Piping Design Hazardous Fluid Service

7/30/2019 Piping Design Hazardous Fluid Service

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Incorporating fire safety into plantdesign takes on two fundamentalgoals: to prevent the occurrenceof fire and to protect the initially

uninvolved piping and equipment

long enough for operations person-nel to perform their duties and foremergency responders to get the fireunder control. While it is impracticalto completely eliminate the potentialrisk of an accidental fire in a complexprocess-plant facility that is expectedto handle and process hazardouschemicals, it is reasonable to assumethat certain aspects of design can beincorporated to reduce that risk.

Designing facilities that use andstore hazardous chemicals requires

a demanding set of requirements, attimes beyond what can practically bewritten into industry codes and stan-dards. It is ultimately the responsi-bility of the engineer of record (EOR)and the owner to fill in those blanksand to read between the lines of theadopted codes and standards to cre-ate a safe operating environment,one that minimizes the opportunityfor fire and its uncontrolled spreadand damage.

This article will not delve into the

various trigger mechanisms of how afire might get started in a process fa-cility, but will instead discuss contain-ment and control of the fuel componentof a fire that resides in piping systemsthat contain combustible, explosive orflammable fluids.

In the design of piping systems con-taining such fluids, there are criticalaspects that need additional consid-erations beyond those involved in thedesign of piping systems containingnon-hazardous fluids. There are twokey safety aspects that need to beincorporated into the design, namelysystem integrity and fire safety.

System integritySystem integrity describes an expecta-

tion of engineering that is integratedinto the design of a piping system inwhich the selected material of con-struction (MOC), system joint design,

valve selection, examination require-ments, design, and installation haveall been engineered and performed in amanner that instills the proper degreeof integrity into a piping system. Whilethis approach is certainly needed forthe piping design of so-called normalfluid service it is absolutely critical forhazardous fluid systems.

The design of any piping system, haz-ardous or non-hazardous, is based, inlarge part, on regulations and industryaccepted standards published by suchorganizations as the American Soci-ety of Mechanical Engineers (ASME)and the American Petroleum Institute(API). The standards published bythese organizations include tables thatestablish joint-pressure ratings basedon MOC and temperature. Where the

joint-design consideration for hazard-ous fluid services departs from that of

non-hazardous fluid services is in gas-ket and seal material specifications.

This is due to the need for sealingmaterial to contain hazardous chemi-cals for as long as possible while sur-rounded by a fire or in close proximityto a fire. The effect of heat from a fire onan otherwise uninvolved piping systemcan only be delayed for a relatively shortperiod of time. And the first thing to failwill be the mechanical type joints.

Depending on the type of fire andwhether the piping is directly in thefire or in close proximity, the window ofopportunity, prior to joint seal failure,for an emergency response team to get

the fire under control is anywhere froma few hours to less than 30 minutes. As

you will see, a number of factors dictatethe extent of that duration in time.

A system in which the gasket mate-rial is selected on the basis of materialcompatibility, design pressure, anddesign temperature may only requirea solid fluoropolymer. In a fire, thisnon-metallic material would readilymelt, allowing the contents of the pipeto discharge from the joint once sealedby the gasket. Specifying a gasket thatis better suited to hold up in a fire fora longer period of time gives the emer-

gency responders time to bring the ini-tial fire under control, making it quitepossible to avoid a major catastrophe.

Fire-safe systemPreventing the potential for a firerequires operational due diligenceas well as a proper piping-materialspecification. However, controllingand restricting the spread of firegoes beyond that. Results of the as-sessment reports of catastrophicevents coming from the U. S. Chemi-

cal Safety and Hazard InvestigationBoard (CSB; Washington, D.C.) haveshown that many of the occurrencesof catastrophic incidents have actu-ally played out through a complexset of circumstances resulting fromdesign flaws, instrumentation prob-lems, pipe modifications, inadequatefire-proofing and human error.

Events, such as a fire, are not neces-sarily then the result of a hazardousfluid simply escaping through a leaky

joint and then coming into contact withan ignition source. There are usually acomplex set of events leading up to afire incident. Its subsequent spread,

Feature Report

36 ChemiCal engineering www.Che.Com June 2010

Feature Report

William M. Huitt

W.M. Huitt Co.

