NEWc ECHNIQUES
T- - -
GUARANTEEIN6 THE LIFE AND SAFE OPERATION OF
PROCESSPRESSUREEQUIPMENT R
d
e. s*
The Travelers Insurance Company, Hartford, Corn. Safety in chemical processing demands that vessels subject to pressure be comctly designed, properly installed, safely operated, and periodfully inspected. The material in process bean an important relationship to the suitability of the vessel, its loution, appurknanar, and control. Continuous safe operation n q u i m a definite inspection routine conducted b y a competent teat engineer, who will apply all required tests after proper preparation of the vessel with respect to
RFBBuRE vessels are in reality magazines of energy and, P unless correctly designed and constructed, properly instdled, eafely operated, and constantly ioepected, may constitute a serious hasard. While operating experience 88 a whole with un6red preasure vessels has beem relatively good, constant watchfuineas is eesmtd. There are nnmerous hazards which should be guarded against if workmen are to be protected from injury and property from damage. One has only to consider the numerous types of vessels used today in the chemical inaustrp to a p p h t e that each must be looked upon 811 a eeparata entity, and operated and inspected sccording to the circ&m surrounding that psrtiaular object. Thia procedure should be based upon common practice for such v d , but ahodd be changed slightly to take cam of the special situation surrounding the individual unit.
necessary cleaning and ventilating.
Such inspec-
tions will provide opportunity for checking corrosion of internal surfaces and clogging or rendering inoperative d pmsure release valves, pressure gages, or reducing valves. The result will be longer life of equipment, better efficiency of process, accurate lonusting of the life of objects, and freedom horn interruptions and shutdowns. The actual cost of such procedure is surprisingly moderate.
There is no Merence in the energy stored in a ateam-jacketd vessel at the m e preasure 88 the steam in the boiler supplying it except in 80 far aa rdse and contents are concerned. The wide variation of mstari$e of construction and the mnltitude of substances handled make the problem one requiring strict supervisionin every chemical plant. There is a dehite relation between the vessels under preb 8nre and the materisl in pmceas. Such a relation must be established prior to the Salection of the v d , for without a clear understanding of what ie to be done under a given Bet of processing conditions, the material for the vessel or for the king cannot be properly selected. While it ia true that cartain unlired v d do not fall within the scope of common pvessel codes, these codee am guides to Batisfaototy m a h a k and meth& of constructing p m u e v d . Ob-
INDUSTRIAL A N D ENGINFmm'NG CHEMISTRY
I
Vd 33, No. 9
ample space must be allowed for all appurtg nancea, connectiom, and control& Room is required to operate, to repair, and to inspwt, and the location of any veseel within any pop tion of a plant should consider these items before the installation is actually made. The cmwding of Y d too C l d y is hardous and not only prevents proper control but may be a factor in the incream of loaaes during an emergenay. V 4 equipped with stjning appliances require service and power plat-
k M 0 N U C O M m O a EXPLOSrON
Note the front of the rampressor blown out in the right-hand photoBraph
vioudy, if the design and theoonetructionof t h e 4 are not
good in thebeejnning, ite operation is bound to be more or less b a d o u a . The plant owrating ocgmhtion can produce the ultimate in eafety if they pmvide the builder with au5cient inetruction% or references. Any d o n preaaure vessel should be jnat as carefulIy built as if it were a steam produm'. Most veesele in the chemical induatty are subjeoted to one primary haaard--meabanicalfailure of nome portion as a rfr d t of corroeion or conditiom incident to continued use. This may include exceaive pmaure. The heet remedy for this situation is to &ot matmiah for comtruation which are not afiected by thematerial to beprooessed. This is often a di65cult problem to solve, but m h haa produced many e.p3pecial d 0 and ~ the material known to give the beet resise ance to cornxion should be considered the proper one to use.. If such a material is not available for the vesseliteelf, then the aame conaideratian ahodd be given to the selection of a lining whicb will &t deterioration. Thin warning is emphssised becaw it in of utmost importance in producing safe wor!dng conditions. Design and Inct.llation Compliance in d& and iwtdation with ~ t i and d local codes is eawmtiaL De&n and inntabtion go hand in hand. Not only mwt the vessel be Suitably located, but
forms to be installed to provide eafety for the operator. Any equipment may need repairs and alwaya repuirae fmquent inspection. If such rapairs and +ctionn are to be adequately made, sdcient epsce must be provided for the workers to function properIy and with parsod safety. Rssotion wmla should alwaya be corredy placed in relation to th& pact in the pmoe~a,thus eliminsting ~tructursland 00nnection di5cultiea. Wen vessels are heated by &?am, pressure gages and d e @ valvee are essential. Readion vessels, where the m a W manufactured may clog the eafety Y ~ Y and E render it uselesr, may repuire frangible diskn to proteot it. In the we of stesm, when the prooesa preseure is reduced below normal boiler pressure, a reduoingvalve in required in the line leeding from the msin to the 4.Pressure gsges and d e t y v a l Bhould ~ be i n s t a l l e d OR the l ~ ~ - p r a s sside ~ r eof the IWducing valve; CBSB should be that the d e t y valve inside the reducing valve has a relief cape.city d c i e n t to taka the main line supply if the reduoiog valve &odd fail to function. In general, it is preferable to install aapsrate set6 of ~and~~valvesforeeohvesselandnotrelyuponsinglp valw aupplying manifold In everg instsnce t h ~ dhbacge from d e t y valvea and other relied de.vim &odd be piped to a safe area. When v~cuumreIief devicas 818 uaed, @hilarpracautionn are eaeential. When speoial aum
&pt.mba.
