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Walworth Check Valves

Jun 02, 2018



Fahad Rocking
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  • 8/11/2019 Walworth Check Valves


    Walworth Check Valves - Overview

    Walworth Cast Steel Swing Check Valves have straight-through flow paths and readily openingdiscs to provide very low friction loss.

    Points to Remember About Check Valves

    1. The function of check valves is to stop reversal of flow automatically and promptly. Usually it isdesirale that they e supplemented y gate! gloe! or other valves for manual control of the fluidin the line.

    " Check valves must e si#ed to meet the flow conditions. $versi#ing of a check valve will resultin increased wear and improper operation.

    %. Check valves should e installed so that the force of gravity will assist in closing and holdingthe disc or clapper on the seats.

    &. Check valves are one-way valves and should e installed so that the line pressure is under the


    '. Swing check valves should not e used in vertical pipe lines handling li(uids at high headsecause severe water hammer may result from reversal of flow.

    Cover)ike the *ody is a sturdy unit designed to ade(uately resist pressure-temperature.

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    Thick Uniform BodTotally fulfills or e+ceeds the re(uirements set forth y , Standard.

    !mooth O"eration/ue to the precision machining of the component parts of the /isc ,ssemly.

    !treamlined Portsermit unostructed flow which reduces turulence resulting in minimum pressure drop andminimum erosion.

    *ack to Check Valve llustrationJump to: navigation, search

    Check valve in a fire hydrant

    A check valveis a mechanical device,a valve, that normally allows fluid(liquidor gas)

    to flowthrough it in only one direction. Check valves are twoport valves, meaning they

    have two openings in the !ody, one for fluid to enter and the other for fluid to leave."here are various types of check valves used in a wide variety of applications. Check

    valves are often part of common household items. Although they are availa!le in a wide

    range of si#es and costs, many check valves are very small, simple, and$or cheap. Checkvalves work automatically and most are not controlled !y a person or any e%ternal

    control& accordingly, most do not have any valve handle or stem. "he !odies (e%ternal

    shells) of most check valves are made of plastic or metal.

    An important concept in check valves is the cracking pressure which is the minimumupstreampressureat which the valve will operate. "ypically the check valve is designed

    for and can therefore !e specified for a specific cracking pressure.

    'eart valvesare essentially inlet and outlet check valves for the heartventricles, since the

    ventricles act as a pump.


    * "ypes of check valves

    + Applications


    - ee also

    / 0%ternal links
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    [edit] Types of check valves

    A closed !all check valve.

    An open !all check valve.

    A Ball check valveis a check valve in which the disc, the mova!le part to !lock the flow,

    is a spherical !all. 1n many !all check valves, the !all is springloaded to stay shut, !ut

    also many do not have a spring inside. 2or those designs without a spring, reverse flow isrequired to move the !all toward the seat and create a seal. "he interior surface of the

    seats of !all check valves are more or less conicallytapered to guide the !all into the seat

    and$or form a positive seal when stopping reverse flow.

    3all check valves are often very small, simple, and cheap (although some are e%pensive)."hey are commonly used in liquid or gel minipump dispenser spigots, spray devices,

    some ru!!er !ul!s for pumping air, etc., manual air pumps and some otherpumps, and

    refilla!le dispensing syringes. Although the !alls are most often made of metal, they can!e made of other materials, or in some speciali#ed cases out of artificial ru!y.'igh

    pressure '45Cpumpsand similar applications commonly use small inlet and outlet !all

    check valves with !alls made of artificial ru!y and seats made of artificial sapphire, !oth

    for hardness and chemical resistance. After prolonged use, such check valves caneventually wear out or the seat can develop a crack, requiring replacement. "herefore,

    such valves are made to !e replacea!le, sometimes placed in a small plastic !ody tightly

    fitted inside a metal fittingwhich can withstand high pressure and which is screwed into

    the pump head.

    "here are similar check valves where the disc is not a !all, !ut some other shape, such as

    a poppet energi#ed !y a spring. 3all check valves should not !e confused with!all

    valves, which is a different type of valve in which a !all acts as a controlla!le rotor tostop or direct flow.

    A diaphragm check valveuses a fle%ing ru!!er diaphragm positioned to create a

    normallyclosed valve. 4ressure on the upstream side must !e greater than the pressure onthe downstream side !y a certain amount, the pressure differential, for the check valve to

    open allowing flow. 6nce positive pressure stops, the diaphragm automatically fle%es

    !ack to its original closed position.

