge-ment attack and increase scrubbing efficiency. Static in-linemixers are sometimes used to give good water/vapor contact(scrubbing) after the injection point.

Multiple injection points are useful when the inlet headersystem is unbalanced. This design ensures that the water isevenly distributed to each cell in the REACs provided the sys-tem is properly monitored and maintained. Water is injectedjust ahead of the air cooler bundles, usually in the inlet noz-zles. Each injection point requires a flow monitoring deviceand a flow controller, usually a manually operated valve. Astrainer is often used in the water supply piping to preventinjector plugging. The proper flow of water to each pointmust be checked daily to prevent a potential buildup of

deposits. The best systems employ a control house indicatoron the process control board.

All the surveyed units with single injection points andunbalanced inlet piping experienced severe corrosion of theREACs. On the other hand, two units with balanced pipingsystems have not experienced corrosion even with single injec-tion points. There is no clear improvement from using multipleinjection points. Severe corrosion has been reported in unitswith both balanced and unbalanced inlet and outlet piping.

It was desired to include in Table 11 a measure of theseverity of corrosion for each of the units so that the influenceof the wash water conditions on corrosion might be assessed.Unfortunately, a suitable system for conveniently expressingthe corrosion experience for each unit as a simple index wasnot realized.

5.6 WATER SOURCES

Several sources of water are used and, depending on avail-ability, are used separately or mixed. These are boiler feedwater, steam condensate, the stripped bottoms from a sourwater stripper and recycle sour water from the downstreamcold separator. Each of these sources varies in purity and maycontain one or several of the following contaminants; oxygen,carbon dioxide, ammonia, chlorides, cyanides and iron.Boiler feed water is a very good source of water since it hasbeen de-oxygenated but may still contain chlorides. Steamcondensate is most likely to contain oxygen and carbon diox-ide and for this reason should be degassed and de-oxygenatedand stored under a nitrogen blanket before introducing intothe process. Not all respondents admitted to using these pro-cedures. Stripped sour water may contain all of the abovelisted contaminants in varying degrees and is often the pri-mary source of wash water with steam condensate or boilerfeed water being used as back up.

Although the role of oxygen in the corrosion process hasnot been clearly resolved, most operators take steps to mini-mize the amount of oxygen introduced with the wash water.Most units target the oxygen level at 100 ppb maximum but anumber try to maintain oxygen between 20 ppb – 50 ppb. Inone unit where corrosion inhibition is used, an oxygen levelof 330 ppb was tolerated. One operator reported that oxygenwas being admitted to the process from air drawn into leakingpump seals. The seals were, therefore, boxed in and an oxy-gen free purge provided.

Because cyanides, when present, are below the limits ofdetection, none of the operators consider them a controllablefactor and have no knowledge whether or not they are harmful.

5.7 INSPECTION

The importance of proper inspection of these units islinked to the inconsistent nature of the corrosion experience.The corrosion experience in REAC systems is usually unpre-dictable and often highly localized. This means that the num-



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

24 API P

UBLICATION

932-A

ber of thickness measurement locations (TMLs) must begreater than for a system where the corrosion is more generaland predictable. The two types of equipment of prime inter-est—air coolers and the associated piping—both have theirown particular set of difficulties with respect to inspection.

The purpose of the survey was to determine the extentand frequency of inspections used by the individual opera-tors interviewed. At the same time, the inspection method-ology was explored to detect any correlation between thequality of inspection and the unit performance from amaterials standpoint.

The information gathered with respect to inspection proce-dures and frequencies is summarized in Table 12. It is appar-ent from the interviews that an internal rotating inspectionsystem (IRIS) is preferred by operators for inspection of aircooler tubing. This consists of traversing the bore of eachtube with a special probe that measures the wall thicknessultrasonically. The method requires the unit to be shut downand the tube surfaces to be well cleaned, usually by high pres-sure water jetting. The inspection frequency depends on theturnaround interval for the unit and in most cases a 2- to 3-year interval is used. The maximum interval between inspec-tions is 5 years. One operator had access to his unit every yearand conducted tube inspections annually.

Piping is especially difficult to inspect because of theextensive surface area that is often difficult to access. The his-

tory of erratic flow patterns in the form of spiraling groovesintroduces a problem of ensuring adequate coverage byselecting a sufficient number of TMLs. Random-spot UT isclearly unreliable and continuous scan techniques are pre-ferred. There is a considerable effort required to conduct100% scans of all the piping. Most operators conduct pipinginspections on a 2

1

/

2-

to 3-year interval but more frequentinspections (6 – 12 months) may be carried out at water injec-tion points or other known problem areas. The relatively lowprocess temperatures allow on-line inspection of piping butnot air coolers. Where experience justifies it, the inspectionperiod can be extended to 5-year intervals. Several plantswith little or no corrosion are able to do this.

Ultrasonic inspection is most frequently used, but manyplants employ radiography to look for localized corrosion orspiral grooving that may have been missed by close intervalUT. Radiography has also been used on fittings, especiallyelbows, to detect localized corrosion. Radiography of largediameter thick wall piping requires a cobalt source whichresults in a grainy image and less reliable interpretation.

Inspection intervals are set by annual turnaround schedulesand the thoroughness of inspection during the turnaround is afunction of available manpower and time restraints. Severalrespondents indicated that upgrading to alloy extended theinspection interval for the items where alloy was employed.

Table 12—Inspection Summary

Plant Equipment Item Inspection Interval (years)

Type of Inspection Operating Factor

B, Q Piping 2.5 (5 max.) UT 98%one time only RT

A/C tubes 2.5 (5 max.) IRIS, RFECR, O, C Wash water injection point 3 UT/B-scan

HP separator; LP separator 3 Ð 6 WFMPT, T-scanA/C tubes 3 Ð 6 IRIS

Piping 3 UT, RT6 B-scan

W All 2A/C tubes 100% IRIS

Piping ells; headers 1 close interval UTX Air coolers 5

Piping 3 Ð 5Y Air coolers 2 100% IRIS

Piping 2 UTWash water injection point 2 RT

CC All 2.5 91%AA, BB All 3 UT, RT

Selected items 0.5 Ð 1 100% B-scan, RTU A/C tubes 1 VT, UT (IRIS)

Feed/effluent exchangers 5 VT, UTSeparators 5 VT, UT, WFMPT

E All 4 VT, UT, RTVessels API 510Piping 1 API 570



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

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25

Wherever possible, on-line inspection should be employedto provide closer monitoring of potential problem areas.Operators have reported avoiding serious consequences byfinding a problem area before a leak occurred. It is expectedthat hydroprocess units will receive a high priority from riskbased inspection planning. These survey results show thatthere is a relatively low incidence of failure but a very seriousconsequence when failure occurs.

