einth012

8/11/2019 EINTH012

http://slidepdf.com/reader/full/einth012 1/32

Rolta Academy:Engineering Design Services

CHAPTER 10

CONTROL AL ES10!0 "NTROD#CT"ONEstimates reveals that a substantial portion approximately 8-10% of totalcapital expenditure of the chemical process industry is used for theprocurement of valves. In terms of the number of units also, valves exceedany other piping component. ence proper thought is given for selection of valves. !he first step in selection is to determine exactly "hat function thevalve is expected to perform after it has been installed. #alves are installed one$uipment piping to perform any one of the follo"ing functions&1.isolation'.regulation(.nonreturn).*pecial purpose.

1.0 I*+ !I+• / !E•• 2/• I*!+• 3I / 4 5• 2!!E46 7• I

'.0 4egulation• /lobe• eddle• utterfly• 3iapghram• iston• inch

(.0 on return• hec9 valves

).0 *pecial purpose• 5ulti port• 6lush bottom• 6loat• ine blind• :nife gate.

$%at are &all alves'

Instrumentation Engineering 1

8/11/2019 EINTH012


Rolta Academy:Engineering Design Services ball valve ;li9e the butterfly valve, one of a family of valves called $uarter turn valves< is a valve that opens by turning a handle attached to a ball insidethe valve. !he ball has a hole, or port, through the middle so that "hen theport is in line "ith both ends of the valve, flo" "ill occur. =hen the valve isclosed, the hole is perpendicular to the ends of the valve, and flo" is bloc9ed.!he handle position lets you >see> the valve?s position.

all valves are durable and usually "or9 to achieve perfect shutoff even after years of disuse. !hey are therefore an excellent choice for shutoff applications;and are often preferred to globe valves and gate valves for this purpose<.!hey do not offer the fine control that may be necessary in throttlingapplications but are sometimes used for this purpose. !he body of ball valvesmay be made of metal, ceramic, or plastic. !he ball may be chrome plated toma9e it more durable.!here are three general body styles of ball valves& split body, top entry, and"elded. !here are three general types of ball valves& full port, standard port,and reduced port. full port ball valve has an oversi@ed ball so that the hole in the ball is thesame si@e as the pipeline resulting in lo"er friction loss. 6lo" is unrestricted,but the valve is larger. standard port ball valve is usually less expensive, but has a smaller ball anda correspondingly smaller port. 6lo" through this valve is one pipe si@esmaller than the valve?s pipe si@e resulting in slightly restricted flo".In reduced port ball valves, flo" through the valve is t"o pipe si@es smallerthan the valve?s pipe si@e resulting in restricted flo".

trunnion ball valve has a mechanical means of anchoring the ball at the topand the bottom.5anually operated ball valves can be closed $uic9ly and thus there is adanger of "ater hammer. *ome ball valves are e$uipped "ith an actuator thatmay be pneumatically or motor ;electric< operated. !hese valves can be usedeither for on off or flo" control. pneumatic flo" control valve is alsoe$uipped "ith a positioner, "hich transforms the control signal into actuatorposition and valve opening accordingly.


8/11/2019 EINTH012


Rolta Academy:Engineering Design Services(ig 10!1 &all alve

$%at are &)tter*ly alves' butterfly valve is a particular type of valve that uses either a circular vane or a disc as the shut-off mechanism. utterfly valves have a $uic9opening closing $uarter-turn mechanism that is used to control the flo" of li$uid through a piping system. !hey typically pivot on axes perpendicular tothe direction of flo" inside the flo" chamber. ompared "ith ball valves,butterfly valves do not have poc9ets to trap fluids "hen the valve is in theclosed position. utterfly valves are fre$uently used as throttling devices,controlling the levels of flo" in various positions& entirely closed, entirely openor partially open. !hey can control various substances of air, li$uid or solidcurrents and are situated on a spindle that allo"s for flo" in a singledirection. utterfly valves are offered in many si@ed diameters, resulting indifferent flo" rates. *maller valve assemblies may be used "here space islimited. :no"ing the pipe dimensions, desired flo" rate and material isimportant "hen considering a valve investment. Installation time is typicallylo" compared to some other types of valves, and additional parts areunnecessary to "or9 and maintain the valve.

utterfly valves have a lever that allo"s the operator to open or close thevalve to control the flo". !hese valves are part of a family 9no"n as rotaryvalves, "hich are defined by the $uarter turn that is used to move from theopen to closed position and vice versa. !his results in a lo"er surface friction,

"hich means that these valves can be smaller than others and still operateefficiently. utterfly valves are available in numerous closure types and bodyconfigurations, depending on the type of flo" control needed. !hese types of valves are commonly composed of metals, li9e aluminum and stainless steel,but also can be made from various plastics. +ne 9ind, flange butterfly valves,can be mounted bet"een flanges. nother, lug butterfly valves, uses metalinserts that are attached to the valve?s boltholes. 2sing an independent set of bolts for each flange, this valve assembly is fixed bet"een t"o flanges.6inally, "afer style butterfly valves are the cheapest and most popular type of butterfly valves because of their simplicity and ease of use. utterfly valvesare used in many food transporting and chemical plants "here controllable

product flo" is re$uired. +ther specific industries include # , tertiarypetroleum recovery and industries that use high-pressure "ater. =henproperly used, butterfly valves offer many benefits. 6irst of all, they are lo"er priced than many other types of valves, and generally have a longer life cycle.

utterfly valves are easy to maintain, are light"eight and compact and areable to handle a "ide range of temperatures. !hese valves are also veryreliable because of their tight shut-off, reducing the amount of lea9age. +ne of the problems "ith butterfly valves, ho"ever, is that their design ma9es itdifficult to efficiently and thoroughly clean all residual contaminants. lso,these valves are not good for use "ith highly abrasive or corrosive materials,because the disc can be easily eroded. +verall, butterfly valves are one of thefastest gro"ing types of valves in the industry.