Piping Design forHazardous Fluid Service

Extra considerations and precautions are needed

beyond the requirements of codes and standards


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into a possible catastrophic event, can

then be the result of inadequate de-sign requirements that extend beyondthe piping itself.

While this discussion touches onlyon piping issues, know that this isonly a part of the overall integrationof safety into the design of a facilitythat handles hazardous fluids. Whatfollows are recommended piping de-sign considerations that are intendedto substantially reduce the risk ofthe onset of fire and its uncontrol-lable spread throughout a facility. Indiscussing the spread of fire, it willbe necessary to include discussion re-garding the needs for disciplines other

than piping, namely fire proofing of

structural steel.

General codes and standards

From a fire-safety standpoint, somerequirements and industry regula-tions are stipulated in the Interna-tional Fire Code (IFC), published bythe International Code Conference(ICC) under IFC 3403.2.6.6. There arealso requirements by the National FireProtection Assn. (NFPA) under NFPA1 and NFPA 30. Test requirements forfire-rated valves can be found under

API 607 Fire Test for Soft SeatedQuarter Turn Valves. Starting withthe 4th edition of this API standard,

it was added that, among other things,

the tested valve has to be operatedfrom fully closed to fully open afterthe fire test. Prior to the 4th editiona soft-seated fire-rated valve had toonly remain sealed when exposed tofire without having to be operated, orrotated. Additional fire test require-ments can be found as published by theBSI Group (formerly known as BritishStandards Institution) as BS-6755-2Testing of Valves. Specification forFire Type-Testing Requirements, andFM Global FM-7440 Approval Stan-dard for Firesafe Valves.

With exception to the specific re-quirements covered in the valve test-

ChemiCal engineering www.Che.Com June 2010 37

IncIdent no. 1

Valero-McKee refInery,

Sunray, tex., feb. 16, 2007

Without going into great detail as to the cir-cumstances that led up to this incident, pipinghandling liquid propane in a propane deas-

phalting (PDA) unit ruptured. The location of the rup-ture was in a section of isolated piping that had beenabandoned in place several years prior. A valve, in-

tended to isolate the active flow of liquid propane fromthe abandoned-in-place piping, had been unknow-ing left partially open due to an obstruction inside the

valve. Water had gradually seeped in past the valveseat over the years and being heavier than the liquidpropane, settled at a low-point control station whereit eventually froze during a cold period. The expand-ing ice inside the pipeline subsequently cracked thepipe. When the temperature outside began to warm,the ice thawed allowing liquid propane to escapefrom the active pipeline, through the partially closed

valve, and out the now substantial crack. The resul-tant cloud of propane gas drifted toward a boilerhouse where it found an ignition source. The flame of

the ignited gas cloud tracked back toward its sourcewhere the impending shockwave from the explosionripped apart piping attached to the PDA extractorcolumns causing ignited propane to erupt from oneof the now opened nozzles on the column at such a

velocity as to create a jet fire.The ensuing jet fire, which is a blow-torch like

flame, discharged toward a main pipe rack approxi-mately 77 ft away, engulfing the pipe rack in the jet fire. As thetemperature of the non-fire-proofed structural steel of the pipe rackreached its plastic range and began to collapse in on itself, thepiping in the rack, which contained additional flammable liquids,collapsed along with it (Figure 1).

Due to the loss of support and the effect of the heat, the pipes in

the pipe rack, unable to support its own weight, began to sag. Theallowable bending load eventually being exceeded from the forceof its unsupported weight, the rack piping ruptured spilling its flam-mable contents into the already catastrophic fire. The contents ofthe ruptured piping, adding more fuel to the fire, caused the flamesto erupt into giant fireballs and thick black smoke.

The non-fire-proofed support steel (seen on the left in Figure 1and on the right in Figure 2) was actually in compliance with APIrecommendations. Those recommendations can be found in Pub-lication 2218 Fireproofing Practices in Petroleum and Petro-chemical Processing Plants; API Publications 2510 Design andConstruction of LPG Installations; and 2510A Fire-Protection

Considerations for the Design and Operation of Liquefied Petro-leum Gas (LPG) Storage Facilities. In these issues of the publica-tions it was recommended that pipe-rack support steel within 50 ftof an LPG vessel be fire proofed. The collapsed support steel wasapproximately 77 ft from the extractor columns, which is beyondthe 50-ft recommended distance.While the EOR was in compliance with the governing code, withregard to fire proofing, there may have been a degree of compla-cency in defaulting to that minimum requirement. This goes backto a point made earlier in which it was said that industry standardsare not intended to be design manuals. They instead provide, the minimum requirements necessary to integrate safety into thedesign, fabrication, inspection, installation, and testing of pip-ing systems Proprietary circumstances make it the imperativeresponsibility of the EOR or the owner to make risk assessmentsbased on specific design conditions and go beyond the minimumrequirements of an industry code or standard when the assessmentresults and good engineering practices dictate.