5.8 CORROSION

It is generally recognized that corrosion in hydroprocess-ing REAC systems can occur at any place between the pointof water injection and the point of water separation. However,this survey also reveals that carryover and entrainment ofwater in the hydrocarbon streams beyond the separators hasbeen a persistent and worrisome problem for some operators.Corrosion has also occurred in bypass piping (deadlegs)around feed/effluent exchangers when the piping is config-ured such that deposited salts can accumulate and becomecorrosive in the presence of water.

The general locations where the survey showed most oper-ators to have experienced problems have been summarized inTable 13. The list of locations is familiar and has been wellcovered in previous surveys. Table 13 shows that most of theunits surveyed have experienced either REAC or piping cor-rosion in their early history. Also listed in Table 13 are thetypes of attack associated with the corrosion experience.These fall into three categories, salt deposit attack, aqueoussalt corrosion and velocity influenced corrosion. These prob-lems have been countered by either carefully implementedprocess controls or by the use of corrosion resistant alloys.

The primary corrosive in the majority of cases was believedby respondents to be ammonium bisulfide, and the severity ofattack dependent on concentration and velocity. However, it isbelieved that oxygen can be an accelerant of the corrosionprocess and is an undesirable component. The majority ofoperators take care to minimize or control the amount ofoxygen entering the system. One unit reported improvement intheir corrosion experience when the oxygen in the water washwas reduced from 1 ppm – 4 ppm to 50 ppb. Some operatorsbelieve that oxygen should be kept below 50 ppb, or even aslow as 20 ppb, in the water injected into the system. There arehowever many units that allow up to 100 ppb oxygen. It isinteresting to note that some of the units that have a relativelybenign corrosion history take no special steps to minimizeoxygen. The role of oxygen and other oxidants in the corrosionprocess is not clearly understood by the corrosion communitybut the effort to control them to low levels appears to bejustified by the experiences reported. Plants that employdeaerated steam condensate, for example (EE) and (I), havereported a relatively good corrosion performance (see Table 7).Plant B maintains low oxygen and was able to use carbon steelREAC outlet piping for 24 years. During a period when the

oxygen levels were higher than usual, corrosion rates of 50mpy on the CS pipe were experienced.

The potential harm of chlorides was recognized in two ways:1. Deposition of ammonium chloride in the effluent cool-ing train with serious corrosion consequences, (see PlantsC, O and R, Table 7); and2. Potential effects on alloy materials performance suchas pitting and stress corrosion cracking (see Plants Q, W,BB and E).

One operator believed that chlorides were always involvedwith bisulfide corrosion and had evidence of a differencebetween 1 ppm and 5 ppm on the severity of attack. Chloridesenter the system with the feed, from the reactor catalyst, withmake-up hydrogen, especially if unscrubbed reformer gas isused, and with the wash water. One or two plants do not havea crude desalter so that the chloride levels are inherently high.Steps to control the ingress of chlorides from all sourcesseems prudent. Unfortunately, insufficient data was availablefrom this survey to correlate any distinctive influence of chlo-rides on the severity of corrosion experienced by any particu-lar unit other than clear cases of NH

4

Cl deposition.The survey was unable to determine the importance of cya-

nides in the corrosion process. Most units reported undetect-able levels of cyanides and, therefore, were unable to determinewhether or not the minute quantities present had any effect ornot; however, the presence of cyanides was reported by severalrespondents who either observed the Prussian blue color of cor-rosion deposits on piping or found it by analysis of the depos-its. One operator reported no apparent problems with cyanidesin the REAC system but a very definite influence on corrosionof a downstream coker unit.

5.9 ALLOY SUBSTITUTION TO PREVENT CORROSION

The alloy solution is costly. A recent study

10

concluded thatto upgrade REAC exchanger and piping materials from carbonsteel to Alloy 825 would cost $1 million for a hydrocrackerunit with 6,160 ft

2

of air cooler surface. Alternative solutions,however, carry more risk. This is because the parametersaffecting corrosion are not quantitatively well defined andthere often is great difficulty predicting the sites where theprocess parameters may fall outside of the guidelines.

Some of the alternative materials selections to replace orupgrade from carbon steel have been discussed by Singh etal.

11

,

and Shargay and Lewis

12

. A few additional items have

10

C. A. Shargay and K. R. Lewis, “Cost Comparison of MaterialsOptions for Hydroprocessing Effluent Equipment and Piping,”NACE, Corrosion 96, Paper 600.

11

A. Singh, C. Harvey and R. L. Piehl, “Corrosion of Reactor Efflu-ent Air Coolers,” Paper 490, Corrosion 97.

12

C. A. Shargay and K. R. Lewis, “Cost Comparison of MaterialsOptions for Hydroprocessing Effluent Equipment and Piping,”NACE, Corrosion 96, Paper 600.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

26 API P

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932-A

been picked up by this survey. Austenitic stainless steels areavoided by most operators because of the potential for stresscorrosion cracking and pitting corrosion. Some units usethem in the form of linings for pipe or as weld overlay wherethe integrity of the pressure boundary is less threatened andthe risk of catastrophic failure is greatly reduced. Experiencewith pitting of Alloy 800 tubes and polythionic acid cracking

of Alloy 800 piping has introduced concerns about the use ofthis alloy. There is a preference for using Alloy 825, whichhas a higher alloy content and stabilizing elements to combatboth these modes of attack.

