Document emKagKOnLEv9yXaYM4aa0rKyE

FILE NAME: Foster Wheeler (FW) DATE: 1957 DOC#: FW001 DOCUMENT DESCRIPTION: Book Excerpt - Safety in Plant Design Project Engineering of Process Plants iASE, H . F. AND BARROW, M . H . Project Engineering of Process Plants IASE, H . F. Piping Design for Process Plants PROJECT ENGINEERING of Process Plants HOWARD F. RASE Associate Professor of Chemical Engineering The University of Texas and M. H. BARROW Project Engineer Foster Wheeler Corporation Original Illustrations by James R. H olmes Assistant Professor of Drawing The University of Texas NEW Y O R K JOHN WILEY & SONS, INC. London Sydney Fourth Printing, October, 1964 Copyright 1957, by John Wiley & Sons, Inc, Aill rights reserved. This book or any part twhietrheoouft mthueswt rnitottenbpeerrmepirsosidounceodf tihne apnuyblifsohremr. Library of Congress Catalog Card Number: 57-5929 Printed In the United Slates of America To our wives for their patience and help CHAPTER SAFETY IN PLANT D E S IG N Safety must be a paramount consideration in the design of a process plant. The mass of steel that is fashioned into a plant will ultimately be operated by human beings whose health and well-being must be protected. Disregard of safety in the design of a process plant is comparable to criminal negligence. Each design engineer should feel a responsibility for providing a safe design. He should ask himself: "Would I be willing to operate this equipment as it is now designed?" If the answer is "no" or if the "yes" is weak, the reasons for the uncertainty should be found and corrected. The designer should, whenever possible, discuss his design with operating men and the plant safety engineers. These men are keenly aware of safety and will have many good suggestions. HAZARDS IN PROCESS PLANTS The hazards to be considered in the design of any process plant can be grouped roughly into three major categories: flammable and explosive, hygienic, and mechanical. A majority of chemical processes are poten tially dangerous because flammable and explosive materials are produced. The design of equipment for such plants must be dictated by character istics of the materials being processed. One's memory on the combustion characteristics or explosive limits of materials should never be trusted. Careful investigation of these characteristics of the materials to be handled in the plant should precede all design calculations. 632 Safely in Plant Design 633 A number of chemical products constitute definite health hazards. Pro longed breathing and/or skin contact over a long period of time of such materials as benzene and carbon tetrachloride can cause permanent im pairment of health or even death. It is surprising that many scientifically trained people are unaware of the hazards to health presented by some common chemicals. Although there are excellent references on industrial toxicology which can be used as guides, all but the most common chemicals should be referred to a trained industrial toxicologist for study before handling techniques are devised. Mechanical hazards include all the hazards created by moving equip ment and arrangement of equipment and structures. These classifications are somewhat arbitrary and overlap, but they serve to emphasize the primary hazards in process plants that create or are capable of creating unsafe working conditions. Careful design combined with well-planned accident prevention pro grams have produced amazing safety records. Manufacturers of tetra ethyl lead, for instance, were faced with an unusual health problem due to the poisonous nature of the raw materials and product. Realizing this problem even before the design of the plant was begun created an aware ness that influenced all design decisions. The result has been an enviable record of safe operation. Sources of Information Many safe practices are no more than application of common sense. The problem is to recognize unsafe situations before they are incorpo rated in plant design. Basically, this involves determination of the hazards that exist. In addition to many books on safety and industrial h y g i e n e , t h e r e are a number of handbooks, codes, and similar publications by numerous agencies interested in safety and fire prevention which are helpful in defining potential hazards. A selection of these is given below: I. 1F. irNeaatinodnaelxpFliorseioPnrsotection Association; 60 Batterymarch Street, Boston 10, Massa..; Satannodnaprrdosf:itwtiedcehlnyicaadloapnteddetdhurcoautgiohnoault oUr.gSa.Ani.zaatsiobna;sispufobrlischoenss:truction. Follow(ii)ngNsautmiomnaalriFesireofCsotadnedsafrodrsFalraemamvaabillaeblLe.iquids, Gases, Chemicals and (ii) ENxaptiloonsiavleFs.ire Codes for the Prevention of Dust Explosions. ((iiivi)) NNaattiioonnaall FFiirree CCooddeess ffoorr BEuxitlidnignugisChionngstarnudctiAonlaramndEEqquuipipmmenent.t. b. (Qv)uaNrtaertiloynoafl tEhleecNtraictaiolnCaoldFei.re Protection Association: upc-.toN-dFaPteAinHfoarnmdbatoiookn oofn Ffiirrees Panrodtefcirteiopnr:evaenhtainond.y reference work. 634 Project Engineering of Process Plants Yo2r.k;Naatnionedaul cBaotiaorndalo, ffaFcitruealUnandderwenrgitienrese,ri8n5gJoorhgnanSitzraeteiot,nNseuwppYorotrekd 7b,yNtehwe capitaa.l Ssttoacnkdafrirdes