Figure 1. A collapsed pipe rack as a result o heat rom a jet fame

Figure 2. The same collapsed pipe rack as Figure 1 seen rom above


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Feature Report


ing standards, the codes and standardsmentioned above provide generalizedrequirements that touch on such keyaspects of safety as relative equip-

ment location, mass volume versusrisk, electrical classifications, valving,and so on. They cannot, and they arenot intended to provide criteria andsafeguards for every conceivable situa-tion. Designing safety into a particularpiping system containing a hazardousliquid goes beyond what should be ex-pected from an industry-wide code orstandard and falls to the responsibil-ity of the owner or EOR. As ASMEB31.3 states in its introduction, Thedesigner is cautioned that the code is

not a design handbook; it does not doaway with the need for the designer orfor competent engineering judgment.

When designing piping systems tocarry hazardous liquids, the designbasis of a project or an establishedprotocol for maintenance needs toincorporate a mitigation strategyagainst two worse-case scenarios: (a)

A leak at a pipe joint containing ahazardous liquid, and (b) The ruptureor loss of containment, during a fire,of surrounding hazardous piping sys-

tems, not otherwise compromised thatwould add fuel to the fire.

The occurrence of those two fail-ures, one initiating the incident andthe other perpetuating and sustain-ing the incident, can be minimized oreliminated by creating a design basisthat provides the following:Addedassuranceagainstthepoten-

tial for joint failureAdded assurance of containment

and control of a hazardous liquidduring a fire

Safeevacuationofahazardousliq-uid from the operating unit underdistress

Fire prevention through designPiping joints. When designing pip-ing systems to contain hazardous liq-uids, one of the key objectives for thedesign engineer should be taking thenecessary steps to minimize the threatof a leak, steps beyond those typicallynecessary in complying with the mini-mum requirements of a code. There arecertainly other design issues that war-rant consideration, and they will betouched on much later. However, while

the pipe, valves, and instrumentationall have to meet the usual criteria ofmaterial compatibility, pressure, andtemperature requirements there areadded concerns and cautions that needto be addressed.

Those concerns and cautions arerelated to the added assurance thathazardous liquids will stay containedwithin their piping system during

normal operation and for a period oftime during a fire as expressed in suchstandards as API-607, FM-7440, andBS-6755-2. Designing a system, startto finish, with the intent to minimizeor eliminate altogether the potentialfor a hazardous chemical leak willgreatly help in reducing the risk of fire.If there is no fuel source there is nofire. In the design of a piping system,leak prevention begins with an assess-ment of the piping and valve joints.

There are specified minimum re-

quirements for component ratings,examination, inspection, and testingthat are required for all fluid services.Beyond that, there is no guidancegiven for fire safety with regard to thepiping code other than a statement inB31.3 Para. F323.1 in which it states,in part: The following are some gen-eral considerations that should beevaluated when selecting and applyingmaterials in piping: (a) the possibilityof exposure of the piping to fire andthe melting point, degradation tem-

perature, loss of strength at elevatedtemperature, and combustibility of thepiping material under such exposure,(b) the susceptibility to brittle failureor failure from thermal shock of thepiping material when exposed to fireor to fire-fighting measures, and possi-ble hazards from fragmentation of thematerial in the event of failure, (c) theability of thermal insulation to protectpiping against failure under fire expo-sure (for example, its stability, fire re-sistance, and ability to remain in placeduring a fire).

The code does not go into specifics onthis matter. It is the engineers respon-

sibility to raise the compliance-levelrequirements to a higher degree whereadded safety is warranted and to definethe compliance criteria in doing so.

