Advances in Reformer Tube Metallurgyand Production

Recent developments in alloys provide better creep resistance,leading to a longer service life and possibility of using smallertube diameters and higher operating pressures.

J. Thuillier and F. Pons,Acieres du Manoir-Pompey,

Paris, France

As in all continuous processing, the economics call for avirtually permanent onstream operation of catalytic reform-ing units. This is related mainly to satisfactory performanceof all parts of the unit, among which the catalytic reformertubes are of major importance. The tube life depends uponthese two main points: 1) the method of design calculationused, and 2) the tube metallurgy.

Methods of calculation differ completely from onedesigner to another, since it is they who generally fix thetube dimensions. Most calculations are based on a theo-retical tube life-time of 100,000 hr. but in some cases, cer-tain modifications may be made. In actual fact, calculationsare based upon formulas intended to determine the appropri-ate thickness with respect to three factors: 1) pressure,2) external diameter, and 3) allowable stress for the antici-pated and calculated temperature including an efficiencyfactor and an "increment," both depending upon the pres-sure used, which varies with the temperature.

Practical application

The reforming furnace is generally operated with caution,considering the size of the 25% investment covered by thereformer and miscellaneous associated heat recovery equip-ment in a plant. (1 ) Operation of a reformer furnace at anonstream factor of 92 to 96% is quite commonplace. Usualoperation during the first year is 85 to 92% followed byrapid stabilization at the maximum operating time, with anannual downtime average of 9 to 24 days.

Operation of five reforming units during a three-yearperiod has recorded an estimated average annual outage ofup to 16 days, resulting from the following causes:

7.0 days downtime for convection section and for high-temperature tube repairs.

0.4 days downtime for the reformer tube failures.

2.5 days for miscellaneous catalyst changes (reformingconversion and methanation).

2.4 days for cleaning heat exchangers.3.7 days for power failures, and "miscallaneous" causes

in the equipment or in operations.Minor troubles arising from the quality of the spun

tubes account for only 2.5% of the downtime. Except forincidental cases that occurred during downtime periods, it isessentially overheating that reduces the life of the tubes.Causes of this overheating have been mainly poor catalystperformance; operations above design conditions; inaccuratepredictions of the operating temperature; and poor burneroperation or fuel contaminants.

It is usually after three to five years service that the firsttubes facing difficulties are withdrawn from the furnace.However, for the most severely damaged furnaces, tubescan be taken from service after only one year. (2)

It is also usual to renew installations after 8 to 10 yearsof operation. This onstream time limit is the life barriergenerally accepted for optimum periods of downtime andrepair expenses. (3) Figure 1 is based on a survey of approx-imately 3,900 tubes of HK-40 alloy. The abscissa showsthe percentage of damaged tubes and the ordinale indicatesthe onstream time.

Some individual cases confirm that it is possible to im-prove considerably on the average slope and that life-timecould be increased, based on the best results representedby the bottom curve. That indicates that repairs or replace-ments would be postponed.

For improved alloys such as Manoir-Pompey's patented' 'MANAURTTE 36 X, " which has been used for more thanrune years for experimental tubes and seven years for pro-duction tubes, it has not been possible to establish a curveof the type representing the bottom curve in Figure 1. In fact,

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'/•'•'• ÏÏS

Figure 1. Reformer tubes — average life and dis-persion.

up to now, we have not yet heard of any case of rupture.For reformer tube applications,' MANAURITE 36 X

offers far higher resistance to overheating than does HK-40and the INCO 519 alloys, which are limited to 950°C. The36 X can withstand 1,100°C.

In Figure 2, the comparison of creep curves is sufficientlyclear to establish the difference existing among the threealloys being discussed, allowing a sizeable reduction ofthe tube wall thickness, under similar conditions for theimproved alloys. This in turn permits a lower thermalgradient through the tube wall, and induces less radialstress.

CREEP CURVES

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Figure 2. Creep rupture curves (rupture in 100,000hr.), for five alloys.

Methanol plants have, for example, replaced their HK-40tubes with tubes of MANAURITE 36 X. The latter have thesame external diameter, but a wall thickness of 11.5 mm.,as opposed to the 17.3-mm. thick walls of the tubes inHK-40. The calculated temperature of both the 36 X and theHK-40 tubes has shown that at a distance of 3 meters fromthe arch, the mid-wall tube temperatures are 887°C forHK-40 tubes and 830°C for the 36 X tubes; ie., a decreaseof 57°C for the latter. (4) Thus increasing tube life ex-pectancy.