8/11/2019 EINTH012



(ig 10!+: &)tter*ly valve$%at are C%ec, alves'

hec9 valves, also referred to as >non-return> or >one-"ay directional> valves,are very simple valves that allo" fluid, air or gas to flo" in only one direction.=hen the fluid moves in the pre-determined direction, the valve opens. !hemoveable portion of the valve prevents any bac9flo". s"inging disc, ball,plunger or poppet moves out of the "ay of the original flo". *ince thesedevices are slightly larger than the through hole, the pressure of bac9flo" "illcause them to tightly seal, preventing reversal of flo". /ravity or a springassists in the closing of the valve.

hec9 valves are indispensable in every area of life. 3omestically, they arefound in devices such as faucets, toilets and dish"ashers. =ithout them,fresh "ater "ould be impossible and common plastics "ould be unheard of.Industries use them to control flo"s of all typesAfrom the thinnest gas toradioactive materials, from molten metal to highly corrosive materials. hec9

valves can range in si@e from less than an inch in diameter to (0 feet across.!he simplest chec9 valves can be purchased at the local hard"are store, butthey may also be precision-designed for highly sophisticated systems. s aforementioned, chec9 valves use different means of preventing bac9flo".*"ing disc valves are typically used "ith li$uids, such as slurries, that caneasily damage the valve seat. !hey may be installed either vertically or hori@ontally, but a lever and counter-balance is recommended "hen verticallyinstalling s"ing disc valves. all chec9 valves use a ball "ith a small hole in


8/11/2019 EINTH012



(ig 10!- C%ec, alve

the middle, "hich can be either free-floating or spring-loaded. !hese valveshave a "ide variety of applications. !hey are more resistant to plugging thanother valves, and thus can be used to handle even fluids that deposit gummyresidue. chec9 valve may also utili@e a plunger, "hich is spring-loaded andusually used to prevent bac9flo" of pressuri@ed gases, or bac9pressure. pop pet in a chec9 valve is spring-loaded and typically installed in systemsthat re$uire prevention of bac9pressure.

ecause chec9 valves have so many applications, they are made of a "idevariety of materials. *ome companies exclusively manufacture plastic chec9valves. +ther companies produce chec9 valves made of brass, electro lessnic9el plated brass and stainless steel. !he components of valves may bemade of different materials, such as cast iron or bron@e, depending on theapplication. *peciali@ed valves may also be constructed of !eflon. Elastomer products may also be used to construct valves.

$%at are .ate alves' /ate #alve, or *luice #alve, as it is sometimes 9no"n, is a valve that opensby lifting a round or rectangular gate "edge out of the path of the fluid. !hedistinct feature of a gate valve is the sealing surfaces bet"een the gate andseats are planar. !he gate faces can form a "edge shape or they can beparallel. /ate valves are sometimes used for regulating flo", but many are not

suited for that purpose, having been designed to be fully opened or closed.=hen fully open, the typical gate valve has no obstruction in the flo" path,resulting in very lo" friction-loss./ate valves are characteri@ed as havingeither a rising or a no rising stem. 4ising stems provide a visual indication of valve position. o rising stems are used "here vertical space is limited or underground.


8/11/2019 EINTH012


8/11/2019 EINTH012


Rolta Academy:Engineering Design Servicesseat to form a lea9-tight seal "hen shut.5any globe valves have a class ratingthat corresponds to the pressure specifications of *I 1G.(). ibcoc9s andsillcoc9s are variations of globe or stop valves used in plumbing. eedlevalves are variations of globe valves "here instead of a separate attacheddisc piece, the internal end of the stem is conically tapered to act as the discto fit into a matching seat for fine flo" adCustment. +ther different types of valve usually are called globe style valves because of the shape of the bodyor the "ay of closure of the dis9. s an example typical s"ing chec9 valvescould be called globe type.

(ig 10! : .lo e valves!he above classification is based on functions. !he valves could also beclassified based on the type of constructionH valves manufactures offer endless varieties of constructions. ased on operation valves can be broadlyclassified as operated valves and self operated valves. 5ainly the chec9valves are self-operated and all the other types come under operated valve.!he valves can further be classified based on end connections. Endconnections mean the arrangement of the attachment of the valves to thee$uipment or to the piping. !he types of end connections are&• *cre"ed ends

• *oc9et "eld ends• 6langed ends• utt "eld ends• soc9eted ends• =afer type ends

!he valves can also be classified based on materials of construction. !herecan be any number of combinations possible "ith the materials of constructions. It is for the piping engineer to select the same in consultation"ith the process engineer to suit the process fluid. !he environment in "hichthe valves are installed is also to be considered for selection of materials of construction. o"ever the most commonly available materials are&