Joints in a piping system are itsweak points. All joints, except for thefull penetration buttweld, will de-ratea piping system to a pre-determinedor calculated value based on the typeof joint. This applies to pipe longitudi-

nal weld seams, circumferential welds,flange joints and valve joints such asthe body seal, stem packing, and bon-net seal, as well as the valve seat.For manufactured longitudinal weldseams, refer to ASME B31.3 Table

A-1B for quality factors (E) of thevarious types of welds used to manu-facture welded pipe. The quality factoris a rating value, as a percentage, ofthe strength value of the longitudinalweld in welded pipe. It is used in wallthickness calculations as in the follow-

ing equations for straight pipe underinternal pressure:

(1)

(2)Where:c = sum of mechanical allowancesD = outside dia. of piped = inside dia. of pipeE = quality factor from Table A-1A

and A-1BP = internal design gage pressureS = stress value for material from

Table A-1t = pressure design thicknessW= weld-joint strength-reduction

factory = coefficient from Table 304.1.1Also found in Para. 304 of B31.3 arewall thickness equations for curvedand mitered pipe.

With regard to circumferentialwelds, the designer is responsiblefor assigning a weld-joint reductionfactor (W) for welds other than lon-gitudinal welds. What we can do, at

PTFEenvelope

Profiledinner ring

Monel*windings

* Monel is a registered trademark of international Nickel

Primarysealingelement

Secondarysealing element

Flexiblegraphite filler

Carbon steelouter ring

Figure 3. I angedjoints are necessary, it issuggested that fre-saespiral-wound type gas-kets with graphite fller

be specifed


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least for this discussion, is to provide,as a frame of reference, some quality

rankings for the various circumfer-ential welds based on the stress in-tensification factor (SIF) assigned tothem by B31.3. In doing so, the fullpenetration buttweld is consideredto be as strong as the pipe with anSIF = 1.0. The double fillet weld at aslip-on flange has an SIF = 1.2. Thesocketweld joint has a SIF= 2.1. Any

value in excess of 1.0 will de-rate thestrength of the joint below that of thepipe. With that said, and assumingan acceptable weld, the weld joint,and particularly the full penetrationbuttweld, is still the joint with thehighest degree of integrity. In a fire,

the last joint type to fail will be thewelded joint.

The threaded joint has an SIF =2.3 and requires a thread sealantapplied to the threads, upon assem-bly, to maintain seal integrity. Withflame temperatures in a fire of around2,7003,000F the thread sealant willbecome completely useless if not va-porized, leaving bare threads with nosealant to maintain a seal at the joint.

The flange-joint-sealing integrity,like the threaded joint, is dependentupon a sealant, which, unlike thethreaded joint, is a gasket. Flangebolts act as springs, providing a con-stant live load so long as all thingsremain constant. Should the gasket

melt or flow due to the heat of a fire,the initial tension that was given the

bolts when the joint was assembledwill be lost. Once the gasket has beencompromised the sealing integrity ofthe joint is gone.

Knowing that the mechanical typethreaded and flange joints are theweak points in a piping system, andthe primary source for leaks, it is sug-gested that their use be minimized tothe greatest extent possible. Considerthe following design points:Donotspecifyflangejointssolelyfor

installation purposesSpecifyflangejointsonlywherere-

quired for equipment connectionsand for break-out spools

IncIdent no. 2: Formosa PlastIcs corP.,

PoInt comFort, tex., oct. 6, 2005

Atrailer being towed by a forklift operatordown a pipe rack alley in the Olefins IIoperating unit of Formosas Point Comfort

facility attempted to back the trailer up into anopen area between pipe rack support columnsin an effort to turn the rig around. When theoperator, in the process of pulling back intothe pathway, began to pull forward the trailer

struck a protruding 2-in. blow-down valve ona vertically mounted Y-strainer that was con-nected to a 4-in. NPS liquid propylene linesubsequently ripping the valve and nipplefrom the strainer (Figure 4). Liquid propyleneunder 216 psig pressure immediately begandischarging into a liquid pool from the 2-in. opening and partially

vaporizing into a flammable cloud.The flammable cloud eventually found an ignition source, ignited

and exploded, in-turn igniting the pool of liquid propylene. Thefire burned directly under the pipe rack and an attached elevatedstructure containing process equipment and piping. About 30 mininto the event, non-fire-proofed steel sections of the pipe rack andthe elevated structure containing process equipment collapsed

