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Brimstone Sulfur Symposium Molten Sulfur Fire Sealing Steam Requirements

© Black & Veatch Holding Company 2015. All Rights Reserved. The Black & Veatch name and logo are registered trademarks of Black & Veatch Holding Company.

Black & Veatch 1 September 2015

MOLTEN SULFUR FIRE SEALING STEAM REQUIREMENTS PROPOSED MODIFICATIONS TO NFPA 655

AlanD.Mosher

Black&VeatchCorporation

SeanM.McGuffieDennisH.MartensPorterMcGuffie,Inc.

BrimstoneSulfurSymposium

Vail,ColoradoSeptember14‐18,2015

Abstract

NationalFireProtectionAssociation(NFPA)655,StandardforPreventionofSulfurFiresandExplosions(currentedition:2012),Chapter5,HandlingofLiquidSulfuratNormalHandlingTemperatures, Section 5.5, Fire Fighting, states that protection of covered liquid sulfurstoragetanks,pitsandtrenchesshallbebyoneofthefollowingmeans:(1)inertgassystem,(2)steamextinguishingsystemcapableofdeliveringaminimumof2.5poundsperminute(lb/min)(1.13kilogramsperminute[kg/min])ofsteamper100cubicfeet(ft3)(2.83cubicmeters[m3])ofvolume,or(3)rapidsealingofenclosuretoexcludeair.TheNFPAsnuffingsteamratestatedinthestandardresultsinalargesteamratebeingfedtosulfurtanksandsulfurpits that typicallyhavea lowdesignpressure. Thesulfur tanksandsulfurpitsaretypicallydesignedwithairsweepsystemstopreventtheaccumulationofhydrogensulfide(H2S) in the vapor space, thereby eliminating the flammablemixture. The air intake andexhaustsystemsaretypicallydesignedwithverylowpressuredropsfornormaloperation.IfsnuffingsteamisfedtosulfurtanksandsulfurpitsattheratespecifiedinNFPA655,thebuilt‐upbackpressuretypicallyfarexceedsthedesignpressureoftheenclosure.Forthesereasons, the refiningandgasplant industryhas tended to choosenot to follow theNFPA655‐specified snuffing steam rate. Actual operatingdata from sulfur fires in sulfur tanksandsulfurpitsindicatethatalowersealingsteamrateisadequatetoextinguishthefirebysealingthesulfurtankorsulfurpitfromairingressandpurgingsomeoftheairasthefireisextinguished by lack of oxygen. Some computational fluid dynamics (CFD)modeling hasbeencompletedthatsupportsthefielddatashowingthatalowersteamrateisadequatetoextinguish the fires. Thispaper focuseson thepotentialproblems causedby the currentNFPA655snuffingsteamrate,analysisofactualfielddataforfiresinsulfurtanksandpits,andarecommendationfortheNFPA655committeetoconsiderregardingasteamratetoseal the enclosure and extinguish the fire in a sulfur tank and sulfur pit. Thepaper alsoincludes commentson goodengineeringpractice resulting from the calculationsandCFDanalysesthatwerecompleted.NFPA655iscurrentlybeingupdatedandwillbereissuedin2017. This paperwas initially prepared to document issueswith the current NFPA 655snuffingsteamrateformoltensulfurandtorecommendareducedratetoNFPAduringthefirstpubliccommentperiodthatendedonJanuary5,2015.




Executive Summary

Onthebasisofthedatacollectedandanalysisperformedforthispaper,inDecember2014theauthorsrecommendedthefollowingchanges(textinred) toNationalFireProtectionAssociation (NFPA) 655, Standard for Prevention of Sulfur Fires and Explosions (currentedition:2012),Chapter5,HandlingofLiquidSulfuratNormalHandlingTemperatures:5.5FireFighting.5.5.1Protectionforcoveredliquidsulfurstoragetanks,pits,andtrenchesshallbebyoneofthefollowingmeans:(1)InertgassysteminaccordancewithNFPA69,StandardonExplosionPreventionSystems(2)*Steam extinguishing system capable of delivering a minimum of 2.5 lb/min(1.13kg/min)ofsteamper100ft3(2.83m3)ofvolume.(3)Rapidsealingoftheenclosuretoexcludeair(3)*Rapidsealingoftheenclosuretoexcludeair. Forsulfurtanksandsulfurpits,theuseofasteamrateof1.0lb/min(0.45kg/min)ofsteamper100ft3(2.83m3)oftotaltankorpitvolumeisexpectedtodevelopapositivepressureintheenclosure,therebysealingthesulfurtankorsulfurpit,preventingairingress,andextinguishingthefire.5.5.2SnuffingSteamandSealingSteamPrecautions.5.5.2.1Theventsystemsonenclosedsulfurtanksandsulfurpitsmustbedesignedtoallow the required snuffing steam rate or sealing steam rate to vent withoutoverpressuring theenclosure. Thevent systemsmustalsobedesigned forproperoperationduringnormaloperation.5.5.23WaterExtinguishingPrecautions.5.5.2.13.1 Liquid sulfur stored in open containers shall be permitted to be extinguishedwithafinewaterspray.5.5.2.23.2Useofhigh‐pressurehosestreamsshallbeavoided.5.5.2.33.3Thequantityofwaterusedshallbekepttoaminimum.5.5.34 Dry Chemical Extinguishers. Where sulfur is being heated by a combustible heattransfer fluid, dry chemical extinguishers complying with NFPA 17, Standard for DryChemicalExtinguishingSystems,shallbeprovided.A.5.5.1(3)Forenclosedsulfurtanksorsulfurpitswithairsweepsystems,thesealingsteamshouldbefed intotheenclosureveryneartheair inlets. Asthesealingsteamventsbackwardthroughtheairinlets,thesealingsteamwillquicklystopairingresstothefire.Sealingsteamshouldbefedintothesulfurtankorsulfurpitforaminimumof15 minutes or until the temperature has returned to near normal. For furtherinformation and good engineeringpractice regarding sealing steam, refer toMoltenSulfurFireSealingSteamRequirements.C.1.2.3OtherPublications.Mosher, A. D.,McGuffie, S.M., andMartens, D.H.,Molten Sulfur Fire Sealing SteamRequirements,BrimstoneSulfurSymposium,Vail,CO,September2015.InFebruary2015,theaboveinformationwasdiscussedwiththeNFPA655Committeethatoversees changes to the document. The committee accepted the concept thatwas beingpresented but some wording changes were made and the statement about 1.0lb/min(0.45kg/min)ofsteamper100ft3wasrelocatedtothedocumentannex.InApril2015the




committeemembers voted to accept the revisedwording. Thenext step in the approvalprocess is posting of the first draft of the revised document followed by a second publiccommentperiodinthefallof2015,withthefinalgoalofissuinganeweditionofNFPA655in2017.1.0 Introduction

NationalFireProtectionAssociation(NFPA)655,StandardforPreventionofSulfurFiresandExplosions,(1) contains information about snuffing steam requirements for extinguishingfiresinsulfurtanksandsulfurpits.TheratethatislistedinChapter5,HandlingofLiquidSulfuratNormalHandlingTemperatures,isexcessiveandcausesproblemswiththeventingsystemsoftheseenclosuresandcancausebuilt‐upbackpressurewithintheseenclosuresthatexceedstheoriginaldesignpressure.Sulfurproducedinoilandgasfacilitiescontainsquantitiesofhydrogensulfide(H2S).ThisH2Sisslowlyreleasedfromthemoltensulfurduringstorage. Toeliminatetheflammablemixture,sulfurtanksandsulfurpitsinthesefacilitiesaretypicallydesignedwithairsweepsystemstopreventtheaccumulationofH2Sinthevaporspace.Continuoussweepingwithair alsoprevents accumulationofpyrophoric iron sulfide (FeS)byoxidizing thematerial.TheFeSisgeneratedbycorrosionofcarbonsteelcomponentsincontactwithH2S.Theairintakeandexhaustsystemsforsulfurtanksandsulfurpitsmustbecarefullydesignedwithconsideration forvery lowpressuredropsduringnormaloperation,mustaccount for thebuoyancyeffectoftheheatedair intakes,andmustaccountfortheeffectofwindblowingacross the vents. Developing a vent design that accounts for all the normal operatingparameterscanbequitedifficultandcanbecomeimpracticalwhentheventsmustalsobecapableofventing thesnuffingsteam fedat theNFPA655rateof2.5poundsperminute(lb/min)per100cubicfeet(ft3)ofvolume.This paper focuses on the potential problems caused by the current NFPA 655 snuffingsteamrateandrecommendationsforarevisedrateby:

Presenting analysis of available actual field data for extinguishing fires insulfurtanksandsulfurpitsprovidedbyownersofoilandgasfacilities.

Providing the basis for a recommendation to the NFPA 655 committee toconsider defining a steam rate to suitably seal the sulfur tanks and sulfurpitstoexcludeoxygenand,thereby,safelyextinguishsulfurfires.

Recommendingasealingsteamratebasedondocumentedindustrypracticeforextinguishingfiresinsulfurtanksandsulfurpitsandcomputationalfluiddynamics(CFD)studies.

Comments on good engineering practice resulting from the analysis of the informationgatheredandfromCFDmodelingthatwascompletedareincluded.




NFPA655iscurrentlybeingupdatedandwillbereissuedin2017. TheoriginalrevisionofthispaperwaspreparedtodocumentissueswiththecurrentNFPA655snuffingsteamrateformoltensulfurfiresandrecommendincorporatingasealingsteamratetoNFPAduringthefirstpubliccommentperiodforNFPA655thatendedonJanuary5,2015.Note: No part of this document addresses fire fighting for solid dust sulfur fires. This

documentonlyaddressessteamrequirementsformoltensulfurfires.2.0 Brief History of NFPA 655 Snuffing Steam Requirements

ThefirefightingtechniqueslistedbyNFPAformoltensulfurhaveevolvedsincetheinitialadoptionofNFPA655in1940.Thefollowinginformationwastakenfromdocumentssuchasrevisionhistory,TechnicalCommitteeReports,etc.,availableonNFPA’swebsite.12.1 1968 Edition

The Technical Committee Report for Amendments to be included in the 1968 edition ofNFPA655includedthefollowingtext:

45.FireFighting.4501.Covered liquidsulfurstorage tanksshouldbeprovidedwithsome inertgassystemforextinguishingfiresthatmayoccurinthetank.Theinertgasmayconsistof carbon dioxide, nitrogen, flue gas or steam. Since the inert must be suppliedrapidly enough todisplace the ventilation air from thevents, steam isusually themosteffectiveandeconomicalchoice.4502.Where liquid sulfur containers are of sufficiently small size to permit suchaction, it is recommended that they be so arranged so that they can be sealedrapidly to exclude air in case of fire; formation of sulfur dioxidewill exhaust theoxygenintheenclosureandsmotherthefire.Thesystemshouldbeallowedtocoolbelow154C(310F)beforereopeningittotheatmosphere.4503.Liquidsulfurstored inopencontainerscanbestbeextinguishedwitha finewaterspray.Avoidtheuseofpressurehosestreamswhichmayscattertheburningliquidsulfur.Thequantityofwaterusedshouldbekepttoaminimum.