Another approach to avoiding these problems was toemploy duplex alloys which have inherent resistance to bothphenomena. 3RE60 has been used successfully (Plant CC)

Table 13—Corrosion Experience

Corrosion Description Plants Matching DescriptionAir cooler tube thinning from bisulfide corrosion Underdeposit attack B, R Flow corrosion C, Z Inlet end erosion-corrosion Y Outlet end erosion-corrosion W, Y Other Q, R Reasons Insufficient wash water B,R High bisulfide concentration at separator C, Z Too high velocity C, W, Y Unbalanced header system Z,R Ammonium fluoride RWater injection point corrosion R, Y No with alloy protection C, RAir cooler inlet piping corrosion Q, R, W, BB Elbows Q Headers W, Y Header boxes Y Reasons Insufficient wash water R, W, Y High bisulfide concentration at separator Too high velocity Q, W, Y Suspect O2 and Cl R, BBAir cooler outlet piping corrosion Q, C Thinning B, C, W, CC, AA Headers Y Header boxes Y Elbows C, AA, E Dead leg C, O Reasons Insufficient wash water Y, AA, E High bisulfide concentration at separator AA, C Too high velocity Q, W, Y, AA, CC Unbalanced header system W, Y Suspect Cl and O2 B, WFeed/effluent exchanger corrosion Bisulfide corrosion B, Q, CC Suspect O2 B Chlorides O, AA, E, BB Insufficient wash water O, AATrim cooler corrosion Bisulfide corrosion B, Q U-bend corrosion B Reasons Insufficient wash water High bisulfide concentration at separator Too high velocity Unbalanced header system Suspect O2 QDownstream of separator C, Y, AA High bisulfide C, AA High velocity C, Y, R O2

By-pass dead legs AA



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downstream of the water injection point for more than 18years with no plans to replace it. Alloy 2205 has been used forboth air cooler tubes and piping with varying success. Prob-lems can arise if the material is not properly specified andfabricated

13

. High residual hardness after welding can lead tosulfide stress cracking and must be avoided. In addition onerespondent reported selective leaching of the ferrite phase byammonium chloride.

Alloy 400 has been used successfully for tubes in watercooled exchangers for more than 20 years but this survey didnot reveal any air cooler applications.

5.10 INHIBITION

Only two units surveyed used chemical inhibition to con-trol corrosion. One unit employs a filming amine type ofinhibitor and the other uses a proprietary chemical. Bothunits, however, continue to exercise control over some of thecritical parameters such as bisulfide concentrations, velocitiesand contaminants. It appears that the application of the inhib-itors does permit the use of carbon steel where it might other-wise have to be substituted. The decision to use an inhibitoris, therefore, an economic choice versus an alloy upgrade.

6 Discussion and Analysis of Survey Responses

The following section is an interpretation of the informa-tion supplied by the respondents from a more generalizedviewpoint. These interpretations include ideas and opinionsexpressed by the engineers interviewed that were not neces-sarily substantiated with actual scientific or engineering data.

6.1 GENERAL EQUIPMENT CONSIDERATIONS

This study has been broadly directed at hydroprocess reac-tor effluent streams. This includes the products of reactionfrom both hydrotreating and hydrocracking processes. Withinthose processes are differences in the equipment arrange-ments that affect where corrosion might most be expected.Hydrocracking may involve two stages of catalytic reactions,the first is essentially a hydrotreating step in which sulfur,nitrogen and chloride contaminants are converted to H

2

S,NH

3

and HCl respectively. These corrosive compounds arenormally removed before the hydrocarbon stream enters thesecond stage so that the second stage effluent would beexpected to be less aggressive than the first stage effluent.This does not preclude the requirement for alloy in these sys-tems, as reported for Plant CC.

Downstream of the reactor there are two flow schemes thatinfluence the amount of equipment to be protected and the

wash water flow scheme. In one scheme, the total reactoreffluent is sent to a single bank of air coolers and the to a highpressure low temperature (HPLT) separator. In the secondscheme, a high pressure high temperature (HPHT) separatoris employed and only its vapor stream is water washed. TheHPHT separator liquid hydrocarbon is unwashed and carriesdissolved H

2

S, NH

3

and HCl to the downstream productstripper tower. This can result in corrosion problems in thestripper.

Clearly, each of the above process schemes has economictrade-offs, which also include the number of equipment itemsto be monitored and inspected, the amount of alloy employed,and the distribution of wash water. The corrosion processesare the same since the reactants are unchanged but the sever-ity may be affected by the distribution in terms of concentra-tions and flows.

Many operators have reported problems in equipmentdownstream of the separators. This includes the separatorvessels, piping and strippers. These items have experiencedvarious modes of attack from general bisulfide and chloridepromoted corrosion to hydrogen related cracking. It is beyondthe scope of this study to detail these incidences but the expe-rience serves to emphasize the aggressive nature of aqueoussolutions of the ammonium salts wherever they occur. Opera-tors should be aware that poor separation or entrainment ofthe aqueous phase can result in the same problems that haveplagued REAC systems. The penalty for allowing the aque-ous phase to carry through the separators is increased corro-sion requiring an increased scope of inspection andmaintenance activities.

6.2 NITROGEN CONTENT OF THE FEED

A number of respondents have reported observing anincrease in corrosion severity when the nitrogen content ofthe feed has been increased, or the throughput increased forthe same size equipment. The relationship between nitrogenin the feedstock and the severity of corrosion is simply thatincreased nitrogen will increase the amount of ammonia afterhydrogenation and since the number of moles of bisulfide isessentially equal to number of moles of ammonia, this willresult in an increase in bisulfide. The bisulfide content inmols/hr is determined by the difference between the lbs/hrnitrogen in the feed and the lbs/hr nitrogen in the productdivided by the molecular weight of 14. To determine thebisulfide concentration in condensed water, the followingrelationships are used;

For wt% H

2

S < 2

×

wt% NH

3

, then wt% NH

4

HS = 1.5

×

wt% H

2

S

For wt% H

2

S > 2

×

wt% NH

3

, then wt% NH

4

HS = 3

×

wt% NH

3

.

The amount of ammonia produced for any given amount ofnitrogen in the feed is a function of both the process and the

13

A. K. Singh, R. J. Gaugler and A. J. Bagdasarian, “Duplex Stain-less Steel in Refinery Hydroprocessing Units, Success and FailureStories,” “Proceedings of 4th International Conference on DuplexStainless Steels,” Paper no.16, Glasgow, Scotland, Nov. 1994.