infosrurafinrcee pcroomtepcatnioiens mpuabnlyishoesf: which are based on NFPA stabn.dRaredsse.arch reports. frece. oSfpecchiaalrgreepuoprtosnonrequuneusstuaolndicsoamstperasn.y (sMtaotisotneorfyt.hisAidnvfoicremaatniodnciosusnesnetl ostnancdearrtadisn cpahnasbees oobftfaiirneedp.r)otection problems not now covered by codes or As3y.luOmthSetrreients,uHraanrcteforodrg2a,nCizoantino.n;s:AsTsohceiaFteadctFoaryctoIrnysuMrauntcuealAFsisroeciIantosunr,an55c5e lCaobmorpaatonryi,es18a4ndHpiguhbliSshtresetat,ndBaorsdtso.n 10, Mass. These also operate research 4. aF.ed1.erUa.lSA. geBnucrieesa,uWoafshiMngintoens--, Dr.eCse.arch reports and recommendations, mibn.erUa.lSs,. dDuesptst,, aonfdAgoirlisc.ulture; dust explosions and fires. recco.mUm.Se.ndDedepftorovfarLioaubsorin, dDusitvriiseiso.n of Labor Standards; safe practices de.. UN.aSt.ioPnuabllBicurHeeaaultohf SSetarvnidcaer;disn;dfuirsetrhiaalzahrydgsieannedpfuirbelirceastiisotnasn.ce research. saf5e.tyNaetdiouncaatlioSna. fetRyesCeaorucnhcilw, o2r0k Nco.ndWucatcekde-radDvircievea, vCaihlaicbalego,(n1o1n1.p;romfiat)in. ly (n6o.npUrnodfietr)w. rTiteerssts Leqaubioprmateonrties(,eleIcntcr.i,ca2l0,7firEe.prOohteioctioSntr,eeettc,.)Cthoiciangsoure11s,af1e1t1y.;. sLuipstpslemofenttess.ted and approved equipment issued annually with bi-monthly Ma7n. yLarwefse;r fteodevraarl,iosutastep,raonvdisilooncsalolfawthsereNlaFtiPnAg tocofdirees.protection and buildings. pr8o.viIdnesuvraalunaceblecoamdvpiacne.ies and local and state fire prevention bureaus can pu9b.lisAhmeserbiucallnetinPsetornoleFuimre PIrnostteictutitoen, i5n0RWefi.ne5r0ietsh. Street, New York, N.Y.; II.1. MInadnuustfraicatluhriynggienCehemists' Association; publishes chemical safety data she2e. tNfoartiothnealsaSfaefehtayndClionugncainl;d puusebloisfhmesanhyeaclthhemaincdalss.afe practices pamphlets rel3a.tiAvemteoricthane hInadndulsitnrgialofHvyagriieonues cAhsesmociciaatliso.n; publishes technical quarterly on4i.nUdu.Sst.rBiaulrheyaguieonfe.Mines, Washington, D.C.; dusts and toxic vapors. 65.. MAteolmlonicInEsnteitrugtye,CInomdumstirsisaiolnH, yWgiaensheinFgotuonnd, aDti.oCn.,;Pirtatsdbiouarcgthiv,ePam.;atreersieaalrs.ch. D.7C..;DpivuibsiloicnatoiofnIsnodfusvtarriiaoluHs ytygpieensea, nUd.Sa.dvPiuceb.lic Health Service, Washington, Ha8z.arAdmouersicNaantuMree. dical Association, Committee on Physical Agents of a ow9n. Cphroedmuicctasl. coPmrpoadnuiceesr;s poufbltieshtraientfhoyrlm-laetaido,n foonr tihnestasnacfee, hraencdolminmg eonfdthaenidr fu1r0n. ishSacxo,mpNle. teI.d, esHiganndfobrooekquiopfmDenatnhgearnodulisngMthaetereitahlys,l fRlueidin.hold Publishing Safety in Plant Design 635 aClosorps.u, gNge.Yst.ion(s19f5or1)s;afgeivheasndhlainzgarodfounsumperroopuesrttioexs,ictmreaattemrieanlst,ofancdomamnetirdcoet.es; III1.. EMqeucihpamniecnatl manufacturers; will recommend safe installation procedures. saf2e. pUr.aSc.tiDceespaarntdmreenctomofmLeanbdoart,ioDnisvifsoironinodfusLtaryb.or Standards; publications on pa3m. pNhlaettiso.nal Safety Council; safe practices, recommendations, presented in 4. aC. oAdSesMfEoracnodnsAtrPuIc-tAioSnMoEf vCeossdeelss,fotarnUksn,fipreipdinPgr,esesleucrter.ical equipment: cb.. AAmSMerEicaBnoiSletarnCdoanrdstsruAcstsioocniaCtioodne.Code for Pressure Piping. de.. NAaPtIioSntaalnEdalercdtrNicoa.l 1C2oCdea.nd 12E for Storage Tanks. de5si.gnLioafbislaitfye pinlasunrtsa.nce companies; publish helpful data which aid in the PLANT LAYOUT The safety record of a plant depends partially upon the arrangement of the various units and the location of equipment within those units. For this reason alone safety considerations should permeate even the early planning stages of plant design. Topography and Weather In selecting a site topography and meteorological phenomena should be carefully considered. H illy areas present certain inherent hazards. Overflowing tanks or vessel failures can send thousands of gallons of flammable materials into surrounding areas. If the area is hilly, these materials can reach a wider area and if ignited cause a rapid spread of fire. Prevailing winds should be considered in the layout of a plant so that flammable and toxic gases can be properly dissipated without endangering the lives or comfort of plant personnel or people in the surrounding areas. The occurrence of such weather phenomena as inversions should be analyzed. The atmospheric temperature normally decreases with altitude. During periods of inversion the reverse is true. In such situations the lower atmosphere remains stagnant, and smoke and noxious vapors hang over the earth's surface and create extreme health hazards. The DeNora, Pennsylvania disaster in which a number of people died was caused by such a condition. Any area that has frequent inversions lasting more than three to four days should be rejected as a possible site for a process plant. Division of Plant into Units The operating units of a process plant should be separated not only for more efficient operation and maintenance but also for safety reasons. By 636 Project Engineering of Process Plants separating the process units it is possible to prevent the spread of fire and explosion. Dividing a plant into city-type blocks has many advantages. The blocks are usually separated by roadways making access to all parts of