2.2 1982 Edition

The Technical Committee Report for Amendments to be included in the 1982 edition ofNFPA655includedthefollowingtext:

4‐4FireFighting.

1NFPA’swebsiteiswww.nfpa.org




4‐4.1Coveredliquidsulfurtanksshallbeprovidedwithagaseousfireextinguishingsystem. (SeeAppendixEofNFPA86A,StandardOvensandFurnaces,AppendixE,andNFPA69,StandardonExplosionPreventionSystems.)4‐4.1.1*Whereafixedinertingsystemisused,thinTeflon®rupturediscsshallbeplacedovertheinertingnozzlessothatsulfurcannotcondensewithinthenozzle.4‐4.2*Where liquid sulfur containers are small enough, theymay be arranged sothat they can be rapidly sealed to exclude air in case of fire. The sulfur dioxideproducedbythefirewillsmotherthefire.Insuchcases,thesystemshallbeallowedtocoolbelow154Cbeforereopening.4‐4.3Liquidsulfurstoredinopencontainersmaybeextinguishedwithafinewaterspray.Useofhighpressurehosestreamsshallbeavoided. Quantityofwaterusedshallbekepttoaminimum.

2.3 1993 Edition

TheReportoftheCommitteeincludedthefollowingrecommendation,whichwasacceptedandbecamepartofthe1993edition:

1.Revise4‐4.1"andaddanExceptiontoread:4‐4.1* Covered liquid sulfur tanks shall be provided with a steam extinguishingsystemoraninertgassysteminaccordancewithNFPA86,StandardforOvensandFurnacesandNFPA69,StandardonExplosionProtectionSystems.Exception:Where liquidsulfurcontainerscanberapidlysealedtoexcludeair, theSO2producedwillsmotherthefire. Insuchcases,steamextinguishingsystemsorinertgassystemsshallnotberequired.Thesystemshallbeallowedtocoolbelow154C(309F)beforereopening.2.Deleteexistingtextof4‐4.2.

2.4 2001 Edition

TheReportoftheCommitteeincludedthefollowingrecommendation,whichwasacceptedandbecamepartofthe2001edition:

RECOMMENDATION:Revise4‐4.1asfollows:4‐4.1Protectionforcoveredliquidsulfurstoragetanks,pitsandtrenchesshallbebyoneofthefollowingmeans:(a)InertgassysteminaccordancewithNFPA69,StandardforExplosionPreventionSystems.




(b)*Steamextinguishingsystemcapableofdelivering8lbs/minofsteamper100cuftofvolume.–Boldaddedbycurrentauthors(c)Rapidlysealtheenclosuretoexcludeair.A‐4‐4.1(b) The steam should preferably be introduced near the surface of themoltensulfur.SeeNFPA86,StandardforOvensandFurnaces,AppendixE‐3.SUBSTANTIATION:Thisrecommendationbringsthesteamfloodingrequirementinline with NFPA 86. The recommendation is based on 1934 FMRC fire test of agasoline firewherethesteamwasappliedabove thegasoline fireandcombustionairwasintroducedbelowthesteaminjectionpoint.Itinessencerequiressupplying200 cu ft/min steam for every100 cu ft of enclosurevolume. Forhot enclosures(above220F)wherethesteamisinjectedatthesurfaceoftheliquidsulfur,asupplycapableof4cuft/minper100cuftofenclosurevolumewouldbesatisfactory.Thisrequirementensuresthattheavailablesteamsupplyisadequatetofurnishenoughsteamataratesufficienttoextinguishthefire.

2.5 2007 Edition

The Report of the Committee shows a recommendation and acceptance of a completerevisionof the2001editionofNFPA655 tobecome the2007edition. The2007editionshowedthefollowingtext:

5.5FireFighting.5.5.1Protection forcovered liquidsulfurstorage tanks,pits,andtrenchesshallbebyoneofthefollowingmeans:(1)InertgassysteminaccordancewithNFPA69,StandardonExplosionPreventionSystems(2)* Steam extinguishing system capable of delivering a minimum of2.5lb/min (1.13 kg/min) of steam per 100 ft3 (2.83m3) of volume – Boldaddedbycurrentauthors(3)Rapidsealingoftheenclosuretoexcludeair5.5.2*Whereafixedinertingsystemisused,thincorrosion‐resistantrupturediscsshallbeplacedovertheinertingnozzlessothatsulfurcannotcondensewithinthenozzle.5.5.3WaterExtinguishingPrecautions.5.5.3.1Liquidsulfurstoredinopencontainersshallbepermittedtobeextinguishedwithafinewaterspray.5.5.3.2Useofhigh‐pressurehosestreamsshallbeavoided.




5.5.3.3Thequantityofwaterusedshallbekepttoaminimum.5.5.4 Dry Chemical Extinguishers.Where sulfur is being heated by a combustibleheat transfer fluid, dry chemical extinguishers complyingwithNFPA17,StandardforDryChemicalExtinguishingSystemsshallbeprovided.

2.6 2012 Edition

The Report of the Committee shows a recommendation and acceptance of a revisedwordingofthe2007editionofNFPA655tobecomethe2012editionasfollows:

5.5FireFighting.5.5.1Protection forcovered liquidsulfurstorage tanks,pits,andtrenchesshallbebyoneofthefollowingmeans:(1)InertgassysteminaccordancewithNFPA69,StandardonExplosionPreventionSystems(2)* Steam extinguishing system capable of delivering a minimum of 2.5 lb/min(1.13kg/min)ofsteamper100ft3(2.83m3)ofvolume(3)Rapidsealingoftheenclosuretoexcludeair5.5.2*Whereafixedinertingsystemisused,thincorrosion‐resistantrupturediscsshallbeplacedovertheinertingnozzlessothatsulfurcannotcondensewithinthenozzle.5.5.2WaterExtinguishingPrecautions.5.5.2.1Liquidsulfurstoredinopencontainersshallbepermittedtobeextinguishedwithafinewaterspray.5.5.2.2Useofhigh‐pressurehosestreamsshallbeavoided.5.5.2.3Thequantityofwaterusedshallbekepttoaminimum.5.5.3 Dry Chemical Extinguishers.Where sulfur is being heated by a combustibleheattransferfluid,drychemicalextinguisherscomplyingwithNFPA17,StandardforDryChemicalExtinguishingSystems,shallbeprovided.

2.7 2017 Edition

ThenexteditionofNFPA655isduetobepublishedin2017.ThepublicinputclosingdatewasJanuary5,2015.




2.8 Equivalency of Various Required Snuffing Steam Rates

Table1providesacomparisonofdifferentsnuffingsteamratesformoltensulfurthathavebeenlistedindifferenteditionsofNFPA655.

Table1.SnuffingSteamRatesforMoltenSulfurListedinNFPA655

lb/minper100ft3

ft3/minper100ft3 Comment

2001Edition 8 214 Basedon1934FactoryMutualResearchCorporation(FMRC)firetestofagasolinefire.

2001discussionpriortoissueofofficialedition

0.14 3.8 Commentmadeaspartofsubstantiationofitemabove.Whensteamisinjectedatthesurfaceoftheliquidsulfur,thisratewouldbesatisfactory.

2007Edition 2.5 67

3.0 Issues with Current NFPA 655 Snuffing Steam Requirement

Most existing and new sulfur recovery units (SRUs)within oil refineries and gas processingfacilitieshaveenclosedbelowgradesulfurpitsand/orabovegradesulfurtanksforstorageofproducedmoltensulfur.ManyofthesesulfurpitsandsulfurtankshavebeendesignedwithairsweepsystemsinthevaporspacetomaintaintheconcentrationofH2Sbelow25percentofthelower flammable limit (LFL) in accordancewithNFPA 69, Standard onExplosionPreventionSystems(2), Chapter 8, Deflagration Prevention by Combustible Concentration Reduction.BecauseofsafetyconcernsregardingpossibleventingofH2Stotheenvironmentthroughleaksinthesulfurpit,manyofthesulfurpitsandsulfurtanksareoperatedwithaslightvacuum.Thevacuum is often createdbymechanical blowersor ejectors thatpull the required amountofsweepair through thevapor spaceof theenclosureand thendischarge that air/vapor to anappropriatedisposallocation/deviceataslightlyelevatedpressure. Becauseofthebuoyancyeffect,thevacuumcanalsobecreatedbytheheightdifferencebetweenaheatedventstackandtheheatedairintake.Thetypicaldesigninternalandexternalpressuresofsulfurtanksarelow(approximately‐3to+10inchwatercolumn[inchWC]orless),andtherefore,theventsystemsmust be carefully designed to prevent excessive vacuum from causing damage to the sulfurtank. Unlike tanks or vessels, sulfur pitsmay ormay not be designedwith the intention ofhavingaspecific internaldesignpressure. However,sulfurpitsaretypicallydesignedwithaconcreteroofthicknessof12to15inches.Basedsolelyontheweightoftheroofslab,thesulfurpitshouldbeabletocontainapressureof1‐1.3poundspersquareinch(psi)(28‐36inchWC).However,sulfurpitroofsfrequentlycontainaccesshatches.Precastconcretehatchestypicallyrange from 4to 6 inches thick and can therefore withstand an internal pressure of onlyapproximately 0.35 to 0.51 psi (10‐14 inchWC) before lifting. Roof hatches or deflagrationhatches can also bemade from thin aluminum sheetmaterial that ismuch lighter than theconcreteandthereforeabletowithstandaninternalpressureofonlyapproximately0.25‐0.50inchWCbeforelifting. Theairinletstothesulfurpitorsulfurtankaretypicallyheatedwithsteam(steamjacketed)topreventsulfursolidificationandtheresultingpluggingwithintheair