    A swing check valveis a check valve in which the disc, the mova!le part to !lock theflow, swings on a hinge ortrunnion,either onto the seat to !lock reverse flow or off the

    seat to allow forward flow. "he seat opening crosssection may !e perpendicular to the

    centerline !etween the two ports or at an angle. Although swing check valves can come in

    various si#es, large check valves are often swing check valves.
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    "his iamese clappered inlet allows one or two inputs into a deluge gun.

    A clapper valveis a type of check valve used in or with firefighting, and has a hingedgate (often with a spring urging it shut) that will only remain open in the outflowing


    A stop-check valveis a check valve with override control to stop flow regardless of flow

    direction or pressure. 7hen the valve is open, it acts as a check valve, !ut the valve can!e deli!erately shut to stop flow.

    A lift-check valveis a check valve in which the disc, sometimes called a lift, can !e liftedup off its seat !y higher pressure of inlet or upstream fluid to allow flow to the outlet or

    downstream side. A guide keeps motion of the disc on a vertical line, so the valve canlater reseat properly. 7hen the pressure is no longer higher, gravity or higher downstream

    pressure will cause the disc to lower onto its seat, shutting the valve to stop reverse flow.

    Adou!le check valveis often used as a!ackflow prevention deviceto keep potentiallycontaminated water fromsiphoning!ack into municipalwater supply lines.

    "here are also double ball check valvesin which there are two !all$seat com!inations

    sequentially in the same !ody to ensure positive leaktight shutoff when !locking reverse

    flow& and piston check valves, wafer check valves, and !allandcone check valves.

    [edit] Applications

    Check valves are often used with some types of pumps. 4istondriven and diaphragm

    pumps such as metering pumpsand pumps for chromatographycommonly use inlet and

    outlet !all check valves. "hese valves often look like small cylinders attached to thepump head on the inlet and outlet lines. 8any similar pumplike mechanisms for moving

    volumes of fluids around use check valves such as !all check valves.

    Check valves are used in many fluid systems such as those in chemical, andpower plants,

    and in many other industrial processes.

    Check valves are also often used when multiple gases are mi%ed into one gas stream. Acheck valve is installed on each of the individual gas streams to prevent mi%ing of the

    gases in the original source. 2or e%ample, if a fuel and an o%idi#er are to !e mi%ed, then

    check valves will normally !e used on !oth the fuel and o%idi#er sources to ensure thatthe original gas cylinders remain pure and therefore nonflamma!le.
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    ome types of irrigationsprinklersand drip irrigationemittershave small check valves

    !uilt into them to keep the lines from draining when the system is shut off.

    Valve0ay e used in vertical or hori#ontal pipe lines. ravity and ack- flow will cause /isc to close.

    !eat Rin#Welded or threaded provides a positive seal against leakage etween ody and seat ring.

    $isc with Welded-On %acin#Sturdily constructed to provide an e+cellent seal despite the varying pressures e+erted on theseat surface


    Check Valve on Minimum Flow Line

    thread124-198861Forum Search FAQs Links Jobs Whitepapers

    ghamsa(Chemical) 2Oct07


    We have 4 running centriugal !um!"# $ach !um! i" e%ui!!ed &ith 8' minimum& rec*cle line# $ach rec*cle line ha" & cntrl valve and chec+ valve(d&n"tream the cntrl valve# ,ll 4 !um!" di"charge int ne cmmn header24'#

    .here &ill /e additinal 5th !um! identical t the ther 4 !um!" and cnnected t

    the "ame 24' di"charge header# nrtunatel* thi" ne& !um! came &ithut chec+valve d&n"tream the minimum rec*cle valve#We need t "tart u! the ne& 5th !um! /ut in ca"e thi" !um! "hutd&n" r tri!" theminimum & rec*cle cntrl valve &ill ull* !en# * %ue"tin i": &ill there /emar !re""ure di"tur/ance in the 24' di"!arage header3 Or the ther running!um!" &ill &r+ t maintain "ta/le !re""ure in the di"charge header3

    te: &e need t "tart u! the ne& !um! /ut &e are !lanning t add chec+ valveater "metime rm !um! "tart u!#