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28 API P

UBLICATION

932-A

efficiency of conversion within the process. In the first case,for example, naphtha hydrotreaters are usually low in nitro-gen and may not require continuous water wash, whereas die-sel hydrotreaters will require a water wash. In addition, thecatalytic efficiency of the process will vary from unit to unitfor the same amount of nitrogen. Some units process feed-stocks with varying nitrogen contents, resulting in fluctuatingammonia concentrations in the reactor effluent. Add to thatthe changes in catalyst activity during its normal cycle and itis clear that ammonia contents of the effluent may vary con-siderably over a period of time. This in turn influences theamount of wash water needed at any one time. It is for thisreason that some operators use adjustable wash water ratesthat are determined by the amount of nitrogen in the feed.

Superficially, the data from this survey appear to indicatethat units that experience severe corrosion are dealing withnitrogen contents in the feed of 1200 ppm or greater, andthose units not experiencing significant corrosion have feed-stocks of less than 600 ppm nitrogen. However, from the fore-going it is clear that such inferences can be grosslyinaccurate.

Careful examination of the UOP survey data showed anon-linear relationship between the nitrogen in the feed andthe ammonia in the air cooler inlet gas. The amount of ammo-nia increased with the increase in nitrogen level as expected.However, the ammonia contents reported for a given nitrogenlevel covered a range of values. Consequently, with such ascatter of data, the correlation between the ammonia contentand the severity of corrosion is weak and at best indicates atrend rather than a specific relationship.

Note: Both parameters in this relationship are variable. This is analo-gous to the relationship between

K

p

factor and corrosion severity.Therefore it appears that the use of nitrogen content as a guide topotential corrosion severity is no more reliable than the

K

p

factor.

Although the severity of corrosion due to increased quan-tity of nitrogen in the feedstock is not predictable, the experi-ence shows a trend towards increased problems. The practiceof changing feed stocks or increasing unit throughput withoutregard for the corrosion consequences has penalized someoperators.

6.3 SALT DEPOSITION AND TEMPERATURE

Corrosion can be caused by deposition of either ammo-nium chlorides or ammonium bisulfides or both (excludingplants where fluorides may be present). The deposition tem-peratures of the two salts are considerably different. Ammo-nium chloride usually deposits at 350°F – 400°F, whereasbisulfides deposit in the range of 80°F – 150°F. Depositiontemperatures increase with the amount of chloride or nitrogenin the feed. The prediction of the precise deposition tempera-tures for any particular set of process conditions has beenstudied extensively. Yiing-Mei Wu

14

has used a thermody-namic approach to predict the temperature at which saltdeposits, the kind of salt that deposits and an approximation

of the amount. Turner

15

and others

16

make use of depositioncharts to predict salt crystallization temperatures.

The temperature at the point of water injection is anothercritical process parameter. When the unit is being designed tomeet a 25% unvaporized water criterion, the amount of washwater can vary by 1.5 – 2.5 times for a 100°F difference. Thisis directly tied to heat exchanger design and performance.

It is clear that there is an important relationship betweenprocess design and operation and control of corrosion. Theinformation needed to provide the best system of corrosioncontrol requires computational understanding and skills nor-mally out of the range of traditional metallurgists and corro-sion engineers. Few respondents to this survey volunteered anunderstanding of the process aspects in this area, althoughseveral indicated awareness of the influence of the processconditions on the corrosion performance of their units.

6.4 MANAGEMENT OF CORROSION

Successful management of corrosion in these units requiresintegrated team effort involving process, mechanical, inspec-tion and corrosion engineers. This is not only for design orexpansion but for day-to-day operating decisions. As dis-cussed below, the effect of the process conditions on washwater management is an important aspect of corrosion con-trol; in addition, a joint effort is needed to resolve the effec-tiveness of changes in ammonia levels and bulk fluid flowvelocities. Some units have been successful in persuadingtheir management to recognize the importance of observingthe process parameter guidelines and keeping the corrosionengineer informed of any changes in operating conditionsthat could affect those parameters. The advent of federalrequirements for process safety management has introducedprocedures that several respondents rely on, namely the man-agement of change. This requires that when a process changeis introduced, all affected disciplines are notified and aregiven the opportunity to discuss the consequences and to pro-pose any necessary remedial action.

6.5 FACTORS INFLUENCING CORROSION

The key factors influencing corrosion in REAC systems areunanimously recognized as bisulfide concentration and veloc-ity. It is also realized that these two parameters interact in theeffect on corrosion severity.

A widely observed influence of velocity has been thesevere, localized metal loss experienced at changes of flowdirection, such as in piping elbows, the u-bends on exchangertubes, or the spiral gouging that occurs in straight piping. It is

14

Y. M. Wu, “Calculations Estimate Process Stream Depositions,”

Oil & Gas Journal

, January 1994, p. 38.

15

J. Turner, “Design of Hydroprocessing Effluent Water Wash Sys-tems,” Paper 593, Corrosion 98.

16

E. F. Ehmke, “Corrosion Correlations with Ammonia and Hydro-gen Sulfide in Air Coolers,”


, July 1975.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

A S

TUDY

OF

C

ORROSION

IN

H

YDROPROCESS

R

EACTOR

E

FFLUENT

A

IR

C

OOLER

S

YSTEMS

29

this writer’s opinion that the mechanism is more properlydescribed as erosion-corrosion. The difference between ero-sion-corrosion and mechanical erosion is that in the first eventit is the susceptibility of the corrosion product film tomechanical disruption that is the controlling factor. In thecase of mechanical erosion, both the corrosion product filmand the underlying metal are removed mechanically. In ero-sion-corrosion, the accelerated loss of metal is caused by highcorrosion rates of the metal surface by constant removal ofthe partially protective corrosion product film as it is formedand re-formed. The importance of this distinction is that it isthe quality of the corrosion product film that is most influen-tial in the resistance of the material to the effect of velocity.Thus, factors that affect the chemical and mechanical proper-ties of the corrosion product film may have a significant effecton its ability to resist breakdown caused by increased veloc-ity. It is for this reason that the presence of contaminants isimportant where they are perceived to affect the protective-ness of the surface film. It is beyond the scope of this study topursue this complex subject further.

Discussion of this subject with corrosion engineers indi-cated that the effects of contaminants, such as oxygen, chlo-rides and cyanides, on the corrosion film properties were notsufficiently understood to allow customized control of theoutcome. However, through empirical experience, many engi-neers have strong convictions regarding the influence ofsome, if not all the contaminants, and use their own guide-lines based on their experience. This study could haverevealed some consistent correlation between differences incontaminant levels and the severity of corrosion experienced.The variability of the aggressiveness from unit to unit and thelack of precision in the measurement of the contaminantsmade this an extremely difficult task.

