Bulletin-BP_Texas_City

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Safety Bulletin U.S. Chemical Safety and Hazard Investigation Board

BP T EXAS C ITY , TX REFINERY F IRE

No. 2005-04-B|October 12, 2006

Summary

On July 28, 2005, 4 months after a devastating

incident in the Isomerization (Isom) Unit that killed15 workers and injured 180, the BP Texas City refineryexperienced a major fire in the Resid Hydrotreater Unit(RHU) that caused a reported $30 million in propertydamage. One employee sustained a minor injury duringthe emergency unit shutdown and there were no fatalities.

The RHU incident investigation determined that an8-inch diameter carbon steel elbow inadvertently installedin a high-pressure, high-temperature hydrogen lineruptured after operating for only 3 months. The escapinghydrogen gas from the ruptured elbow quickly ignited.

This incident occurred after a maintenance contractoraccidentally switched a carbon steel elbow with an alloysteel elbow during a scheduled heat exchanger overhaulin February 2005. The alloy steel elbow was resistant tohigh temperature hydrogen attack (HTHA) but the carbonsteel elbow was not. Metallurgical analyses of the failedelbow concluded that HTHA severely weakened thecarbon steel elbow.

The U.S. Chemical Safety and Hazard Investigation Board(CSB) issues this Safety Bulletin to focus attention onprocess equipment configuration control and positive

material verification of critical alloy steel pipingcomponents. The CSB recommends that the refining,petrochemical, and chemical industries review materialverification programs to ensure that maintenanceprocedures include sufficient controls and positivematerial identification (PMI) testing to prevent impropermaterial substitutions in hazardous process systems.

BP Texas City Refinery The Texas City refinery is the third-largest inthe United States with a capacity in excess of450,000 barrels per day of crude oil. More than1,600 BP employees and hundreds of contractpersonnel operate and maintain the facility.

Residual material from the crude oil processingunit is processed in the RHU to removenitrogen, sulfur, and metals. Hydrogen ispressurized to about 3000 psi, and then pre-heated in the RHU heat exchangers (Figure 2) to

about 600o

F. The preheated hydrogen nextpasses through a furnace to increase thehydrogen temperature, and then is injected intothe reactor feedstock. Hydrogen combines withnitrogen compounds and sulfur within thefeedstock in the presence of the catalyst insidethe RHU reactors to form hydrogen sulfide andammonia. Light hydrocarbon, such asgasoline, is then processed in downstreamrefinery units.

P OSITIVE M ATERIAL V ERIFICATION : P REVENT E RRORS

D URING A LLOY S TEEL S YSTEMS M AINTENANCE

Figure 1. Carbon steel RHU heat exchanger outletpipe (arrow) ruptured after operating only 3

months in high-temperature hydrogen service.


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CSB Investigation Reports areformal, detailed reports onsignificant chemical accidents andinclude key findings, root causes,and safety recom-mendations. CSB HazardInvestigations are broader studiesof significant chemical hazards.CSB Safety Bulletins are short,general-interestpublications thatprovide new or noteworthy information onpreventing chemicalaccidents. CSB

Case Studies are short reports onspecific accidents and include adiscussion of relevant goodpractices for prevention. Allreports include safety recommendations whenappropriate. CSB InvestigationDigests are plain-languagesummaries of InvestigationReports.

IncidentDescriptionOn July 28, 2005, at about 6:00 pm,an RHU hydrogen gas heatexchanger process pipe ruptured.The venting hydrogen gas ignitedand a huge fireball erupted in theunit. One employee sustained aminor injury while assisting withthe RHU emergency shutdown.The RHU sustained major damagefrom the hydrogen-fed fire thatburned for two hours. There were

no offsite impacts but, as a

precaution Texas City ordered ashelter-in-place for nearbyresidents until the fire wascontained.

BP personnel examined the exten-sively damaged unit and deter-mined that an 8-inch diameter pipeelbow on an RHU heat exchangerhydrogen gas outlet pipe ruptured(Figure 3). The BP investigationteam recovered the elbow segments

that remained attached to the pipeand three pieces found in thedebris (Figure 4).

The venting hydrogen gas ignitedand a huge fireball erupted in theunit.