(Figure 5). The collapse caused the rupture of equipment and ad-ditional piping containing flammable liquids, adding more fuel toan already catastrophic fire. The flare header was also crimped inthe collapse and ruptured, causing flow that should have gone tothe flare stack to be discharged into the heart of the fire. The fireburned for five days.Again, as in Incident No. 1, you can see in Figure 5 the result

of insufficient fire proofing of steel beams and columns in closeproximity to process units. And fire protection does not applyonly to vertical columns. As you can see, it is not sufficientlyeffective to have the vertical columns protected while the hori-zontal support steel is left unprotected and susceptible to the heatfrom a fire.Another key factor in the Formosa fire was the ambiguous deci-

sion by the designer to orient the Y-strainer blow-down in such aposition of vulnerability. While there is absolutely nothing wrongwith installing the Y-strainer in the vertical position, as this onewas, they are normally installed in a horizontal position with theblow-down at the bottom, inadvertently making it almost impos-sible to accidentally strike it with enough force to dislodge the

valve and nipple.However, orienting the blow-down in such a manner, about the

vertical axis, should have initiated the need to evaluate the risk andmake the determination to rotate the blow-down about its verticalaxis to a less vulnerable location, or to provide vehicle protection

around the blow-down in the form of concrete and steel stanchions.Both of these precautionary adjustments were overlooked.The plant did perform a hazard and operability study (HAZOP)

and a pre-startup safety review (PSSR) of the Olefins II operatingunit. In the CSB report, with regard to process piping and equip-ment, it was stated that, During the facility siting analysis, thehazard analysis team [Formosa] discussed what might occur if a

vehicle (for instance, fork truck, crane, man lift) impacted processpiping. While the consequences of a truck impact were judgedas severe, the frequency of occurrence was judged very low(that is, not occurring within 20 years), resulting in a low overallrisk rank [The ranking considered both the potential consequencesand likely frequency of an event]. Because of the low risk ranking,the team considered existing administrative safeguards adequateand did not recommend additional traffic protection.

4-in. Propyleneproduct line

Strainer

Pipenipple

2 ft

Column

Figure 5. Collapse o non-fre-prooed structural steel

Figure 4. The impact point (let)showing the damaged Y-strainer


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Feature Report


If a lined pipe system is required,usethetyperequiringthelinertobefused, a coupling installed and onethatissuitableformulti-axisbending

Threaded joints should be limitedto instrument connections and thenonly if the instrument is not avail-ablewitha flangeorweldedconnec-tion.Ifathreadedconnectionisused,itshouldbeassembledwithoutthreadcompoundthenseal-welded.Thismayrequirepartialdismantlingofthein-strumenttoprotectitfromtheheatoftheweldingprocess.Itis recommendedthatpiping sys-

temsbeweldedasmuchaspossibleandflangedjointsbeminimizedasmuchas

possible. That includes using weldedend valves and inline componentswhere possible. If flanged joints arenecessaryforconnectingtoequipmentnozzles, flanged valves, inline compo-nents,orneededforbreak-outjoints,itissuggestedthataspiral-woundtypegasketwithgraphitefillerbespecified.Thismaterialcanwithstandtempera-tures upwards of 3,000F. There arealso gasketdesigns that are suitableforwhena fluoropolymermaterial isneeded forcontactwiththechemical,

while also holding up well in a fire.ThesearegasketssimilarindesigntothatshowninFigure3.Valves.Afire-ratedvalvemeetingtherequirements of API 607 (Fire TestforSoftSeatedQuarterTurnValves)isdesignedand tested toassure thepreventionoffluidleakagebothinter-nallyalongthevalvesflowpath,andexternallythroughthestempacking,bonnet seal, and body seal (where amulti-piecebodyisspecified).TestingunderAPI607subjectsavalvetowell

definedandcontrolledfireconditions.Itrequiresthatafterexposureto thefire test the valveshall bein a con-ditionthatwillallowittoberotatedfrom its closed position to its fullyopen position using only themanualoperatorfittedtothetestvalve.Quarter turn describes a type of

valve that goes from fully closed tofullyopenwithinthe90degrotationofitsoperator.Itincludessuchvalvetypesasball,plug,andbutterflywithavalveseatmaterialoffluoropolymer,elastomer,orsomeothersoft,non-me-tallicmaterial.Standards such as FM-7440 and

BS-6755-2, touched on earlier, applytovirtuallyanyvalvetypethatcom-plieswiththeirrequirements.Under

the FM and BS standards, valvetypessuchasgates,globes,and pis-tonvalveswithmetal seats canalsomakeexcellentfire-ratedvalveswhenusinga bodyand bonnet gasket andstempackingmaterialsimilarintem-peraturerangetothatofagraphiteorgraphitecomposite.