    Find A Job or ost a Job !pening Click "ere#

    $eap(Chemical) 2Oct07


    i gham"a

    ir"t all a %ue"tin: ,""uming that each *ur !um!" ha" a chec+ valve and a"hut valve (a" m"t the "*"tem" have)# .he rec*cle line "tart" u!"tream r
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    d&n"tream thi" chec+ valve3 it "tart" d&n"tream the !um! chec+ valve cn"ider it i" nt nece""ar* t in"tall an additinal chec+ valve d&n"tream r thecntrl valve#

    the rec*cle line "tart" u!"tream the !um! chec+ valve *u need t in"tall anadditinal chec+ valve d&n"tream the cntrl valve in rder t avid the recicling

    uid t run thrugh the !um! &hen it i" "t!!ed# .hi" a""um!tin i" cn"ideringthat *u have a minimum & cntrl that !en" the cntrl valve i it i" "en"ed al& & at the !um! di"charge#

    ,n additinal !tin i" t !rgram an interlc+ t cl"e the cntrl valve &hen the!um! "t!"# t culd /e dne /* venting the air eed line t the cntrl valve &ith athree &a* "lenid valve r cur"e i *ur cntrl valve i" ail t Cl"e#


    dcasto(Chemical) 2Oct07


    ea! d have th"e /ac+&ard"3

    ghamsa(Chemical) Oct07


    & can !redict the !re""ure dr! in the 0; header (570 !"ig) in the

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    !lea"e l+ the diagram:


    t i" !""i/le under the cnditin" "h& *u###

    eed t +n& a "cheme the gham"a ca"e#

    %ig&nch(=etrleum) 4Oct07


    ight# .he diagram hel!ed a lt#

    ir"t %ue"tin i" &h* i" the rec*cle line !en &hen the !um! i" "t!!ed3 t "huld/e cl"ed (and interlc+ed i remte cntrlled) until the !um! /egin" "!inningu!# Once the !um! /egin" "!inning u! !re""ure /uild" ver* ra!idl* and an* &rm an* ther rec*cling !um! int that "egment &uld nrmall* /e ver* "mall and%uic+l* rever"e directin a" the "tarting !um! /uild" !re""ure# it tri!" n the"tart-u! cl"e the valve#

    dn?t "ee an* ther ;!r/a/le; rea"n that & &uld cme rm the rec*cle line"int the

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    dcasto(Chemical) 4Oct07


    can?t "ee the diagram /ut *u are in gd hand" &ith ignch#

    ghamsa(Chemical) 4Oct07


    .han+ *u all r !artici!atin"# &ill tr* t de"cri/e m* ca"e again# >rr* culdnt ma+e a clear !icture#We have ur centriugal !um!" each i" 10000 //l !er da* in ca!acit*# rmall*three !um!" are running and ne i" n "tand/*# .he !um!ing "*"tem i" cn"i"ting the ll&ing cm!nent":3 >uctin header: "iFe i" 16'&ith !erating !re""ure 270 !"ig (all ur !um!"ta+e "uctin rm thi" cmmn 16'header)3 Bi"charge header: "iFe 0'&ith !erating !re""ure 570 !"ig (all ur !um!"di"charge int thi" cmmn header)3 =um! !re""ure at ange di"charge i" 590 !"ig3 =um! di"charge line i" 14' "iFe3 $ach !um! di"charge i" e%ui!!ed &ith a chec+ valve ll&ed /* a /lc+ valve#3 $ach !um! i" "erved &ith 8'minimum rec*cle cntrl valve that i" /ranchedrm each !um! 14'di"charge line and ha" chec+ valve u!"tream the minimumrec*cle cntrl valve#3 $ach 8'minium rec*cle line i" cnnected int the 16'"uctin header3 .he intent the adding a chec+ valve u!"tream the 8'minium rec*cle valvei" t avid relea"e di"charged & t 16'"uctin header during !um! "hutd&n

    tri!" r minimum cntrl valve air "ignal i" a ne& 5th !um! /eing added t the"e ur !um!"# nrtunatel* the 5th!um! came &ithut a chec+ valve u!"tream the 8'minium rec*cle line# Withadditin thi" ne& !um! &e &ill have running !um!" and 2 "tand/* !um!"#* %ue"tin: i the 5th !um! tri!" the cntrl lgic &ill ull* !en" u! the 8'miniumrec*cle cntrl valve /ac+ t the 16'"uctin header &hich i" at 270 !"ig# & thi""udden !ening the 8'minium rec*cle cntrl valve &ill di"tur/ the !erating!re""ure in the 0'di"hcarage !re""ure &hich i" at 570 !"ig in the