Upper left & top arrow --Alloy steel elbows 2 and 3

Lower left arrow --Carbon steel elbow 1

Figure 2. Dimensionally identical piping elbows on RHU heat exchangers A and B.


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BP personnel examinedthe extensively damagedunit and determined thatan 8-inch diameter pipe

elbow on an RHU heatexchanger hydrogen gasoutlet pipe ruptured . . .

The BP investigation teamrecovered the elbow segments that remainedattached to the pipe andthree pieces found in thedebris . . .

Figure 3. Ruptured 8-inch pipe elbow on heat exchanger A outlet.

Figure 4. Ruptured 8-inch carbon steel pipe elbow piecesrecovered after the fire

Upper left -- Carbon steel elbow segments (view of inside surface)

Above -- Flange segmentsLower left -- Close-up of fissure on

middle elbow segment


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5 The cost of 1.25 percent chrome lowalloy pipe is approximately three to fourtimes more expensive than carbon steelpipe.

4 The metallurgical analysis did notcalculate the actual service life of thecarbon steel elbow. Rather, it comparedthe observed extent of carbon steeldegradation to total time in service ofmore than 100,000 hours or fewer than3,000 hours assuming the carbon steeland low alloy steel elbows were swappedeither in the 1991 or 2005 maintenanceoverhaul.

Incident Analysis

HTHA Failure Mechanism

Incidents involving HTHA dateback to the 1940s. Carbon steel inhydrogen service at temperaturesabove about 450 oF and pressuresabove 100 psia is susceptible toHTHA. At these operatingconditions, atomic and molecularhydrogen permeates the steel andreacts with dissolved carbons orcarbides 1 to form methane gas. Theloss of carbon in the steel, ordecarburization, significantlydegrades the steels mechanicalproperties, including tensilestrength and ductility. 2 Themethane gas creates high localizedstresses, which combine with thenormal piping system stresses tocreate voids and fissures in thesteel, which ultimately causes thepipe to rupture (API, 2004).

The American Petroleum Institute(API) Recommended Practice 941,Steels in Hydrogen Service at ElevatedTemperatures and Pressures inPetroleum Refineries andPetrochemical Plants, recommendsoperating limits for carbon steeland low alloy steel piping systemsin hydrogen service. Experimentsand operating plant data show thatHTHA is typically avoided byusing low alloy steels 3 containing

1 A carbide is a chemical compoundformed between carbon and a metal ormetals (e.g., chromium carbide, ironcarbide).2 Ductility is the ability of a metal toplastically deform without breaking orfracturing.3 Low alloy steels typically contain lessthan 0.3% carbon and 2-8% total alloyingelements.

1.253.0 percent chrome, as thechrome combines with carbon toform chromium carbide, which isresistant to reacting with hydrogen.

Failed Elbow MetallurgicalAnalyses

The elbow segments recovered fromthe damaged unit were examinedto identify the steel and the failuremechanism. Chemical analysisand microscopic examinationdetermined that the elbow wasmade from carbon steel.Microscopic examination alsorevealed that the segments wereseverely decarburized and haddeep fissures on the inside surface(see Figure 4). The decarburizedsteel and severe fissuringconfirmed that HTHA caused thecatastrophic elbow failure.

Detailed metallurgicalexaminations and micro-hardnesstesting quantified the extent ofhydrogen damage to estimate thetotal time the elbow could havebeen in the high-temperature, high-pressure hydrogen service before itfailed. The results, compared toexisting experimental data andempirical service life predictions,concluded that the elbow failedafter being in service for fewer than3000 hours. 4

RHU System Design

Designed in the early 1980s, theRHU has three parallel operatingsystems. Each system contains aheat exchanger assembly thatconsists of two series-connectedheat exchangers to preheat thehydrogen. For high-temperaturehydrogen gas service piping toresist HTHA, the piping designspecification requires 1.25%chrome low alloy steel, but forpiping in hydrogen service at lowtemperatures, or those below 450 oF,non-HTHA resistant carbon steel isspecified, as using this materialminimizes material cost.

5Because

heat exchanger B inlet piping andcomponents operate attemperatures below 450 oF, they arecarbon steel; heat exchanger Boutlet and all downstream pipingand components (Figure 5) arerequired to be low alloy steelbecause they operate attemperatures above 500 oF.