the plant convenient. These roadways may vary from 50 to 100 ft in width. Dead-end roads should be avoided and convenient access from any part of the plant should be possible in at least two different directions. Although roads provide a definite service to the plant, they also can constitute a safety hazard if they are not properly arranged. Many seri ous fires and explosions have been caused by vehicles striking tanks or vessels containing flammable materials. The possibility of such accidents should be considered in planning roadways. In Table 24-1 some suggestions are made for spacing between process units and other facilities in a plant. Any such list to be complete would be extremely voluminous since each type of process unit should be con sidered individually. TABLE 24-1. Suggested Minimum Spacings for Hazardous Process Units Description of hazard Diostpaenrcaetintgo uanditj,acfetnt 21.. HOrigdhinfalraymfmlaambmiliatybialintyd ahnigdhlopwrestsoumreedium pressures 12050--17550 43.. BDliorwecdtofwirnedstbaociklesrswaitnhdflfaurrenaces 110000--210500 65.. PLuobadliicnrgofaadcsilaitnieds railroads 110000 78.. SCtooorlaingge ttaonwkesr* 17050-150 Co*dMesi.n10imuWmherdeivstearncpeosssibbeletwaeednistsatonrcaegeequtaivnaklsenatreto g1ivoern 1i%n ttihmeesNtahteiondaialmFetireer of the larger tank should be used. Utilities The generation and distribution of utilities in a process plant is such an important function that these facilities should be located as remotely as practicable from the operating units. During emergencies caused by fire or explosions, the continued operation of the power plant, the water pumping station, and the steam generating unit is imperative. Open flatties and sparking equipment are common in power plants and boiler houses, and location near a unit processing flammable products constitutes a real hazard. Buildings Plant offices, mechanical shops, and laboratories should be located as far as possible from the operating units. There is no real need for prox- Safety in Pic ` Design 637 unity of offices to process units, and since open flames and sparks arc common to both shops and laboratories it is also advisable to separate these buildings from the process units. Warehouse and loading facilities should be located at the plant property line so that they are easily accessible from the public roads and are as far as possible from the areas of possible danger. Equipment Layout A primary rule for equipment layout is that an operator should have at least two different routes of escape from any point in a unit. Extremely hazardous areas should be provided with escape chutes for rapid exit from the area. Since a clean well-maintained area is often a safe area, equipment must be arranged to enable ease of maintenance and housekeeping. Equipment should be located well above the average height of a man or considerably below since any location near usual height can result in frequent head injuries. As a rule the simpler the arrangement, the safer the unit. A simple arrangement facilitates prompt action in emergencies. Firewalls When it is necessary to space units closer than is considered safe, shield ing walls can be constructed of concrete. High walls of this type are also used to protect personnel from operating equipment which could ex plode or cause a severe fire. Dikes or firewalls should be provided around all storage tanks contain ing flammable material. These walls are especially necessary for flam mable materials which tend to boil over. In these cases, the firewall should be high enough to contain the entire contents of the tank and thus prevent the spread of fire to other tanks or other portions of the plant. Rules and suggestions for firewall construction have been outlined in detail.5'8'9 Units processing large quantities of flammable material are often paved and enclosed by low walls 12 to 24 in. high. Should a vessel rupture, these walls will contain the flammable material and prevent the spread of fire. In some process areas the sewage system can be designed to drain off such spillages rapidly in which case firewalls are not needed. MECHANICAL DESIGN AND SAFETY To insure safe construction all mechanical design should be in accord ance with existing codes. All practices important to safe design, however, are not specified by the codes. Some of the more common practices are listed below in the form of a safety check list for designers. A.1.VCesasreelsfualndselteacntikosn of materials to withstand corrosion. Use adequate corrosion allowances on materials that are expected to corrode. 638 Project Engineering of Process Plants an2d. mPraoinvtiedneanaces,ufafnicdienotriennutmtboerpeormf imt aenahsyoleasccaenssd. handholes for inspection 43.. NAvoozzidlesscarbewoveed 1n%ippilne.s sfhooruslmd ablel nfloaznzgleesd.. UFsoer 6h0i0g0h lpbrecsosuuprlei,nguss.e flanged no5zz. leCsarfoefrulallinssipzeesc.tion during and after fabrication; a competent inspector can dao. Imnuspchecttioonproefvemnattevreisaslesl. failures. cb.. IWnsitpneecstsiionng ooff wraodrikomgraanpshhiinpg (aensdpesctiraellsys rqeuliaelvitiyngo.f welds). bre6a.thAitnmgos(pthheerticaksintograingeotrandkisschsahroguinldg boef vparpovoirdeads twhiethpraedsesuqrueatien vtehnetstafnokr increaa.seUssoervdaeccurueams-epsrdesuseurtoe bchreaantgheerinvtaelmvepser(acotunrseerovratliioqnuidvelnetvse)l.). cb.. UCosennfelocattivnagp-ororosfptaacnekssofforgavso-tlaigthilte tliaqnukisdst.o a single tank serving as a 7g.asAhuotlodmera.ticPtrootvaildlye erneclileofsevdaltvaenskfloerveallgl atgaenskps,rehfoewrreevde;r.manual gage hatches sBu.bHjeectatopexecrhaatonrgetros needless hazards. val1v.eIfonboctohldinsliedteatnodporuetvleent tofexecxecshsiavnegethrerremqaulireexbplaoncskiovna.lveMs,anpyrovexidpelorsieolinesf ha2v.e Soucfcfuicrireendt bdercaainusseshoofusludchbeexppraonvsidioend. for maintenance. 