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    thin+ the cntrl lgic i" /ac+&ard"#

    When the !um! tri!" C@O>$ the valve# /elieve that the valve "huld /e !enO@A &hen the !um! ha" a G "tatu" and &hen = "tatu" i" @OW (!r/a/l*H 20I $= rev")#

    When *u !u"h the "tart /uttn the valve need" t

    1#) O=$ and "ta* !en until

    ,#) .he !um! reache" "mething arund 20I !erating "!eed at &hich time *uneed t /e cl"ing that valve t /uild "uJcient !re""ure t !en the !um!di"charge chec+ and get int the header# t i" du/tul *u &ill /uild "uJcient!re""ure i a rec*cle valve i" !en#


    #) .he !um! de" nt get a G "tatu" immediatel* ater "tarting then


    When the !um! tri!" the rec*cle valve "huld /e cl"ing#

    > t ma+e the lgic ver* "im!leC!/0

    IF 0 B" *u !r/a/l*dn?t need an* rec*cle line i *u have the cntrl lgic crrect " in that ca"e u"ttr* it &ith the rec*cle valve C@O>$B all the time#


    ghamsa(Chemical) 5Oct07


    .here are O K>B" n the"e !um!"# We cl"e minimum rec*cle cntrl valveduring !um! "tart u! in rder t /uild !re""ure# agree ur lgic i" /ac+&ard# .he valve "huld cl"e &hen !um! "hutd&n" rtri!"#

    Lu"t r the "a+e an"&ering m* thread can *u !lea"e an"&er the %ue"tin in thela"t !aragra!h n the thread3'* %ue"tin: i the 5th !um! tri!" the cntrl lgic &ill ull* !en" u! the 8'miniumrec*cle cntrl valve /ac+ t the 16'"uctin header &hich i" at 270 !"ig# & thi""udden !ening the 8'minium rec*cle cntrl valve &ill di"tur/ the !erating!re""ure in the 0'di"hcarage !re""ure &hich i" at 570 !"ig in the

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    minute"33 Will thi" verlad the ther 2 running !um!" r &uld the0'di"charage header !re""ure 570 !"ig dr! t cl"e the "uctin !re""ure33

    an* than+"

    %ig&nch(=etrleum) 5Oct



    OM# ut ince *u have cntrl valve" in the !um!di"charge the* &ill al" aect h& lng the d&n"tream !i!eline

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    (&TRO$UCT(O&,The centrifugal pumpis one of the simplest pieces ofe(uipment from the controls andinstrumentation point of view. t is a twoport device with a well defined

    characteristic. ts purpose is to providethe necessary pressure to move li(uid atthe desired rate from point , to point * ofthe process. 7igure 1-1 shows a ;generic;process with a centrifugal pumpconnected to deliver li(uid from , to *.

    7igure 1-" shows the characteristic curveof an actual pump 5a single stage verticalturine pump6 together with thecharacteristic curve of the process!

    known as the system curve. Theintersection of the two curves defines theoperating point of oth pump andprocess. t would e fortunate indeed ifthis operating point is the one actually specified for the process. t is impossilefor one operating point to meet all desired operating conditions since theoperating point is! y definition! e+actly one of an infinity of possile operatingpoints. n fact the entire point of controlling the pump is to modify itscharacteristic so that its actual operating point is the one that is re(uired at everyinstance in time.

    Several definitions are presented in order to discuss the diagramo= 7low rate! at operatingpoint! of the pump and also ofthe process.

    pm= 0a+imum differentialpressure across the pump 5atshutoff6.

    >pm= 0a+imum dischargeflow of the pump.

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    lm= Static 50inimum6 differential pressure etween points * and , of theprocess.

    The minimum static differential pressure of the process is fre(uently #ero! as in aclosed! circulating system. f the pump is in parallel with other pumps that aremaintaining the system pressure! then lmis greater than #ero. t is clear fromthe outset that if lmis greater than pm! no amount of process control can forcethe two curves to intersect. The pump is simply inade(uate. ?ow is processcontrol like cutting off a rope@ Aou can always cut off more! ut you can;t cut offless.