Construction costs may have beensaved by making elbows 1, 2, and 3on each heat exchanger assemblydimensionally identical, as doingso requires fewer pipe assemblyfabrication drawings and weldjoints in each assembly. Becausethe elbows are dimensionallyidentical, the piping contractor hadto ensure that the low alloy steelelbows 2 and 3 were installed inthe correct locations when the RHUwas built. Had the elbow 1 design


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6 Portable hand-held test devices, such asan x-ray fluorescence instrument, quicklydistinguish between carbon steel andalloy steel piping materials withoutdamaging the test article.

dimensions been different, elbow 1would not have beeninterchangeable with elbow 2 or 3(see Figure 2).

Because of this componentinterchangeability, any heatexchanger piping disassembly/reassembly for maintenance orrepair requires the maintenancecrew to be careful to install eachelbow in the correct location.Because carbon steel and low alloysteel are visually indistinguishable,special test equipment 6 is neededto distinguish the two low alloysteel elbows from the carbon steelelbow. Otherwise, to preventswitching elbow 2 or 3 withincompatible carbon steel elbow 1,the crew must clearly label or markeach elbow before removing them,then confirm that each has beenreinstalled in the correct location.

The results, compared toexisting experimental dataand empirical service lifepredictions, concludedthat the elbow failed afterbeing in service for fewerthan 3000 hours.

Figure 5. RHU hydrogen heat exchanger piping material requirements.


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RHU Heat Exchanger

MaintenanceThe RHU heat exchangers wereplaced in service in 1984.Scheduled heat exchanger cleaningand inspection were performed in1989 and 1991. Elbows 1, 2, and 3were removed from the heatexchangers, set aside, and thenreinstalled following themaintenance.

The next scheduled heat exchanger

maintenance overhaul began inJanuary 2005. The three elbowswere removed, stored temporarily,and then reinstalled 39 days later.The maintenance contractor, JVIndustrial Companies, wasunaware of the material differencesin the elbows and BP did notrequire the contractor to implementany special precautions to preventinadvertently switching the elbowsor any post-reassembly testing toconfirm the alloy elbows werereinstalled in the correct locations.

Metallurgical analyses after theincident concluded that the carbonsteel elbow could withstand thehigh-temperature, high-pressureservice for only a few thousandhours. X-ray fluorescence testingconfirmed that an alloy steel elbowwas installed in the carbon steelelbow position on the heat exchan-

ger B inlet. Therefore, the CSBconcluded that carbon steel elbow

1 was inadvertently switched withalloy steel elbow 3 when themaintenance contractor reassem-bled the piping during the winter2005 heat exchanger overhaul.

Alloy Piping MaterialVerification

The BP Texas City refinery has amaterial verification program, andPMI test equipment that quicklydifferentiates carbon steel fromalloy steel piping components. TheBP procedure requires alloy steelcomponents to be verified whenthey are received in the warehouseand when alloy steel componentsare shipped from the warehouse foruse in new construction. However,it does not require PMI duringmaintenance, even when there is arisk of inadvertent substitution ofthe wrong material with alloypiping components.

As this incident demonstrated,merely disassembling andreassembling piping componentsduring maintenance can result inunacceptable system modifications.Lacking post-installation PMItesting, or positive identification ofthe alloy steel components beforeand after installation (e.g.,component tagging beforedisassembly), the maintenancecrews reassembly error wentundetected until the pipe failed.

Key Findingsl Piping systems can be designed

such that incompatiblecomponents cannot beinterchanged. All three elbowscould have been made from thesame low alloy steel material,even though this would havemeant additional materialexpense. Alternatively, elbow 1could have been dimensionallydifferent from elbow 2 and 3,

although this would have meantadditional construction costs.

In February 2005, a carbon steelelbow was installed in the hightemperature, high pressurehydrogen line instead of therequired 1.25 percent chromelow alloy steel elbow. HTHAcaused the carbon steel elbow torupture after the unit operatedonly a few months.

The BP Texas City refinerymaterial verification proceduredid not require critical pipingcomponent PMI testing duringequipment maintenance, eventhough the incompatiblecomponents could beinadvertently switched. 7 The testis simple to perform and quicklydifferentiates between carbonsteel and alloy steel.