inlet.Theheatingoftheairintakescausesabuoyancyeffecttooccur,andthesulfurpitorsulfurtankmustbeoperatedunderaslightvacuumtoovercomethisbuoyancyeffectandensurethatairisflowingintothesulfurpitorsulfurtank.Windblowingacrosstheroofofasulfurtankalsocausesupliftontheleewardsidethatcancauseareversalofairflowfromtheairintakesandvent the vapor space out through the air intakes rather than out through the exhaust. Thetypicaloperatingpressureofthevaporspaceofasulfurpitorsulfurtankisintherangeof0.01‐1inchWCvacuum.Thenumberandsizeoftheairinletsmustbecarefullyselectedsothattheairinletswillprovideenoughairflowtoachievetherequiredairsweepbutalsoinduceenoughpressure drop so that the vapor space remains at or below the required vacuumneeded tooffsetthebuoyancyeffectandupliftcausedbywind.Ifthereistoomuchairinletarea,theairintakes can draft backward and allow the air intakes to exhaust H2S to the atmosphere.Therefore,theairinletareaissetonthebasisoftherequiredairrateforapropersweepofthevaporspacebut,at thesametime,makingsure that thevaporspacestaysatavacuumlevelgreaterthanthebuoyancyeffectcausedbytheheatedairintakes.After theair intakearea is set fornormaloperation (and in theabsenceof a relief valve,whichmostsulfurpitsandsulfurtanksdonothave),thenthatistheonlyareaavailabletoventsnuffingsteamwhensteamisusedtoextinguishaninternalfire.ThecurrentsnuffingsteamextinguishingsystemrequirementsinNFPA655of2.5lb/minper100ft3ofvolumebecomeaveryhighsteamflowrateformostsulfurtanksandsomesulfurpitssothatthisratecaneasilyoverpressurethesulfurtankandsulfurpit,causingthesulfurtankorsulfurpit to rupture. It isnotpractical todesigna sulfur tankair sweepsystem toprovide thenecessaryvacuumtoprevent flowreversals in theair inletsduringnormaloperationandprovidethenecessaryairintakeareatoventtherequiredsnuffingsteamrateduringafirescenario.Toillustratetheissueswithoverpressureofsulfurtanks,theauthorsevaluatedsixexistingsulfur tanks that were designed for air sweep of the vapor space to reduce the H2Sconcentration and the resulting back pressure if steam is fed to snuff a fire according toNFPA655’srateof2.5lb/minper100ft3(refertoTable2).As shown in Table 2, the NFPA 655 snuffing steam rate ranges from approximately30to200times theoriginal sweepair rate. This snuffing steamrate results inabuilt‐upback pressurewithin the sulfur tank of 0.23‐59.3 inchWC. This built‐up back pressurewouldexceedthedesignpressureof4outof6ofthesulfurtanks,asshownbytheredtextinTable2.TanksCandDhavelowerbuilt‐upbackpressure,andtheyhaveatotalvolumeless than 6,000 ft3. All the tanks with a volume of approximately 50,000 ft3 and largerwould have built‐up back pressure far higher than their design pressure. A generalstatement can likely be made that the vents on smaller tanks may be able to vent thecurrentNFPA655snuffingsteamrate,butitislikelythattheventsonlargertankscannotvent the current NFPA 655 snuffing steam rate without the built‐up back pressureexceedingthetankdesignpressure.Figure 1 is a scaled comparison of the size of the six existing sulfur tanks that wereevaluated.

Brimstone Sulfur Symposium

Molten

Sulfur Fire Sealing Steam Requirem

ents

© Black & Veatch Holding Company 2015. All Rights Reserved. The Black & Veatch nam

e and logo

are registered tradem

arks of Black & Veatch Holding Company.

Black & Veatch

10

September 2015

Table2.ExistingSulfurTankswithSteam

at2.5lb/m

inper100ft

3

TankA

TankB

TankC

TankD

TankE

TankF

Dimensions

66‐6"Diax

35'‐0"H

59‐6"Diax

48'‐0"H

19'‐8"Diax

17'‐9"H

14'‐5"Diax

16'‐4"H

42'‐8"Diax

32'‐10"H

47'‐6"Diax

24'‐10"H

Volum

e,ft

3 138,500

148,635

5,800

2,833

48,885

49,435

DesignPressure,inchWC

‐3/+10

‐1.5/+2.5

‐3/+10

‐3/+10

‐1.1/+3.2

‐0.9/+3.5

AirInlets

Six‐10"

Two‐16"

Three‐6"

Three‐6"

Six‐6"

Six‐6"

AirOutlet

One‐10"

One‐16"

One‐8"

One‐8"

One‐18"

One‐8"

AirMovem

entM

ethod

Blowersucksair

throughtankand

pushesthrough

causticscrubber

Naturaldraft

2inlets,1outlet

Naturaldraft

3inlets,1outlet

Naturaldraft

3inlets,1outlet

Naturaldraft

6inlets,1outlet

Naturaldraft

6inlets,1

outlet

NormalOpPressure,

inchWC

‐0.05

‐0.0031

‐0.00005

‐0.00008

‐0.029

Unknown

AirSweepObjective

<16%

H2Slower

flammabilitylimit

(LFL),with150

partspermillion

(weight)(ppm

[wt])

H2Sinsulfurfeed

<21%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<50%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<50%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<25%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

Unknown

PressureinchWCwith

steamat2.5lb/m

in/

100ft3

59.3

50.8

1.05

0.23

8.1

29

Steamvolum

etricrate

(@2.5lb/m

in/100ft

3 )

comparedtosweepairrate

29x

190x

198x

79x

53x

Unknown


Molten


ents


e and logo



Black & Veatch

11

September 2015

Figure1.SulfurTankSizeComparison




Toillustratetheissueswithoverpressureofsulfurpits,theauthorsevaluatedfiveexistingsulfur pits that were designed for air sweep of the vapor space to reduce the H2Sconcentration and the resulting back pressure if steam is fed to snuff a fire according toNFPA655’srateof2.5lb/minper100ft3(refertoTable3).AsshowninTable3,theNFPA655snuffingsteamraterangesfromapproximately10to31times the original sweep air rate. This snuffing steam rate results in a built‐up backpressurewithin the sulfurpits of 17.6 inchWC to1,585 inchWC (50poundsper squareinch gauge [psig]). This built‐up back pressurewould exceed the pressure rating of thehatchcoversonall fivesulfurpits,asshownbytheredtextinTable3. Forall fivesulfurpitsanalyzed,thehatchcoverswouldliftifsteamwasfedatthecurrentNFPA655rateof2.5lb/minper100ft3.Nocalculationswereperformedtodeterminetheresultingbuilt‐upbackpressurethatwouldremaininthesulfurpitafterthehatchcoverslifted,becausethatwould require extensive and difficult calculations. The one sulfur pit that wouldtheoretically achieve 50psig pressure in the pit (if the hatch covers did not lift) is large,withatotalcapacityof60,665ft3.ThissulfurpitisassociatedwithanSRUwithacapacityofapproximately675longtonsperday(LTPD),whichisabigplant,butcertainlythereareotherexistingplantsthataremuchlarger.Thisparticularsulfurpithasalargereductionininletlinesize‐‐8inchesdownto4inchestofeedintoaflowmeter.Theflowreachessonicvelocity in this4inchdiameter section, resulting inveryhighbuilt‐upbackpressures. AgeneralstatementcanlikelybemadethattheventsonmostsulfurpitsassociatedwithanSRU cannot vent the current NFPA 655 snuffing steam rate without the built‐up backpressure exceeding thepressure that canbe containedby the hatch covers. When thesehatchcoverslift,steamcontainingH2Sandsulfurdioxide(SO2)willventtotheatmosphereatgroundlevelandcauseaserioussafetyconcern.Figure 2 is a scaled comparison of the size of the five existing sulfur pits that wereevaluated.


Molten


ents


e and logo



Black & Veatch

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September 2015

Table3.ExistingSulfurPitswithSteam

at2.5lb/m

inper100ft

3

PitA

PitB

PitC

PitD

PitE

Dimensions

24'‐0"W

x35'‐0"Lx

10'‐6"D

47'‐7"W

x72'‐2"Lx

17'‐8"D

14'‐0"W

x34'‐0"Lx

11'‐0"D

13'‐2"W

x49'‐10"Lx

9'‐2"D

21'‐0"W

x30'‐0"Lx

8'‐0"D

Volum

e,ft

3 8,820

60,665

5,236

6,015

5,040


‐unknown/+9.5setby

precastconcreteaccess

hatchcovers


alum

inum

hatchcovers



hatchcovers

‐unknown/+14.2set

byprecastconcrete

accesshatchcovers

‐unknown/+0.42set

byaluminum

hatch

covers

AirInlets

One‐6"

Two‐8"

One‐4"

One‐4"

One‐6"

AirOutlet

One‐6"

Two‐8"

One‐4"

Two‐3"

One‐6"

AirMovem

entM

ethod

Ejectorsucksairthrough

airinletandpushesto

thermalreactor


airinletsandpushesto

thermalreactor


airinletandpushesto

thermalreactor

Ejectorsucksair

throughairinletand

pushestoincinerator

Ejectorsucksair

throughairinletand

pushestoincinerator

NormalOpPressure,

inchWC

‐0.22

‐6.9

‐1.2

‐0.30

‐0.22

AirSweepObjective

<15%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

<25%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

<25%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

Unknown

Unknown

PressureinchWCwith

steamat2.5lb/m

in/

100ft3

57.8

50psigifhatchcovers

donotlift,sonicvelocity

backthroughairinlets

266

230

21.4/17.6(Note1)

Steamvolum

etricrate

(@2.5lb/m

in/100ft

3 )


16X

31x

17x

30x

9.7x

Notes:

1. Thesecondvaluelistedisthebuilt‐upbackpressureforthissulfurpitbasedontheejectorstayinginoperation.Allothersulfurpitsanalyzed

haveasafetyinstrumentedsystem

(SIS)thatwilltripejectorswhenfireisdetected.