    ,ssuming the pump is more than ade(uate for the process re(uirements at themoment! what is the est way to trim it ack to the desired operating point! 1!>1@ There are three possile locations to place a valve< ,t the discharge! at the

    suction! and as a recycle valve. 3ach will e discussed in turn.

    $(!CAR)* TROTT'(&), Since the pump e+ists to serve the re(uirements ofthe process! and one of the primary purposes of instrumentation is to adapt thee(uipment to the process! let us consider the pump from the point of view of theprocess. t can e viewed as a constant pressure device with an internalrestriction. t is the restriction that gives it the BcurveB. t seems natural to put avalve on the discharge to further restrict the pump. This has the effect of rotatingthe curve of the pumpvalve system clockwise around pm! as can e seen in7igure 1-%.

    ,t this point must warn thereader that we are aout toencounter a paradigm shift. 5D6The comination of pump andvalve will e presented as aBlack o+B with a singlecharacteristic curve which shall term the BmodifiedB pumpcurve.

    The more traditional way of

    looking at the situation is fromthe point of view of the pump. tsees the process system curveas having rotated counterclockwise around lm. 7igure 1-% shows that the flow! >1! is thesame for oth cases. The difference etween the two pressures is the /elta across the valve. Since the purpose of the pump is to serve the process

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    re(uirements! and the purpose of the valve is to adapt the pump to the process! itmakes sense to consider the valve to e part of the pump system and to use themodified pump curve rather than the modified system curve in our discussion. nany case it can e seen that a discharge valve can e used to achieve anyoperating point on the system curve so long as the point is elow the pump


    !UCT(O& TROTT'(&), The second possiility for control using valves is toplace the valve in the pump suction line. This would have an identical effect onthe characteristic curve! ut the method has a fatal flaw E cavitation. Cavitation isa phenomenon that occurs when the pressure of a li(uid is reduced elow itsvapour pressure and rought ack up aove the vapour pressure again. *ulesof vapour form in the li(uid and then collapse upon arriving at the higher pressureregion. The collapse occurs at sonic speed eFecting minute Fets of e+tremely highvelocity li(uid. Wherever these Fets impinge on a solid surface e+treme erosionoccurs. $ver time even the hardest materials will e destroyed. Therefore it is of

    utmost importance that this pressure reduction never occurs. t is prevented yhaving sufficient pressure availale at the pump suction so that the pressuredrops that occur as the li(uid is drawn into the eye of the impeller are at alltimesaove the vapour pressure of the li(uid at its current temperature.

    ,n e+planation of the term Get ositive Suction ?ead 5GS?6 is in order. This isthe pressure of the li(uid at the pump suction in terms of feet or meters of li(uidhead aove the vapour pressure of the li(uid. The actual GS? under operatingconditions is called GS?, and the minimum re(uired y the pump to preventcavitation is called GS?H. Clearly GS?, must e greater than GS?H toavoid cavitation. t is safe to leave a margin of aout one meter.

    These peculiar definitions are very reasonale in terms of the pumps actualcharacteristic ut they cause some prolems to the controls engineer. t meansthat the gauge pressure e(uivalent of a given GS?, is proportional to thedensity of the li(uid and is also affected y its temperature. The vapour pressurecan rise dramatically as the temperature rises. This means that the GS?, canfall without a noticeale change in pressure.

    ,nything that would reducethe net positive pressure atthe pump inlet elow theGS?H must e asolutelyavoided. Thus suctionthrottling is never used tocontrol pump flow.

    R*C.C'* CO&TRO',Thethird remaining possiility forpump control with valves is to

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    leed some of the discharge flow ack to the pump suction or to some otherpoint on the supply side. $nce again we can view the result as a modified systemcurve or as a modified pump characteristic. 7igure 1-& shows oth. 3ach curve isa rotation of the original< The modified system curve as a clockwise rotationaround lm. Gote the little BtailB at the left of the modified system curve. This

    represents the flow through the recycle valve efore the discharge check valveopens to the process. The modified pump curve has a counter clockwise rotationaround the hypothetical intersection of the pump curve with the flow a+is.

    This family of curves shows several prolems with recycle control. 7irstly! thepump is not rated to discharge more than the flow rate at the end of the curve. t

    is"ossible! of course! to run the pump with a wide open discharge! minimum

    ! ut it is unhealthy for this particular pump to run at such a high rate. 3+cessiveflow may cause cavitation damage. 53+cess flow cavitation is not caused y

    GS? prolems ut y high velocity within the internal passages of the pump.6This restriction means that the minimum discharge pressure may not e lowerthan the one corresponding to the ma+imum flow. n other words! the modifiedpump curve cannot reach all points on the system curve.

