Download - Basic Control Valve and Sizing and Selection
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Basic Control Valve
Sizing andSelection
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DeZurikOperation
October 2005
Valve Sizing
What is Valve Sizing?
It is a procedure by which the dynamics o a process system
are matched to the perormance characteristics o a valve!
"his is to provide a control valve that will best meet the needs
o managing low within that process system!
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Flow Coefcient (CV)
The valve ow coefcient, CVis the number o
US gallons !er minute o water at "# $egreesF which will !ass through a given ow
restriction with a !ressure $ro! o % !siFor e&am!le, a control valve which has a ow
coefcient, or CV,o %' has an eective !ort
area that it !asses %' gallons !er minute o
water with % !si !ressure $ro!
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ValveSizing#asic inormation re$uirements or eective valve sizing%or the system&
'ressure beore and ater the control valve( P
%low rate( $uantity and units( Q'rocess temperature with units( T
'roperties o the media( )viscous( iber suspension( gaseous
its vapor pressure( sometimes molecular weight*
%or the control valve&
%low capacity )+v*( inherent throttling curve(,c )+avitation Inde-*( %./ )+ritical %low actor*
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elta * (*)* Sizing is the !ressure $ro! across the valve
use$ or control valve sizing at a s!eci+c ow rate
ost a!!lications have multi!le ows or sizingwith $ierent -* or each
.n a given s/stem, higher ow rates generall/result in lower !ressure $ro! across the valve
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Flow CharacteristicsControl valve ow characteristics are $etermine$ !rinci!all/
b/ the $esign o the valve trim
The three basic ow characteristics available are0Quick Opening Linear Equal Percentage (=!
1 mo$i+e$ characteristic (sometimes calle$ mo$i+e$!ercentage) generall/ alling between the linear an$ e2ual!ercentage characteristics ma/ also be available
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3h/ Use 42ual *ercentage
*rovi$es e2ual!ercentage increases inrate o ow or e2ual
increments o !lugmovement
*rovi$es the best choice
o ow characteristic ormost s/stems
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DeZurikOperation
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Valve Sizing 5CavitationWhat is cavitation and what does it do to valves?
+avitation is a 2stage activity where a portion o the li$uid
media drops below vapor pressure! "his part will boil )vaporize*!
In stage two( slightly downstream( pressure recovery ta1es place
and the vapor bubbles collapse!
"he condition 1nown as critical low3 is an e-tension o cavitation
in that it simply gets worse as the pressure drop increases! "his is
to the point that changes )reductions* in downstream pressureno longer inluence low rate!
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Flui$ 6ecover/ Factor (F7)
3hen a ui$ !asses through the valveori+ce, the velocit/ increases Thisvelocit/ increase is accom!anie$ b/ a!ro!ortional $ecrease in !ressureVelocit/ reaches a ma&imum an$!ressure a minimum at the smallestcross sectional ow area $ownstreamo the ori+ce (the vena contracta)
4ownstream o the vena contracta the luid decelerates andconse$uently the pressure increases or recovers )giving us the termpressure recovery*! 4ierent valve types e-hibit dierent recovery
actors which becomes an important consideration in valve selection!
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Flui$ 6ecover/ Factor (F7)
The ui$ recover/ actor
(F7) is eectivel/ an in$e&
o !ressure recover/ in a
control valve 8igh F7values in$icate low
!ressure recover/ where a
low F7value is an
in$ication o high !ressure
recover/ 8igher F7values result in better
resistance to cavitation
%.)'6'2*7)'6 'V+*
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Valve sizing 5 Cavitation
Colla!se o the bubbles create$ b/ cavitation !ro$uce noise,li9e gravel in the line
:ubbles that colla!se in contact with the valve or !i!e create
$amage
Cavitation can be controlle$ with s!ecial valve trims in somet/!es o valves
Cavitation shoul$ be avoi$e$ in all circumstances
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DeZurikOperation
October 2005
ValveSizingWhat is %lashing3?It is a condition where the downstream pressure is below the
vapor pressure o the incoming luid and allows some o the
li$uid to become vapor!
%lashing does not create noise or damage in the valve as with
cavitation! It can( however( create damage to the downstream
piping due to high velocity!
8 control valve will unction $uit well under these circumstances! 9es:
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ValveSizing+haracteristicsSystems driven by centriugal pumps lose pressure two ways
8s the low increases the pump pressure declines
8s the low increases pipeline riction losses increase
;sing a valve with e$ual percentage or parabolic characteristics
will produce an installed characteristic that is more linear!
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.nherent vs .nstalle$
Characteristics"n#erentow characteristic is $e+ne$ as the
relationshi! between ow an$ valve stro9e at constant!ressure $ro! .t is uni2ue to valve construction an$$e!en$s !rimaril/ on geometr/ o the throttling trim an$
bo$/ ow !assages an$ is $etermine$ b/ ow testingThe inherent ow characteristic changes when installe$
in a !i!ing s/stem The result is the installedowcharacteristic 1s s/stem !ressure $ro! absorbe$ b/ thecontrol valve is re$uce$, the greater the $eviation romthe inherent state
.$eall/ a control valve shoul$ be size$ to absorb %;
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DeZurikOperation
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Valve
SizingWhat is desired ?