Tube metallurgy

The only tube manufacturing process used presently iscentrifugal casting. It is actually the only means by whichthe most refractory alloys can be manufactured. The cold,or hot, drawn alloys compared to the same spun materialhave shown a loss in their creep resistance capability, therebyslightly increasing their ductility. The continuous castingmethod would perceptibly incur the same drawbacks. Conse-quently, it is the centrifugal casting method which representsthe only acceptable process owing to: 1) its very great flexi-bility now that tubes of 6 meters hi length are being made;and 2) its more economical price.

Over many years, tubes were supplied free of internalmachining, i.e. with a porous layer that resulted from thenatural conditions of alloy solidification that propagatesoxide pits on the unmachined tube wall surface.

This is the reason why most European users now choosebored surface tubes, which result in a minimum expense.The manufacturer can accurately control the depth andscattering of the porous layers resulting from solidification;and thus he can use 100% Eddy Current testing of the in-ternal surfaces. This test method allows successful detectionand localization of faults such as porous layers remainingafter machining, internal inclusions, bad surface roughness,cracks, and faults resulting from tool scratches. Some ofthese faults can obviously cause a reduction in tube life. Toensure highest quality, Manoir-Pompey uses eddy currenttesting for machined tubes.

Influence of scrap materials

It has of course been proved beyond doubt that creepbehavior improves through the creation of increasinglysophisticated materials. However, melting is an importantparameter that should not be neglected. In fact, the questionremains: is it enough to depend upon using the chemicalcompositions defined as correct for attaining the requiredcreep characteristics?

To reply to this question, we carried out creep tests ontest-bars taken from tubes of type HP 40 and MANAURITE900 manufactured as follows:

HP 40: One tube manufactured from 20% virgin mater-ials and 80% scraps; and one tube manufactured from 90%virgin materials and 10% scraps.

MANAURITE 900: One tube manufactured from 20%virgin materials and 80% scraps; and one tube manufacturederom 100% virgin materials, according to the usual method

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of manufacture for this grade of material. This latter alloyis generally used for parts external to the furnaces, likeheaders, manifolds, transfer lines, reducers, tees, etc. It isa patented material, low-carbon, stabilized 32 Ni 20 Cr,cast material that competes with INCO 800 H.

Two comments need to be made. Reverts are not consid-ered as scraps. They are cropped parts of as-cast tubes,which were by necessity cut to adjust the total length ofassemblies. To comply with the ladle analysis, additions ofvirgin material are made only before pouring, and onlywhen necessary.

Comparative creep rupture test results are illustrated bythe curves in Figures 3 and 4. It can be seen that, comparedwith 90% or 100% virgin materials, the time till rupture forall materials manufactured with 20% virgin material isclearly shortened. This explains why the slope of the stressto time and temperature curve based on the Larson & Millerparameter is more abrupt than for the reference curve of itsrespective basic alloy.

For this parameter, it can be noted that for the low valuesand short-time tests at low-temperatures, the points are closeto the scatter band; whereas for the higher values (longertime tests at elevated temperatures), the gap is accentuatedin comparison to the minimum of the scatter band. Sucha result gives good evidence that short-time tests cannottransmit a true indication of the effect of creep behavioron any alloy.

This phenomenon can be explained by the fact that, whena large percentage of scraps is used, it is equal to a simplemelting where there is practically no bubbling in the bath,which greatly impedes deoxidizing efficiency. However, theexistence of inherent oxides in the material components,

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must not be disregarded; due either to their small dimensionswhich do not allow sufficient décantation, or to the irreduci-bility of some of them (e.g. complex oxides, titanium,aluminium etc.).

The alloys manufactured with 80% scraps also have lessweldability than the 90% or 100% virgin materials.

One must conclude that the results prove and confirmthat scraps should strictly be prohibited materials.

It appears at present that the maximum admissible valueis approximately 30%, but for the MANAURITE 900 noscrap is used in fact; and it is manufactured with 100% virginstock. For the other heat-resisting materials only revertsare used in our manufacturing method, to a maximum of30%, or less, according to the customer's requirements.

Choosing suitable alloys

Selection of an adequate alloy has already been frequentlydiscussed. (5) We should now like to point out presenttrends in reasons for selection. The basis of this choicedepends obviously on the operating factors, which are:

1. Internal and external temperatures, their variationsin time, and along the reformer tube wall.

2. Internal pressure and its evolution.3. Miscellaneous mechanical stresses.4. The media concerned.5. Chemical reactions produced, and the resulting vari-

ous reactions and the combinations of oxidation andreduction.