Process systems.Attheonsetofafirewithinanoperatingunit,initiallyun-affectedprocesspipingsystemsshouldnotbeacontributortosustainingandspreadingwhatisalreadyapotentially

volatilesituation.There arebasic de-signconceptsthatcanbeincorporatedintothephysicalaspectsof aprocesssystemthatwill,attheveryleast,pro-vide precious time for operators andemergencyresponderstogetthesitu-ationundercontrol.Inreferringtothesimplifiedpipingandinstrumentationdiagram(P&ID)inFigure6,therearesevenmainpointstoconsider:1.Flow supply (LineA), coming fromthefluidssourceoutsidetheoperat-ingunit,needstoberemotelyshutoff

totheareathatisexperiencingafire2.Theflowpathatthesystemsusepointvalves(VA-1)needstoremainopen

3.Theflowpathatdrainandventvalves(VA-2)needstoremainsealed

4.The external path through stempackingandbodysealsneedstore-mainintactduringafire

5.Thebottomoutletvalve(XV-2)onavessel containing a flammable liq-uidshouldhaveanintegralfusiblelinkforautomaticshut-off,withitsvalve seat,stempackingand bodysealsremainingintactduringafire

6.PipelineAshouldbeslopedtoallowallliquidtodrainintothevessel

7.The liquid inthe vessel should bepumpedouttoasafelocationuntilthefusiblelinkactivates,closingthe

valve.Thereshouldbeaninterlocknotifyingthecontrolroomandshut-tingdownthepump

Those sevenpoints, with thehelp oftheP&ID inFigure6,are explainedasfollows:Point 1. The supply source, or anypipelinesupplyingtheoperatingunitwithaflammableliquid,shouldhavean automated, fire-rated isolationvalve(XV-1)locatedoutsidethebuild-ingoroperatingunitareaandlinkedtotheunitsalarmsystemwithremote

on/offoperation(fromasafelocation)ataminimum.Point 2.Anypoint-of-usevalve(VA-1)ata vessel should remain open dur-inga fire. The area orunit isolationvalve(XV-1)willstopfurther flowtothe system, but any retained or re-sidual fluiddownstream of the auto-maticshut-offvalveneedstodraintothevesselwheretheincreasingover-pressure, due to heat from the fire,willberelievedtoasafelocation,suchasa flarestack, throughRD-1. Ifthe

Valves,XV-1andVA-1,areclosedinafiresituationtheblocked-influidinaheatedpipelinewillexpandandpoten-tiallyrupturethepipeline;firstatthemechanical jointssuchas sealsandpacking glandson valves andequip-ment,aswellasflangejoints,andthenultimatelythepipeitselfwillrupture(catastrophicfailure).Duringafire,ex-pandingliquidsandgasesshouldhaveanunobstructedpaththroughthepip-ingtoavesselthatissafelyvented.Point 3. Valves at vents and drains(VA-2&VA-6)needtobefire-ratedandremainclosedwithsealsandseatintactforaslongaspossibleduringafire.

Dischargeto safe area

SG-1

XV-2

VA-2

VA-3

VA-5

VA-6

XV-4XV-3

PG-1

VA-4

VA-1

LT-1

XV-1

RD-1

Line D

Line C

Operating unit

battery limits

Line B

Pump

Line ASlope Flammableliquid in

Flammable

liquid torecovery

Flammableliquid out

Figure 6. A simplifed P&ID used in the discussion about process systems


6/7 ChemiCal engineering www.Che.Com June 2010 41

Point 4. During a fire, another sourcefor valve leakage is by way of stempacking and body seal, as mentionedearlier. Leakage, at these seal points,can be prevented with valves that arenot necessarily fire-rated, but containstem packing and body seal gasketmaterial specified as an acceptableform of graphite (flexible graphite,graphoil and so on). This is a fire-safematerial which is readily available innon-fire-rated valves.

Point 5. The valve on the bottom ofthe vessel should be fire-rated with afusible link or a fail closed position.

