hrsg corrosion

7/28/2019 Hrsg Corrosion

1/8OH-14 COMBINED CYCLE JOURNAL, Third Quarter 2005

HEAT-RECOVERY STEAM GENERATORS 2006 OUTAGE HANDBOOK

G

a s - s i d e c o r r o s i o naffects all heat-recov-ery steam generators(HRSGs). Consequenc-

es range from unsightliness andreduced performance to reli-ability problems and potentialsafety hazards. Presented hereare thumbnail sketches of sev-eral corrosion mechanisms thatyou may find when inspectingyour boilers. They explain howto identify the different types ofcorrosion, the consequences ofinaction, problem correction, andwhen to call for outside assis-tance. In most cases, a relatively

small investment is all it takesto assure long life for your HRSGand maximize its availability.

1. Dewpoint corrosionMoisture contained in gas-tur-bine (GT) exhaust gas will con-dense on HRSG heat-transfersurfaces when that metal is belowthe gas dewpoint temperature,which typically ranges from 112Fto 120F. Dewpoint corrosion usu-

ally is found in low-pressure (l-p) economizers and condensateheaters that receive water froma relatively cool sourcethe con-denser hotwell, for example.

Piping, headers, and tubes inthese HRSG components operatevery close to the water tempera-tureespecially in upper andlower crawlspaces, where there isrelatively little gas flow and heattransfer. The inlet piping, head-ers, and tubes are at the lowesttemperature and most likely toshow signs of attack.

Some OEMs (original equip-ment manufacturers) specify

alloy materials to protect againstcorrosion. Others provide a recir-culation system or external heatexchanger to increase the tem-

perature of incoming feedwater.However, many HRSGs in ser-vice were built with carbon-steelmaterials in areas where metaltemperature is well below thegas dewpoint. Do you have one ofthese units?

What to look for. Dewpoint cor-rosion is easy to identify visually

given access to the gas side of theHRSG where feedwater enters.Compare the condition of inletpipes, headers, and tubes to thatof nearby piping, headers, andtubes: If components at the feed-water inlet exhibit greater mate-rial wastage, dewpoint corrosionis the likely cause (Figs 1-3).

Consequences. Dewpoint cor-rosion has caused numerous tubefailures in HRSGs. Experienceindicates that carbon-steel tubesoperating in a condensing envi-

Identify, correct HRSG gas-side

corrosion problemsBy Bryan Craig, HRST Inc

Inlet header

Inlet pipeSplitter platelocation

First pass tubes

Second pass

Third pass

Inlet (first pass)tubes are corroded

Second pass tubesare not corroded

Inlet header

Inlet tubes

3. Inlet tubes and header exhibitwastage; other tubes are not corroded

1. Dewpoint corrosion attacks coolfeedwater inlet components

2. Damage is in evidencewhere first-pass tubes intersect with inlet headerand metal is coolest; theres no dewpoint corrosion of second-pass tubes


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2000 Day Hill Road Windsor, CT - USA 06095-0500 Phone: 860.285.5478 Fax: 860.285.9666 www.alstom.com


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OH-16 COMBINED CYCLE JOURNAL, Third Quarter 2005


ronment have a service life rang-ing from two to 15 years. Actualwater inlet temperature, operat-ing hours, the presence of ammo-nia salts, and other variablesdetermine the actual service life.

Corrective action. Prevent

carbon-steel components fromoperating below the GT exhaust-gas dewpoint. If your system isequipped with a recirculationsystem or other means for main-taining feedwater inlet tempera-ture above the dewpoint, makesure it is operating properly. If nosuch system is installed, considerretrofit of a recirculation system,an external heat exchanger, ora preheat coil designed for con-densing service to prevent corro-

sion (Fig 4).

2. Offline corrosionThis is a relatively recent phe-nomenon because HRSGs aremore likely to spend extendedperiods offline today than in thepast. The large thermal mass ofHRSG components causes themto lag behind naturally fluctuat-ing ambient temperatures. If aperiod of relatively cool weatheris followed by warm humid condi-tions, HRSG metal surfaces mayremain below the ambient dew-point for several days, causingthe metal to sweat like a coldbeverage on a summer day (Fig5). This, of course, increases thecorrosion rate of those surfaces.

What to look for. HRSGs thatoperate in a cycling mode typi-cally exfoliate more fin scale thanthose in base-load service (Fig 6).The scale is flaky in appearanceand may fill the gaps betweenthe fins. It also finds its way toaccess lanes and lower crawl-space floorsoften resulting in

deep piles of debris (Fig 7). Often,offline corrosion is most severe inthe middle of the HRSG or one ortwo access lanes upstream of thestack.

Consequencesinclude reducedperformance caused by foulingof heat-transfer surfaces. Such

performance reduction oftenis difficult to quantify, and insome instances may be less thanexpected considering the appear-ance of the finned tubes. Debristends to foul surfaces in low-velocity zonesfor example, thedead spot on the downstreamside of each tube, where littleheat transfer occurs.