For catalytic reformer tubes, most present operating unitsare provided with tubes in alloy 25 Cr - 20 Ni, HK-40. Forseveral reasons, and because of its creep behavior, gradeHK-40 has failed to satisfy operators. Furthermore, in newinstallations, the engineering companies are eager to supply

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Table 1. Proprietary alloys ami additive elements

Proprietary alloys Additional elements

MORE1 W (tungsten)MANAURTTE36X Nb(columbium)HOM Mo (molybdenum)MANAURITE36XS w + NbMANAURITE36D,etc Mo + miscellaneous

The following group shows richer contents of components(their use is reserved for very special cases):

SUPERTHERM 35 Ni + 10 Co + 25 Cr + 5 WSUPERNa22H 50 Ni + Co + 27 Cr + 5 WINCO807 40 Ni + 8 Co + 20 Cr + 5 W + 1 Nb

the best appropriate alloy to users who have already experi-enced a possible improvement.

Based on 25-20 (HK) alloy containing Columbium, Inter-national Nickel has developed a new material designatedIN 519 with 24-24 Ni-Cr Niobium content. The basic com-ponents of the improved alloys are mainly 35-25 Ni-Cr; i.e.HP. type alloys of the Alloy Casting Institute.

It is from this basis, however, that foundries have devel-oped derivative materials essentially by adding elements thatproduce carbides. Table 1 shows the proprietary alloys andthe additive elements.

Practically the most important development is the MAN-AURITE 36 X alloy which, by its technological qualities,gives the best performance/cost ratio, rendering it the mosteconomical alloy. This is the reason why more than severalthousand tons of tubes have already been fabricated inthe alloy.

Welding considerationsCreep phenomena wholly apply to weld beads because

the reformer tubes involve several butt-welded sections. Theweld joints are obviously subject to operating conditions(stress and temperature) similar to those exerted on the tubes.This fact implies investigation of a weld quality compatiblewith tee quality of the base materials.

Stege joints can be welded according to the various weld-ifif* procedures usually employed. These include: 1) themanual welding method with coated rods, a method usuallyused for filling; 2) the T. I. G. welding method (TungstenInert Gas), perfonäÄln the tube material, either manually,with or without filler metal, or automatically with or withoutfiller metal; and 3) M. I. G.. welding method (automaticprocedure using bare wire).

The arc welding is performed with the material preparedand under a shielding gas. The research center laboratoriesof Aciéries du Manoir-Pompey have conducted differentcreep tests on welded test samples. (6)

To check the results of the different types of welded joints,samples have been taken from three defined areas for eachwelded joint. A test bar has been taken in: 1) the parentmetal taken as a reference; 2) the filler layers of the weldjoint; and 3) the root run of the weld joint.

0 50 80 100%

ROOT PASS

SIMPLE MELTING

COATED ELECTRODES

SHIELDED METAL ARC

FILLING WITH DARE WIRE

GAS TUNGSTEN ARC

FILLING WITH RARE WIRE

GAS METAL ARC

Figures. Creep strength of weld metal vs. basemetal.

The results revealed that in a weld joint:1. The tendency to resist creep of the root run is the lowest

in comparison with that of the entire weld joint. This disad-vantage results mainly from the structure of the weld deposit.

2. The creep strength of the welded joints confirm thatthe automatic T. I. G. and M. I. G. procedures were superiorwhen compared to the creep resistance of the beads weldedby the manual method with coated electrodes. This strengthcould reach more than 80% of the creep resistance of thebase material (see Figure 5).

These substantiated assumptions have influenced the ex-tent of the use of automatic procedures, especially theM. I. G. method, for which macrographs of weld joints areshown in Figure 6.New tube developments

Today, however, materials with high resistance to creepare also required to withstand corrosion by the ash productfrom residual fuel oils. It is, in fact, common knowledge thatfor fuel for reformer furnaces, efforts are now being made toreplace natural gas (becoming scarce and thus expensive) byfuel oil. Such a change is not without problems however,because if ash corrosion is occurring, this could result in asubstantial reduction in tube life.

To fulfill the conditions necessary for good resistance toboth creep and ash corrosion, Manoir-Pompey has perfected

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Figure 6. Macrograph of an M.I.G. weld.

the manufacture of bi-metallic tubes, as explained in thefollowing section.

Various companies face the need to switch from gas to oilfiring fro high-temperature reforming or for preheating oper-ations.

As for corrosion due to ash products of residual fuel oil,there is controversy over the maximum content of metallicimpurities in oil which will not induce some attack at thetemperatures of 800 and 1,000°C. Some suggestions are forthe following maximum levels (in parts/million): Na f K,1.0: V, 0.5; S, 0.5. Others think that if the vanadium con-tent is less than 5 parts/million fuel ash corrosion will beavoided.