1M1
IN DU 8 T R I A L A N 0 1 Y QI I IlESBL# Q C H P Y I 8 T R Y
1093
and diefdevieeumunt be made from theesmematdrid. Rupave dike, if used,may slso bsplatedorpmtestsd by the specisl alloy, snd where the relief demand is exoegiW, noh qui& outletsmaybeprmriaedinmultiph. The looation and marking of control vdvm is important for the safe operation of prpssure vwnela. Vdm may be msrlrsdby numb or by the um of diehctive oolor systems When oolora IVB used and the d v m rn painted a oertain color,at tips^ of the sama color &odd indiratetheline or vfesel upon whieh the valve is located and owrate. Buoh spatems will give proteotion to the operation of the equip men$. When veesela IVB shut down for rep&n or for inape0tion, the dvea or lines should be m marlred, and dl control valves should belocLed in the cloead ponitionwith thekey in the paaaession of the department supervhor. &xiow injuriea have wcurred through the inadvertent opening of d v e a while a vesselw ~ being 8 repaired or inspected.
to the end that safety is innessed and longer vessel life exInspection Personnel Thelifeandsafeopmthnof~premmequipmentm peded Proper safety control,test, Md ilI@fdom 08IlUot be made mananted by four lelated W m . The E& thrmuulem the iwpedru hae a thoragh undmtmding of the pope? conatruotion and design, proper lofation, sad propa ohsmid aspects of the prorm# carrisd on within a given VBBiwtdhtion, including the poovision of relie$ and contml desel. Thin is true w h e k ib he a b @ , openmkiyvat or.m vi-prepste the way for the mod important item whioh is e x t m d y large or d v e bemb capable of W Y ~ ~ ~ ~ U I B propet inspection routine. Given the fint three, thafaurtb in in &@ solely mapodble for tha dlife of the VraSaL enonnow pressurecl. without aroh hrowledge the ilwwtor determine the eqwtwoy of linea, liniogs, control w h i l e t h e ~ o n o f i n d p s M i t y i n s u r a n c e ~ o n a M i r s op~~tlot vdvea, relief desta; thinb true if during the with it a certdu amount of inapeotion mrvice, inhmdiste procpasmstQisls areintaudu~intotale unit whioh may be mthefe&andinapactionamudbe.u.wbbyplantpermnM)rroLbvB, ermive, 5 m d J l e # pOinmOua, or otherwise innel. When a plant is d i e i e n t d y b , it is preferablethat the tent and inspeotion pewonnel be aeperste from manutactqring juriow to hedth. It fquently b.ppeols that in developing new matariala theresepmh department fsils to realire the imoperation. The dirsction of such a stetlshould be independent of thw prhwily in charge of production or equipment, portauoe of the inspection p m b b and dom not give the inspector such aential infomation until trouble hss h. for free judgment o ~ then n be had conceaping the oondition All poesible riakn attgched b the p r o m should be considered ofthe equipment and tale ohmwter of the maintenance found. jhtlyby the ddeperhaemt with the inspeotoras tha Such hdqmdenca eliminates possible irduerw from pmmm ~ be inwhom inteest oentm upon maximum production or mainte- Work prograaws, and appmpi& p W a ~ t i 0 1should atitated. mea at the lowe& possible ood. The inspsotion abfl must have sbsolute frasdom to steta oonditim exaotly IYI found Theinap6otorehouldbeempdbclwdoantheo~ and to comment c.mhuctively upon the data developed tion of any VPBBd failing to pna hm i test or found openting cantrary to insbuotedroutine. On the other hand,he ahould ~ t b i r ~ o n a n d b l i n g p m c e d u r eTheisstelle. tion of Rloh a statr, or even of an illdividd Workmm, lhould be held mapomiblefor the appmvd of a 4when it pasvv h M Wag h t h e d s p s r t m p m t a U p m ” f h ? BWplanteneOnhieexaminstion. T b ~ ~ & ~ ~ l d b s ~ l a b b f f l k ~ ti- of the Hsponsibillty for the d e opesstion or maintemeh instrnas except throu& aonaultation with ewinedw expartainthepflekliu-fold nuumofthseguipmsnt,but rhould augment suah pmcadum
I W D U S T KIA L A N D E N Q 1 N E E R I N 0 C H E H IS TR Y Installation Testing Probably the moet important inspeOtion is & one made irpmedistely following inatsllation and prior to a test run to msLe nue there baa been striot wmplianm with original safety stsndards. &me of the more important iterms for such an inapeationmutine are as follows:
Y.%2%Zwordingdsrioes' elements been so p~ m to show accurately the hl eat temperature liahle to be reached in the reaction, and are rewrdera capable of varying over the rsngerequid? 4. Have the
du*g
&
per relief vdvea, explosion dish, prpssure redEv 'lw with necemav slarms been pro-
and m s % ~
Videdl
6. It any hedth basarda are involved thmngb the material
k w yd, is proper ventilacinn installed?