Molten


ents


e and logo



Black & Veatch

14

September 2015

Figure2.SulfurPitSizeComparison




4.0 Attempted Oxygen Concentration Dilution Calculation

Whenconsideringanewapproachtodetermininganadequatesteamratetoextinguishasulfur fire, the authors first considered a dilution calculation for oxidant concentrationreduction or combustible concentration reduction as described in NFPA 69, Standard onExplosionProtectionSystems. Tobeable tocomplete thedilutioncalculation, the limitingoxygenconcentration(LOC)orLFLofthevapormixturemustbeknown.Surprisingly,theLOCandLFL formolten sulfur couldnotbe found. Several searcheswere conducted: aninternetsearch,asearchofcommonreferencebooks,andasearchofsulfurspecificbooks.Several research organizations and numerous engineering/operating/simulationcompanies thatspecialize inSRUswerecontacted. TheLOCandLFLofmoltensulfurarenotreadilyavailableatanyofthesesources.Somematerialsafetydatasheet(MSDS)werelocatedthatshowedanLFLofmoltensulfur,butfurtherinvestigationshowedthattheLFLlistedwasactuallyforH2S.SomeMSDSactuallyaddedafootnoteindicatingthevaluelistedis forH2S, and others did not include the detail. At the normal temperature thatmoltensulfuristypicallystored,itcouldbearguedthatthesulfurvaporpressureissolowthattherealdangerforfireisbasedontheconcentrationofH2Sthathasevolvedfromthesulfurandnotthesulfuritself.TheLOCandLFLforH2Sareavailable.It has been shown experimentally that the LFL of a substance typically decreases withincreasing temperature. Zabetakis(3) developed some correlations for predicting thetemperatureeffectontheLFLforparaffinhydrocarbons.TheLFLofaparaffinhydrocarbonapproaches zero at temperatures above approximately 2,192F (1,200C). The LOC of asubstance also typically decreases with increasing temperature. The LFL and LOC of asubstancealsovarywith thespecific inert that ispresent. Withoutexperimentaldata forH2Sandmoltensulfurwithsteamastheinert,itcanonlybespeculatedonwhathappenstothe LFL and LOC in gasmixtures near an existing fire. However, it could reasonably bespeculatedthat theLFLandLOCofH2Sandmoltensulfurwill fall tonearzero, if the fireraises localized temperatures to where they approach 2,192F (1,200C). Even at lowerlocalizedtemperatures,theLFLandLOCconcentrationwillbeverylow.Therefore,onceafirehasstarted,consideringLFLandLOCasparameterstotargetforsnuffingsteamdilutioncalculationsisnotproductive.5.0 Actual Operating Data for Molten Sulfur Fire Extinguishing Steam

Theauthorswereunabletofindanypublisheddataontestingcompletedonmoltensulfurfires ordata collected fromactual fires in industrialmolten sulfur applications. Throughvarious molten sulfur production forums (technical conferences, sulfur productionspecialistusergroups,etc.)andpersonalcontacts intheindustry,theauthorscontactedabroad spectrum of sulfur production specialists at the major refining and gas plantcompaniesinNorthAmericathatoperateSRUstorequestdataonanysulfurfiresthathaveoccurred in sulfur pits and sulfur tanks at the owner’s facilities. The sulfur productionspecialists contacted should have had access to data from approximately 75 locations inNorthAmerica thatoperateSRUs. Some locationsoperatemore thanoneSRUand somelocationshaveasmanyas10sulfurpitsorsulfurtanks.Ontheconservativelylowside,therequestshouldhavebeenabletogatherdatafrom100to200sulfurpitsandsulfurtanks.




The request was made with the promise that the sources of all data would remainanonymous.The request elicited some useful information about actual molten sulfur fires that haveoccurred in sulfurpits and sulfur tanks. Manyowner responses indicated theyhadbeenfortunateenoughtoneverhaveexperiencedafire.Severalownerresponsesindicatedthatthey have had fires, but they occurred more than 10 years ago and therefore cannotremembermuchaboutthem. Someownerresponsesindicatedthattheyhadexperiencedfires,buttheircorporatepracticeissuchthattheycouldnotgetcorporatelegalapprovaltoallowdatareleasedoutsideoftheorganization.Someownerresponsesindicatedthattheyhad experienced sulfur fires, but they were unable to find sufficient documentation toindicatehowthefireshadbeenextinguished,ortheyknewthatsteamhadbeenused,buttheyhadnoway todeterminehowmuch steamwasused and thedurationof the steamflow. One owner indicated that fires had occurred in three separate sulfur pits at onelocationandthefireswereextinguishedwithacombinationofsteamandnitrogen,butnorateinformationwasavailable.Theauthorsanalyzedthesulfurfiredatathatweremadeavailablebyfourowners.Therestofthissectiondescribesthedatathatwerecollectedandtheanalysisofthatdata.5.1 Detection of Sulfur Pit and Tank Fires

As a side note, one outcome of the data collection was to discover how some ownersdetectedandrespondedtosulfurpitfires.Fornewerunitdesigns,itisrelativelycommontohaveaSafetyInstrumentedSystem(SIS)that receives inputs from instrumentation around the sulfurpit. If unsafe conditions aredetected,theSISwillisolatecertainsystemsaroundthesulfurpitinanattempttopreventdamage to equipment, environmental releases and exposure of operators to toxic ordangerous environments. It is not uncommon for the SIS to monitor the vapor spacetemperatureofasulfurpit,andifthetemperatureincreasesaboveahighsetpoint,theSISwilltriptheejectorsystemandstopdrawingairthroughthesulfurpit.Thistripsystemisinplacetopreventairfrombeingdrawnthroughthesulfurpitandintensifyingthefire.Oneownerrespondedthatitspecificallydidnotwantitsejectorsystemtoshutdownonhighsulfur pit vapor space temperature. That ownerwanted the ejector system to stay onlineduringafiresothatthemajorityoftheSO2generatedcouldberoutedbytheejectortotheSRUincinerator,allowingtheemissionstobetrackedandrecorded.Iftheejectorsystemwasshutdown,theownerwouldneedtoestimatetheemissionstobeabletoreportthemtotheregulator agency. This particular owner experienced a number of sulfur pit fires in twosulfur pits over a 5 year period before the systems could be analyzed and modified toeliminate the sources of the fires. During this 5 year period, the owner’s operations staffdetermined that whenever they experienced a rapid increase in the incinerator stack SO2emissions(theconcentrationwouldincreasefromanormalvalueof75‐90partspermillionby volume [ppmv], through the alarm point at 125 ppmv, to 1,000‐1,200 ppmv), it wastypically a sulfur pit fire. This causal relationship was so strong that procedures weremodifiedtotraintheoperatorstoimmediatelystartsteamtothesulfurpitwhenevertheysawtheincineratorstackSO2emissionsclimbrapidly.Theoperatorswouldstartsteamto




thesulfurpitandallow thesteamto flow for15minutesbeforeshutting itoff. After thesteamwasshutoff,theoperatorswouldmonitortheincineratorstackSO2emissions,andinmostcases,theemissionslevelsreturnedtonormal.Iftheemissionslimitsdidnotreturntonormal, theoperatorswould then lookatothercauses, suchasanupset in the tailgastreating unit (TGTU). This owner stated thatwhen a fire occurred in the sulfur pits, theresultscouldbeseenintheincineratorstackfirst,then,about30secondslater,thesulfurpitvaporspacetemperaturewouldbegintorise. Thesulfurpitvaporspacetemperaturewouldonlyincreasebyapproximately50to60F(28to33C)duringa fire. Theoperatorsconsidered the sulfur pit vapor space temperature increase as confirmation of a fire butusedtheincineratorstackemissionsincreaseastheirindicationtoimmediatelystartsteamtothesulfurpit.Figure3isaplotofthedistributedcontrolsystem(DCS)datashowingtheincineratorstackSO2emissionsandthesulfurpitvaporspacetemperatureduringatypicalsulfurpitfireforthisowner. ThedataonFigure3showa30 to40seconddifference in the timebetweenwhen the incinerator stackSO2emissions start to increaseandwhen the sulfurpit vaporspacetemperaturestartstoincrease.Figure3isare‐plotoftherawdataandnottheactualplottheoperatorswouldsee.WhenoperatorslookataplottrendgeneratedbytheDCS,itwillhavedifferentscalesassociatedwitheachparameter.Infact,theincineratorstackSO2emissions are reported by both a low range analyzer 0to 500 ppmv and a high rangeanalyzer,greaterthan500ppmv.Therefore,itmakessensethattheoperatorswouldnoticetheinitialrapidincreaseintheincineratorstackSO2emissionsmoreeasilythantheslowerincreaseinthesulfurpitvaportemperature.Becauseofthequickactionbytheoperators,theincineratorstackSO2emissionsdecreasedbelowthealarmsettingof125ppmvwithin25minutesoftheinitialalarm.Itshouldbeconsideredthat,fortheexampledescribedabove,thethermocouplethatwasinthesulfurpitvaporspaceneverindicatedasignificanttemperatureincreaseduringfireevents.Thisislikelyduetothelocationofthethermocouplecomparedtothelocationofthefirewithinthevaporspace.Itcouldbeconcludedthat,duringthefiresexperiencedinthesulfurpitexamplesdescribedabove,thefiresweresomewhatlocalizedanddidnotconsumetheentirevaporspace.Forconfigurationsthatintendtoleavetheejectorsoperatingthroughafireevent,locatingthetemperatureindicatorintheejectorsuctionlineislikelybetterthanlocatingitinthesulfurpitvaporspace.Atleastwiththetemperatureindicatorintheejectorsuctionline,thethermocouplewillseean“average”temperatureofthegaspassingthroughthesulfurpitvaporspaceratherthanseeingapointtemperatureatasinglelocationwithinthevaporspace.SulfurtankstypicallydonotincludeanSISwithhightemperatureinput,andtherefore,thetypicalmethodforsulfurfiredetectionisavisualindicationofayellowplumebeingventedfromthetank.Sometanksdohaveatemperatureindicatorthatoperatorscanusetoindicatethatafireisoccurring.




Figure3.IncineratorStackSO2andSulfurPitVaporSpaceTemperatureDuringaSulfurFire

5.2 Sulfur Pit Fires

OwnerNo.1experiencedanumberofsulfurpit fires in twosulfurpitsovera5yearperiodbeforethesystemscouldbeanalyzedandmodifiedtoeliminatethesourcesofthefires. Thisownerprovideddataonhowsulfurpitfiresareextinguished. Thesulfurpitsinquestionare21'‐0"W x 30'‐0" L x 8'‐0"D, and each has a single 2 inch steam line from the header thatreducesto1½inchesneartheconnectionontheroofofeachsulfurpit. Thesulfurpitsaretypicalconcretepitsthatarecompletelyenclosed,withoneairinletlineandoneairexitlinethatfeedsanejectorforeachsulfurpit.Theejectorsweepsairthroughthevaporspaceofthesulfur pit and discharges to an incinerator. When a sulfur fire is suspected, the operatorsimmediatelyopenthe2inchgatevalveinthesteamlinetostartsteamflowtothesulfurpit.The owner reported that the operatorswill open the gate valve to a pointwhere they seesteamflowingoutofthesingle6inchairintakelineonthesulfurpit,whichindicatestheyhaveput inenoughsteamtoovercomethecapacityofthesulfurpitejectorandsealedtheenclosure/sulfur pit. The owner reported that the 2 inch gate valve is approximatelyone‐thirdopenwhentheoperatorsseesteamexitingfromtheairintakeline.Thesteamisleft flowingforaperiodof15minutes,thentheflowisstopped,andtheoperatorslookattheincineratorstackSO2emissionstoseeiftheyhavereturnedtonormal.Inalmostallcasesthatthisownerexperienced,after15minutesofsteamflow,thefirehadbeenextinguished.Theownercouldonlyrecalloneincidentwhenthefirereturnedafterthesteamwasstopped.Theowner speculated that for thisonecase, a re‐ignitioneventmayhaveoccurred, ratherthanlackofsuppressionofthefirstfire.