43.. IEfauchnitfsluaidrestpoabcee sotfacaknede,xcthhaeyngmerussthobueldstabcekegdivdenurainngitnhdeivtiedsutianlgtepsrto.cess. des5i.gnIfamndixwtuorrekmofanshshe'ilpamndusttubbee efxlueirdcsisecdo.nstitutes a hazard, extreme care in C.1M. Porvoinvgideeqguuiparmdesnat t(palulmcpous,pleitncg.)s, belts, and chain drives. 23.. PAruotvoimdeataicdeoqvueartloeasdp,aocevebrestpweeeden, ourniottshfeorrdseavfeiceasndarceongovoednieinnvtessetrmviecnintsg.on ma4c.hPinreortyecstuivceh daesviccoems psruecshsoarss.strainers on pumps and liquid knockout drums fDo.r Pcoipminpgressors are essential to safe operation. pla1c.edAbaotvger-oguronudnldeveplroocnesssleelpineerss. arBeurpireedfelrinreeds.conTshtietsuetemaahyazbaerdelseinvcaeteldeakors are2.nIoft lrienaedsilayredeitnescttaeldle.d in trenches, fire stops and drains must be provided at3r.eAgurlraarnginemterevnatlso.f piping and valves is important to safe operation. Apap.rLoxairmgeatvelaylv7esfti0n inp.aristitchuelamr amxiumstumbedirsetaadnicley farcocmescsiebnlteerflrionme toplpaltafotfromrms. levbe.l.UsAebdovoeubtlhei-sblcohcakinvaolpveersatfoorrshaarzearudseodu.s equipment which must be re imnobveetdwweehniledoruemblaeinbdloecrkosfinudniictactoesntlienaukeasgeto. operate. A small tell-tale valve pocr.tioAnrrcaanngbeeloisnoglaytaedrdfrloinmesthtoe raevmoaidindcoenrgbeystciolonsisnogtahabtloicnk cvaasleveo.f fire any roadd. sH; e6adftro6omin.foarboovveerphleaatdfolrimne;s 9shfotu'0dibne. aabpopvreoxgirmaadtee.lyI1f5aftlin0einm. uasbtobvee Safety in Plant Design 639 loovcearteidt. below 6 ft 6 in., locate considerably below and provide a walkway /e.. EFmueerlgseunpcpylyduvmalpvevsalfvoersfushronualcdesbsehlooucladtebde aitn aa sraefme odtiestalonccaetiofrno.m the eqgu.ipImnsetnatlltovablevedsudmispcehdaragnindgpirnetfoeroapbelyn btaehniknsdsao htheaatvyopwearlaltoorr bcaunildoipnger.ate w4.itChohuectkunvdaluveesexlpeaoksu;reantdo ivfapsuorcsh olreakspinlagshiisngin. tolerable, provide a block v/)5]v.6Ainvoaidddaintigolne tvoaltvhess.check valve. 67.. FDorrainstsrufocrtulroawl preoainsotsnsanadvovidentpsipfeorsmhiaglhlerpotihnatsn s1hoiunl.dfobre povroervhideeadd. linUesse. %8i.n.Woratlearrgleinressi,zeess.pecially fire lines, should be looped. fro9m. Lseovoeprasltedaimrectliinoness ianndcasperoovfidfaeilubrloeckof voanlevepsorstoionsteoafmthecasnysbteemp. rovided V1e0n.tiSnegpoafrastuechsewsyesrtesmysstemmusstmbaey cbaerenfeuclelyssaprlaynfnoerd.certain hazardous materials. of11f.ireF.ire stops are necessary in all surface drainage systems to prevent spread to12.avLoiindesosvheorustldresbseinpgrecsesrutraeintesptieedcepsrioofr teoquoippemraetnitono. r Cfoaurendmautisotnsbedtuarkienng hEy.droEslteacttircictaelstse.quipment and grounding byTthhee sNafaetioinnsatlalElalteioctnricaanldCdoedseig. n Tohf eelpecrtorbicleaml erqeuqiupimreesntthies actatreenftuilolny oouftlcionmed phaetzeanrdt seilnecteraicchalpeanrgtinoefetrhs.e pTlahnety. must be supplied with information on the mo1u. nMteedanstsafrotrerrsemisodteessihraubt-lde.own of operating equipment in addition to locally rem2.oCterlyiticaasl pitreamctsicsaubclhe farsomswiotcphegraeatirnganudnittrsa.nsformers should be located as cas3i.ngI.dentify underground conduit by including a dye in the concrete for the 45.. PPrroovteidcte oevmerehrgeeandcycopnodwueitr fsruopmpleiexsp.osure to excessive heat. av6o.idAbllopdoilrytiionnjsuroifesthferopmlanbtummpusstobrefaaldlse.quately lighted so that operators can op7er. aStitoanti.c eNleecittrhiecritycaanndbleighptrneivnegntceodnsbtiutut tperroetaelchtiaoznaridssptoosssiabflee.proWceastschplafonrt statica.eMlecotvriincigtymaaccchuinmeurlyat(iboneltasn, dcondvisecyhoarrsg,eestcin.).the following situations: b. Flowing fluids or dust. cd.. Human beings. Lightning. whPerroetescttaitoinc iesleactcrciocmitypliissheadntbicyipparteodp.er Wgrhoeunndainllg mofetaalll eeqquuiippmmeenntt rinestasreoans concrete foundations, for instance, grounding must be installed. Instructions tfhore pNraotpieornaglroEulnecdtirnigcaolfCeoqdueisp,maenndt vaarreioguivsegnoivnerHnamnednbtopokubolficFatiiroenPs9ro(tSeecetioanls,o8 sources of information). 640 Project Engineering of Process Plants F. 1S. tPrurcottuercatlfsrtoemel failure due to fire by covering with concrete to the main soufpfpiroer.t level or by providing emergency water sprays for cooling steel in event fer2e.ncAevsoiadreinstoemrfeertiemnceess roefmsetrduiecdturinal tshteeelfiwelidthbpyiprinegmoavnidngeqourip"mceunttt.ingI-notuetr" po3r.tioPnlastofofrsmtesels,htohuelrdebbyerperdouvciidnegdthaet satlrleonpgethraotifntgheorstirnuscpteucrteio. n areas with a a2roftun6dinm.amnhinoilmesu.m width. 