6.6 CORROSION CONTROL

The history of corrosion in REAC systems has been a suc-cession of surprise events. Among the first problems encoun-tered were tube end erosion-corrosion and tube thinning inthe air coolers. This was followed by corrosion of the REACinlet piping and subsequently outlet piping. These problemslacked universal consistency. Some units had problems, andthe location of each problem varied. Differences in bisulfideconcentration and velocity accounted for much of the vari-ability and the broad guidelines developed encompassedmany of the variations. Problems persist because the condi-tions at the locations where corrosion occurs cannot beobserved and monitored and are difficult to forecast. Oneexample is plugging occurs in air cooler tubes, causingincreased flow and subsequent attack on unplugged tubes.Monitoring such an event requires frequent on-line inspec-tion, which is costly and runs the risk of being unsuccessful.In addition to limits on bisulfide concentration and velocity,the crux of corrosion control has been to make the system as

balanced as possible with respect to stream compositions andflow distribution. Thus, balanced flow paths into, through andout of the air coolers, sufficient quantity and uniform distribu-tion of the wash water have been an essential part of the cor-rosion control strategy.

6.7 WATER WASHING

Water washing is an essential control measure but requiressufficient water to be effective. It is important that water beintroduced into the system in such a way that it does not cre-ate additional problems. Liquid water must be available at alllocations where salts condense from the effluent stream insufficient quantity, to dissolve the solids and move themthrough the system. A sufficient quantity of water is needed tomaintain a mildly corrosive solution throughout its passage tothe separators. This survey did not find any consistency in theamount of water used by the various units, some units usingas much as six times as much as others in proportion to thefeed rate.

A detailed discussion of the subject has been presented byJ. Turner

17

, who indicated two guidelines that are commonlyused for determining the amount of wash water to be injected.

1. The maximum bisulfide concentration in the cold sepa-rator, and

2. The amount of washwater required to ensure that 25%water remains in the aqueous phase at the point ofinjection.

The two guidelines differ greatly in their influence on thedesign requirements to achieve each goal. The amount ofammonia that goes into solution at the point of injection is afunction of temperature, and it is assumed that almost totalsolubility is achieved below 150°F – 200°F. The concentra-tion of the bisulfide solution depends then on the ammoniaproduced by de-nitrification. Simulation analyses have shownthat the bisulfide concentration at the point of injection is lessthan in the downstream separator water. It is likely that corro-sion will be worse downstream of the air coolers and bisulfideconcentrations will be higher. In this case, it does not seem asimportant to maintain 25% unvaporized at the point of injec-tion. There is in fact an advantage to distributing the water asvapor rather than liquid to the point of condensation withinthe air cooler tubes. Insufficient water, however, could pro-duce high concentrations of salts at the point of condensationdownstream, especially chlorides. For this reason, it is pru-dent to maintain a minimum of 25% unvaporized water, aspracticed unanimously by the respondents to this survey.

The actual amount of water required for any particular unitis not, therefore, a matter of simple rules of thumb butrequires a careful assessment of the process parameters. It

17

J. Turner, “Design of Hydroprocessing Effluent Water Wash Sys-tems,” Paper 593, Corrosion 98.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

30 API P

UBLICATION

932-A

requires knowing the chemical equilibria and the temperatureat the wash water injection point.

The water quality is also important. The contaminants intro-duced with the wash water should not have any significantcontribution to downstream corrosion. A few of the respon-dents mentioned control of iron entering the system with washwater. Insoluble sulfides will quickly form from any iron ionsentering these sour water systems and will lead to deposits.For example, 50 gpm of wash water containing 1 ppm iron candeposit as much as 300 lbs iron sulfide per year in the system.

In an effort to obtain good distribution of the wash water,there has been a progression of improvements in the injectiondevices starting with an open tee, to quills and spray nozzleswith downstream inline mixers. Experience with these differ-ent devices has varied from unit to unit. The use of single ver-sus multiple injection points is controversial. To improveuniformity of distribution through the air coolers, many oper-ators use multiple injection points. This has not worked infal-libly and corrosion of tubes has been experienced in spite ofmultiple injections showing that corrosion may not depend onwash water distribution alone, assuming each injection pointis working properly.

6.8 CORROSION ASSESSMENT

A major difficulty in assessing the performance of individ-ual process units is the problem of quantifying corrosionexperience. A unit that performs generally well can suffer alocal failure that is disastrous. This unit is then classified as a“serious” corrosion case and the process and engineeringparameters used for comparison with “moderate” or “mild”cases. Attempts to correlate individual process parameterswith severity of corrosion are inherently flawed by the inclu-sion of extreme behavior for the stated process conditions,whereas the conditions at the point of failure are probablyquite different. Plotting corrosion severity against anothergeneral parameter, such as

K

p

factor or nitrogen content in thefeed, only compounds the error and the resulting plots areextremely scattered.

The precise conditions prevailing at the location of corro-sion may be difficult, if not impossible, to quantify and con-trol. The options available to rectify the problem ofteninclude a lesser but continued risk. The distinct trend towardsuse of expensive alloys to replace carbon steel probablyreflects the desire to minimize the risk. There are definitetrends toward the use of higher alloys such as 2205, 800 and825 for air coolers and related piping especially for replace-ments and expansions. At the same time, alloy substitutionallows more process flexibility, including higher nitrogenfeedstocks and higher conversion efficiencies, reducedinspection and reduced maintenance. As an example, unitsthat employ carbon steel air cooler tubes strive to maintain4% – 8% bisulfide in the separator water but allow increasing

amounts of bisulfide depending on the alloy used. Typically,for 8% – 15% bisulfide, Alloy 800 would be employed.

6.9 FLOW EFFECTS

Velocity limits also depend on the alloy being used. Withcarbon steel tubes an upper velocity limit of 20 ft/s is generallyobserved, whereas with alloy tubes, higher velocities are per-mitted. For example, velocities up to 40 ft/s have been usedwith Alloy 800 tubes. Another aspect of flow through the sys-tem is the importance of maintaining high enough velocities tominimize phase separation between the vapor, water andhydrocarbon phases. The importance of flow regime has beendiscussed by Ehmke

18

, whose experience indicated that leastcorrosion was experienced in a stratified or annular flow pat-tern where the liquid was in laminar flow at the wall. Thepresent survey showed that most operators try to maintainannular flow in the air cooler tubes to avoid possible problems.