6. f volatile solvent. or other Bammable materials are employed, is all electrical equipment installed rwrding M code? 7. Are outleta and inlet.of eu65ment capacity? a Have all of the 8pecScatiom oven the manufacturer for building the 4 bean roanplied with and hsa the veseel been
proprly installed by the countruetion engineer?
Wbib 801118 of the items W above may ~ e e msuperfluous, eacb has a d a t a bearing upon the d e operation and life of the equipment. Numerous injuries and property 1b v e been recorded through failure to comply with the sim-
plat inetruaeioaa. Cnm connection of supply lines, r e v d electrid polarity, failme to remune ddpflu@ fmm 014E-, and 0 t h ~similar ~ items have owurred and do continue to pmdqce loss exmpt where the inapeation service w E&Cimtly active to detect the situation prior to aatual operetion. It h the mspomibility of the inspeotor to discover suoh negligenca. Testing and Inspecting Routine
The mtine inspsotion of equipvmt inwlvea both t d h g inspection. The subjedion of a 4 to prm3tmj
V d 33, No. @
usually hydrcetatio, in exof the normal operationrequire menta whioh am ordinarily one and a half timeu working prea a m for riveted wmtruction and twice working prsawve for welded oonstruction to be within the factor of d e t y limita, is considered a test. The visual obearvation of any apparatus, either in operation, statio, or dismantled, together with a hammerkstof theequipment, wmtitutesavisuainopection. The actual t e e t i of the equipment can be waived when the p& service record or operating conditions are known to j u s tify this part of the procedure. The pttrpcw of routine inspection h not only to reduce the possibility of acudenta but to reduce mechanioal failures and fire hacards,and at the eame time to inplant &ciency and thereby rtmm wntinuity of production. Consequently, it is well to establish a oertain definite system that should be followedby the inapeotion stafl. An example of such a routine follows:
should have the approval of the ci6c in requiremenL and efficient UL operation. Reports c.hod??be made of all raplira m well M teas and inspeetiom and these reports become a permanent part of a record ~yeie.m. kvery piece of pressure equipment should he identi6e.d and tbe identi6cation remain a# lo M ths vm¶el in in edstenea in the plant. AU e quipmvt sho3d be prepared for teat and inspeetion by the operatq departmsnt personneL Thin arrangement eliminates duplicahon of dorts and at ths Bame time re uires the personnel to be wmpletely familiar with the makeup the in their control. The v m ~must l be thoroughly clegneb;' ventilated, or wsshed until no noxlous or 1.