In any case, up to now, the major factor for catalyticreformer tubes has been the usage of suitable material offer-ing a very good resistance to creep at high temperatureswithout problem of corrosion.

Fabrication of bi-metallic tubes

To meet the conditions required for good resistance tocreep and ash corrosion, Manoir-Pompey has developed thefabrication of bi-metallic tubes using the following alloys:IN-657, to resist fuel ash corrosion, on the outer wall of thetube, 5 to 6 mm. thick layer of the nickel 50% chromiumalloy and Manaurite 36 X on the inside wall of the tube, 10to 15mm. thick after boring, to obtain high creep-resistance.

After the technique for producing these bi-metallic tubeshad been perfected, tests were carried out - mechanical andchemical. An etch test was made on a dross section of abi-metallic tube, and macrographs in Figure 7 show thewell-defined thicknesses of the adjacent alloys. Their dilu-tion boundary at the interface was micrographed and is seenin Figure 8

A flattening test, seen in Figure 9, demonstrated the goodbinding between the two alloys at their rupture point.

The tubes have also undergone the following tests:1. Chemical analysis made every 1.5 mm. Results can be

seen in the curves in Figure 10. It should be noted that thechemical analysis of the inside layer of the tube correspondsto the analysis of the 36X.

2. Mechanical tests at room temperature at the locationsshown in Figure 11. The basic mechanical properties con-sidered are those of the 36 X alloy; i.e. tensile strength -45kg./sq.mm., yield strength - 25 kg./sq.mm., and elonga-tion - 8% (4d.).

3. Creep to rupture tests at 1,100°C under 2.0 kg./sq.mm., and at 1,050°C under 2.5 kg./sq.mm. stressapplied, seen in Figures 12, 13, and 14.

4. Tests of weldability performed on welded tubes asfollows:

a) Melted root pass following T.I.G. automatic proce-dure without weld gap and filler metal, and shield gas99.9% pure argon.

b) The first filler passes have been executed accordingto the M.I.G. automatic procedure using Manaurite 36 X

COLD END

X 0,7

POURING END

X0,7

Figure 7. Macrographs of bi-metallic tube structureafter etching test.

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IN 657 AREAx 250

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Figure 8. Micrographs of the different areas of a bi-metallic tube.

Figure 10. CurveoftheC.Ni.Cr.Nb.inthebi-metallictube vs. depth.

Pompey bare wire, and up to the dilution area. Manually-coated electrodes 50/50 Mb have then helped toward thecompletion of the covering runs.

The complete welded joints have been radiographicallyinspected, revealing no visible defect at all. Figures 15 and16 refer to the macrograph and the micrograph of one buttjoint, taken at random in the fabrication and showing com-plete absence of fissuring or hot tear.

Progress has been made recently in the fabrication ofsmall-diameter tubes, which are the most difficult to obtainby spinning. It is now possible to centrifugally-cast tubeswith an internal diameter as small as 1 inch. A reductionin this diameter allows the use of very high pressures in thereformer tubes, such as 700 Ib./sq.in.

Dilution

AFTER FLATTENING TESTBROKEN PARTS AFTER

FLATTENING TEST

MACROGRAPH AFTER ETCHINGINCO 657

DILUTION BOUNDARYMAN 36X

Figured. Photograph of a bi-metallic tube afterflattening test, showing broken parts in upper pic-ture; and macrograph after etching (in lower pic-ture).

Figure 11. Locationsofthetestbarsformechanicaltests at room temperature in the bi-metallic tube.

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Dilution area

Figure 12. Location of the test bars for creep to rup-ture tests in the bi-metaflic tube.

Conclusions

1. For much better creep resistance than HK 40 alloy,Manoir-Pompey has developed its Manaurite 36 X withconsiderable success. This quality alloy allows higherservice temperatures and reduced wall thicknesses, whichleads to longer performance life.

Sim"Wun-i

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Figure 13. Creep to rupture results of the test barspicked up from Area No. 1.

Figure 14. Creep to rupture results of test barspicked up from Areas No. 2 and No. 3.

2. Firing with fuel oils has led to research alloys withgood ash-corrosion resistance but also high creep-resistance.The problem can be solved with bimetallic tubes in whichInco 657 is placed on the corroded side, and where Manau-rite 36 X withstands the creep stresses.

3. Creep tests reveal the necessity of using clean metal,thus limiting the use of scraps for the melt bath.

4. New possibilities of small diameter tubes allow an in-crease of pressure in reformer tubes. #

Literature cited1. Westenbrink, D.J., "Operational Problems with Re-

forming Furnaces," presented at Madrid, Spain, meeting

FigurelS. Macrograph of a bi-metallic butt-jointtube weld.