Additionally, reduced heat-transfer in one section typically isoffset by increased heat transfer

downstream, because those tubessee hotter gas than they wouldnormally. However, cleaning offouled heat-transfer surfaces byCO2 blasting or other methodsstill may be warranted based onperformance improvement.

Be sure to collect and disposeof corrosion products that pile upon the floor of the HRSG. Thisdebris usually is absorbent andhygroscopic. Thus any moisturethat enters the HRSG casing is

retained, accelerating corrosionof the floor liner, drain piping,and casing penetration seals.

Correction action. Clean heat-transfer surfaces and floors annu-ally or more frequently. Offlinecorrosion can be prevented orreduced by maintaining a low-humidity environment in theboiler while offlineprovidedthe problem is severe enoughto justify the cost. This can beaccomplished by adding heat orby dehumidification. In eithercase, a stack damper or balloonis recommended to seal off thespace being conditioned.

3. High-temperatureoxidationTwo important points to remem-ber here: Metal oxidizes rapidlywhen exposed to excessive tem-peratures, and practical tem-perature limits for oxidationresistance vary with the alloy.High-temperature oxidation typ-ically is a threat to liner systems,

To low-pressureevaporator



Gas flowGas flowGas flow

Coldcondensate

Coldcondensate

Coldcondensate

Add apreheatercoil fabricatedfrom materialsuitable forcondensing

service

CONDENSING PREHEATEREXTERNAL HEATEXCHANGER

RECIRCULATION

4. Alternatives for maintaining inlet feedwater temperature above the GTexhaust gas dewpoint

5. Condensate drips from lowerreheater header while unit is shut

down

7. Offline corrosion causes debris toaccumulate in lower crawlspace

6. Scale between fins is a conse-quence of offline corrosion


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fin tips, duct-burner components,and baffles (Figs 8, 9). For exam-ple, inlet ducts and firing ductsexperience temperatures thatare too high for carbon steelscommonly used for other com-ponents and must be fabricatedfrom stainless steel. By contrast,high-temperature oxidation ofpressure parts is highly unlikelybecause these components havematerial strength limits that

require operating temperaturesto be below those at which oxida-tion becomes a factor.

High-temperature oxidationoccurs in HRSGs for these rea-sons:Incorrect material selection.Specifically, the temperature isas expected, but the material isnot suitable for use at that tem-perature (Fig 10). This can occurbecause of an engineering error,or a material mix-up in the fabri-cation shop or during field instal-lation.

Temperature exceeds thedesigners expectation. Thisrarely occurs in HRSGs withoutduct firing because GT exhausttemperature is very predict-able. Units with duct firing mayhave areas where temperaturesexceed the designers expecta-tion because of unaccounted-forradiation from burner flames,poor fuel-gas flow distribution,and poor exhaust gas flow dis-tribution. The last is most com-mon.

What to look for. Conspicuous

color changes in a liner systemcan indicate a problem (Fig 11).For example, if a duct liner is sil-ver in color except for a few redsheets or washers, it may be thatthe wrong material was used forthose parts. Note that a red coloris not necessarily bad; some liner

materials are supposed to be red.Focus your attention on colorvariations.

A magnet often can provide aquick verification as to whether amaterial is what its supposed tobe. However, this doesnt alwayswork, because multiple grades ofboth magnetic and non-magneticmaterials are used in HRSGs.Consider sending a small sampleto a metallurgical lab to learnexactly what materials were

used.Perhaps the most obvious signof high-temperature oxidationis burned-up, swollen, sagging,or crumbling metal. Such condi-tions generally indicate the pres-ence of flow distribution problemsin supplementary-fired units.Keep in mind that duct burnersrequire a relatively uniform GTexhaust gas flow profile acrossthe burner grid for trouble-freeoperation.

Consequences.High-tempera-ture oxidation can lead to compo-nent failure and other operation-al problems if the cause of theproblem is not addressed. Thespecific consequences dependon the component affected. Forexample, in liner systems, high-temperature oxidation mayresult in (1) loss of insulation, (2)hot spots, (3) excessive pressuredrop through the SCR (selectivecatalytic reduction) because cata-lyst is blinded by liberated insu-lation, etc.

Corrective action depends onwhy high-temperature oxidationoccurred in the first place. Com-ponents failing because incor-rect materials were used in theirfabrication obviously must bereplaced. For local areas wheregas temperature exceeds thedesigners expectations, correc-tive action specific to the situ-ation can be identified by wayof an engineering evaluation.Consider calling in the OEM or athird-party expert to conduct thesophisticated analysis required.

8. High-temperature oxidation damaged this igniter box

10. Incorrect material use was causeof high-temperature corrosion here

9. Oxidized burner componentsrequire replacement

11. Conspicuous color changes inliner system help identify problem


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4. CasingpenetrationsPipe penetrations that pierce theHRSG casing represent a uniquechallengeactually two chal-lenges because roof and floor pen-

etrations each are subject to theirown set of corrosion problems.