7 The Mechanical Integrity QualityAssurance element in the OSHA ProcessSafety Management Standard requiresappropriate checks and inspections toassure equipment is installed properly(OSHA, 1992)


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Lessons Learned

Human Factors BasedDesign

Designers should consider theentire process system life cycle,including planned maintenance, toavoid piping configurations thatallow critical alloy pipingcomponents to be interchangedwith non-compatible pipingcomponents.

Positive MaterialVerification Programs

In-situ alloy steel materialverification using x-rayfluorescence, or other non-destructive material testing, is anaccurate, inexpensive, and fastPMI test method. Facility owners,operators, and maintenancecontractors should ensure that theverification program requires PMItesting, such as specified in APIRecommended Practice 578, orother suitable verification process,for all critical service alloy steelpiping components that areremoved and reinstalled duringmaintenance.

At a minimum, piping componentsand their respective locationsshould be tagged or marked beforeremoval, and the correct installedlocation is verified afterreinstallation.

Recommendations

BP Texas City Refinery 2005-04-B-R1

Revise the maintenance qualitycontrol program to requirepositive material identificationtesting or another suitablematerial verification process forall critical service alloy steelpiping components removed andreinstalled during maintenance,and inform work crews of specialmaterial handling precautions.

JV Industrial Companies2005-04-B-R2

Develop/update the writtenpiping component installationquality control procedure torequire positive materialidentification testing or othersuitable verification or trackingprocess for all alloy steel pipingcomponents removed duringmaintenance.

BP did not alert the maintenancecontractor that two of the threeelbows were alloy steel pipingcomponents and must not beinterchanged with the carbonsteel elbow.

The maintenance contractor, JVIndustrial Companies, did notadequately control thedisassembly, storage, andreassembly of the pipingcomponents to ensure that allthe piping components werereturned to their originalinstalled locations.

The maintenance contractorinadvertently switched carbonsteel elbow 1 (Figures 2 and 5)with the alloy steel elbow 3during the January 2005 RHUheat exchanger overhaul.

As this incidentdemonstrated, merelydisassembling andreassembling pipingcomponents duringmaintenance can result inunacceptable system

modifications.

At a minimum, pipingcomponents and theirrespective locations should

be tagged or marked . . .


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The U.S. Chemical Safety and Hazard Investigation Board (CSB) is an independent Federal agency whose

mission is to ensure the safety of workers, the public, and the environment by investigating and preventingchemical incidents. The CSB is a scientific investigative organization; it is not an enforcement or regulatorybody. Established by the Clean Air Act Amendments of 1990, the CSB is responsible for determining theroot and contributing causes of accidents, issuing safety recommendations, studying chemical safetyissues, and evaluating the effectiveness of other government agencies involved in chemical safety.

No part of the conclusions, findings, or recommendations of the CSB relating to any chemical accident maybe admitted as evidence or used in any action or suit for damages. See 42 U.S.C. 7412(r)(6)(G). The CSBmakes public its actions and decisions through investigation reports, summary reports, safety bulletins,safety recommendations, case studies, incident digests, special technical publications, and statisticalreviews. More information about the CSB is available at www.csb.gov.

CSB publications can be downloaded at www.csb.gov or obtained by contacting:U.S. Chemical Safety and Hazard

Investigation BoardOffice of Congressional, Public, and Board Affairs

2175 K Street NW, Suite 400Washington, DC 20037-1848

(202) 261-7600

ReferencesAmerican Petroleum Institute (API),

1999.Material VerificationProgram for New and ExistingAlloy Piping Systems,Recommended Practice 578, May1999.

API, 2004. Steels for HydrogenService at Elevated Temperaturesand Pressures in PetroleumRefineries and PetrochemicalPlants , Recommended Practice941, March 2004.

Oriani, Richard A., John P. Hirth,and Michael Smialowski, Ed.1985. Hydrogen Degradation of Ferrous Alloys, NoyesPublications, 1985.

U.S. Chemical Safety and HazardInvestigation Board (USCSB),2004. Investigation Digest: DPC Enterprises Chlorine Release.

U.S. Department of Labor,Occupational Safety and HealthAdministration (OSHA), 1992.Process Safety Management, 29CFR 1910.119(j)(6)(ii).

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