Rst-152 Problems & Failures - Boiler Piping & Supports

8/20/2019 Rst-152 Problems & Failures - Boiler Piping & Supports

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AA DDBB RRIILLEEY Y TTEECCHHNNIICCAALL PPUUBBLL IICCAATT IIOONN

Post Office Box 15040Worcester, MA 01615-0040

http://www.dbriley.com

RST-152

CURRENT EXPERIENCE IN

TYPICAL PROBLEMS AND FAILURES WITH

BOILER PIPING COMPONENTS AND SUPPORTS

by

James P. King, Design ManagerDB Riley, Inc.

Presented at the1998 ASME Pressure Vessels and Piping Conference

San Diego, CaliforniaJuly 26-30, 1998

DB Riley, Inc. is now Babcock Power Services Inc., aBabcock Power Inc. company.

www.babcockpower.com


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© D B Riley, I n c. 1998

CURRENT EXPERIENCE IN TYPICAL PROBLEMS AND FAILURESWITH BOILER PIPING COMPONENTS AND SUPPORTS

by

J ames P. King, Design ManagerDB Riley, Inc.

ABSTRACT

Th i s paper w i l l pr ovi de some cur r ent exper i ence wi th t ypical pr oblems and f ai l ur es of

boi l er pi pi ng component s and suppor ts, togeth er w i th a descr i pti on of the r ecommended

actions and r esolut i ons. Th e focus of t he paper wi l l be on recent exper i ence i n t he areas of

water i nd ucti on in sloped steam l i nes and fl ow accel er ated cor r osi on in w ater car r yin g li nes.

I n add i ti on, some pr obl em ar eas wi th super heater crossover an d outl et pi pi ng component s

w i l l be descr i bed.

Water i nd ucti on from condensin g steam can occur at l ow poin t l ocati ons i n pi pi ng li nes,and l ead to corr osion, pit t ing, and th erm al quenchin g with resul tant di stort ions, crackin g,

and fai lu r es. Fl ow acceler ated cor r osion is a ver y topical subject. I t i nvolves the thi nni ng of

pi pi ng component s from th e i nt er nal effects of w ater vel oci ti es and chem i cal make-up etc.

A detai led over view of w ater in ducti on and fl ow acceler ated cor r osion i n boil er pipi ng

component s wi l l be pr esent ed, i n add i ti on to recent exampl es in th e form of case hi stori es,

together wi th their appli cable find in gs, conclusions, and r ecomm endat ions.

INTRODUCTION AND BACKGROUND

Over the pa st fifteen yea rs, condition assessment progra ms ha ve routinely been sched-uled for fossil fired boiler piping components and supports as part of annual or major out-ages.

Init ially, the prima ry focus of such progra ms w a s on the current condition of the highenergy stea m lines routed from t he boiler to the turbine. This at tention wa s driven by theoccurrence of several catastrophic failure events in hot reheat steam lines fabricated withlongitudina l seam w elds. This pa per focuses on recent experiences w ith the phenomenaknown a s wa ter induction which ca n occur in sloped steam lines including th eir sma ll diam-eter branch and drain piping lines.


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In more recent times, increased at tent ion ha s been given to the feedwa ter piping lines tothe boiler. This bas ed on fa ilure events r ecorded for such piping systems in both fossil andnuclear power plant s. These failures have been a t tr ibuted to a phenomena known as f low accelerated corrosion.

Over the years, increa sed at tent ion ha s been given to the boiler proper piping lines.

Problem areas have been identified on systems such as the superheater crossover and out-let piping, including th e branch feedwat er piping to the at tempera tor nozzles on thecrossover lines. The problem ar eas include cra cking in th e piping component w elds whichhave been a t tr ibuted to over t emperature condit ions, therma l fat igue, an d non-functioningsupports.