8/11/2019 EINTH012


Rolta Academy:Engineering Design Services• ast iron• 3uctile iron

•ron@e

• /un metal• arbon steel• *tainless steel• lloy steel• *pecial alloys

10!1 CONTROL AL E SELECT"ON

ontrol valves handle all 9inds of fluids at temperatures from the cryogenic

range to "ell over 1000 °6 ;B(8° <. *election of a control valve body assemblyre$uires particular consideration to provide the best available combination of valve body style, material, and trim construction design for the intendedservice. apacity re$uirements and system operating pressure ranges alsomust be considered in selecting a control valve to ensure satisfactoryoperation "ithout undue initial expense. 4eputable control valvemanufacturers and their representatives are dedicated to helping select thecontrol valve most appropriate for the existing service conditions. ecausethere are fre$uently several possible correct choices for an application, it isimportant that all the follo"ing information be provided&

• !ype of fluid to be controlled• !emperature of fluid• #iscosity of fluid• *pecific gravity of fluid• 6lo" capacity re$uired ;maximum and minimum<• Inlet pressure at valve ;maximum and minimum<• +utlet pressure ;maximum and minimum<• ressure drop during normal flo"ing conditions• ressure drop at shutoff • 5aximum permissible noise level, if pertinent, and the measurement

reference point• 3egrees of superheat or existence of flashing, if 9no"n• Inlet and outlet pipeline si@e and schedule• *pecial tagging information re$uired• ody 5aterial ; *!5 '1G grade = , *!5 '1 grade = ,

*!5 (B1 685, etc.<• End connections and valve rating ;scre"ed, lass G00 46 flanged,

lass 1B00 4!J flanges, etc.<• ction desired on air failure ;valve to open, close, or retain last

controlled position<

• Instrument air supply available• Instrument signal ;( to 1B psig, ) to '0 m , art, etc.<


8/11/2019 EINTH012


Rolta Academy:Engineering Design ServicesIn addition the follo"ing information "ill re$uire the agreement of the user andthe manufacturer depending on the purchasing and engineering practicesbeing follo"ed.

• #alve type number • #alve si@e• #alve body construction ;angle, double-port, butterfly, etc.<• #alve plug guiding ;cage-style, port-guided, etc.<• #alve plug action ;push-do"n-to-close or push-do"n-to open<• ort si@e ;full or restricted<• #alve trim materials re$uired• 6lo" action ;flo" tends to open valve or flo" tends to close valve<• ctuator si@e re$uired

• onnet style ;plain, extension, bello"s seal, etc.<• ac9ing material ; !6E #-ring, laminated graphite, environmentalsealing systems, etc.<

• ccessories re$uired ;positioner, hand "heel, etc.<

10!1!1 AL E SELECT"ON PROCESS

1.0 3E!E45I E *E4#I E + 3I!I+ *• ; 1, ∆ K, L, ! 1, 6luid roperties, llo"able oise, etc<.• *elect appropriate *I ressure lass re$uired for

valve body and trim.'.0 2 !E 4E I5I 47 v 4EL2I4E3

• hec9 noise and cavitation levels

(.0 *E E ! !4I5 !7 E• If no noise or cavitations indication, choose standard trim.• If aerodynamic noise is high, choose =hisper !rim• If li$uid noise is high and or cavitation is indicated, choose

avitrolM III trim.

).0*E E ! # #E +37 3 !4I5 *INE

• *elect valve body and trim si@e "ith re$uired v.• ote travelH trim group, and shutoff options.

B.0 *E E ! !4I5 5 !E4I *

• *elect trim materials for your applicationH ma9e sure trimselected is available in the trim group for the valve si@eselected.

G.0 + !I+ *


8/11/2019 EINTH012



onsider options on shutoff, stem pac9ing, etc.

10!1!+ Control alve (lo2 C%aracteristics!he flo" characteristic of a control valve is the relationship bet"een the flo"rate through the valve and the valve travel as the travel is varied from 0 to100%. Inherent flo" characteristic refers to the characteristic observed "ith aconstant pressure drop across the valve. Installed flo" characteristic meansthe one obtained in service "here the pressure drop varies "ith flo" andother changes in the system.

haracteri@ing control valves provides for relatively uniform control loopstability over the expected range of system operating conditions. !o establishthe flo" characteristic needed to match a given system re$uires a dynamicanalysis of the control loop. nalyses of the more common processes havebeen performed, ho"ever, so some useful guidelines for the selection of theproper flo" characteristic can be established. !hose guidelines "ill bediscussed after a brief loo9 at the flo" characteristics in use today.

(lo2 C%aracteristics6igure 10.G illustrates typical flo" characteristic curves. !he $uic9-openingflo" characteristic provides for maximum change in flo" rate at lo" valvetravels "ith a nearly linear relationship. dditional increases in valve travelgive sharply reduced changes in flo" rate, and "hen the valve plug nears the"ide open position, the change in flo" rate approaches @ero. In a controlvalve, the $uic9 opening valve plug is used primarily for on-off serviceH but it isalso suitable for many applications "here a linear valve plug "ould normallybe specified. !he linear flo" characteristic curve sho"s that the flo" rate isdirectly proportional to the valve travel. !his proportional relationshipproduces a characteristic "ith a constant slope so that "ith constant pressuredrop, the valve gain "ill be the same at all flo"s. ;#alve gain is the ratio of anincremental change in valve plug position. /ain is a function of valve si@e andconfiguration, system operating conditions and valve plug characteristic.< !helinear valve plug is commonly specified for li$uid level control and for certain

flo" control applications re$uiring constant gain. In the e$ual-percentage flo"characteristic, e$ual increments of valve travel produce e$ual percentagechanges in the existing flo". !he change in flo" rate is al"ays proportional tothe flo" rate Cust before the change in valve plug, dis9, or ball position ismade. =hen the valve plug, dis9, or ball is near its seat, the flo" is smallH "itha large flo", the change in flo" rate "ill be large. #alves "ith an e$ualpercentage flo" characteristic are generally used on pressure controlapplications and on other applications "here a large percentage of thepressure drop is normally absorbed by the system itself, "ith only a relativelysmall percentage available at the control valve. #alves "ith an e$ualpercentage characteristic should also be considered "here highly varying

pressure drop conditions can be expected.