7 ConclusionsThe cause of corrosion in reactor effluent air cooler systems

is reasonably well understood and has been attributed to theformation of ammonium chloride and ammonium bisulfidesalts. To prevent plugging of the flow paths by condensed soliddeposits, water is injected into the process to solubilize thesalts. The aqueous solutions thus formed may be extremelyaggressive to carbon steel and even cause problems for somealloys. Experience has shown that the problem can be con-trolled by strict limitations on aqueous salt concentration andvelocity. This simple strategy, however, is complicated bynumerous factors such as the even distribution of flow throughthe equipment and the complexity of controlling the flow pat-terns to avoid high velocity turbulent conditions. Maintaininguniform concentration of the salt solutions throughout theequipment by avoiding deposit build-ups or evaporative condi-tions is another complication. The corrosion history of theseunits has been characterized by highly localized failures.Severe thinning has occurred over relatively small areas wherelocal conditions have been very aggressive. This has made thetask of inspection of the equipment more difficult. The inspec-tion intervals have been shortened and the coverage increased.But in spite of this, serious failures have occurred.

The extent to which these factors can be manipulated forthe best control is often dictated by economic considerations.There is evidence that a hydraulically balanced piping systemaround the air coolers alleviates corrosion in some but not allsystems. For that reason, there has been a trend toward theuse of Alloy 825 for air cooler tubes and related air coolerpiping. This alloy has been used successfully under some pro-cess conditions for up to 15 years with no adverse experience.Currently, users allow up to 40 ft/s velocity. There is no

18E. F. Ehmke, “Corrosion Correlations with Ammonia and Hydro-gen Sulfide in Air Coolers,” Materials Performance, July 1975.



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A STUDY OF CORROSION IN HYDROPROCESS REACTOR EFFLUENT AIR COOLER SYSTEMS 31

known limit with respect to bisulfide concentrations, but labo-ratory research has indicated low corrosion up to 45% bisul-fide under very low flow conditions. No pitting corrosion orcracking problems have been reported. The prospects of pro-viding trouble-free equipment with minimal risk of a cata-strophic failure, that allows flexibility in operating conditions,is to some companies worth the considerable investment.

The design and control of the water wash addition to theprocess is one of the most critical factors in controlling thecorrosion problem, especially with carbon steel equipment.The proper amount of water needed to keep the systemflushed of salt deposits and to control its distribution throughsplit flow paths to ensure uniform washing of all the equip-ment. The water quality is also important and is suspected toaffect details of the corrosion process such that the corrosionproduct films can vary in their degree of protectiveness. Thisin turn affects the tolerance to bisulfide concentration andvelocity. For this reason, care is exercised by limiting oxygenand chloride contents of the wash water, though the maxi-mum permissible levels of both these elements are notknown. Other contaminants, such as mineral salts and heavymetal ions, can cause plugging problems in the water systemand contribute to fouling in the process equipment.

The most critical task is to determine the amount of waterneeded at the point of injection. One criterion is to maintain atleast 25% water in the liquid state after injection. The amount ofvaporization is a function of the temperature and pressure. Theamount of water injected also depends on the ammonia concen-tration which is a function of the nitrogen content of the feedand the efficiency of de-nitrification. The amount of ammoniathat goes into solution depends on the temperature at the washwater injection point and may be calculated using thermody-namic modelling. Many operators base the quantity of injectionwater on the concentration of bisulfide in the low pressure sepa-rator. Typically, they try to maintain a level of 4% – 8% bisulfidein the sour water. This apparently has been a successful strategyfor those who use it according to the reported performance oftheir units. It is not clear whether the target concentration isachieved simply by adjusting the water addition or whether themore sophisticated analyses just discussed are employed. It isrecommended that a proper process evaluation be used, takinginto account conditions at the point of water injection to main-tain 25% unvaporized water, as well as the bisulfide concentra-tion at the separator.

It is clear that chemical engineering techniques are neededto determine the water wash requirements and also to predictthe deposition temperature of the ammonia salts and theamount of salt depositing. These factors bring into play theheat balance of the process and affect equipment perfor-mance. This is especially true during the design of the unit butshould also apply to any process changes that are subse-quently made. For this reason, cost-effective corrosion con-

trol of these units requires a continuous team effort byoperating engineers and those responsible for corrosion per-formance, to monitor and control the process parameters andto implement effective change management procedures whensignificant adjustments are made.

8 Future Research

The survey/interview approach used in this study and inprevious studies has clearly exhausted the possibility ofuncovering new or unique information leading to resolutionof some of the unanswered questions. It is evident that thereis a need for a new approach, such as the generation of reli-able laboratory data that serves as a baseline to predict mate-rials performance under the various conditions encountered inthese processes.

The referenced literature used in this report includes sev-eral laboratory studies 5,6,9 that cover different aspects of theproblem. The tests by Damin and McCoy5 were conductedunder essentially stagnant conditions while those of Scherreret al.6 utilized a simulated process flow scheme. Bonner etal.9 studied the effects of oxidants and explored the effect ofpH. A combination of the latter two studies might be mostuseful in providing realistic corrosion rate data and providinginsight into the mechanisms of the corrosion processes. Twotypes of study are suggested, possibly in sequence.

The first study would consist of a series of tests under simu-lated process conditions reproducing the natural process con-ditions as closely as possible. The objective would be togenerate corrosion on various alloys including carbon steelthat matched the mode and severity of that observed in plantequipment under the same conditions. Assuming that thiscould be achieved, the studies would then be broadened tointroduce the effects of different variables, especially velocityand oxygen, chlorides and cyanides. The objective would be toprovide a clearer picture of the influence of velocity on corro-sion and determine the effect of contaminants on both corro-sion and erosion-corrosion performance of the various alloys.

In a second series of studies the more fundamental aspectscould be explored. The following is a suggested list of topics.

1. The corrosion product film needs to be characterized interms of chemical composition and properties and differ-ences in protective and nonprotective films determined.This could include studies of alloys as well as carbon steel.