    Secondly! although many pumps are capale of operating near #ero dischargepressure! the very flat pressure vs. flow curve for much of the lower range formost pumps means a change of flow has very little effect on the dischargepressure. Thus it would take a very large amount of flow to produce a small dropin pressure. n control terms this means that control would e very ;sloppy;./ischarge throttling on the other hand! allows the pump to develop the head that

    ;suits; it. The unwantedpressure is dropped acrossthe valve. 5Gote that thecurve for this particularpump rises rather steeply. twill e more easilycontrolled than most.6Thirdly! this method is ofteninefficient. 7igure 1-' showsa system curve! a pumpcharacteristic! a discharge

    modified characteristic! anda recycle modifiedcharacteristic. ,ove theseis a pump powerre(uirement curve. n thecase of discharge control!the pump is adapted to theprocess y dropping its

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    discharge pressure. f one follows the flow line vertically to the actual pump curveand then eyond to the power re(uirement curve one arrives at its powerre(uirement. n the case of recycle control! the pump is adapted y reducing thedischarge flow. 7ollowing the pressure line to the right to the actual pump curveand then upwards to the power re(uirement curve one arrives at the power

    re(uirement for recycle control. Gote that the power re(uirement curve tends toslope upward as flow increases. Therefore recycle control consumes more pumphorsepower than discharge throttling when oth achieve the same operatingpoint. This is not always so. f the power re(uirement curve were flat! there woulde no difference. Gotice on the curve that there is a slight drop in horsepowernear the right hand end. f circumstances were such that the operating pointcorresponded to a downward sloping power curve! recycle control would e moreefficient. This is rare.

    !P**$ CO&TRO', There is!of course! one other means of

    adapting a pump to thechanging demands of theprocess< Speed control. Thevirtue of this method is that itreduces the energy input tothe system instead ofdumping the e+cess. 7igure 1-I shows a system curvesuperimposed on a family ofcurves for a variale speedpump. The curves reach all

    parts of the system curveelow the full speed curve.Therefore this is an effectivemeans of control. Gote!

    however! that these curves have one feature in common with recycle control< ,tthe far left end of the system curve the pump curve and the system curve arealmost parallel. 5The particular pump chosen for this e+ample has a rathersteeply rising curve near shutoff. 0ost are consideraly flatter.6 n mathematicalterms this means that the intersection is poorly defined. n practical terms thismeans that it is difficult to maintain a precise operating point and that control is;loose; at high turndown.

    n practice! variale speed drives for centrifugal pumps are still relativelyuncommon. 7or small pumps the power savings are not significant and for largepumps the associated electronics ecome very e+pensive. ,lso! they do not havethe high reliaility of valves. Variale speed steam turine drives are (uitecommon in the larger horsepower ranges. 3lectric variale speed drives are usedin certain speciali#ed applications such as pumps that are emedded inside ahigh pressure vessel. n such cases there are no alternatives.

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    R($(&) O& T* CURV*, )ast ut not least< Go control at allD The fact is that themaFority of pumps in the world run with no control at all. The e+act flows andpressures are not critical and the pump has een reasonaly well selected. Thedischarge pressure will rise to partially compensate for increased ack pressure.t falls as the ack pressure decreases so that the flow does not increase as

    much as it otherwise might. The pump is allowed to Bride on its curveB. When thissituation is acceptale! leave well enough alone and don;t try to fi+ what ain;troke. 5*e careful though! the machine may still re(uire minimum flow and otherprotections as detailed in the section on 0achine rotection.6

    +*A!UR*+*&T, The appropriate measurement for the controller depends onthe demands of the process. 7low control is a fre(uent re(uirement. Two rulesguide the location of the flow measurement< 0ake sure that side streams areincluded or not! as re(uired! y the measurement and make the measurement atthe highest convenient pressure. The latter re(uirement is to avoid any possiilityof flashing or cavitation within the measuring device. n general the est place to

    measure flow from a centrifugal pump is etween the recycle Tee and thedischarge throttling valve. The e+ception is when the discharge is at an e+tremelyhigh pressure and the suction has ade(uate GS?,. n that case a suctionmeasurement may e est.