8/11/2019 EINTH012



(ig 10!3: (lo2 C%aracteristics10!1!- Selection o* (lo2 C%aracteristic*ome guidelines "ill help in the selection of the proper flo" characteristic.4emember, ho"ever, that there "ill be occasional exceptions to most of theseguidelines, and that a positive recommendation is possible only by means of acomplete dynamic analysis. =here a linear characteristic is recommended, a$uic9 opening valve plug could be used, and "hile the controller "ill have tooperate on a "ider proportional band setting, the same degree of controlaccuracy may be expected. !he tables belo" give useful guidelines for selecting valve characteristics

Ta le 10!1 (lo2 Control Process


8/11/2019 EINTH012



Ta le 10!+ Li4)id Level Systems

10!+ alve Si5ing

+ver si@ing of valves sometimes occurs "hen trying to optimi@e processperformance through a reduction of process variability. !his results from usingline-si@e valves, especially "ith high-capacity rotary valves, as "ell as theconservative addition of multiple safety factors at different stages in theprocess design

+ver si@ing the valve hurts process variability in t"o "ays. 6irst, theoversi@ed valve puts too much gain in the valve, leaving less flexibility inadCusting the controller. est performance results "hen most loop gain comesfrom the controller.

otice in the gain curve of figure '-B, the process gain gets $uite high in theregion belo" about 'B% valve travel. If the valve is oversi@ed, ma9ing it moreli9ely to operate in or near this region, this high gain can li9ely mean that thecontroller gain "ill need to be reduced to avoid instability problems "ith theloop. !his, of course, "ill mean a penalty of increased process variability.

!he second "ay oversi@ed valves hurt process variability is that an oversi@edvalve is li9ely to operate more fre$uently at lo"er valve openings "here sealfriction can be greater, particularly in rotary valves. ecause an oversi@edvalve produces a disproportionately large flo" change for a given increment of valve travel, this phenomenon can greatly exaggerate the process variabilityassociated "ith dead band due to friction.4egardless of its actual inherent valve characteristic, a severely oversi@edvalve tends to act more li9e a $uic9-+pening valve, "hich results in highinstalled process gain in the lo"er lift regions ;figure '-B<. In addition, "henthe valve is oversi@ed, the valve tends to reach system capacity at relatively

lo" travel, ma9ing the flo" curve flatten out at higher valve travels ;figure '-B<.6or valve travels above about B0 degrees, this valve has become totally


8/11/2019 EINTH012


Rolta Academy:Engineering Design Servicesineffective for control purposes because the process gain is approaching @eroand the valve must undergo "ide changes in travel "ith very little resultingchanges in flo". onse$uently, there is little hope of achieving acceptableprocess variability in this region.

(ig 10!6 "nstalled *lo2 c%aracteristics and gain!he valve sho"n in figure '-B is totally misapplied in this application because ithas such a narro" control range ;approximately 'B degrees to )B degrees<.!his situation came about because a line-si@ed butterfly valve "as chosen,primarily due to its lo" cost, and no consideration "as given to the lost profitthat results from sacrificing process variability through poor dynamicperformance of the control valve.

2nfortunately, this situation is often repeated. rocess control studies sho"that, for some industries, the maCority of valves currently in process controlloops are oversi@ed for the application. =hile it might seem counterintuitive, itoften ma9es economic sense to select a control valve for present conditionsand then replace the valve "hen conditions change.=hen selecting a valve, it is important to consider the valve style, inherentcharacteristic, and valve si@e that "ill provide the broadest possible controlrange for the application.

*tandardi@ation activities for control valve si@ing can be traced bac9 to the

early 1 G0Os "hen a trade association, the 6luids ontrol Institute, publishedsi@ing e$uations for use "ith both compressible and incompressible fluids.!he range of service conditions that could be accommodated accurately bythese e$uations "as $uite narro", and the standard did not achieve a highdegree of acceptance. In 1 G , the I* established a committee to developand publish standard e$uations. !he efforts of this committee culminated in avalve si@ing procedure that has achieved the status of merican ational*tandard. ater, a committee of the International Electro technical ommission;IE < used the I* "or9s as a basis to formulate international standards for si@ing control valves. ;*ome information in this introductory material has beenextracted from *I I* * B.01 standard "ith the permission of the publisher,the I* .< Except for some slight differences in nomenclature and procedures,the I* and IE standards have been harmoni@ed. *I I* *tandard


8/11/2019 EINTH012


8/11/2019 EINTH012



Ta le 10!- A reviations and Terminologies

6or rotary valves "ith reducers ;s"aged installations<, 6 p factors are includedin the appropriate flo" coefficient table. 6or other valve designs and fittingstyles, determine the 6 p factors by using the procedure for 3etermining 6p, the

iping /eometry 6actor.