Relying on an air or electric operatedvalve actuator may not be practical. Afusible link is most certainly neededon a manually operated valve. Thecontents of a vessel containing a haz-ardous liquid needs to get pumped toa safe location during a fire until suchtime as the fusible link is activated,closing the tank bottom valve, or thepump fails. All valved gage and instru-ment connections (SG-1) mounted ona vessel should have a graphite-typestem packing and body-seal-gasketmaterial at a minimum. Flange gas-kets at these gage and instrument con-

nections should be of a spiral-woundfire-safe gasket type similar to thosementioned earlier. Specialty tank-bottom valves (XV-2) should be givenspecial consideration in their designby considering a metal-to-metal seat,or a piston valve design along withfire-rated seal material.

Point 6. As mentioned in Point 2, theresidual fluid in Line A, after flow hasbeen stopped, should be drained tothe vessel. To help the liquid drain,the pipeline should be sloped towardthe vessel. The intent, as mentionedabove, is to prevent sections of any

IncIdent no. 3: BP RefIneRy,

texas cIty, tex., July 8, 2005

In the design layout of a duplex heat-exchanger arrangement (Figure 7) in theresid-hydrotreater unit of the BP Refinery in

Texas City, Tex., the designer duplicated thefabrication dimensions of the 90-deg fabri-cated elbow-spool assemblies shown in Fig-ure 7 as Elbows 1, 2, and 3. While the pipesizes and equipment nozzle sizes were the

same, permitting an interchangeability of thefabricated elbow spool assemblies, the serviceconditions prohibited such an interchange.

The shell side conditions on the upstreamside (at Elbow 1) were 3,000 psig at 400F.The shell side conditions on the downstreamside (at Elbow 3) were 3,000 psig at 600F.The intermediate temperature at Elbow 2

was not documented. In the initial design,the material for Elbow 1 was specified ascarbon steel, Elbow 3 was specified as a1 - 1/4 chrome/moly alloy. The reason forthe difference in material of construction(MOC) is that carbon steel is susceptible to

high temperature hydrogen attack (HTHA)above ~450F at 3,000 psig, therefore thechrome/moly alloy was selected for thehigher temperature Elbow 3.At 3,000 psig and temperatures above

450F hydrogen permeates the carbon steeland reacts with dissolved carbon to formmethane gas. The degradation of the steelstensile strength and ductility due to decarburization, coupled

with the formation of methane gas creating localized stresses,weakens the steel until it ultimately fatigues and ruptures.

In January 2005, scheduled maintenance was performed on theheat exchanger assembly. The piping connected to the heat ex-changers was dismantled and stored for the next 39 days. After

maintenance was completed, the piping was retrieved from stor-age and reinstalled.The elbows of different material were not marked as such and

the maintenance contractor was not warned of the differentMOC for the elbows. Elbows 1 and 3 were unknowingly in-stalled in the wrong locations. On July 8, 2005, approximatelyfive months after re-installing the piping around the heat ex-changers, the elbow in the #3 position catastrophically failed asshown in Figure 8.As you can see in Figure 9 the carbon steel, after becoming

progressively weakened by HTHA, fractured on the inside ofthe pipe and catastrophically failed. The incident injured oneperson in operations responding to the emergency and cost thecompany $30MM.

The one thing you can takeaway from this incident is: Donot dimensionally replicatepiping spools or assemblies ofdifferent materials. The otherunderlying, but significant

component you can also takeaway is this: In the initial de-sign of a plant facility the en-gineer of record will routinelyhold formal design reviewsthat will include all key personnel with vested interest in the proj-ect. In doing so, include, among the attendees, key operationsand management plant personnel from one of the owners op-erating facilities, if available. These individuals typically bring alot of insight and knowledge to a review. Whereas the designersmay not have the wherewithal to think along the lines of issuesthat might pertain to a facility turnaround, the plant personnel

will. These are issues that they normally think long and hardabout. Make use of this resource.

Elbow 3(failure location)

Elbow 1carbon steel

Elbow 2

High-temperaturehydrogen to furnance

Low-temperature3,000 psig

hydrogen feed

Preheat gas

Preheat gasto separator

Heatexchanger A

11/4 chromealloy piping

11/4 Chrome alloy pipe

Heatexchanger B

Bolted flange(typical)

Carbon steel pipe

Figure 8. Severed 8-in.NPS hydrogen piping

Figure 7. Heat exchanger fow diagram

Figure 9. Fragments othe ailed 8-in. NPS carbon-steel spool


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Feature Report


pipeline that do not contain a reliefdevice from being blocked and isolatedduring a fire. If the piping system forflammable fluid service is designed

properly, the contents will be able todrain or expand into a vessel whereover-pressurization can be relievedand safely vented.