    )evel control of a vessel is one of the most common re(uirements 1. The vesselmay e either upstream or downstream. t is (uite possile to connect the )evelController directly to the discharge valve. 7re(uently! however! the vessel servesto uffer a downstream process from upstream flow variations. n that case it isnot desirale for level control to e precise. erfect level control implies that theflow out is e+actly e(ual to flow in at all times. $ften it is desired that the

    downstream flow remain as uniform as possile while keeping the level withinounds. n simple terms! it is desired that the flow out is the average of the flowin. The vessel asors the instantaneous differences. This simple re(uirement ismore difficult to accomplish than it may seem and deserves a discussion entirelyof its own. , simple arrangement that is often satisfactory and is widely used is tohave the )evel Controller cascade to a 7low Controller on the pump discharge.The flow loop keeps the discharge ;constant; while the )evel Controller graduallyraises or lowers the setpoint as the level in the vessel rises or falls.

    ,nother common re(uirement is to control the pressure of either upstream ordownstream e(uipment. The tap for the pressure transmitter should econnected at the point where it is desired to control the pressure. Gote that apressure tap etween the pump and a discharge throttling valve is proalymeaningless. , careful look at many pump curves will show that thecharacteristic near shutoff is (uite flat and may even slope downward. ressurecontrol cannot e accomplished when the pressure curve is flat. f the slope isthe ;wrong; way! control will work ackwards and drive the valve away from theset point. n this case the minimum flow should e set so that the pump cannotoperate in the positive slope region of the curve. 5t is! of course! possile to

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    reverse the action of the controller so that it can operate to the left of the peak.*ut in that case! what will happen if the operating point moves to the right@ t ise+tremely difficult to design control systems that can operate continuously alonga characteristic curve that has a local minimum or a ma+imum in it.6

    There is a second! more serious! prolem with pressure control. Centrifugalpumps are essentially constant head machines. The discharge pressure for agiven pump rotating at a fi+ed speed is proportional to the density of the li(uid.This means that if the li(uid has a constant density! the discharge pressure isconstant. The BcurveB of the pump curve is produced y losses and other affectscaused y flow. Unless there is a flow through the system! there is only onepressure and that is the shutoff pressure. f it is desired to control the pressure ofa vessel eing charged y a pump! it is est to pressure control a valve at theoutlet of the vessel and let the pump ride on its curve. f the vessel must e deadended! only recycle flow at the pump can control pressure to a setpoint.

    O& / O%% CO&TRO', $noffcontrol is used in manysituations where the oFect issimply to move a li(uid frompoint , to point * and thee+act pressure or flow rate isunimportant. , typicale+ample is the sump pump.The simplest arrangementemploys a level switch with avery road deadand. This is

    used together with a?and$ff,uto switch to turnthe pump on and off. The schematic is shown in 7igure 1-J. The )S?) contactopens when the level is elow its setpoints. B0B represents the motor contactorwhich energi#es the motor whenever the contactor is energi#ed. B0B alsorepresents the au+iliary contact that is closed whenever the contactor isenergi#ed.

    f it is important that the levelnever goes eyond the upperor lower setpoints! theStartStop arrangement ispreferred. t is illustrated in7igure 1-K. The processsensing switch has aseparate output for the uppersetpoint 5$n6 and the lowersetpoint 5$ff6. 5Two switchesmay e re(uired.6 The

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    manual switch consists of a Start and a Stop utton or a cominedStartHunStop selector with a spring return to centre. The operator may start orstop the pump whenever the level is etween the two setpoints. ?e cannot stop itwhen the level e+ceeds the high setpoint unless he locks it out. ?e cannot startthe pump elow the low setpoint. , variation of the circuit places the left

    connection of the start utton to the left of the low level switch. With thisarrangement it is possile to drain a vessel elow the low set point y holding thestart utton on. The pump will stop as soon as the utton is released.

    With oth of these arrangements! there must e sufficient deadand etween thehigh and low setpoints to make certain that the pump does not cycle on and offtoo rapidly. 3+cessive wear of oth the motor and its starter will result if thisoccurs. Hapid cycling is a sign of an over-si#ed pump.

    +AC(&* PROT*CT(O&, $nce the process re(uirements have een met! theattention of the process control engineer turns to protecting the e(uipment.