8/11/2019 EINTH012



Ta le 10!/ E4)ation Constants

). 3etermine $ max ;the maximum flo" rate at given upstream conditions< or P max ;the allo"able si@ing pressure drop<.

!he maximum or limiting flo" rate ;$max<, commonly called cho9ed flo", ismanifested by no additional increase in flo" rate "ith increasing pressuredifferential "ith fixed upstream conditions. In li$uids, cho9ing occurs as aresult of vapori@ation of the li$uid "hen the static pressure "ithin the valve

drops belo" the vapor pressure of the li$uid.!he IE standard re$uires the calculation of an allo"able si@ing pressure drop;P max<, to account for the possibility of cho9ed flo" conditions "ithin the valve.!he calculated P max value is compared "ith the actual pressure drop specifiedin the service conditions, and the lesser of these t"o values is used in thesi@ing e$uation. If it is desired to use Pmax to account for the possibility of cho9ed flo" conditions, it can be calculated using the procedure for determining $max, the 5aximum 6lo" 4ate, or P max, the llo"able *i@ing

ressure 3rop. If it can be recogni@ed that cho9ed flo" conditions "ill notdevelop "ithin the valve, max need not be calculated.

B. *olve for re$uired v, using the appropriate e$uation&• 6or volumetric flo" rate unitsA


8/11/2019 EINTH012



• 6or mass flo" rate unitsA

In addition to v, t"o other flo" coefficients, : v and v, are used, particularly outside of orth merica. !he follo"ing relationshipsexist&: v Q ;0.8GB<;v< v Q ;'.)0 R 10 -B<; v<

G. *elect the valve si@e using the appropriate flo" coefficient table and thecalculated v value.

10!+ + Determining ( p 7 t%e Pi8ing .eometry (actor 3etermine an 6 p factor if any fittings such as reducers, elbo"s, or tees "ill bedirectly attached to the inlet and outlet connections of the control valve that isto be si@ed. =hen possible, it is recommended that 6 p factors be determinedexperimentally by using the specified valve in actual tests. !he 6p factors for rotary valves used "ith reducers have all been determined in this manner, andtheir values are listed in the flo" coefficient tables.6or 6p values not listed in the flo" coefficient tables, calculate the 6p factor using the follo"ing e$uation.

' Q umerical constant found in the E$uation onstants tabled Q ssumed nominal valve si@e.

v Q #alve si@ing coefficient at 100-percent travel for the assumed valve si@e

In the above e$uation, the Σ: term is the algebraic sum of the velocity headloss coefficients of all of the fittings that are attached to the control valve.Σ K = : 1 S : ' S : 1 - : '

=here,: 1 Q 4esistance coefficient of upstream fittings: ' Q 4esistance coefficient of do"nstream fittings: 1 Q Inlet ernoulli coefficient: ' Q +utlet ernoulli coefficient!he ernoulli coefficients, : 1 and : ' , are used only "hen the diameter of thepiping approaching the valve is different from the diameter of the piping leavingthe valve, "hereby&


(10.1)

(10.2)

(10.3)

(10.4)

8/11/2019 EINTH012



=here,d Q ominal valve si@e3 Q Internal diameter of piping

If the inlet and outlet piping are of e$ual si@e, then the ernoulli coefficients arealso e$ual, : 1 Q : ' , and therefore they are dropped from the e$uation.

!he most commonly used fitting in control valve installations is the short-lengthconcentric reducer. !he e$uations for this fitting are as follo"s&

9 6or an inlet reducerA

• 6or an outlet reducerA

• 6or a valve installed bet"een identical reducersA

Determining 4ma ;t%e <a im)m (lo2 Rate= or Pma ;t%e Allo2a leSi5ing Press)re Dro8=3etermine either $max or ∆ max if possible for cho9ed flo" to develop "ithinthe control valve that is to be si@ed. !he values can be determined by usingthe follo"ing procedures.

Determining 4ma ;t%e <a im)m (lo2 Rate=

#alues for 6 6, the li$uid critical pressure ratio factor, can be obtained from thefollo"ing e$uation&

#alues of 6 , the recovery factor for valves installed "ithout fittings attached,can be found in the flo" coefficient tables. If the given valve is to be installed

"ith fittings such as reducer attached to it, 6 in the e$uation must bereplaced by the $uotient 6 6p, "here&


(10.5)

(10.6)

(10.7)

(10.8)

(10.9)

8/11/2019 EINTH012



nd: 1 Q : 1 S : 1

=here,: 1 Q 4esistance coefficient of upstream fittings: 1 Q Inlet ernoulli coefficient

Determining Pmax ;t%e Allo2a le Si5ing Press)re Dro8=∆ max ;the allo"able si@ing pressure drop< can be determined from thefollo"ing relationships&6or valves installed "ithout fittings-

6or valves installed "ith fittings attached-

=here,1 Q 2pstream absolute static pressure' Q 3o"nstream absolute static pressurev Q bsolute vapor pressure at inlet temperature

#alues of 6 6, the li$uid critical pressure ratio factor, can be obtained from6igure B-' or from the follo"ing e$uation&

#alues of 6 , the recovery factor for valves installed "ithout fittings attached, can be found in the flo" coefficienttables. n explanation of ho" to calculate values of 6 , the recovery factor for valves installed "ith fittings attached, is presented in the procedure for determining $max ;the 5aximum 6lo" 4ate<.