Historically, electric ut il it ies have scheduled unit outa ges on a n a nnua l basis, typicallyduring the spring or fa ll months w hen the load demand is lower. Today the util i t ies ha veincreased the intervals between ma jor outa ges up to as long a s thirty -six months. This pat-tern of scheduling less frequent outa ges requires much better record keeping, a nd moredetailed planning for the inspection and maintenance tasks performed during the outage.

For a typical condit ion a ssessment progra m of a boiler st eam piping system, the w ork

ta sks w ould include the following items.• R eview of R ecor ds• H ot a n d C old Wa l kd ow n I n s pect ion s• O.D. and Circumferen t ia l P ipe/Header Measurements• U lt rasonic Thickness and Shear Wave Test ing• Rep lica t ion Meta l lography and Ha rdness Test ing

If deemed necessar y, a current st ress an a lysis progra m w ould be performed for a pipingsyst em. This due to revisions in the piping geometry, changes in unit opera tion or the exis-tence of deteriorated supports. For th e lat t er i tem, proof test ing of support assemblies canbe done on-site t o determine if th e component is st ill ca pable of ca rrying its origina l designloading.

WATER INDUCTION

Water induction from condensing steam can occur in boiler piping lines and cause corro-sion , pit t ing a nd thermal q uenching, with resul t ing d is tor t ions, cracking and subsequentfailures. This condition is most likely to occur in sloped, horizonta l steam lines at low points.Wa ter can a lso be intr oduced into steam lines from back flow t hrough dra ins an d sa mplelines. For boilers with horizonta l seconda ry superheat er elements; steam w hich becomesentra pped in t he tubes during a cool dow n event w ill quickly condense a nd overflow into theoutlet header a nd ma in stea m piping, depending on the routing or loca tion of the compo-nents.

Therma l quenching ca n occur when t he condensat e comes into conta ct wit h a head er orpipe bottom section. Repea ted cycles of this resulting therm a l shock ca n eventu a lly lea d toexcessive bending, perma nent distort ions, such a s sa gging, and cra cking of the components.A sa gging pipe condition can be initia ted a nd/or agg ra va ted by t he existence of support s th a tar e not functioning or carrying t heir load, as designed.

Remedial measures used to prevent or lessen the effects of thermal quenching are:a dherence to specified boiler cool dow n ra tes, tempera tur e monitoring of the t op a nd bottomsections of components by t hermocouples, a nd n ondestructive exa mina tion of components toesta blish their current condition. Retrofit modifica tions have included stea m line rerouting,


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enlarging of drain a nd vent l ines, an d the a ddit ion of valves in dra in lines to prevent ingressof the condensa te.

The subject of water induction and thermal quenching of steam piping components hasbeen a ddressed in t he Reference 1 and 2 papers.

DB Riley, Inc. has been involved in two studies, wh ich include the assessment of steam

piping lines with w a ter induction problems, w hich a re described as follow s.

Main Steam Piping Line

A full condition assessment program was recently performed by DB Riley on the mainstea m piping line of a 1970’s vinta ge fossil fired utility boiler. All of the work ta sks, a s list-ed earlier in this pa per, were performed on the piping components, welds, an d supports.

B oth hot a nd cold wa lkdown s of the piping system w ere performed. Severa l anomalieswere visually apparent . First , sagging wa s evident in the lower sect ion of a long horizonta lrun of piping. The maximum am ount of sag wa s later measured to be seven inches. (See theFigure 1 P hotograph). Nondestructive test ing ta sks, including metallographic replicat ion,w ere performed on th e outside surfa ce of the sagged portion of pipe. They revealed a more

adva nced sta ge of spheroidizat ion tha n a t other piping locat ions. However, there wa s no evi-dence of creep dama ge, loss of har dness, or component thinning. A near t erm recommenda-tion wa s given to monitor th e condition of the sagged portion of piping, including th e ta kingof a core sample at t his location, in order to record the microstr uctura l an d surfa ce condi-tions on the inside of the component.

Figure 1 Main steam piping general view of sagged horizontal portion.