The inspection 8
tern
v e n t ; it ehoulfie
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1941
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I N D U S T R I A L A N D E N G I N E E R I N G CBBMISTRY
the exact condition of the uipment based on pm%%%? where codes exint, the code%odd be followed. 6. Thems~-?A~ on shwld produce a detsiled analye of the t con& on of the equipment. This wiu permit the estab ent of the rate of wnvaion or detezioration and Bstimation
K
logs
This outline msy appear to the nninitiated to be eskuwive, complex,and d y , but when auah a p r o d u r e is made s definite part of the entire plant production progmm, it is readii ahsorbed and, according to published information, can be held well within 0.6 pm cent the totsl plant man-horn. It is a ranall price to pay for protecting plant and psnOnna and guarsnteeing the life and safe operation of prooese pres-
sure equipment. There is Little new in this Buggested routine. It b the correlation of many i d w , definitely attached to the operation edted company executives. of chemical proasae d. For t h m who desire to 7. Through the correlation of tests and inspection data, the obtain sdditional information for guidsnoe in handling the of design, comtruction, and meintenance standt&.blishment u d s f o r t u t u r e e q u i ~ t m a y b e ~ ~ , a n d t h r o u g h t h e c o n -problems in their own plant, I would draw attention to t of defective equipment the pmoees may be the d e s d e d o p d within the American Sooiety of Me %%E=uay l new. chanid Engineers, the d e for d e d preeaure veaaels of 8. The iaspeation pmnud should inveatigata all accidents Mean PetruIe.um Institute and American sooiety of the involving perarms or e uipment. while this may ~ e e mpost Mechanical Engineers, the National Wety Counoil’s publicamorttrm treatment, the lata obtsined ma h o m e important in tion, “Prmre Vessels,Fired and Unlired”, Parts I and 11. devainping future construction and c o n t r o ~ p d u r e . 6. Autest‘ and I’epsirs, when remrded on suitable report forma f o r z f e r e n m , &odd be distributed to inter-
GETTING THE MOSTFROM AUTOMATIC CONTROL a. e. P&
Leeds BE Northrup Company, Philadephia, Penoa.
in auionutic conbol it is the combined charutuirtics d
w&oller and pronr that count. They may be suited to
&.
UnuUdadwy m u l b may mean h i eithn a mom “mlined‘ mode d conboi or additional conbol may be q u i d A m i n some rimple change in the p w e a may solve (he problem. ClVrKtrrhticr d conboi equipmrnt are gennrliy dmple and udly w i l e d . Charas(.risUo d pfocesws a n infinitely vaded. They not only depend upon &e parUculrr application but en frequently subied tu wide *u*Uon rl(h Ume, The UICI d autonutic eonbol equipment should be familiar with cehin general pdnciples which a n
uch
control has long heen to pressure, AUTOMATIC flow, level, and temprature, and recently, to an inaxtent, to such variables as electrolytic conductivity applied
ing
and pH. The great majority of applications have been mcd u l , and, besides freeing operators for other duties, have often paid for themnelvea many fold in in& production and i m p r o d product. An the um of instruments and automatic controllers has i n c d , many plank have assigned s p c i d men to c ~ n tfor them. These men have usually become very proficient, often understanding a mechanism just as well as the manufacturer who supplied it. Generdy they have had a leas complete picture of important relations between controller characteriatiw and p r o w characteristics. To get the most out of automatic control they should be provided with this knowledge, in EO far powile, and, what is just as important, should be given m5cient authority to apply it. Further, they ahould be consulted when new pmcesa equipment is being designed to avoid mistakea which may be dif6cuIt to correct later. What special knowledge should the p h t automatic control expert have in addition to knowing the mechanismsemployed? FW he should understand the modes of control whioh these
helpiui in padlcubr sonbol prcblemr The idul time to apply these to a pmcer is during the M o d of design whcn c ~ U midaka y can be avoided. A brief mview daubnrUc control theory is gira, with examples horn the fields d hmperatumand pH conboi. The impohnce of a suitable relation &em (he con. bol-valve setling and the resultant llow is dmud. C u m am given showing the chanckdrtics d wme pmentday valves when b t e d at consbnt pam drop. The dkd 01 line d m p in ddormining durachhUo in tnrice i s shown by families of gmerrlly applicable cunes plotkd on a percentage bsir
meobanims produce. Then he should know how the results obtSinable with those mcdea depend upon process ahmmtw iatics. FinalIy, he should undmtand the particular prows under consideration well enough to anaIye it from the point of view of ohsracteristicsfavorahIe or unfavorable to control. oenerslly speaking, a d c i e n t knowledge for moat practical purpoees w q u i ~ little ~ or no mathermatics and is well within the grasp of the average instrument department head. A brief review of the general theory will be given, followed by a discussion of what is called “the &ective valve characteristic”, a subject of considerable practid importanm in both existing and projected &omastic control applications. Examples of M
An
A of Control
a basis for discuening the combined dwt of controller
characteristics and pmcass characteristics, three of the most important mcdea of control w i l l be considered: two-position control, proportional-position control, and p i - ~ p ~ i ~ ~ l position plus proportional-aped-floatingcontrol. For brevity the latter will be referred to 88 proportionalplusfloating control in further discussion. In explanation of these mcdea, consider a furnace heated by gas to be controlld at about 600’ C. The temperature is