3 ft to 3 ft 6 in. in preferred in maintenance areas lar4g.erStuaniritws.aysUsaeregenprtleefesrlaobplee, 4t5o loarddleesrss, aanndd waitdtlhesasotf t3wfot 0arien.roerqumiroerde. for 56.. HSaafnedtyracilasgeasrearreeqrueiqreudireadrofuonrdlaadldl eprlsatofvoerrms15antod 2s0taifrtw. ays. pr7o.teTctoepeprlsaotnensel(6froinm. hfaigllhin)gaorboujencdts.the edges of platforms are necessary to G.1.BuFiilrdeipnrgosof construction is essential in hazardous areas. Op2e.raUtniolnessofaobustodluooterlyplannetcsesissarmyucphroscaefsesr.equipment should not be housed. 43.. PPrroovviiddee eaxuptolomsiaotnicdsoporrisntkolehrasnydslteemansywphoesnsibplreacetxipcalol.sions that may occur. H.5.InAsut laletaiosnt two exits are required. ma1t.elIyns1uilna.tetoal1l%hoitnl.inoefs8w5%ithmwahgicnhespiaerissoandneeqlumaateyfcoormperointeccotinotnacotf. pAerpsponronxeil. aMreoraev, aoiflacboluersfer,ommaiynbsuelarteiqounirmedanfourfahcetautrecrosn.servation. Detailed specifications of 2e.xcIensssuivlaetehepaotrtoironasreoafscwrihtiecrael fiinrsetrisumpreonbtalbealed.s and electrical conduit in areas VENTILATION When process equipment must be located indoors, ventilation becomes a major problem. Both toxicity and flammability of a material must be considered in design of a ventilation system. Toxic Concentrations Materials such as aniline, the halogens, hydrogen fluoride, and phosgene are toxic in concentrations as low as 5 parts per million, whereas toxic concentrations of such familiar materials as ammonia, benzene, chloro form, hydrogen sulfide and carbon tetrachloride range from 20 to 100 parts per million. Maximum allowable concentrations of various chemi cals have been published as tables.2 The user of such tables should be cautioned, however, that research on toxic properties of some materials is incomplete, and, as new information becomes available, the values of allowable concentrations may be revised either upward or downward. Limits of Flammability Information on flammability limits of gases and vapors and methods of estimation have been published.3,11'15 The limits of flammability are Safety in Plant Design 641 expressed as upper and lower limits. The lower limit represents the least amount of the material which will sustain flame propagation when mixed with air, and the upper limit represents the greatest amount. These limits of flammability as determined experimentally are affected by the type of testing apparatus used. Extremely small apparatus in particu lar produces narrow limits of flammability, and results from such equip ment should be used with caution. The flammability limits for mixtures of several combustible gases and air can be estimated from an equation which has been verified experimentally: 100 L ^+ Uc? + Lcz, + where L m = limit of flammability of mixture. Mole % (upper or lower) L\, 2, Lz = limits of flammability (upper or lower) for gases 1, 2, and 3, mole % (e.g., moles of gas/mole of gas plus air) Ci, Cz, Cz = mole % on air and inert free basis of gases 1, 2, and 3 The units of this expression are consistent since the summation of C /L terms has units of moles of air plus moles of combustible per mole of combustible. Required Ventilation Rate Knowing the rate of release of hazardous vapor and the allowable con centration of contaminate in an enclosure, the minimum fresh air rate can be calculated from a simple material balance. Ty _ L a TA + T v 100 where rr = moles of contaminate entering enclosure per unit time rA -- moles of fresh air per unit time L a = lower flammability limit or toxic limit whichever is smaller, mole % contaminate per mole of air plus contaminate The practice of expressing ventilating capacity in air changes per hour should be avoided. If 15 air changes per hour proves the correct air rate for one installation, it does not follow that this same rate will produce satisfactory results in an enclosure of smaller size. A..ir c,hanges = --E--nclo--sA-u--ir-re--r-v-a-ot;-el-u,-m-c--ue---fc-tu-/-h--fot/u--crh--a--n--g-e-- Local Ventilation General room ventilation can be supplemented by localized ventilation over the equipment from which offending vapors are being emitted. Such 642 Project Engineering of Process Plants local units are arranged as exhaust systems, and consist of hoods designed to catch the maximum amount of vapor or dust without interfering with the operation or servicing of the equipment. Hoods can be placed over filters, at pump packing glands, around crushing and grinding operations, and at other similar installations. Positive Air Pressure It is often desirable to maintain a higher air pressure in a hazardous area than in the surrounding rooms or the outdoors. The electrical equip ment, for instance, which cannot be designed for explosion-proof operation can be installed in a hazardous area by placing it in a room under positive air pressure. Inert G as Purge The use of inert gas is another valuable aid to ventilation. Inert gas is manufactured by burning fuel gases in conventional furnaces or, prefer ably, specially designed inert gas generators. The gas can be used to purge tanks and vessels of toxic and flammable vapors before purging with air so that men may enter the vessel. Men have died after entering inefficiently purged equipment. Every