Owner No. 1 provided the piping isometric for the steam line. Hydraulic calculationswerecompletedforthesteambasedonthesteamheaderpressure,theone‐thirdopengatevalve,andinformationfromthepipingisometric.Theanalysisshowedthatthesteamflowachievessonicvelocityinthe1½inchsectionofpipenearthesteamlineexitintothesulfurpit.Theflowofthesteamwascalculatedtobe2,644poundsperhour(lb/hr).Thetotalvolumeofthesulfurpitis5,040cubic feet (ft3). Calculating the steamrate tosulfurpit volumeratio shows that thesteamratethisownerhasbeenusingis0.87lb/minper100ft3oftotalsulfurpitvolume.Theownerreportedthatmultiplesulfurfireshavebeensuccessfullyextinguishedinthesulfurpitswiththisratewhenthesteamhasflowedfor15minutes.Thisrateis2.87timeslessthan(orabout35percentof)thecurrentNFPA655specifiedvalueof2.5lb/minper100ft3forsnuffingsteammethodology.Theownerreportedthatnodamagehasbeenexperiencedinthesesulfurpitsbythenumerousfiresbecauseofthefactthatthefirecanbeextinguishedin15minutes.It is difficult to accurately determine the resistance coefficient (K) for a partially openedgatevalvewithouthavingtrueflowtestdatafromthemanufactureroftheactualgatevalve.The authors determined that with a completely open full port gate valve (K values areavailable)theflowofsteamwouldonlyincreaseto2,900lb/hrbecausethesteamflowhitssonicvelocityinthe1½inchsectionnearthesulfurpitroof.Therefore,themaximumflowof steam possible with the existing piping configuration is 2,900 lb/hr. Calculating thesteam rate to sulfur pit volume ratio shows the maximum steam rate this owner couldpossiblyfeed,assumingacompletelyopengatevalve,wouldbe0.96lb/minper100ft3oftotalsulfurpitvolume. Thisnumberisnotsubstantiallydifferentfromtheestimatedratewiththegatevalveonlyone‐thirdopen.It should be considered that for the sulfur pit evaluated above, the operators specificallyadjustthesteamflowuntiltheyseesteamcomingoutoftheairintakepiping.Theflowofsteam effectively seals the air intake and prevents air from entering the sulfur pit. Thesteamisalsodilutingtheairthatisinthevaporspaceandpushingsomeofitoutoftheairintakepiping.IntheComputationalFluidDynamicsModelingofaSulfurPitsection,CFDisusedtodeterminetheoxygenconcentrationduringthe15minuteperiodwhensteamisfed.Asthefireconsumessulfurandoxygenfromtheair,SO2isproduced.ThisSO2isheavyandcould possibly sit on the surface of themolten sulfur, helping to limit the fire access tooxygen in the vapor space. Alberta Sulphur Research Ltd (ASRL) has completed somepreliminary evaluations of fires in rail cars. According to Clark [Clark, P.D. personalcommunication October 8, 2014], ASRL completed experiments that simulated whathappensifarailcarofsolidsulfurignited.Whattheyobservedwasthatthesulfurstartedtoburnonlyafteritbecameliquid.Inaddition,theysawthattheflametemperatureneverreachedthemaximumvalueinair(ca.1,200F[650C]butstalledaround840F(450C),afterall of the solid had liquefied. Theypreliminarily concluded that the SO2 produced at thesulfursurfacepreventedmasstransferofairtothesulfur,limitingtherateofcombustion.Theseexperimentsweredoneinanopenboxwithoutalid.Clarkalsospeculatedthatsincethesurfaceoftheliquidsulfurwasadjacenttothehotvaporat840F(450C),evaporationofliquid sulfur to the gas phase may be impeded by a viscous sulfur layer at or near thesurface. Thus, the equilibrium vapor pressure at the bulk liquid sulfur temperature (ca.356F [180C]) might not be obtained during a fire in a pit due to the kinetic effects ofevaporation.




Even if the SO2 is well dispersed within the vapor space, by effectively sealing all airentrances to the sulfur pit with steam, the fire becomes self‐limiting and is quicklyextinguished.OwnerNo.2experiencedasulfurpitfireinonesulfurpitrecently.Thisownerprovidedtheauthorswithdataonhowthesulfurpitfirewasextinguished.Thesulfurpitinquestionis19'‐6"W x 57'‐0" L x 10'‐0" D and has a single 2 inch steam line from the header thatbranches into three separate 2 inch lines, which each feed a 3 inch connection with arupturediskon3inchconnectionsontheroofofeachsulfurpit.Thethreeconnectionsonthe sulfur pit roof are roughly 20 feet apart and feed the common vapor space abovedifferent sections within the sulfur pit. The sulfur pit is a typical concrete pit that iscompletelyenclosedwithoneairinletlineandoneairexitlinethatfeedstwoejectors(oneejectorcanbeusedtofeedthesulfurpitsweepairtotheincinerator,andtheothercanbeusedtofeedthesulfurpitsweepairbacktothefrontendoftheSRUatthethermalreactor).Theownerreportedthatwhenanincreaseintemperatureofthesulfurpitvaporspacewasnoticed(120F[66.7C]injustafewminutes),thesteam2inchballvalvewasopenedaboutonequarter.Thesteamwasleftflowingforaperiodofapproximately2minutes,thesulfurpit vapor space temperature began to decrease, and the steam flow was stopped. Theoperators then monitored the sulfur pit vapor space temperature and noticed that,approximately 30minutes after stopping the steam flow, the temperature started toincreaseagain. Atthatpoint,theoperatorcrackedopenthe2inchballvalveinthesteamlineandletthesteamflowforanadditionalminuteuntiltheysawthesulfurpitvaporspacetemperaturestarttodecrease.Thesteamvalvewasthenclosed,andthefiredidnotreturn.Figure4isDCSdatashowingthetemperatureofthesulfurpitvaporspaceandthe liquidsulfurtemperatureduringthefireinOwnerNo.2’ssulfurpit.Ascanbeenseen,thesulfurpitvaportemperatureincreased120F(66.7C)approximately8minutesafterasulfurpumptrippedduetohighviscosity(moltensulfurviscosity increasesastemperatureincreases).Steamwas started for 2minutes and,when the operators saw the temperature begin todecrease,thesteamwasstopped.Thesulfurpitvaporspaceandliquidsulfurtemperaturecontinued to decrease to normal values over an approximate 30minute period. Then asecondtemperaturespikeoccurredinthesulfurpitvaporandliquidsulfur.Asmallamountof steamwas added for about 1minute, and the temperatures again started to decreasetowardnormalvalues.Whenthesteamflowwasinitiallystarted,onlyoneofthethreerupturedisksactuallyburst.Therupturedisk thatwasclosest to thesteamsupplyvalvewas thedisk thatburst. Theothertworupturedisksremainedintactthroughbothsteamevents.Althoughnotperhapsapparentduringtheoriginaldesign,butobviousinhindsight,havingmultiplerupturedisksonasinglesteamsupplylinewilllikelyresultinonlyonediskbursting.Therupturedisksarelocateddirectlyonthesulfurpitnozzlestopreventsulfurpitvaporsfrombackingintothe steam line, condensing, and then solidifying and plugging the line during normaloperation. The rupture disks are located at practically the lowest pressure in the pipingsystem. Therefore,with slight differences in the actual bursting pressure of the rupturedisksandslightlydifferentpressuresineachsectionofpipingfeedingtherupturedisks,itislogical that only one of the rupture disks would burst, and all the flowwould enter thesulfurpitatthatlocation.




Figure4.SulfurPitFireDCSTemperatureDataOwnerNo.2providedthepipingisometricforthesteamline.Hydrauliccalculationswerecompleted for the steam based on the steam header pressure, the one‐quarter open ballvalve,andinformationfromthepipingisometric. Theflowofthesteamwascalculatedtobe 891lb/hr. The analysis shows that the steam flowonly achieves about 10percent ofsonicvelocity intheupsized3inchsectionofpipenearthesteamlineexit intothesulfurpit.Thetotalvolumeofthesulfurpitis11,115ft3.Calculatingthesteamratetosulfurpitvolumeratioshowsthesteamratethisownerusedwas0.13 lb/minper100 ft3of totalsulfurpitvolume. Thisrate likelywassuccessful inextinguishingthe initialsulfurfirein the sulfur pit when this rate was used for only about 2minutes. The owner didexperience a second fire 30 minutes later, but it was likely a re‐ignition event. Theauthors did not calculate a steam rate for the second event because the operators saidtheyonlycrackedthevalveopenslightlyfor1minute.Therefore,thesteamratefedthesecondtimewaslessthanthefirstrateshownabove. Thesteamrateaboveis19timeslessthan(orabout5percentof)thecurrentNFPA655‐specifiedvalueof2.5lb/minper100ft3.Theratediscussedaboveof0.13lb/minper100ft3ispracticallythesamevalueaslistedinTable1ofSection2.8regardingastatementmadeinsubstantiationofchangestothe2001editionofNFPA655,namely,that,ifsteamisinjectedatthesurfaceofthesulfur,4ft3/minper100 ft3 of enclosure volume would be satisfactory (4 ft3/min per 100 ft3 is equal to0.14lb/minper100ft3).