2. The effect of process contaminants, such as oxygen,chlorides and cyanides, should be studied with respect tothe films characterized in item 1 above. Studies of filmformation under truly anaerobic conditions are needed asa baseline for comparison with the presence of oxidants.

3. Determine the desirable film properties that provideresistance to the effects of flow velocity, and how best toachieve those properties.



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

33

APPENDIX A—PLOTS OF CORROSION SEVERITY VERSUS VARIOUS PARAMETERS FROM THE UOP SURVEY DATA FOR

ALL COOLER TUBES (REFERENCE 7)



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

34 API PUBLICATION 932-A

Figure A-1—Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Tube Velocity (ft/s) on Corrosion Severity

Figure A-2—Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentrationin the Downstream Separator and Calculated Maximum Tube Velocity (ft/s)

on Corrosion Severity for Balanced and Unbalanced Headers

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50

Calculated maximum tube velocity (ft/s)

Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

BalancedHeaders

No corrosion

Severe

Medium

Low

Balanced InUnbalanced Out

No corrosion

Severe

Medium

Low

UnbalancedHeaders



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---


Figure A-3—Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Tube Velocity (ft/s)

on Corrosion Severity for Balanced Header Systems

Figure A-4—Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Tube Velocity (ft/s) on Corrosion Severity

for Balanced Inlet and Unbalanced Outlet Header Systems

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

BalancedHeaders

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low




--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---


Figure A-5—Corrosion of Air Cooler Tubes—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Tube Velocity (ft/s)

on Corrosion Severity for Unbalanced Header Systems

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

UnbalancedHeaders



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---

37

APPENDIX B—PLOTS OF CORROSION SEVERITY VERSUS VARIOUS PARAMETERS FROM THE UOP SURVEY DATA FOR

REAC PIPING (REFERENCE 7)



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---


Figure B-1—Corrosion of REAC Outlet Header—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum

Outlet Header Velocity (ft/s) on Corrosion Severity

Figure B-2—Corrosion of REAC Outlet Header or Piping—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum

Velocity (ft/s) in the Outlet Header or Piping on Corrosion Severity

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50

Calculated maximum outlet header velocity (ft/s)

Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50

Calculated maximum outlet header or piping velocity (ft/s)

Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---


Figure B-3—Corrosion of REAC Outlet Piping—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum

Outlet Piping Velocity (ft/s) on Corrosion Severity

Figure B-4—Corrosion of REAC Outlet Header—The Combined Effect of Calculated Ammonium Bisulfide Concentration in the Downstream Separator and Calculated Maximum Outlet Header Velocity (ft/s)

on Corrosion Severity for Balanced and Unbalanced Header Systems

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50

Calculated maximum outlet piping velocity (ft/s)

Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

BalancedHeaders

No corrosion

Severe

Medium

Low


No corrosion

Severe

Medium

Low

UnbalancedHeaders



--`,``,,```,`,``,```,,,,```,`-`-`,,`,,`,`,,`---



on Corrosion Severity for Balanced Header Systems


on Corrosion Severity for Balanced Inlet and Unbalanced Outlet Header Systems

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

BalancedHeaders

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low




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on Corrosion Severity for Unbalanced Header Systems

0

2

4

6

8

10

12

14

16

18

20

0 5 10 15 20 25 30 35 40 45 50


Calc

ula

ted b

isulfid

e c

oncentr

ation (

wt%

)

No corrosion

Severe

Medium

Low

UnbalancedHeaders



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43

APPENDIX C—QUESTIONNAIRE



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44

SURVEY OF HYDROPROCESS REACTOR EFFLUENT SYSTEM CORROSION

To: All selected recipientsThank you for your participation in this important task to determine recommended practices to minimize corrosion through

design, operation and maintenance of hydroprocess effluent systems. From your response to the preliminary questionnaire itappears that your experience will be of great value to this effort and we would like to obtain more detailed information. We wouldlike to follow up with a visit to your office or refinery to conduct personal interviews and discussions with the people most knowl-edgeable about corrosion in the hydroprocess unit(s) you have identified.

We ask that the following information be available at the time of the visit as a minimum basis for the discussions. As much aspossible should be available as hard copies to turn over to the consultant for subsequent analysis. Any or all of the documents fur-nished can be returned to the engineers at the conclusion of the project.

SECTION 1—SCHEMATIC DRAWINGS

Please provide the following schematic drawings. Single page (11'' × 17'' max.) reductions of process flow diagrams areacceptable provided, they are legible. Mechanical flow sheets and piping layout drawings should not be submitted at this time butmay be requested later.

a. Process flow schematic from the reactor to low pressure separator (show design pressures and temperatures and major controlinstruments).b. Overhead air cooler piping arrangement with location of water injection points.

Comments (use a separate page if necessary):____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

SECTION 2—OPERATING CONDITIONS

(a) Temperature and Pressure

Normal RangeMaximum Upset

High/Low

TemperaturesReactor effluentAir cooler inletAir cooler outletHot high pressure separatorCold high pressure separatorHot low pressure separatorCold low pressure separator

PressuresReactor effluentAir cooler inlet Air cooler outletHot high pressure separatorCold high pressure separatorHot low pressure separatorCold low pressure separator



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45

Annual operating cycle—scheduled annual operating hours__________actual operating hours (history) ___________scheduled outage ______________________

Comments (use a separate page if necessary):____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

(b) Process Conditions

It is desirable to know as much as possible about the chemical composition of the process streams from the outlet of the reactorto the discharge of the low-pressure, low-temperature separator. It is generally recognized that compounds of H

2

S, NH

3

, halo-gens, cyanides and oxidants such as oxygen are the principal contributors to corrosion. We need to identify the presence and con-centration of each salt to match with the corrosion experience for each of the units reported.

Please provide the following flow stream compositions (typically found on the bottom of the PFD). Please give moles/hr orppm, mol wt of component, and total moles in the stream. If available please also provide gals/hr or scfm. Include all componentsin the stream and indicate the phases present (e.g., liquid, vapor).