(10.10)

(10.11)

(10.12)

(10.13)

8/11/2019 EINTH012



(ig 10!> Li4)id critical 8ress)re ratio c)rve

+nce the ∆ max value has been obtained from the appropriate e$uation, itshould be compared "ith the actual service pressure differential ; ∆ Q 1 -' <. If∆ max is less than ∆ , this is an indication that cho9ed flo" conditions

"ill exist under the service conditions specified. If cho9ed flo" conditions doexist ;∆ max T 1 - ' <, then step B of the procedure for *i@ing #alves for

i$uids must be modified by replacing the actual service pressure differential; 1 - ' < in the appropriate valve si@ing e$uation "ith the calculated∆ max

value

10!+!- Li4)id Si5ing Sam8le Pro lem ssume an installation that, at initial plant start-up, "ill not be operating atmaximum design capability. !he lines are si@ed for the ultimate systemcapacity, but there is a desire to install a control valve no", "hich is si@ed onlyfor currently anticipated re$uirements. !he line si@e is 8 inches, and a lass(00 globe valve "ith an e$ual percentage cage has been specified. *tandardconcentric reducers "ill be used to install the valve into the line.3etermine the appropriate valve [email protected]. *pecify the necessary variables re$uired to si@e the valve&

• 3esired valve designA lass (00 globe valve "ith e$ualpercentage cage and an assumed valve si@e of ( inches.

• rocess fluidAli$uid propane• *ervice conditionsA$ Q 800 gpm


8/11/2019 EINTH012


Rolta Academy:Engineering Design Services1 Q (00 psig Q (1). psia' Q ' B psig Q '8 . psia

∆ Q 'B psi! 1 Q 0°6/ f Q 0.B0

v Q 1').( psiac Q G1G.( psia

'. 3etermine an 1 value of 1.0 from the E$uation onstants table.

(. 3etermine 6 p, the piping geometry factor.

ecause it is proposed to install a (-inch valve in an 8-inch line, it "ill be

necessary to determine the piping geometry factor, 6 p, "hich corrects for osses caused by fittings attached to the valve.

=here,' Q 8 0, from the E$uation onstants table

d Q ( in., from step 1v Q 1'1, from the flo" coefficient table for a lass (00, ( in. /lobe valve "ith

e$ual percentage cage

!o compute Σ: for a valve installed bet"een identical concentric reducers&

=here,3 Q 8 in., the internal diameter of the piping so,

). 3etermine ∆ max ;the llo"able *i@ing ressure 3rop.<


(10.14)

8/11/2019 EINTH012


Rolta Academy:Engineering Design Servicesased on the small-re$uired pressure drop, the flo" "ill not be cho9ed ; ∆ max

U∆ <.

B. *olve for v, using the appropriate e$uation.

G. *elect the valve si@e using the flo" coefficient table and the calculated v

value. !he re$uired v of 1'B. exceeds the capacity of the assumed valve,"hich has a v of 1'1. lthough for this example it may be obvious that thenext larger si@e ;) inches< "ould be the correct valve si@e, this may notal"ays be true, and a repeat of the above procedure should be carried out.

ssuming a )-inch valve, v Q '0(. !his value "as determined from the flo"coefficient table for a lass (00, )-inch globe valve "ith an e$ual percentage

cage.4ecalculate the re$uired v using an assumed v value of '0( in the 6pcalculation.

"here,


8/11/2019 EINTH012



Q1'1.

!his solution indicates only that the )-inch valve is large enough to satisfy theservice conditions given. !here may be cases, ho"ever, "here a moreaccurate prediction of the v is re$uired. In such cases, the re$uired v shouldbe redetermined using a ne" 6 p value based on the v value obtained above.In this example, v is 1'1. , "hich leads to the follo"ing result&

!he re$uired v then becomes&


8/11/2019 EINTH012



ecause this ne"ly determined v is very close to the v used initially for thisrecalculation ;11G.' versus 1'1. <, the valve si@ing procedure is complete,and the conclusion is that a )-inch valve opened to about B-percent of totaltravel should be ade$uate for the re$uired specifications.

10!- Si5ing alves *or Com8ressi le (l)ids6ollo"ing is a six-step procedure for the si@ing of control valves for compressible flo" using the I* standardi@ed procedure. Each of these stepsis important and must be considered during any valve si@ing procedure. *teps( and ) concern the determination of certain si@ing factors that may or maynot be re$uired in the si@ing e$uation depending on the service conditions of the si@ing problem. If it is necessary for one or both of these si@ing factors tobe included in the si@ing e$uation for a particular si@ing problem, refer to theappropriate factor determination section;s<, "hich is referenced and located inthe follo"ing text.1. *pecify the necessary variables re$uired to si@e the valve as follo"s&

• 3esired valve design ;e.g. balanced globe "ith linear cage<H refer tothe appropriate valve flo" coefficient table

• rocess fluid ;air, natural gas, steam, etc.< and• ppropriate service conditionsA

$, or ", 1, ' or ∆ , ! 1, / g, 5, 9, N, and γ 1

!he ability to recogni@e "hich terms are appropriate for a specific si@ingprocedure can only be ac$uired through experience "ith different valve si@ingproblems. If any of the above terms appear to be ne" or unfamiliar, refer tothe bbreviations and !erminology table for a complete definition.