Itisdifficulttoaccuratelydeterminetheresistancecoefficient(K)forapartiallyopenedballvalvewithouthavingtrueflowtestdatafromthemanufactureroftheactualballvalve.Theauthorsdeterminedthatwithacompletelyopenfullportballvalve(Kvaluesareavailable),theflowofsteamwouldonlyincreaseto2,779lb/hr.Thevelocityatthisratewasstillonly29percentofsonicvelocityinthe3inchsectionnearthesulfurpitroof.Themaximumflowof steam possible with the existing piping configuration is 2,779 lb/hr. Calculating thesteam rate to sulfur pit volume ratio shows the maximum steam rate this owner couldpossiblyfeed,assumingacompletelyopenballvalve,wouldbe0.41lb/minper100ft3oftotalsulfurpitvolume.Thisnumberismorethan3timestheestimatedratewiththeballvalveonly one‐quarter open. However, this steam rate is still 6 times less than (or about 16percentof)thecurrentNFPA‐655specifiedvalueof2.5lb/minper100ft3.OwnerNo.3experiencedasulfurpitfireinonesulfurpitinDecember2013.Thatfirewasthefirstknownsulfurpitfireinthe21yearsofoperationofthatunit.Thisownerprovideddataonhowthesulfurpitfirewasextinguished.Thesulfurpitinquestionis12'‐0"Wx36'‐6"Lx8'‐0"Dandhasasingle2inchsteamlinefromtheheaderthatbranchesintothreeseparate2inchlines,eachofwhichfeedsa3inchconnectionontheroofofthesulfurpit.The three connections on the sulfur pit roof feed the vapor space in the sulfur pit. Thesulfurpitisatypicalconcretepitthatiscompletelyenclosed,withoneairinletlineandoneairexit line that feedsanejector. Thesteamdrivenejectorsweepsair throughthevaporspaceofthesulfurpitanddischargestoanincinerator.Theownerstatedthattheirwrittenprocedureforasulfurpitfireistoopenthe2inchvalveinthesteamline100percentandkeepthesteamonforatleast15minutes.Theownerreportedthat,whentroubleshootinghighincineratorstackSO2emissions,theyreducedthemotivesteamflowtotheejector,andalmost immediately (less than5minutes) the incineratorstackSO2emissionsreturned tonormal. By temporarily reducing themotive steam flow to theejector, theejectorpulledlessairthroughthesulfurpit,andthefireextinguisheditself.Nosteamflowwasrequiredforthisparticularsulfurfire.5.3 Sulfur Tank Fires

OwnerNo. 4 experienced a few sulfur tank fires. This owner provided data on how thesulfurtankfireswereextinguished.Thesulfurtankinquestionis20'‐0"Diax32'‐0"Handhasa4inchsteamlinefromtheheaderthatfeedsfour2inchconnectionsontheroofofthesulfur tank. The sulfur tanks are typical carbon steel tanks with a fixed roof and withmultiple air inlet lines around the periphery of the roof and one center air exit line thatvents to theatmosphere. Theownerreportedthat,whentheynoticeda fire in thesulfurtank,theyopenedthesteam4inchballvalveaboutone‐quarter.Thesteamwasleftflowingfor a period of 30minutes and then the flowwas stopped. The owner stated that theynoticed the temperature in the tank decreased after 5 to 10 minutes, but they kept thesteamonfor30minutestobesurethefirewascompletelyextinguished.The authors completed hydraulic calculations for the steam based on the steam headerpressure, theone‐quarter openvalve, and information regarding thepiping routing. Theflowof thesteamwascalculatedtobe10,840 lb/hr. Theanalysisshowedthat thesteamflowachievesonlyabout60percentof sonicvelocity in thedownsized2 inch sectionsofpipeat thenozzleson the sulfur tank. The totalvolumeof the sulfur tank is123,150 ft3.Calculating thesteamrate tosulfur tank volumeratioshowedthesteamrate thisowner




hasbeenusing is0.15 lb/minper100 ft3of total sulfurpit volume. Theownerreportedsuccessfully extinguishing the sulfur fire in the sulfur tankwith this ratewhen the steamflowedfor30minutes(firewaslikelyoutin5to10minutes).Thisrateis17timeslessthan(orabout6percentof)thecurrentNFPA655‐specifiedvalueof2.5lb/minper100ft3.Theownerreportedthatnodamagewasexperiencedinthesulfurtankbythisfire,becauseitwaslikelyextinguishedin5to10minutes.It is difficult to accurately determine the resistance coefficient (K) for a partially openedvalve without having true flow test data from themanufacturer of the actual valve. Theauthorsdeterminedthatwithacompletelyopenfullportvalve(Kvaluesareavailable),theflowofsteamwouldonlyincreaseto14,836lb/hr. Thevelocityatthisratewasstillonly80percentofsonicvelocity in the2 inchsectionofpipeat thenozzleson thesulfur tankroof. The maximum flow of steam possible with the existing piping configuration is14,836lb/hr. Calculating the steam rate to sulfur pit volume ratio shows themaximumsteam rate this owner could possibly feed, assuming a completely open valve, would be0.20lb/minper100ft3oftotalsulfurpitvolume.Thisnumberisnotsubstantiallydifferentfromtheestimatedratewiththevalveonlyone‐quarteropen.OwnerNo.4reportedthatfireshadoccurredinsulfurtanksatanotherlocation,butithadbeen10yearsormoresincethelastone.Forthissite,whenafireoccurredsteamvalve(s)wouldbeopenedforaperiodof20to30minutes.6.0 Computational Fluid Dynamics Modeling of a Sulfur Pit

ACFDmodelofthefirstsulfurpitdescribedinSection5.2wascreated.TheCFDmodelwassetuptodetermineconcentrationsofoxygenandsteamaswellasthevelocitiesthroughoutthemodel.Themodeldidnotincludetheeffectsofactualcombustionofoxygenwithsulfur.Therefore, all changes in oxygen concentration are adirect result ofdilutionof theoxygenwithsteamandexhaustingtheoxygen‐containingairfromthesulfurpitthroughtheejectorsuctionlineandbackwardthroughtheairintakeline.Allreportedoxygenconcentrationsare,therefore,conservative,andtheactualvalueswouldbelowerbecauseoftheconsumptionofoxygen by combustion of sulfur. Two flow conditionswere consideredwith themodel: acurrentconfigurationmodelandamodelthatconsideredarelocationof thesteaminlet. Itshouldbenotedthatthedisturbanceofthesulfur’ssurfacebypossiblehigh‐speedjets,whichwouldresultinmoresulfuravailableforapitfire,wasnotconsideredintheseanalyses.Ashasbeenshowninapreviouspublicationbyoneofthispaper’sauthors(4),thefollowingstepsareinvolvedinallCFDanalyses:

Selectionandconstructionofcomputationaldomains.

Developmentofcomputationalgrid.

Selectionofdomainphysics.

Applicationofboundaryconditions.

Solution.




The following subsections detail how each of these steps was implemented for the CFDanalyses,theresultsandgeneraldiscussionfromtheanalyses.6.1 Selection and Construction of Computational Domains

Themodelwasbasedon theoverall internal dimensionsof the sulfurpit, alongwith thedimensionsof theair inletandejectorsuction lines. Theoveralldomain forbothmodelsincludedthepitvaporspaceata45percentfill level. Figure5showsthethreegeometricdomains created for the analyses, the main pit space (steel blue light, 40 percenttransparent)andthetwoabandonedspargerboxes(turquoisegreen).

Figure5.DomainsUsedforCFDAnalyses6.2 Development of Computational Grid

The computational gridwas constructed using Star‐CCM+’s automatic polyhedralmesherwithwallprismlayersenabled.Thefinalcomputationalgridcontained1,001,116cellsandisshownonFigure6.




Figure6.ComputationalGridDevelopedforCFDAnalyses

6.3 Selection of Domain Physics

Thefollowingphysicsmodelswereenabledfortheanalyses:

Space–3‐dimensional.

Time–Implicitunsteady.

Material–Multispecies,gas(H2S,H2O,O2andN2).

Equationofstate–Idealgas.

Turbulence‐RANS,Realizablek‐,Ally+WallLaw.

Segregatedfluidtemperature.

Gravity(‐9.81m/siny‐direction).




6.4 Selection of Boundary Conditions

Figure7showstheboundaryconditionsappliedforthecurrentconfigurationanalysis.Thesteaminletisdenotedbythedotintheredcircle,andtheairinletisdenotedbythetopofthe pipe in the blue circle. The ejector outlet is shown in yellow. The steam inlet wasdefined as amass flow inlet,with an inlet flow rate of 2,456 lb/hr. The steam inletwassizedtohaveaninletvelocityof0.7Mach,assonicflowwouldrequireconsiderablymorecomputational resources to solve. The air inlet was defined as a stagnation inlet atatmosphericpressure. Theejectorwasdefinedasavelocityoutletwithaflowratebasedonthedesigndatasheetvalueof350actualcubicfeetperminute(acfm).

Figure7.BoundaryConditionsAppliedforCurrentConfigurationAnalysisFigure8showstheboundaryconditionsappliedtothesteaminletmovedmodel. Theairinlet is circled inblue, the steam inlet is circled in redand theejectoroutlet is circled inyellow.Sincethismodelwasoriginallyrunwithapressureboundaryattheejectoroutlet,there isacontractiontopreventbackflow. Thesamevalues listedabovewereappliedtothismodel.




Figure8.BoundaryConditionsAppliedtoSteamInletMovedModelFigure 9 shows the common boundary conditions applied to themodels. The unheatedspargerwallsareshown inmagenta. Theheatedsulfur level is shown inyellow,and thesulfur rundowns are shown inwhite. Both the sulfur level and rundownswere set to adefinedtemperatureof315.5F(157.5C).

Figure9.CommonBoundaryConditionsAppliedtoModels




6.5 Solution

The transientsolutionswereconductedonahigh‐performancecomputingcluster (HPCC)using 120 processors. An adaptive time‐stepwas used so that themaximum convectiveCourantnumberwasbelow1foreachtime‐step.6.6 Results

Thefollowingsubsectionsdetailtheresultsforthetwoconfigurationsthatwereanalyzed.6.6.1 Current Configuration Results

Themaximumand theaverageoxygenconcentration in the sulfurpit vaporspaceversustime, for the current configuration case, areplottedonFigure10. As canbe seenon thefigure,attheendofthe15minutesteamperiod,themaximumoxygenconcentrationinthesulfurpitvaporspaceisabout0.24mole%.However,theaverageoxygenconcentrationisonly about 0.015mole%. The average oxygen concentration is 16 times lower than themaximumvalue,indicatingthatthereareveryfewlocationswithinthevaporspaceofthesulfur pitwith oxygen concentrations near themaximum. The very low average oxygenconcentrationsupportsthefielddatathatthesulfurpitfireisextinguishedbeforethesteamisstoppedafter15minutes.TheactualoxygenconcentrationinthesulfurpitvaporspacewillbeevenlowerthanindicatedonthefigurebecausetheCFDmodeldoesnotaccountfortheoxygenconsumedbythesulfurfire.

Figure10.OxygenConcentrationsinSulfurPitVaporSpace




DatawerealsoextractedfromtheCFDmodeltoshowthemaximumoxygenconcentrationinthesulfurpitairinletlineandinthesuctionlinetotheejector.Thesedata,alongwiththemaximumoxygenandtheaverageoxygenconcentration in thesulfurpitvaporspace,areplotted on Figure 11. The figure shows that themaximum oxygen concentration in theejector suction line immediately starts to decrease after the steam is started. This is asexpectedsincethesteamisfedintothesulfurpitnearthenozzleonthesulfurpitthatfeedsthe ejector suction. The figure shows that themaximumoxygen concentration in the airinlet line remains at the ambient value of 21mole% for about 15 seconds as the steampassesthroughthevaporspaceofthesulfurpittoreachtheairinletonthefarsideofthesulfurpit. Afterabout15seconds,themaximumoxygenconcentrationintheairinletlinebegins to decrease (as steam begins flowing backward through the air inlet line) andbasically matches the maximum oxygen concentration in the ejector suction line after90seconds. Themaximumoxygenconcentration intheejectorsuctionandthemaximumoxygen concentration in the air inlet linepractically lieon topof the line for the averageoxygen concentration in the sulfur pit vapor space, making it difficult to distinguish thethree lines in the figure. Although not shown on the figure, the average oxygenconcentration and themaximum oxygen concentration in the ejector suction line do notdifferbymorethan1percentafterabout25secondsofelapsedpurgetime. Thefactthatthemaximumoxygenconcentrationintheejectorsuctionline,air inlet lineandthevaporspace are basically the same value indicates that the sulfur pit vapor space is fairlywellmixedandventingthesameconcentrationfrombothendsofthesulfurpit.