In addition, please address the following:1. NH

4

HS Concentration in liquid phase________%Location of measurement _____________________Temperature __________ Pressure _____________

K

p

Factor (mol% H

2

S

×

mol% NH

3

) ____________2. Chlorides Concentration ___________ ppm

Location _____________ Temperature _______ Pressure _________3. Fluorides Concentration ___________ ppm

Location _____________ Temperature _______ Pressure _________

(c) Flow Conditions

In addition to the above information on the quantities of vapors and liquids moving through the system, it is desirable to definethe actual flow conditions at certain critical areas. Of particular interest is the dispersion of the injected wash water and distribu-tion to the air cooler tubes, the flow through the air cooler tubes and the flow in the outlet manifolds, especially the elbows. Theinjection and distribution of wash water is explored in a later section. This section will address conditions around the air coolers.

Component

Reactor A/C High Pressure Separator Low Pressure Separator

Inlet Outlet In* In*

Temperature °FPressure psigC

1

– C

7

(total hydrocarbons)HydrogenHydrogen SulfideHydrogen CyanideAmmoniaWaterOxygen

* give both hot & cold



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46

Please answer the following:

What is the condition of the stream entering the air coolers? Is it all vapor or does it have some liquid? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________ At what point does condensation of a) water and b) hydrocarbon take place in the cooler tubes? _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________What are the flow characteristics in the cooler tubes? Please indicate the bulk velocity and flow regime (annular, stratified, plug,etc.?). _____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Give the flow characteristics of the outlet piping. __________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Additional Comments:____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

SECTION 3—METALLURGY

Please provide a Materials Selection Diagram or marked-up flow sheet indicating the principal materials of construction forvessels and piping.

Manufacturers Vessel and Exchanger Data Sheets may be submitted provided the information is up-to-date, including anychanges and the approximate date of the change, and that all items exposed to the operating stream are included.

Inspection Data Sheets and computer graphics may also be used with the same provisions as above. If inspection documentsare submitted in response to Section 4, they need not be duplicated for this section provided they contain all the required informa-tion as indicated next.

• All materials should be properly identified with a) a specification number such as ASTM, ASME or API, b) the materialdescription (e.g., alloy name), c) thickness, and d) corrosion allowance.

• Piping and tubing should be designated as welded or seamless.• Any stress relief heat treatment should be noted.

In general, only materials exposed to the operating environment need be described in detail. The following equipment must beincluded:

Reactor exit piping Reactor feed/effluent exchangers (effluent side only)Piping to the air coolersAir coolers (header boxes and tubes)Trim coolerPiping to the separatorsHot high pressure separatorCold high pressure separatorHot low pressure separatorCold low pressure separator



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47

SECTION 4—WATER WASH

Source of wash water_____________________________________________________________________________________List all chemical contaminants and their concentrations. The following are suggested as a starter list:Ammonium salts or ammonia; hydrogen sulfide, other sulfides or sulfur compounds, cyanides, chlorides, HCl, fluorides, oxygenand oxidizing agents.________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________pH?__________Quantity of water injected (gals/min) _______________Number of injection points _______________ One for each bundle? _______________Based on number of air cooler bundles, what quantity of water is provided per cooler?_________________________________________________________________________________________________________________________________________If not covered in Section 1, please provide a sketch or describe the air cooler piping arrangement from the inlet line through themanifolds and individual inlets to the cooler headers, as well as the outlet arrangement. _________________________________ ____________________________________________________________________________________________________________________________________________________________________________________________________________What chemical analyses are performed on the air cooler effluent?______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Describe the method of determining ammonium bi-sulfide concentration in the AC effluent. Include the sampling procedure andanalytical method. ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

SECTION 5—INSPECTION

To avoid accumulation of a large number of documents, it is preferable that, where possible,

summary

sheets of the inspectionrecords for each equipment item be submitted. The purpose of gathering this information is to determine whether the level andquality of inspection has had a bearing on the performance of the unit. It is also important to learn if any indications of a potentialproblem developed over a long period of time or if they developed suddenly—the time frame can be used to relate to the processconditions in the same time period.

It is suggested that a separate sheet or sheets be provided for each item of equipment discussed. The items listed below are theminimum information required. Name of item (reactor, piping, etc.) ___________________________________________________________________________Frequency of inspection___________________________________________________________________________________Last inspected___________________________________________________________________________________________Type of inspection (visual, UT, eddy current, etc.)______________________________________________________________________________________________________Inspection method—please indicate the extent of inspection for each item, especially piping (i.e., whether it is random, fixed loca-tion or grid readings). Inspection record—year, observations, readings.Please attach Inspection Data Sheets for each item reported.



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48

SECTION 6—CORROSION

Please provide a summary narrative of the corrosion experience with the unit.This section should include corrosion data acquired by coupons, probes or test pieces, as well as visual observations and

inspection thickness data.In answering the following questions please address each item of equipment separately. Do not be confined by the space pro-

vided, attach additional sheets if needed.

Since the unit was commissioned, has any item experienced severe corrosion requiring it be replaced? If so, describe.____________________________________________________________________________________________________________________________________________________________________________________________________________Has the same item been replaced more than once? ______________________________________________________________Were there any changes made in the metallurgy, such as a different corrosion allowance, or different material?___________________________________________________________________________________________________________________________________________________________________________________________________________________________Have any non-metallurgical changes been made, such as inhibition, or process modification? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Have the changes been successful in controlling the problem? __________________________________________________________________________________________________________________________________________________________Has the problem diminished without making changes? ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________If so, what was responsible for the improvement? ___________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Were process changes made for reasons other than corrosion control? ____________________________________________________________________________________________________________________________________________________Has corrosion ever led to an unscheduled shutdown? If “yes” describe the incident. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Has there ever been a catastrophic incident? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________What changes have been made to avoid a repetition of the event? ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

SECTION 7—INHIBITION

The following information is requested with respect to the use of chemical inhibitors in the effluent system.

Type of inhibitor (trade name or generic description) _________________________________________________________________________________________________________________________________________________________________Water soluble? __________________ Hydrocarbon soluble? __________________Point of addition________________________________________________________________________________________________________________________________________________________________________________________________Method of injection____________________________________________________________________________________________________________________________________________________________________________________________Continuous? __________________ Intermittent? __________________Dosage? __________________________________________________Number of years in use ______________________________________



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49

Effectiveness—is corrosion rate reduced or is corrosion essentially eliminated? ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Have other inhibitors been tried which were unsuccessful? Please describe. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________



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