process plant which handles toxic or flammable materials must be equipped with inert gas generators or air or steam eductors for removing the toxic vapors and flammable vapors from tanks and vessels which must be entered by personnel. Ventilation for Comfort In addition to removing flammable or toxic materials generated in enclosed areas, ventilation and conditioning of the air in operating build ings is also important to workers' comfort and morale. It may be con sidered safe for men to work under hot and humid or extremely cold conditions, but the worker's efficiency and dexterity are reduced under such conditions and accidents may develop merely because of fatigue, discomfort, or low morale. Design of Ventilating Systems In process plants extreme care should be exercised in locating inlet and exhaust ducts to avoid short-circuiting of fresh air (entrance of fresh air into exhaust ducts). The degree of mixing of the toxic or flammable gas with the room air must be known in order to decide upon location of exhaust openings. If possible, vapors removed from rooms should be treated before releas ing to the surrounding atmosphere. Centrifugal or cyclone separators, spray or packed columns, air filters, and electrostatic precipitators can Safety in Plant Design 42 render exhaust gases less objectionable. In any event, the gases should be discharged from the building in the same direction as the prevailing wind. Revolving type roof ventilators permit such a discharge even with variable winds. PRESSURE-RELIEVING DEVICES It is impracticable to design process equipment for every possible con dition that may arise through abnormal situations. Fortunately, how ever, abnormal situations can be handled by the use of pressure-relieving TABLE 24-2. Conditions Condition tFoaiflruarcetioofncaotionlgingtowwearter supply Freafilluuxre of fractionating tower Etilnetrmanatceeroiaflforeign highly vola Exposure to fire Requiring Relieving Devices Design Basis oVvaeprohreaedqruaivtealent to maximum total tMotaaxlimovuemrhepaodssribaltee flow equivalent to Cstaalnctualnaeteousvavpaoprorfizloawtionbased on in 1. PCAalWculdataetahweahtichinipnuteqounatibonasifsormof becomes approximately: log q = (0.796) log S - 1.7 where q = hmeialltioinnspouft Bttou vpeesrsehlouinr S = sfluarmfaeceinasrqeaftexposed to 2. Use for exposed surface area lHesosritzhoannta2l 0vefstseilns Ocanle-hsaulrffacceylinpdlruis diam area of heads mHoorreizotnhtaanl v2es0seflts aAbllovareegaruopuntdo 20 ft in diam lVesesrttihcaanl v2e0ssfetls Aarlelacirpcluums febroetntotimal high head gVreerattiecralthvaesnse2l0s ft Lower 20 ft of area high Finosrulaintisounlatoerdabvoevssee)lsred(2ucien.caflicrueplartoeodf input by 20% '644 ' Project Engineering of Process Plants TABLE 24-2. Conditions Requiring Relieving Devices (continued) Condition Exposure to fire (continued) Design Basis 3. Calculate vapors generated where WII == llabt/ehnrtvhaepaotrs (Alternate method) TUhseisqa/Slter=na2t2e,0m00ethod is far more mcoentsheordv.atiOvethertmhaenthodtshgeive iPnAteWr mediate values. Closed valves Thermal expansion of confined liquids Thermal expansion of confined gases nAomrmouanl toopferflautiidngleacovnindgitvioensssel under W = Bq Cv where B = ecxopefafnicsiieonnt, 1o/fT thermal Cqp == hheeaatt csuapppacliietdy to fluid W = lb/hr GCaalscuLlaawtes increase in volume using oEpxecreastsiiovne heat loads during abnormal Vinappuotr generated by additional heat devices such as relief valves or rupture disks. The design of pressurerelieving systems requires careful study. In a badly planned system, too many or too few are installed or those installed are not well placed. Sylvander and K atz14 have prepared an excellent discussion on the design and construction of pressure-relieving systems. Some typical ap plications for relieving devices are given in Table 24-2, with suggested design basis for each application. This table is based in part on the work of Sylvander and Katz. Safety in Plant Design 645 Discharge Rates of Relief Valves and Rupture Disks The rate of discharge for a relief valve or rupture disk is calculated by the following formulas for vapors and liquids. The vapor formula is derived assuming an ideal gas, adiabatic expansion, a value of 1.001 for the ratio of specific heats, and vapor flowing at the acoustic velocity. Higher values of specific heat ratio will give a lower discharge rate; and, therefore, the simplified formula given below is conservative. The formula for liquids is based on the simple orifice equation. Manufac turers furnish handy charts and tables which facilitate the calculations of relief valve and rupture disk sizes. Discharge rates of relief valves and rupture disks Vapor: A = P = C = M = T = Liquid: area of orifice, sq in. upstream pressure, psia nozzle coefficient, 0.97 for relief valves (10% 0.81 for rupture disks (consult manufacturer) molecular weight absolute upstream temperature, F accumulation), Q = liquid capacity in gpm A = orifice area, sq. in. C = coefficient of discharge, 0.40 for relief valves (10% accumula tion), 0.61 for rupture disks S.G. = specific gravity Relief and Blow-down Discharge Systems The proper disposal of materials discharged by automatic relieving devices or manually operated blow-down valves is an important phase of plant safety. Relief valves which are located on top of high fraction ating towers can often be vented to the atmosphere. Such atmospheric venting, however, is not always possible when large quantities of flam mable vapors or liquids must be discharged at a safe distance from the plant for disposal. The discharges of relief valves and blow-down valves can be manifolded into a single discharge line connected to the disposal section (a burning pit, flare stack, or a quench tower). 