A second finding was the current overall condition of the support system of the mainsteam piping line. Several hangers were visibly dam a ged, and others had set t ing indicat ionswh ich showed the support to be malfunctioning or bottomed out . Of part icular concern w asthe th ree pa irs of dua l consta nt force support a ssemblies, on the horizonta l run of pipe,w hich were found to be skewed from the norma l horizonta l and vert ica l axes, in both t he hot


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Figure 2 Main steam hanger in cold condition. View shows the severely skewed support which indicates that the constant supports

are not functioning properly.

Figure 3 Main steam line. Constant force supports in hot condition.It can easily be seen as skewed.

and cold condit ions. (See Figures 2, 3, and 4 P hotographs). Recommendat ions were givenfor a re-evaluat ion of the ma in steam piping ha nger system to ensure its integrity, and forrepair or replacement of han gers/supports, as necessary, to assure tha t t hey are carryingth eir design loa ds.


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Cold Reheat Piping Line

DB Riley and others were involved in the failure evaluation of a ruptured portion of acold reheat st eam lead t o a boiler. This is a mid 1960’s unit . The rupture init ia ted in a hor-izont a l piece of the pipe an d propa ga ted int o pa rt of a bend a t t he inlet to the boiler.

Due to the low er operat ing pressures and tempera tur es of a cold reheat line, the pipingcomponents are typically ma de from plain carbon steel ma terial . Quite often, an d for thispart icular case, the pipe fabricat ion w as from r olled plate w hich is longitudinally sea m w eld-ed. The area of cra ck init ia t ion was at a low point in the horizonta l length of seamed pipingwhere condensing steam had collected during boiler cool down events over the past years.For t his sect ion of pipe, the longitudinal w eld was locat ed on the bottom. Corrosion a nd pit-ting h a d occurred along the w eld due to the cyclica l presence of the condensa tion. Over time,the pit t ing had aligned with eventua l cra ck forma tion, propaga tion, and ult imat ely a rup-ture.

A check for th is potentia l condensa te corrosion da ma ge on cold rehea t piping lines sh ouldbe a pa rt of scheduled piping inspection and a ssessment programs. A visual wa lkdown willlocat e potential condensate entra pment regions. Interna l video inspections ca n determine

the existence of a ny corrosion, pitting, or cracking.

SUPERHEATER CROSSOVER PIPING

Superheater crossover systems are part of the boiler piping and are designed to conveythe superheated steam from the primar y to ra diant , an d radia nt to seconda ry superheatersections. Att emperat ors located in the superheat er crossover piping are used to contr ol thesteam tempera tures by introducing spray w at er to the steam flow.

Figure 4 Main steam constant force support in hot condition.They are also damaged and observed to be skewed.


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Condition assessment programs should be scheduled for components of superheatercrossover piping components a nd w elds including the a tt emperat or assemblies, since crack-ing ha s been found in these components dur ing routine exa mina tions. The condition a ssess-ment of a tt emperat ors is discussed in deta il in the Reference 3 pa per.

The cra cking is typica lly due to therm a l fat igue, a r esult of over tempera tur e conditions

an d cyclic or load sw ing operat ion. Distress in components locat ed down stream of the at tem-pera tors can addit ionally be at t ributed to the frequency a nd dura t ion of wa ter spray events.

The condition of crossover piping supports should also be documented on a scheduledbas is, since such hangers ha ve been found to be non-functioning or ina dequa te, resulting inover st ress conditions on th e piping components. A ca se study of crossover piping problemsis described in the Reference 4 paper. A typical ra dian t superhea ter crossover piping syst emis shown in F igure 5.

Figure 5 A typical radiant superheater crossover piping system

For such condition a ssessment progra ms, the following crossover superheat er pipingcomponents should be evalua ted.

• The nozzles and welds a t t he piping to the superheat er in let a nd out let headers.• The a t tempera tor components including the spray nozzle, l iner and a t ta chment

welds, and feedwa ter pipe welds.• P ipe elbow w elds.• R a diogr a ph ic plug s.• P i pe su ppor t a ss em blies.