'. 3etermine the e$uation constantH . is a numerical constant contained ineach of the flo" e$uations to provide a means for using different systems of units. #alues for these various constants and their applicable units are givenin the E$uation onstants table.

2se either or if si@ing the valve for a flo" rate in volumetric units ;scfh or m( h<. =hich of the t"o constants to use depends upon the specified serviceconditions. can be used only if the specific gravity, /g, of the follo"ing gashas been specified along "ith the other re$uired service conditions. can beused only if the molecular "eight, 5, of the gas has been specified.2se either G or 8 if si@ing the valve for a flo" rate in mass units ;lb h or 9g h<. =hich of the t"o constants to use depends upon the specified serviceconditions. G can be used only if the specific "eight, γ 1, of the flo"ing gashas been specified along "ith the other re$uired service conditions. 8 can beused only if the molecular "eight, 5, of the gas has been specified.

(. 3etermine 6 p, the piping geometry factor. 6p is a correction factor that

accounts for any pressure losses due to piping fittings such as reducers,elbo"s, or tees that might be attached directly to the inlet and outlet


8/11/2019 EINTH012


Rolta Academy:Engineering Design Servicesconnections of the control valves to be si@ed. If such fittings are attached tothe valve, the 6p factor must be considered in the si@ing procedure. If,ho"ever, no fittings are attached to the valve, 6 p has a value of 1.0 and simplydrops out of the si@ing e$uation.

lso, for rotary valves "ith reducers, 6 p factors are included in the appropriateflo" coefficient table. 6or other valve designs and fitting styles, determine the6p factors by using the procedure for 3etermining 6p the iping /eometry6actor, "hich is located in the section for *i@ing #alves for i$uids.

). 3etermine 7, the expansion factor, as follo"s&

=here,69 Q 9 1.), the ratio of specific heats factor 9 Q 4atio of specific heatsx Q∆ 1, the pressure drop ratiox! Q !he pressure drop ratio factor for valves installed "ithout attachedfittings. 5ore definitively, x! is the pressure drop ratio re$uired to producecritical, or maximum, flo" through the valve "hen 69 Q 1.0

If the control valve to be installed has fittings such as reducers or elbo"s

attached to it, then their effect is accounted for in the expansion factor e$uation by replacing the x! term "ith a ne" factor x ! . procedure for determining the x! factor is described in the section for 3etermining x! , the

ressure 3rop 4atio 6actor.

NoteConditions o* critical 8ress)re dro8 are reali5ed 2%en t%e val)e o*

ecomes e4)al to or e ceeds t%e a88ro8riate val)e o* t%e 8rod)ct o* eit%er ( , T or ( , TP at 2%ic% 8oint:

lthough in actual service, pressure drop ratios can, and often "ill, exceed theindicated critical values, this is the point "here critical flo" conditions develop.!hus, for a constant 1, decreasing ' ;i.e., increasing ∆ < "ill not result in anincrease in the flo" rate through the valve. #alues of x, therefore, greater thanthe product of either 69x! or 69x! must never be substituted in the expressionfor 7. !his means that 7 can never be less than 0.GG . !his same limit onvalues of x also applies to the flo" e$uations that are introduced in the nextsection.

B. *olve for the re$uired v using the appropriate e$uation&6or volumetric flo" rate unitsA


(10.15)

8/11/2019 EINTH012


Rolta Academy:Engineering Design Services• If the specific gravity, /g, of the gas has been specified&

• If the molecular "eight, 5, of the gas has been specified&

6or mass flo" rate unitsA• If the specific "eight, γ 1, of the gas has been specified&

• If the molecular "eight, 5, of the gas has been specified&

In addition to v, t"o other flo" coefficients, : v and v, are used, particularlyoutside of orth merica. !he follo"ing relationships exist&: v Q ;0.8GB<;v< v Q'.)0 R 10 - B; v<


Note

Once t%e valve si5ing 8roced)re is com8leted7 consideration can emade *or aerodynamic noise 8rediction! To determine t%e gas *lo2 si5ingcoe**icient ;Cg= *or )se in t%e aerodynamic noise 8rediction tec%?ni4)e7 )se t%e *ollo2ing e4)ation:

Determining TP7 t%e Press)re Dro8 Ratio (actor

If the control valve is to be installed "ith attached fittings such as reducers or

elbo"s, then their effect is accounted for in the expansion factor e$uation byreplacing the x! term "ith a ne" factor, x ! .


(10.16)

(10.17)

(10.18)

(10.19)

(10.20)

8/11/2019 EINTH012


8/11/2019 EINTH012


Rolta Academy:Engineering Design Servicesecause both / g and 5 have been given in the service conditions, it is

possible to use an e$uation containing either or . In either case, the endresult "ill be the same. ssume that the e$uation containing / g has beenarbitrarily selected for this problem. !herefore Q 1(G0.

(. 3etermine 6p, the piping geometry factor. *ince valve and line si@e areassumed e$ual, 6p Q 1.0.

). 3etermine 7, the expansion factor.

It is assumed that an 8-inch 3esign #'B0 valve "ill be ade$uate for thespecified service conditions. 6rom the flo" coefficient table, x! for an 8-inch3esign #'B0 valve at 100-percent travel is 0.1( .

x Q 0. 0 ;!his "as calculated in step 1.<

*ince conditions of critical pressure drop are reali@ed "hen the calculatedvalue of x becomes e$ual to or exceeds the appropriate value of 6 9x! , thesevalues should be compared.