646 Project Engineering of Process Plants If there is a large area far enough from the process section and smoke is not objectionable to the surrounding community, burning pits with pilot flames can be employed for the disposal of both flammable liquids and gases. As industrial areas grow and plants expand, however, the burning pit becomes more and more objectionable. Flare Stacks Flare stacks are constructed of pipe or boiler plate of sufficient diameter to satisfy the overall pressure-drop requirements for the system and high enough that the burned vapors will not constitute a nuisance or hazard and will be diffused readily by the prevailing winds. The stacks usually range in height from 100 to 200 ft. A knock-out or entrainment separator is installed for the separation of liquids contained in the vapor stream. The gases then pass through a seal drum, the inlet of which extends below a water or oil level in the drum. The gas bubbles through this liquid and then enters the flare stack. The sealing liquid prevents flame flash-back from the flare. The upper 8 to 10 ft of the flare stack is best constructed of an alloy such as Type 304 stainless steel since this upper portion becomes extremely hot and ordinary steel will not withstand the high temperature. A pilot light is provided at the top of the flare which burns continuously and is thus able to ignite gases as they escape. To prevent extinguishment of the pilot flame, a mixture of air and gas is piped to the pilot so that a strong, blue flame will result. This flame is difficult to observe from the ground, however, and an additional small luminous burner is also used so that a ground observer can easily detect failure of the pilot. The pilot is lighted by a sparking device inside of the pipe through which the mix ture of gas and air travels to the top of the flare. The flame is, thereby, propagated to the pilot. Other methods are used. One employs a series of gas jets located inside an open pipe spaced at regular intervals starting at the location of ignition and terminating at the pilot. Quench Towers Large amounts of hot heavy liquids can be discharged to a quench tower where condensation of a majority of the liquids will take place. The small amount of vapor yet remaining can be vented to the atmosphere or to a flare stack depending upon its quantity. The heavy liquids can be recovered in an oil separator. Relief Valve Piping Relief valves should be installed so that they are easily accessible for servicing. Dual relief valves can be employed so that one may be serviced while the other is in place for operation. In such installations Safety in Plant Design 647 plug cock valves* are installed on the inlet side of the relief valve and arranged with an interlocking mechanism so that one valve must be opened when the other is closed. Piping upstream from the relief valve should be as short as possible to avoid excessive pressure drops. The pressure drop in the suction line should not exceed one per cent of the allowable pressure for full capacity relief. Discharge piping should also be designed to prevent excessive pressure drop, and for most installations the pressure drop should not exceed 10 per cent of the set pressure of the relief valve. The line should never be smaller than the outlet size of the relief valve. The discharge piping must be well supported at all points of strain, especially that pro duced by the impact of the discharging gases. Common relief valve and blown-down headers should slope gradually toward the knock-out drums so that liquids may readily drain from the system. Pockets in discharge lines cannot be tolerated. Discharge lines venting to the atmosphere must be provided with weather protection to prevent accumulation of water in the vertical pipe. Steam snuffing lines venting to the atmosphere are also needed so that discharging gases might be diluted to prevent ignition. The steam can also be used in case of fire in the discharge line. Though relief and blown-down systems are vital, an overdose of caution in their design may result in a costly and cumbersome system. Each sec tion and item of equipment must be considered separately and the most probable abnormal condition or conditions selected for the design basis. Careful consideration will usually show that all possible abnormalities will not occur simultaneously on one item of equipment or one section of the plant. Experience plays an important part in such decisions and the engineer should seek the advice and counsel of the safety department of his organization. FIRE EXTINGUISHING EQUIPMENT Extinguishing process plant fires is a highly specialized type of fire fighting, the know-how for which has evolved from long experience. For