SUPERHEATER OUTLET PIPING

Several years ago DB Riley performed an evaluation of an elbow with severe inter-na l gouging, located in the outlet st eam piping on an indu str ial boiler.

This 1983 boiler is ra ted a t 275,000 lbs per hour. This elbow component is loca ted inthe superheat er outlet piping between the superheat er outlet header a nd a n a t temperatorsta tion in the penth ouse of the boiler. The at tempera tor is used to cool the outlet st eam t othe turbine.


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The inside surfa ce of the elbow w a s char a cterized by loca lized deep gouging a t a loca-tion corresponding to the extra dos. Associat ed with some regions of gouging w ere interna ldeposits. La bora tory measurements confirmed that the deepest gouge had penetra ted about75%of the elbow wa ll . As part of a deta iled evaluat ion of this condit ion, DB Riley performedoptical meta llogra phy, scanning electron microscopy, x-ra y diffra ction, a nd alloy a nd chemi-cal a nalysis.

The conclusion to t his comprehensive a na lysis of th e gouged elbow is t ha t the condi-tion was due to a combination of fluid impingement coupled with a corrosion mechanism.The pattern of gouging indicated some physical impingement consistent with water spray,especially during low steam flow condit ions. High sodium content found in t he inside sur-face deposits suggested th a t t he corrosion mecha nism w a s caust ic gouging.

Recommendations were given for inspection of the remaining steam line and attem-perat or components, replacement of the subject elbow, possible re-routing of components,and review of the water chemistry and the steam temperature versus water spray f low requirements.

FLOW ACCELERATED CORROSION

In recent yea rs th ere has been a serious industr y concern w ith a number of insta ncesof failures in feedwater and wet steam piping components due to thinning by a phenomenaknown a s flow a ccelera ted corrosion.

Flow accelerated corrosion (FAC) is relatively slow thinning of an inside pipe or com-ponent w a ll w hich can lead to break-before-leak type failures. It occurs in piping systemsw ith h igher flow velocities a t m ore turbulent regions. This w ould include piping sections fol-lowing valves, tees, reducers and bends.

The follow ing par a meters h ave been found to influence wa ll thinning of plain carbonsteel piping by flow accelerated corrosion.

• P ip ing Con figura t ion /Geomet r ic Loca t ion• F luid Chemist ry• Tempera ture• P ipe Ma teria l• F luid Velocit y

Condition assessment ta sks for feedwa ter piping system components include ultra -sonic or eddy current t hickness test ing, alloy t est ing, an d interna l video inspection in ar easor component s of su sceptibility.

The pow er indust ry a wa reness to th e potent ial for piping fa ilures due to flow a ccel-erated corrosion began as a result of a pipe rupture at the Surr y U nit No. 2 nuclear pla nt in1986. After this fa ilure, a study w as underta ken by the Nuclear Power G roup of the ElectricPow er Resea rch Inst itut e (EP RI), to better underst a nd the flow accelera ted corrosion mech-an ism and prevent i ts occurrence. In contrast , the phenomena wa s almost unknown a mongfossil plants until the mid 1990’s.

A fossil plant conta ins hundr eds of feet of wa ter piping components, a nd flow a ccel-erated corrosion can occur anyw here, under the right circumsta nces. EP RI has issued theReference 5 Technica l Report to help identify t he most s usceptible syst ems a nd component sfor inspection. The report a lso a ddresses the development of long term st ra tegies to avoidfuture problems, including changes in water chemist ry, system design , and ma ter ia ls . Oneof the key inhibitors in the resistance of FAC in carbon steel piping components is theam ount of chromium present in t he ma terial .


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In conjunction with t he report , EP RI offers a predict ive softwa re progra m calledCH EC U P, wh ich models FAC wea r in susceptible fossil plan t systems a nd components ba sedon flow a nd operat ing conditions.