Roof casingpenetrationsRoof penetrations can allow rain-

water to leak into the HRSG (Figs12, 13), corroding the pipe andsurrounding liner plate, whenthe seal at the penetration/roof

joint is inadequate.What to look for. Check areas

where penetrations come throughthe roof following a heavy rain to

see if water pools, which meansthat the potential for leakageexists. Additionally, penetrationsthat do not seal properlyforexample, fabric seals with looseband clampsare problematic.If there is access to the spacesabove the tube bundles insideyour HRSG, look for pipe andliner corrosion near liner pen-etrations.

Consequences. Pipe and linerfailure can result from this type of

corrosion; but not likely, based onfield experience. In some cases,rainwater leakage into offlineHRSGs has led to the mistakendiagnosis of a pressure-part leak.Such improper conclusions wasteoutage time and resources.

Corrective action. This is a rela-tively easy problem to fix and shouldbe addressed. Rain-water pools can becorrected by bor-ing holes through

stiffeners or otherstructure to allowwater to drainaway. Periodicallyremove debris fromdrain holes. Repairor replace leakingpenetration seals.

Floor casingpenetrationsFloor penetra -tions are at lows p o t s w h e r edebris and mois-ture conducive topipe corrosion canaccumulate. Drain penetrationscan be particularly problematicbecause there normally is noflow through the drains duringoperation to maintain warmth.Thus that portion of the drainpipe inside the penetration sealmay be cooled to below the gasdewpoint when cool ambienttemperatures prevail, creating acorrosion site.

What to look for. Remove a

few drain penetration seals fromsuspect locations and inspectthe pipe both visually and withultrasound (Fig 14). This is rela-tively easy with clamped-on fab-ric penetration seals, but is morechallenging with stainless steelbellows.

Visually inspect pipe penetra-tions from inside the HRSG cas-ing, to ensure they are protectedfrom debris ingress by a function-al liner donut (metal flashingaround the pipe). If there are nodonuts, or if some are out of posi-tion so that you can see into theinsulation space around the pipe,check for debris, moisture, andpipe corrosion.

Consequences. Drain-pipefailures caused by external cor-

rosion of drain pipes inside thepenetration seal are common.Corrective action. Bore a

small hole through the metalring at the bottom of a fabricor metal-bellows penetrationseal (Fig 15), permitting a smallamount of exhaust gas to leakthrough and warm the pipe.

Controlled-leakage mechanicalseals do not require this. Repairor replace failed and missingliner donuts.

5. Stack corrosionStack corrosion is not unusual,particularly in HRSGs that areoffline for extended periods. Dur-ing operation, the exhaust gastemperature typically is high

Drain holes drilled to locate leak

12. Rainwater leaking in throughpenetration seal corroded economizervent pipe

13. Leakage through riser penetra-

tion reaches tube bundle below

14. Measure thickness of drain-pipewall to determine corrosion rate.Access is easier if penetration seal isremoved from casing exterior

15. Drill holes in base of penetration seals to detectleaks. Allow holes to remain, enabling GT exhaust gasto heat seal and minimize opportunity for corrosion


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enough to keep the stack dry andminimize corrosion. However,it is relatively common to seestanding water in the back of astack during an outage. This vir-tually guarantees rapid corrosionattack.

Mostperhaps allHRSGstacks have low-point drains.But they often plug with debris.

In addition, stack drains nor-mally come with a blind flangeor a valve to seal the stack drainduring operation. Occasional-ly, these drains are not openedwhen the unit is out of servicebecause of oversight or concernsover effluent discharge. In otherinstances, the drain is not at atrue low point, or there is improp-

erly sloped piping that preventswater from draining as intended.What to look for. Inspect for

standing water or mud in thestack base. If water is not pres-ent, check for signs of a previouswaterline. Heavy scaling on thestack interior near the base is anindicator of previous corrosion.If stack-wall corrosion appearsto exist, check metal thicknessin the affected area and above it(Fig 16).

Consequences.Most stacks in

the US are self-supportingthatis they rely on the stack shell forstructural integrity rather thanon external supports. This meansthat significant corrosion of thestack wall could result in failure.

Corrective action. Avoid hav-ing standing water in the stackbase (Fig 17). Check stack drainsopen when the unit is offline andverify that drain piping is slopedproperly. Remove any debristhat could restrict flow of rain-water from the drain. Shovelany lose debris out of the stackbase. CCJOH

Performance analysis

Root cause failure analysis

Outage inspections and service

Design upgrades and retrofits

Training

Design review and specification

HRST, Inc.7510 Market Place Dr, Suite 102Eden Prairie, MN 55344

Phone: 952-833-1428 www.hrstinc.com

HRST, Inc.7510 Market Place Dr, Suite 102Eden Prairie, MN 55344

Phone: 952-833-1428 www.hrstinc.com

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Palm Desert, CaliforniaJanuary 18-19, 2006

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Learn where to prioritize future action.

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16. Corroded stack base. Num-

bers on wall are ultrasonic thicknessmeasurements. Material wastage issignificant

17. Inspector found approximately 8in. of standing water in stack base

COMBINED CYCLE JOURNAL, Third Quarter 2005 OH-21

hrsg corrosion

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