Figure11.OxygenConcentrationsinSulfurPitVaporSpace,AirInletandEjectorSuction




6.6.2 Relocated Configuration Results

Themaximumand theaverageoxygenconcentration in the sulfurpit vaporspaceversustimeforthecurrentsteamlocation,andwiththesteamrelocatedneartheairinletlineareplottedonFigure12. Ascanbeseenonthe figure,relocatingthesteaminletneartheairinletlinesignificantlylowerstheaverageandmaximumoxygenconcentrationinthesulfurpitvaporspacecomparedtothosesameparameterswiththesteamatitsexistinglocation.The model was stopped after about 12 minutes (720seconds) because the oxygenconcentrationsweresolow.Attheendof12minutes,themaximumoxygenconcentrationin the sulfur pit vapor space was less than 0.0005 mole%, and the average oxygenconcentrationinthesulfurpitvaporspacewaslessthan0.00003mole%.Again,theactualoxygenconcentrationinthesulfurpitvaporspacewillbeevenlowerthanindicatedonthefigurebecausetheCFDmodeldoesnotaccountfortheoxygenconsumedbythesulfurfire.Itisobviousfromthedatathatintroducingthesteamneartheairinletlinewillsealtheairinlet line quicker and thereby drive the oxygen concentration in the vapor space to lowlevelsmuchfasterthanlocatingthesteamonthefarsideofthesulfurpitneartheejectorsuction line. Thedata indicate thatbyrelocating thesteamfeedtonear theair inlet line,after about 5minutes themaximum and average oxygen concentrations in the sulfur pitvaporspacecanbedecreasedtovaluessimilartothevaluesachieved in15minuteswiththeexistingsteamlocation.

Figure12.OxygenConcentrationsinSulfurPitVaporSpacewithSteamRelocatedNearAirInlet




6.6.3 Velocity Streamline Comparisons

Figure13showsthevelocitystreamlinesforthecurrentconfigurationanalysis. Ascanbeseen, thehigh‐speed jet impingesonthe liquidsulfursurfaceandthentravelsaroundtheperimeter. Inthisflowcondition, littlepurgingisoccurringinthecenterofthepit,whichresultsinthelongerpurgetimesshownonFigure10throughFigure12.Figure14showsthevelocitystreamlinesforthesteaminletrelocatedconfiguration.Ascanbeseeninthiscase,alargeportionofthesteamentersthepitandisimmediatelyexhaustedthroughtheairinlet.ThiswillresultinrapidlystoppingallO2flowintothepit.

Figure13.VelocityStreamlinesforCurrentConfigurationAnalysis




Figure14.VelocityStreamlinesforMovedSteamInletCase6.7 General Discussion

AdditionalCFD studies areneeded toverify the impactsof locating the steamconnectionneartheairinletlinesforothersulfurpitconfigurations.However,itislogicaltoconcludethatbylocatingthesteamneartheairinlets,thesteamwillquicklysealtheairinletsandthereby stop ingress of oxygen that would maintain the sulfur fire. It is also logical toassume that locating the steam near the air inlets rather than near the ejector suctionallowsmoretimefortheejectortoremovetheexistingsulfurpitvaporspacegasmixture(air,sulfurvapor,H2S,SO2)beforethemixturebeginstobedilutedwithsteam.Therefore,theoxygenisremovedfromthesulfurpitvaporspacemorequickly.AdditionalCFDstudiesarealsoneededtoconfirmhowtheoxygenconcentrationisreducedfordesignswhere the sulfurpit ejector is trippedoff on thebasis of the sulfurpit vaporspacetemperature.PorterMcGuffieInc.estimatesthatitwouldtakeapproximately3man‐daystosetupanewsulfurpitmodelandexecuteonesetofconditions.Eachadditionalsetofconditionswouldrequireabout1.5man‐daystoexecute.




7.0 Potential Rapid Sealing Steam Rate

On the basis of the data collected and the analysis completed in Sections 5.0 and 6.0, itappearsthatasteamrateof1.0lb/minper100ft3wouldbesufficienttoextinguishsulfurfires inenclosedsulfurtankandsulfurpits. Thisvalueisstillhigher(moreconservative)than theratescollected fromanyof theowners thatprovideddata. It ispossible thatanevenlowersteamratecouldbeusedsuccessfully,butnotenoughdatacouldbecollectedtojustifyalowerrate.Itisapparentthatindustryhassuccessfullyextinguishedfiresinsulfurtanksandsulfurpitswithout using the NFPA 655 snuffing steam rate recommendation but by using a lowersteamratethateffectivelysealstheenclosures,preventsairingressandextinguishesfiressafely.The authors completed evaluations of the sulfur tank vent systems for six existing sulfurtankswhensteamwasfedatarateof1.0 lb/minper100ft3ofvolume. ThesulfurtanksevaluatedwerethesamesulfurtanksthatwereevaluatedinSection3.0.Table4showsthesamedatapresented inTable2 inSection3.0,except that twoadditional rows(shown inyellow) have been added to the bottom to show the built‐up back pressure with steamflowingata rateof1.0 lb/minper100 ft3andhowthevolumeofsteamcompares to theoriginalairsweeprate.As can be seen in Table 4, the steam rate of 1.0 lb/min per 100 ft3 ranges from about12to80 times the original sweep air rate. This steam rate results in a built‐up backpressurewithin the sulfur tank of 0.04‐9.1 inchWC. This built‐up back pressurewouldexceedtheactualdesignpressureoftwooutofsixofthesulfurtanks,asshownbytheredtext inTable4. However, if the sulfur tankshadbeen specifiedwith a consistentdesignpressureof‐3/+10inchWC,thenthesteamrateof1.0lb/minper100ft3couldbeventedfromthesulfurtankswithoutexceedingthedesignpressureandwithoutneedingtomodifytheventingsystemsonthesulfurtanks.


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Table4.ExistingSulfurTankswithSteam

at1.0lb/m

inper100ft

3Com

paredto2.5lb/m

inper100ft

3

TankA

TankB

TankC

TankD

TankE

TankF

Dimensions

66‐6"Diax

35'‐0"H

59‐6"Diax

48'‐0"H

19'‐8"Diax

17'‐9"H

14'‐5"Diax

16'‐4"H

42'‐8"Diax

32'‐10"H

47'‐6"Diax

24'‐10"H

Volum

e,ft

3 138,500

148,635

5,800

2,833

48,885

49,435

DesignPressure,

inchWC

‐3/+10

‐1.5/+2.5

‐3/+10

‐3/+10

‐1.1/+3.2

‐0.9/+3.5

AirInlets

Six‐10"

Two‐16"

Three‐6"

Three‐6"

Six‐6"

Six‐6"

AirOutlet

One‐10"

One‐16"

One‐8"

One‐8"

One‐18"

One‐8"

AirMovem

entM

ethod

Blowersucksair

throughtankand

pushesthrough

causticscrubber

Naturaldraft

2inlets,1outlet

Naturaldraft

3inlets,1outlet

Naturaldraft

3inlets,1outlet

Naturaldraft

6inlets,1outlet

Naturaldraft

6inlets,1

outlet

NormalOpPressure,

inchWC

‐0.05

‐0.0031

‐0.00005

‐0.00008

‐0.029

Unknown

AirSweepObjective

<16%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<21%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<50%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<50%

H2SLFL,

with150ppm(wt)

H2Sinsulfurfeed

<25%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

Unknown

PressureinchWCwith

steamat2.5lb/m

in/

100ft3

59.3

50.8

1.05

0.23

8.1

29

Steamvolum

etricrate(@

2.5lb/m

in/100ft

3 )


29x

190x

198x

79x

53x

Unknown

PressureinchWCwith

steamat1.0lb/m

in/

100ft3

9.1

7.9

0.17

0.04

1.3

5.0

Steamvolum

etricrate(@

1.0lb/m

in/100ft

3 )


11.7x

76x

79x

32x

21x

Unknown




Theauthorsevaluatedthesulfurpitventsystems for fiveexistingsulfurpitswhensteamwas fedatarateof1.0 lb/minper100 ft3ofvolume. Thesulfurpitsevaluatedwere thesamesulfurpitsthatwereevaluatedinSection3.0.Table5showsthesamedatapresentedin Table 3 in Section 3.0, except that two additional rows (shown in yellow) have beenadded to the bottom to show the built‐up back pressurewith steam flowing at a rate of1.0lb/minper100ft3andhowthevolumeofsteamcomparestotheoriginalairsweeprate.AscanbeseeninTable5,thesteamrateof1.0lb/minper100ft3rangesfromapproximately4to 12 times the original sweep air rate. This steam rate results in a built‐up backpressurewithinthesulfurpitof2.1to49inchWC.Thelargestsulfurpit(totalcapacityof60,665ft3)couldreachapressureof366inchWC(13.2psig)ifthecoversdidnotlift.Thisparticularsulfurpithasalargereductionininletlinesize,8inchesdownto4inchestofeed into a flowmeter. The flow reaches sonic velocity in this 4 inch diameter section,resultinginveryhighbuilt‐upbackpressures.Thebuilt‐upbackpressurewouldexceedthepressureratingofthehatchcoversonfouroutoffivesulfurpits,asshownbytheredtextinTable5. However, if the sulfurpitshadbeenspecifiedwithprecast concretehatchcoversinsteadofaluminumandaslightadjustmentmadeintheairinletlinesize,thesteamrateof1.0lb/minper100ft3couldlikelybeventedfromthesulfurpitswithoutexceedingthedesignpressure.Itappearsthattobeabletoventasteamrateof1.0lb/minper100ft3,thetypicalsulfurpitdesignpressureandsulfurpitvent systemwouldrequiremoremodifications thanthose thatwouldbe required for typical sulfur tanks. However, it does appear possible todesignasulfurpitsystemtoaccommodatethelowersteamrateof1.0lb/minper100ft3.Additionalengineeringstudies,includingtheuseofCFD,maybeappropriatetoestablishanevenlowersealingsteamrateforanexistingsulfurpit.