Feedwater Piping

Over the past three years DB Riley, Inc. has been involved in the assessment a nd fail-

ure evalua tion of feedwa ter piping component s of fossil plant s.

The assessment t asks involved ultra sonic thickness test ing, wh ere a grid pa t tern ofmeasurements w as ta ken on t he uncovered outside surfaces of elbow, bend, reducer, branchconnection, tee and str aight pipe locat ions in higher flow a nd turbulent regions. Wherea ccessible, interna l inspections w ere performed a nd r ecorded by use of videoscope equip-ment. An exa mple of the inside sur face condition of a section of pipe exposed to flow a ccel-era ted corrosion is show n in t he Figure 6 photogra ph.

Figure 6 Typical example of a piping section with evidence of flow accelerated corrosion on the inside surface.

DB Riley and oth ers w ere involved in the failure evalua tion of a r uptured section of feed-wa ter piping in a fossil fired boiler. The rupture occurred in a str a ight portion of pipe, down-str eam of the check an d contr ol va lves, in the bra nch leg, from a t ee junction to the boiler’seconomizer inlet hea der. A thorough investigat ion confirmed the presence of thinning byFAC, localized to the ar ea of ruptur e. Ext ensive inspection and t esting of the other pipe sec-

tions and fittings of this system revealed no other locations with severe thinning.

SUMMARY

Cur rent experience w ith t ypica l problems and failures of boiler piping and su pports ha sbeen presented herein. Empha sis has been focused on the effects of wa ter induction a ndflow a ccelerated corrosion on piping components. In a ddit ion, some problem a reas w ithsuperheat er crossover a nd outlet piping ha ve been described.


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It ha s become a common practice with electric utilities to schedule less frequent outa gesfor th eir fossil fired boilers. This pat tern of scheduling requires much more deta iled plan-ning for the inspection a nd ma intena nce ta sks wh ich a re performed during the less frequentboiler outages.

Wa ter induction from condensing s tea m occurs mostly in low points of sloped horizonta l

runs of stea m piping lines. Therma l quenching ca n occur w hen th e condensat e comes in con-ta ct w ith t he pipe bottom. Repeated cycles of the result ing thermal shock can lead t o exces-sive bending, perma nent distort ions such a s sa gging, a nd cracking of the component .

Two case studies ha ve been presented. One involves a ma in steam piping line with asagged horizontal sect ion, an d a number of ma lfunctioning a nd bottomed out supports. Thesecond case presents the details of a rupture in a section of cold reheat piping upstream ofthe boiler. The findings, conclusions and r ecommenda tions from these studies are describedherein.

Superheat er crossover piping system components, including a t temperators, should bescheduled for inspection and a ssessment during outa ges, bas ed on insta nces of cra ckingfound by DB Riley during routine examina tions. The cra cking is typically due to thermal

fat igue, a result of over temperat ure conditions a nd cyclica l boiler opera tion.A case study ha s been presented of a da ma ged elbow found in the superhea ter outlet pip-

ing on an industrial boiler. The results of a detailed metallurgical invest igat ion showed tha tthe severe internal gouging was due to a combination of fluid impingement (spray water)and a corrosion mechanism.

There have been some recent failures and distress in feedwater system piping compo-nents due to interna l thinning, caused by a phenomena know n as flow accelera ted corrosion.Parameters which have been found to influence the wall thinning of plain carbon steel pip-ing components include piping configura tion, fluid chemistr y and velocities, tempera tur e,a nd pipe ma teria l. The presence of only a sma ll percenta ge of chromium in the carbon steelcan lessen t he severity of the t hinning process.

DB Riley an d others w ere involved in th e failure evalua tion of a r uptured section of feed-wa ter pipe, located in a br a nch leg betw een the upstream va lves and th e boiler’s economiz-er inlet header. A thorough investigat ion confirmed the presence of thinning by flow a ccel-erated corrosion, but localized to the a rea of rupture. Recommendat ions a re given herein forthe a ssessment of feedw a ter piping components, a s a check on the presence of flow a cceler-a ted corrosion.