69x! Q;0. )< ;0.1( <Q 0.1'ecause the pressure drop ratio, x Q 0. 0 exceeds the calculated critical

value, 69x! Q 0.1' , cho9ed flo" conditions are indicated. !herefore, 7 Q0.GG , and x Q 6: R! Q 0.1' .

B. *olve for re$uired v using the appropriate e$uation.

!he compressibility factor, N, can beassumed to be 1.0 for the gas pressure and temperature given and 6 p Q 1because valve si@e and line si@e are e$ual.*o,


!he above result indicates that the valve is ade$uately si@ed ;rated v Q'1 0<. !o determine the percent valve opening, note that the re$uired v


8/11/2019 EINTH012


Rolta Academy:Engineering Design Servicesoccurs at approximately 8( degrees for the 8-inch 3esign #'B0 valve. otealso that, at 8( degrees opening, the x ! value is 0.'B', "hich is substantiallydifferent from the rated value of 0.1( used initially in the problem. !he nextstep is to re"or9 the problem using the x ! value for 8( degrees travel.

!he 6 9 x! product must no" be recalculated.x Q 69 x!

Q ;0. )< ;0.'B'<Q 0.'(

!he re$uired v no" becomes&

!he reason that the re$uired v has dropped so dramatically is attributablesolely to the difference in the x! values at rated and 8( degrees travel. v of 1118 occurs bet"een B and 80 degrees travel.!he appropriate flo" coefficient table indicates that x ! is higher at B degreestravel than at 80 degrees travel. !herefore, if the problem "ere to bere"or9ed using a higher x ! value, this should result in a further decline in thecalculated re$uired v.

4e"or9ing the problem using the x ! value corresponding to 8 degrees travel;i.e., x! Q 0.('8< leaves&x Q 69 x!

Q ;0. )< ;0.('8<Q 0.(08

and,


8/11/2019 EINTH012


Rolta Academy:Engineering Design Services!he above v of 80 is $uite close to the B degree travel v. !he problemcould be re"or9ed further to obtain a more precise predicted openingHho"ever, for the service conditions given, an 8-inch 3esign #'B0 valveinstalled in an 8-inch line "ill be approximately B degrees open.

Com8ressi le (l)id Si5ingSam8le Pro lem No! +

ssume steam is to be supplied to a process designed to operate at 'B0 psig.!he supply source is a header maintained at B00 psig and B00M6. G-inchline from the steam main to the process is being planned. lso, ma9e theassumption that if the re$uired valve si@e is less than G inches, it "ill beinstalled using concentric reducers. 3etermine the appropriate 3esign E3valve "ith a linear cage.

1. *pecify the necessary variables re$uired to si@e the valve&a. 3esired valve designA lass (00 3esign E3 valve "ith a linear

cage. ssume valve si@e is ) inches.b. rocess fluidAsuperheated steam

c. *ervice conditionsA" Q 1'B,000 lb h

1 Q B00 psig Q B1). psia' Q 'B0 psig Q 'G). psia

∆ Q 'B0 psix Q∆ 1 Q 'B0 B1). Q 0.)! 1 Q B00°6

γ 1 Q 1.0)() lb ft( ;from roperties of *aturated *team table<9Q 1.'8 ;from roperties of *aturated *team table<

'. 3etermine the appropriate e$uation constant, , from the E$uationonstants table.ecause the specified flo" rate is in mass units, ;lb h<, and the specific "eight

of the steam is also specified, the only si@ing e$uation that can be used is that"hich contains the G constant. !herefore, GQ G(.(

(. 3etermine 6 p, the piping geometry factor.

=here,' Q 8 0, determined from the E$uation onstants table

d Q ) in.v Q '(G, "hich is the value listed in the manufacturerOs 6lo" oefficient table

for a )-inch 3esign E3 valve at 100-percent total travel.and


8/11/2019 EINTH012



6inally&

). 3etermine 7, the expansion factor,

"here,

x Q 0.) ; s calculated in step 1.<ecause the )-inch valve is to be installed in a G-inch line, the x! term must

be replaced by x ! .

=here,B Q 1000, from the E$uation onstants table

d Q ) in.

6p Q 0. B, determined in step (x! Q 0.G88, a value determined from the appropriate listing in themanufacturerOs 6lo" oefficient table

v Q '(G, from step (and: iQ : 1S: 1

=here3 Q G in.


8/11/2019 EINTH012


Rolta Academy:Engineering Design Services*o&

6inally&

B. *olve for re$uired v using the appropriate e$uation.

G. *elect the valve si@e using the appropriate manufacturerOs 6lo" oefficienttable and the calculated v value.

4efer to the manufacturerOs 6lo" oefficient tables for 3esign E3 valves "ithlinear cage. ecause the assumed )-inch valve has a v of '(G at 100-percent travel and the next smaller si@e ;( inches< has a v of only 1)8, it canbe surmised that the assumed si@e is correct. In the event that the calculatedre$uired v had been small enough to be handled by the next smaller si@e or if it had been larger than the rated v for the assumed si@e, it "ould havebeen necessary to re"or9 the problem again using values for the ne"assumed si@e.

einth012

Documents