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AREA n°1(50 / 50 )

x 250

AREA n°2( DILUTION )

x 250

"•\;- •'•': •-••:'~ ̂ l^^Y^/Wtf?:';. -:V .;•'.• ;fc-:--'--:- ;" ;:; "-''':^':'!'^•'. ^^//.i^SSw'.SJ^-ï''--''.':-.^^.;,;^

;/: : !''"V: '::.'. V'ïh:?«;.^S='^'j'-'AV^iAREA n°3(MAN 36x)

x 250

Figure 16. Micrographs of the bi-metaliic butt-jointtube welds at three indicated areas.

on Metallurgical Advances in Design of ReformingFurnaces, May, 1975 (Edition, Inginiers Quimica,Madrid.).

2. Estruch, B., "Life Estimating for Reforming Tubes,"presented at Madrid, Spain, meeting on MetallurgicalAdvances in Design of Reforming Furnaces, May, 1975(Edition, Inginiera Quimica, Madrid).

3. Salot, W.J., "The Trouble with HK 40 High-PressureReformer Tube Operation — Reformer InformationNetwork," CEP Technical Manual 16, 1974.

4. Jansen, J., "Lessons Learned from Radiant Tube Crack-ing," Unie van Kunstmestfabrieken.B.V., Pernis,Netherlands.

5. Hubert, R., and J. Thuillier, "Choice of Heat ResistingAlloys for Centrifugal and Statically Cast PetrochemicalFurnaces Components," Edition Acieres du Manoir-Pompey, 1970.

6. Hubert, R., M. Hugo, J. Thuillier, andF. Pons, "WeldStrength of Reformer Tubes in Heat Resisting Alloys,"Edition Acieres du Manoir-Pompey, 1972.

PONS, F. THUILLIER, J.

DISCUSSION

KEES A. VAN GRIEKEN, UKF: Question: How do youget your samples to determine the creep strength ofthat used material. Is the extrapolation based on highertempeature or higher load?THUILLIER: The test bars taken for stress to rupturedetermination all come from normal manufacture andare cut at the end of tubes. Various heats were sub-jected to test. Several temperatures and loads wereused to determine the curves and tests longer than10,000 hours were performed to assume a good extra-polation as feasible. This is confirmed when the rangeof the scatter band of the Larson & Miller curve isnarrow.VAN GRIEKEN: One remark: you said that Manaurite36X tubes could get lower temperatures and you referto certain literature. I suppose it was a former presen-

tation of results based on investigation of our company.I am not sure. Part of this lower temperature in thatcase was due to the higher internal diameter, thatmeans more catalyst. Is that taken from the paper ofMr. Jansen?THUILLIER: Yes. It is based on that reference of thelist. But I could say that many people know that wedeliver a great number of tubes in that Manaurite 36Xsince several years. Up to now we have not had anyfailures at all. Generally, failures occur during the first3 years of service of a plant built in custom alloy. Withthis better grade we are able to greatly postpone thistime.VAN GRIEKEN: Question: You said it is preferable todo M.I.G. welding as to manual welding; does it applyonly for the HK 40 or also for your proprietary grade?

96

THUiULIER: We have matching wire for both qualities.JACK BLACKBURN: A.P.V. Paramount: I waspleased to see actually a manufacturer of tube hereon the platform because there aren't many of us about.Those of you who think making ammonia is difficultshould try to make first class tube. You will find you'vegot a very easy job in comparison. Quite rightly,Jacques Thuillier talked about the importance of rawmaterial in making a tube but I believe he rather over-simplified the case.

When he presented the difference in creep ruptureproperties between virgin and scrap charges, I couldn'thelp thinking that if he looked around the walls of hisfactory at Le Manoir, he could have found some evenworse examples of alloy SH 1T, melted them, andstress rupture tested them and got even poorer results.Similarly, I think that there is a danger in using the word

virgin in a non biological context. Fred Jones used tosay in this gathering, about the young lady - you can'tbe slightly pregnant. You can't be quite so dogmaticabout metals, which in fact have little concept ofheredity about them.

The point is that most advanced manufacturers thesedays take full advantage of secondary materials, butrefine them first. And having refined them, they areproducing materials indistinguishable from the socalledvirgin materials. The message I would like to pass on is,in writing specifications, don't make a burden for your-self. Say what you want. Say waht you want in terms ofproperties. Say what you want in terms of residualelements, but accept that your tube manufacturerknows his business as well as you know yours. Givehim a chance to do it.

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