tunately most of the useful regulations that aid in the selection and appli cation of fire fighting equipment have been published by such agencies as American Petroleum Institute, National Fire Protection Association, National Board of Fire Underwriters, and others (see section on sources of information). As well as taking advantage of such information, the project engineer should obtain the advice of men skilled in the art before attempting the design of fire extinguishing facilities for a process plant. * Gate valves are also used. 648 Project Engineering of Process Plants Water as a Fire Extinguishing Agent Water continues to be the major fire fighting agent, and its uses ex panded after the introduction of water fog nozzles and sprinkler systems. A steady stream of water on a flammable liquid often does more harm than good, but the use of a fog or mist of water which tends to smother the fire in addition to producing a cooling effect has proved extremely successful. The great dependence upon water as a fire fighting agent suggests the careful planning of fire water distribution systems. Such a system invari ably requires fire pumps to boost the normal water pressure to the higher pressures required for the fire hose and sprinkler systems. Fire pumps must be carefully selected and have an alternate drive to insure operation under the most adverse conditions. A typical dual drive fire pump is equipped with an electric motor and a gasoline engine drive. The gasoline engine is used during power failure. Other Extinguishing Agents Another common method of extinguishing fires quickly in confined areas is by smothering the fire. This can be done by the use of foam or inert gases such as carbon dioxide. Steam smothering is also valuable and is particularly useful in extinguishing small fires around furnaces. Emergency Valving Main shut-off valves for lines carrying flammable materials should be located so they can be operated in case of fire. It may be preferable in some fires to simply stop the flow of flammable material and allow that already escaped to burn. This avoids large concentrations of explosive gas and vapor. Portable Extinguishers In addition to the permanent installations of fire hydrants, automatic sprinkler systems, and automatic carbon dioxide extinguishing systems, portable fire extinguishers should be supplied throughout a process plant since a small fire can often be extinguished with ease by the rapid use of an extinguisher. Most large fires begin as very minor conflagrations, and immediate attention to a small fire will often prevent a disaster. Selecting Fire Extinguishing Equipment The project engineer should be cautious in using handy fire extinguish ing tables designed to indicate the exact extinguishing method for various types of flammable material. They give general suggestions and are not intended to be specific for all situations. These tables can be a helpful guide, but advice of authorized agencies should be obtained. Safety in Plant Design 649 When a new plant is being designed all materials which constitute a fire hazard in the operation of the plant should be carefully studied. Case histories of previous fires for similar plants should be obtained so that fire extinguishing equipment can be intelligently selected and placed. The National Board of Fire Underwriters, The National Fire Protection Asso ciation, The Associated Factory Mutual Insurance Agency, and other such groups will supply reliable information of this type. Plant Alarm System An alarm system should be installed in all process plants so that per sonnel can be alerted in case of fire. In most plants the alarm system can be manual, although in some process areas normally requiring little attention, automatic alarm is preferable. REFERENCES 1. Armistead, George, Jr., Safely in Petroleum Refining and Related Industries, John 2. BGr.aSnidmt,mAo.ndDs.,anIndduCsot.r,iaIlncH.,eaNlethw EYnogrikn,ee1r9i5n0g., John Wiley & Sons, Inc., New York, 1947. ( 3. Burgess, M. J., and R. Y. Wheeler, J. Chem. Soc., 99, 2013 (1911). 4. 5. DFiurgegaPnr,oJte.cJti.o, nChienmR. eEfninge.,ri5e8s,, No. 3rd 6, 125 (1951). Ed., American Petroleum Institute, New York, 1941. 6. Gafafer, W. M., Manual of Industrial Hygiene, U.S. Public Health Service, W. 7B. .HSeaautnindge,rsVCeno.t,ilPathinilgadaenlpdhiAai,r1C94o3n. ditioning Guide, Yol. 31, Amer. Soc. Heating Ventilating Engrs., 8. Moulton, R. SN.,ewNFYPoArk,H1a9n53d.book of Fire Protection, Crosby-Fiske-Forster, 10th 9. ENda.,tioNnaatlioBnuarl eFauireofPSrotatencdtaiornds AHssaoncd.,boBookstoHnJ,fO1,9S4u8.pt. of Documents, Washing ton10, .DN.Ca.t,io1n9a4l5.Fire Codes, National Fire Protection Association, Boston (1945). 11. Patty, Frank A., Industrial Hygiene and Toxicology, Interscience Publishers, Inc., New York, 1948. 12. Reigeluth, R. J., Safety and Economy in Heavy Construction, McGraw-Hill Book 13. Co., New Robinson, York, C. S., 19E3x3p. losions, Their Anatomy and Destructiveness, McGraw- Hill Book Co., New York, 1944. 14. Sylvander, N. E., and D. L. Katz, The Design and Construction of Pressure Relieving Systems, Engr. Research Inst., Bulletin No. 31, Univ. of Michigan KPr1e5ss.,TAhnonrntAornb,orW, .MMic.h, .P, h1i9l4.8M. ag., 33, 140 (1917).