Point 7. It will be necessary to evacu-ate as much of the hazardous fluid aspossible from tanks and vessels in thefire area to a safe location. The pump-out should continue until there is in-adequate pump suction head, or untilthe fusible link on XV-2 is activated.

At that time the pump interlockswould shut down the pump.

With regard to tank farms, the fol-lowing is a suggested minimum con-sideration for a safe design: Drain

valves should be of a fire-rated type.Tank outlet valves should be of a fire-safe type with a fusible link. Tanknozzles used for gages or instrumentconnections should have, at a mini-mum, valves containing stem pack-ing and seal gasket material specifiedas an acceptable form of graphite, asmentioned above, or some other fire-safe material. Gaskets used at nozzle

flange joints should be a fire-safe gas-ket similar to the spiral wound gas-kets mentioned earlier or the gasketshown in Figure 3.

Inline valves in piping downstreamof the tank outlet valve, such as pumptransfer lines and recirculation lines,do not necessarily need to be fire-rated, but should have stem packingand seal gasket material that is fire-safe as mentioned earlier.

Situations will arise that do not fallneatly into what has been described

above. If there is any doubt with regardto valving then default to a fire-rated

valve. Each piping system identifiedas needing to be fire-safe should bedesignated as such. Where individualfire-safe valves are to be strategicallylocated in a system, they should bedesignated on their respective P&IDseither by notation or through the as-signed pipe material specification.The pipe-material specification shouldbe indicated on each pipeline of theP&ID. The specification itself shouldtherefore be descriptive enough forthe designer to know which valve toapply at each location.

Lessons learned from incidents

While this particular discussion is spe-cific to piping leaks and joint integrityit bares touching on a few subjects that

are integrally associated with pipingsafety: pipe rack protection, protectingpiping from vehicle traffic, and design-ing for disaster (HAZOP).

In Incident Number 1 (box, p. 37),the onset of a fire that might otherwisehave been quickly controlled becomesa catastrophic event because pipingmounted on the unprotected structuralsteel of a pipe rack, outside the extentof the initial occurrence, becomes col-lateral damage adding more fuel to thefire causing it to sustain itself, increase

in intensity and continue to spread.In Incident Number 2 (box, p. 39), an

unprotected and protruding pipelinecomponent (Y-strainer) is damaged,causing a major leak that operatingpersonnel were unable to stop. The en-suing fire lasted for five days.

In Incident Number 3 (box, p. 41),two dimensionally identical spoolpieces were designed for a system inwhich the two were fabricated fromdifferent materials because their ser-

vice conditions were very different. It

can only be assumed that this was anerroneous attempt at trying to achieveduplication of pipe spools in an effortto assist the fabricator in their pro-ductivity of pipe fabrication. Instead itultimately caused injury to one personand cost the plant owner $30MM.

Edited by Gerald Ondrey

Author

W. M. (Bill) Huitt has beeninvolved in industrial pip-ing design, engineering andconstruction since 1965.Positions have included de-sign engineer, piping designinstructor, project engineer,project supervisor, pip-ing department supervisor,engineering manager andpresident of W. M. Huitt Co.(P.O. Box 31154, St. Louis,

MO 63131-0154; Phone: 314-966-8919; Email:[email protected]; URL: www.wmhuitt.com),a piping consulting firm founded in 1987. Hisexperience covers both the engineering andconstruction fields and crosses industrial linesto include petroleum refining, chemical, pet-rochemical, pharmaceutical, pulp and paper,nuclear power, biofuel, and coal gasification.He has written numerous specifications, guide-lines, papers, and magazine articles on thetopic of pipe design and engineering. Huitt isa member of ISPE (International Society ofPharmaceutical Engineers), CSI (Construction

Specifications Institute) and ASME (AmericanSociety of Mechanical Engineers). He is a mem-ber of three ASME-BPE subcommittees, severaltask groups, an API task group, and sits on twocorporate specification review boards.

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Documents

Piping Design Hazardous Fluid Service