RECOMMENDATIONS

Condit ion a ssessment programs should be performed a t t hree to f ive year interva ls. Thefrequency would be dependent on the age of the unit , the ty pe of operat ion, an d severity of

problems experienced to date.

B a sed on the recent experience w ith problem a rea s due to wat er induction, flow a cceler-at ed corrosion a nd t hermal fa t igue, described herein, it is recommended tha t inspection anda ssessment progra ms be ca rried out for the high energy stea m piping, superheat er crossoverand outlet piping, and feedwat er piping lines.

Remedial measures used to prevent or lessen the effects of thermal quenching due towa ter induction in steam lines a re adh erence to specified boiler cool dow n ra tes, tempera -


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tur e monitoring of the top and bottom sections of components by therm ocouples, a nd non-destructive examina tions of components to esta blish their current condition. Retrofit mod-ificat ions ha ve included stea m line rerouting, enlarging of drain a nd vent l ines, and the a ddi-tion of valves in dra in lines to prevent ingress of the condensa te.

For hot and cold reheat steam line piping fabricated from rolled plate with longitudinal

seam w elds, it is recommended tha t intern a l video inspections be performed a t low pointlocations on horizont a l sections, especia lly if the longitudin a l sea m is located towa rds th ebottom of the pipe.

There is a listing of components in superhea ter crossover lines, presented herein, w hichshould be included in a condition a ssessment progra m. This essentially involves the pipingcircumferential w elds an d a t temperator components. For th e circumferentia l welds, wet f lu-orescent magnetic particle and ultrasonic shear wave testing are recommended as a checkon outside a nd subsur face cra cking.

For feedwater piping components which could be susceptible to flow accelerated corro-sion, recommendat ions a re offered for ultrasonic or eddy current thickness test ing, alloya nalysis, an d internal video inspection. This would typically be at more turbulent locat ions

a nd those with higher fluid velocities, including piping sections follow ing valves, tees andbends.

In general, for a ll the boiler piping systems a ddressed herein, should hot and cold w alk-down inspections note a number of dama ged or ma lfunctioning pipe supports, then remedi-al a ct ions are needed. They can include a current stress a na lysis of the system and repairor replacement of suspect support s. If necessar y, proof testing of support a ssemblies can bea ccomplished on-site t o determine if t he component is capa ble of carr ying it s origina l designloadings.

The data contained herein is solely for your information and is not offered,

or to be construed, as a warranty or contractual responsibility.


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REFERENCES

1. Mandke, J .S ., “Ma in S team Lines - Quench ing and Rela ted Distor t ions,” Canad ianElectrical Associat ion Spring Meeting, Va ncouver, B.C., Ma rch 1992.

2. Fox, M.E. , “Condensa te Flow E vents in Main Steam P iping,” T.U. Elect r ic Company,Da l las , TX.

3. King, J . , “Recent Experience in Condit ion Assessment of B oiler P iping Componentsan d S upports,” ASME P VP - 95-OAC2, ASME P ressure Vessels a nd P iping Conference,Honolulu , HI , J uly, 1995. DB Riley, Inc. Pa per No. RST-135.

4. Hendrix , R. , King, J . , Ryder, B. , “Condit ion Assessment Program for Boi ler and PipingComponen t s a t t he B ig Ca jun I I P ower S t a t ion ,” Power-Gen ’97 In terna t iona lConference, Da l las , TX, December, 1997. DB Riley, Inc. Pa per No. RST-149.

5. Electric Power Research Inst itut e Report No. TR-108859 entit led, “Recommendat ionfor a n E ffective Flow-Accelera ted C orrosion P rogra m for Fossil Power P lan ts,” November,1997.

Documents

Rst-152 Problems & Failures - Boiler Piping & Supports