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Table5.ExistingSulfurPitswithSteam

at1.0lb/m

inper100ft

3Com

paredto2.5lb/m

inper100ft

3

PitA

PitB

PitC

PitD

PitE

Dimensions

24'‐0"W

x35'‐0"Lx

10'‐6"D

47'‐7"W

x72'‐2"Lx

17'‐8"D

14'‐0"W

x34'‐0"Lx

11'‐0"D

13'‐2"W

x49'‐10"Lx

9'‐2"D

21'‐0"W

x30'‐0"Lx

8'‐0"D

Volum

e,ft

3 8,820

60,665

5,236

6,015

5,040




hatchcovers


alum

inum

hatchcovers



hatchcovers

‐unknown/+14.2set

byprecastconcrete

accesshatchcovers

‐unknown/+0.42set

byaluminum

hatch

covers

AirInlets

One‐6"

Two‐8"

One‐4"

One‐4"

One‐6"

AirOutlet

One‐6"

Two‐8"

One‐4"

Two‐3"

One‐6"

AirMovem

entM

ethod


airinletandpushesto

thermalreactor


airinletsandpushesto

thermalreactor


airinletandpushesto

thermalreactor

Ejectorsucksair

throughairinletand

pushestoincinerator

Ejectorsucksair

throughairinletand

pushestoincinerator

NormalOpPressure,

inchWC

‐0.22

‐6.9

‐1.2

‐0.30

‐0.22

AirSweepObjective

<15%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

<25%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

<25%

H2SLFL,

with300ppm(wt)

H2Sinsulfurfeed

Unknown

Unknown

PressureinchWCwith

steamat2.5lb/m

in/100ft

3 57.8

50psigifhatchcovers



266

230

21.4/17.6(Note1)

Steamvolum

etricrate

(@2.5lb/m

in/100ft

3 )


16X

31x

17x

30x

9.7x

PressureinchWCwith

steamat1.0lb/m

in/100ft

3 8.9

13.2psigifhatchcovers



49

39

3.5/2.1(Note1)

Steamvolum

etricrate

(@1.0lb/m

in/100ft

3 )


6.4x

12x

6.9x

12x

3.9x

Notes:

1.

Thesecondvaluelistedisthebuilt‐upbackpressureforthissulfurpitbasedontheejectorstayinginoperation.Allothersulfurpitsanalyzed

haveanSISthatwilltripejectorswhenfireisdetected.




8.0 Good Engineering Practice for Use of Sealing Steam

Onthebasisof thedatacollectedforthispaperandtheanalysiscompleted, the followingpoints(notlistedinanyparticularorder)couldbeconsideredgoodengineeringpracticeforsulfurtanksandsulfurpitstoaddressextinguishingfires:

Sulfur tanks should be designedwith design pressure of at least ‐3 / +10inchWC.

Sulfurpitsneedtobecarefullyevaluatedanddesignedtoallowthechosensealingsteamratetoventfromtheairinletswithoutliftingthehatchcovers.

Thevalvethatisusedtosupplysealingsteamtothesulfurtankorsulfurpitshould be located at a safe location (typically at least 50 feet away) awayfrom thesulfur tankorsulfurpit. Thevalveshouldbe locatedneargradeelevations,easilyaccessibleandclearlymarked.

Sealing steam lines should contain a drip leg and steam trap ahead of thevalve usedby operators so that the linewill remainwarmandnot collectcondensate ahead of the valve. Feeding built‐up condensate through thesealing steam line to the sulfur tank or sulfur pitwhen the steamvalve isopenedwillresultinaverylargeincreaseinpressureintheenclosureandpossiblyresultinoverpressuringtheenclosure.

Sealingsteamshouldbefedneartheairinletlinestosulfurtanksorsulfurpits so that steamwillquicklybackflowoutof theair intakesandpreventfurtheroxygeningressduringafire.

PreliminaryCFDresultsshowthatslowingthesteamdown(oversizedinletnozzle)priortotheentrancetothesulfurtankandsulfurpitmayhelplimitagitationoftheliquidsulfursurfaceand,thereby,reducetheamountoffuelthat is available to burn in the vapor space. Additional CFDwork is stillneededtoclarifytheeffectofthesteamentrancevelocity.

Sealingsteamsystems thatuserupturedisks topreventsulfurvapor frombacking into the line, condensing and solidifying should have a separatesteamsupplyvalve foreachrupturedisk insteadofmultiplerupturedisksfed fromonesteamsupplyvalve. Analternative tohavingseparatesteamsupplypiping/valve foreachrupturediskwouldbe tooversize thesupplypipesothatthevastmajorityofthepressuredropisacrossthesectionthatcontains the rupture disks. A careful hydraulic study would need to beperformedtoensurethatevenifonerupturediskburst,therewouldstillbeenough pressure in the steam supply line to burst all remaining rupturedisks.




If rupturedisksarenotused tohelppreventplugging in thesteamsupplysystemneartheconnectionstothesulfurtankorsulfurpit,theopensealingsteamlineshouldbetestedonaroutinebasis(atleastmonthly)bypassingsteamthrough thesystemtoensure thesteamsupply linesandnozzlearefreeofsolidifiedsulfur.

Thelocationofthethermocouplethatisusedtodetectafireinasulfurpitneedstobeevaluatedcarefully. Forconfigurationsthatintendtoleavetheejectorsoperatingthroughafireevent,locatingthetemperatureindicatorintheejectorsuctionlineislikelybetterthanlocatingitinthesulfurpitvaporspace.Atleastwiththetemperatureindicatorintheejectorsuctionline,thethermocouple will sense an “average” temperature of the gas passingthrough the sulfur pit vapor space rather than a point temperature at asinglelocationwithinthevaporspace.Forsystemsthatwilltriptheejectorsduringafireevent,itissuggestedthatmorethanonethermocouplebeusedasinputtotheSIS.Onethermocoupleshouldbeclosetotheejectorsuctionlinetosensethetemperatureofthevapormixtureasitexitsthesulfurpit,andotherthermocouplesshouldbelocatedinsuspectedlowvelocityareaswhere temperature increases may be noticeable. A CFD model could beusedtohelpplacethethermocouplesinoptimumlocations.

A CFDmodel can be used to evaluate flow patterns in the sulfur tank orsulfur pit vapor space both during normal operation and during sealingsteam injection. The results of the CFD model can help the designer todetermine proper placement of air inlet nozzles, vent nozzles,thermocouplesandsealingsteaminjectionlocations.

9.0 Conclusions

The following conclusions can be drawn from the results of this paper and the authors’experienceswhilepreparingthispaper:

1. The vent systems on typical sulfur pits and sulfur tanks (for storage ofmolten sulfur) must be carefully designed to ensure that the properenvironment is maintained within the vapor space to prevent flammablemixturesfromforming.

2. For sulfur pits and sulfur tanks that use an air sweep system to prevent

flammable mixtures from forming, the enclosures are typically operatedundera slight vacuum toprevent strayemissionsofH2Sand sulfurvapor.This vacuum level is set according to the required air sweep rate and airinlet line sizes. The vacuum level needs to be set so that the buoyancyeffectsoftheheatedairintakelinesareaccountedfor,andforsulfurtanks,the vacuum level should consider the effects of wind blowing across theoutersurfaceofthetank.




3. It isextremelydifficultandimpracticaltodesigntheairsweepsystemsforsulfur tanks and sulfur pits without exceeding the design pressure of theenclosure if they alsomustbe able to vent the currentNFPA655 snuffingsteamrequirementof2.5lb/minper100ft3oftotalvolume.

4. Itappearsthatasealingsteamrateoflessthan1.0lb/minper100ft3oftotal

volumehasbeensuccessful inextinguishingnumerousfiresinsulfurtanksandsulfurpits.

5. Newsulfurtanksandsulfurpitscanbedesignedtoventasealingsteamrate

of 1.0 lb/min per 100 ft3 of total volume without major changes to thetypicaldesignparametersalreadybeingfollowedbyanumberofcompanies.Typical sulfur pits may need more changes than typical sulfur tanks;however,thechangesrequiredarenotonerous.

6. Sealingsteamshouldbefedintothesulfurtankorsulfurpitforaminimum

of15minutesoruntilthetemperaturehasreturnedtonearnormal. Ifthesulfurdoesnot cool downbefore the sealing steam is stoppedand the airsweepsystemreestablished,reignitioncanoccur.

7. Thesealingsteamwillalmostimmediatelysealtheairintake(s)toprevent

furtheroxygeningressandwillbegintopurgeoxygenoutoftheenclosure.Theactualoperatingvaporspacevolumeinthesulfurtankorsulfurpitwillonlyaffectthetimeitwilltakeforthefiretobefullyextinguished.

10.0 Recommendations

Thefollowingrecommendationscanbedrawnfromtheresultsofthispaperandfromtheauthors’experienceswhilepreparingthispaper:

1. TheauthorsrecommendthatNFPAmodifythesealingmethodinNFPA655,Standard for Prevention of Sulfur Fires and Explosions (current edition:2012), Chapter 5, Handling of Liquid Sulfur at Normal HandlingTemperatures,Section5.5,FireFighting,torecommendasealingsteamrateof1.0lb/minofsteamper100ft3ofvolume.

2. The authors also recommend that the good engineering practice points

defined inSection8.0beconsideredand implementedonsulfur tanksandsulfurpitsasapplicable.

11.0 Acknowledgements

The authors wish to thank the efforts of all the sulfur production specialists that werecontactedfordataforthispaper.TheauthorsalsothankLonStern(SternTreating&SulfurRecoveryConsulting,Inc‐ [email protected])forpostingtheirrequestfordataontheAmineBestPracticesGroupDataExchangeNetwork.




12.0 Author Contact Information

AlanD.MosherPrincipalProcessEngineerBlack&VeatchCorporationOverlandPark,KS,[email protected]

SeanM.McGuffieSeniorEngineerPorterMcGuffie,Inc.Lawrence,KS,USAsean@pm‐engr.com

DennisH.MartensConsultantandTechnicalAdvisorPorterMcGuffie,Inc.Lawrence,KS,USAmartensdh@pm‐engr.com

13.0 References

(1) NFPA 655, Standard for Prevention of Sulfur Fires and Explosions, National FireProtectionAssociation,2012Edition.

(2) NFPA 69, Standard on Explosion Prevention Systems, National Fire Protection

Association,2014Edition.(3) Zabetakis, M.G., Flammability Characteristics of Combustible Gases and Vapors,

Bureau of Mines Bulletin 627, U.S. Department of the Interior, TN23.U4 no. 627,622.06173,1965.

(4) McGuffie,S.M.,Porter,M.A.,Hirst,T.H.,UseofCFD inDesign,Presentedatthe2012

American Society of Mechanical Engineers Pressure Vessels & Piping Conference,Toronto,Canada.

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