    Centrifugal pumps are fairly undemanding. n general they have only twore(uirements< that the GS?H is met at all times and that a certain minimum flowis maintained. To meet the first re(uirement is generally a piping design prolem.n cases of dout! a low pressure shutdown switch may e added to the suctionline. , second look at the e+planations of GS?! aove! shows that determiningthe setpoint of the switch is not necessarily a simple matter if there is anypossiility of the li(uid density changing. Things get even more complicated if thevapour pressure is very sensitive to temperature. , rise in temperature thatcauses the li(uid to oil will cause the net positive pressure to fall to #ero eventhough there is an increase in actual pressure. ) and )G pumps arenotorious for GS?, prolems. 7ortunately most pumps can tolerate rief

    periods of cavitation without noticeale damage.

    When a pump is taking suction from a vessel! a low level shutdown switch isessential. The switch! or transmitter! must e separate from any level controldevices.

    To meet the secondre(uirement! minimum flow! issomewhat more difficult. ,centrifugal pump adds energyto the li(uid that the movingli(uid carries away. f flow islocked! the temperature withinthe pump will rise steadily untilthe li(uid oils 5net positivepressure is now #ero6. /amageto the pump is (uite likely. 7orthis reason some form ofminimum flow is almost always

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    included on larger machines. The simplest arrangement is a fi+ed restrictionorifice on a line leading ack to the supply side of the pump. The preferreddestination of the recycle flow is ack to the vessel from which it came. Thisallows the heat to dissipate efore it is recycled ack into the machine.Hestriction orifices have two drawacks< They waste energy when the process

    demand is sufficiently high to meet all minimum flow re(uirements and also theylimit the ma+imum pump output.

    , more efficient method of recycle control re(uires that the discharge flow of thepump itself is measured! and that a valve in the recycle line is opened when theprocess does not draw the re(uired minimum flow. The most straightforward wayto accomplish this is shown in 7igure 1-9. Gote that the recycle line tees offupstream of the control valve. t is precisely when the control valve is closed thatthe recycle is needed. There is a small prolem with controlling the minimum flowin this way< The measurement orifice in the discharge consumes energy and alsoslightly reduces pump capacity. , second prolem is that the actual signal eing

    measured is the

    across the orifice plate. Since flow varies as the s(uare rootof ! a minimum flow of &4L of ma+imum flow implies a controller whose set

    point is only 1IL of the measurement range. , typical instrument accuracy is1L. Therefore an error of JL of the setpoint can e e+pected. 7ortunately theminimum flow need not e held very accurately. Hecycle control is sometimesaccomplished using a local pneumatic controller mounted directly on the valve.Gote< Alwasuse a fail-open valve.

    Various schemes have een devised to infer the re(uired valve setting from thenet discharge flow measurement. These re(uire the flow downstream of therecycle Tee to e sutracted from the re(uired minimum flow. The recycle valve

    is then opened in proportion to the difference! if it is positive. To do this accuratelyone must know the valve and actuator characteristics. There is no feedack toconfirm that the correct flow is occurring. Since the flow is usually aove theminimum flow! the valve is usually closed. This will cause the controller to windup and e slow in responding when a low flow condition suddenly arises.7ortunately pumps can tolerate short periods of low flow so this is not a prolem.

    $ne method of minimum flow control that is occasionally proposed is to put aflow control loop on the recycle line with the set point e(ual to the minimum flow.This solution is worse than a fi+ed restriction. When discharge flow is high! thedischarge pressure falls. 7low through a fi+ed orifice will reduce somewhat. ,

    flow control loop will open the valve further to maintain constant flow preciselywhen it is not needed. ,t this point the operator will e tempted to manually closethe valve. Then! when a discharge lockage occurs! there will e no minimumflow at allD

    There are a numer of devices availale! called ,utomatic Hecirculation Valves!or ,HC valves! that comine the functions of net discharge measurement!recycle control! recycle valve and discharge check valve all in one device. These

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    devices can e very effective ut they suffer from one drawack< lack of fle+iility.n cases where the pump and process characteristics are well known! they cane an ideal solution. ipelines! for e+ample! have many identical pumpsoperating under steady conditions. $nce the correct components are known!application is routine. t must e kept in mind! however! that oth the process and

    pump data provided to the controls engineer for a new facility are often tentative.,HC valves have very little margin for error when the reality turns out differentlyfrom the theory. $ne particular prolem that can occur with the older style ,HCvalves that operate in an openclose mode! and even with some that modulate! isinstaility. t occurs as follows