8ter installation(
generally it should
be near linear!
Why?
"o provide a more
predictable low
change in response
to each incremental valve position change
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Control Valve Seat 7ea9age
.t is becoming more common to e&!ect a control valve to!rovi$e isolation $ut/, thus allowable lea9age becomesmore im!ortant
Control valve seat lea9age is $esignate$ b/ theclassi+cation o .S1=1>S.5?#5'
7ea9age classes are class .5V.
Classes .5V are measure$ using water, class V. is measure$using air
The most common class or metal seate$ control valves isclass .V (##%< o rate$ Cv)
Class V. generall/ a!!lies to resilient seate$ valves onl/
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Flui$ Velocit/4&cessive ui$ velocit/ can create erosion, an$
accelerate corrosion $amage .t also contributesto noise level an$ vibration
Valve users will generall/ $e+ne acce!tablevelocit/ in valves an$ !i!es or the above reasons7i2ui$ velocit/ o u! to %; t=sec is common@ orgas or steam A## t=secor higher, $e!en$ingu!on size, is !erectl/ acce!table
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>oise an$ Vibration
>oise results rom the conversion o the mechanical energ/ o theow into acoustic energ/ as the ui$ !asses through the valverestriction
Vibration is a $irect result o noise an$ can create issues with valve
lieBS81 has $esignate$ oise can be re$uce$ through two means, internal at the valve trim,an$ e&ternal such as !i!e insulation or $ownstream silencers
Cage gui$e$ globe valves oer the wi$est range o noise an$vibration attenuation solutions
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6angabilit/6angabilit/ is $e+ne$ as the ratio o minimum
to ma&imum controlla$leow rates
42ual !ercent characteristic oers the bestrangabilit/ o common t/!es
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Flow irection
an/ valve t/!es have a !reerre$ ow $irection tominimize the eects o velocit/, to create the tightest
shut5o, an$ to o!timize the res!onse to in!ut signal
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DeZurikOperation
October 2005
lobe Valves
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lobeValves>ow do they wor1? #y linear )stro1e* action
8 contoured plug is lited
out o a seat ring to allow
low to pass
In most )not all* cases
low comes in
under the seat3
+onversely( pushing the
plug down into the seat causes closure
%lowin
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DeZurikOperation
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lobeValves;nbalanced 7 #alanced
"rim
;nbalanced has system
pressure acting againstthe ull area o the seat
Single seated plug styles
are unbalanced
#alanced has provisions
or osetting systempressure
8vailable in double seated
plug or balanced cage styles
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lobeValves#odies #onnets +onigured as re$uired
to meet valve style and
8@SI +lass considerations
Aemovable bolted bonnets
retain and7or support the
trim components
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DeZurikOperation
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lobeValvesB Way conigurations or
Ci-ing and diverting!
"emperature control
#lending
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lobeValves"rim +omponents Stem( plug( seat(
cage )i that style*
guiding components Cade o alloy materials
Optional hardened seating
suraces when re$uired
"op guided style
+age guided style
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>oise an$ cavitation control
trims
#y directing low through a serieso staged drops( these trims
eliminate cavitation in li$uid low
and provides multiple pressure
brea1down or noise attenuation
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DeZurikOperation
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6otar/ ControlValves
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6otar/ControlValves
4esign +onstruction
8@SI IS8 %ace to %ace
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6otar/ Control Valves
V ported valves&
;ses a ball segment that is
8lways in contact with the seat!
;p to 600&6 rangability!
Eccentric type:
;ses a plug that cams away
rom the seat during opening
Ideal solution or erosive servicesSuch as slurries and steam!
8vailable with a range o low
capacities!
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>igh perormance
#utterly Valve
Ideal economic choice or
larger line sizes
50&6 Aangeability
Codiied e$ual percent low
+haracteristic
%ast accurate response to
+ontrol signal
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1ctuators
The actuator !rovi$es the muscle or the controlvalve an$ is res!onsible or moving the valvecontrol element to the re2uire$ stro9e !osition
S!ring $ia!hragm actuators are the most wi$el/recognize$ an$ use$ b/ control valve su!!liers6everse acting@ ail close$ an$ $irect acting@ ail o!enSim!listic $esign ,ew moving !arts, an$ eas/ to
maintain
ouble acting !istonsSmaller, lighter, an$ less e&!ensive than $ia!hragmFail in last !osition
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1ctuators
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1ccessories
*ositioners D !neumatic in!ut E5%; !si 4lectro5!neumatic A5'# ma
(816T, Fiel$bus, *ro+bus)
7imit switches
*osition ee$bac9
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6eview material
.S1 *ractical ui$e *ublications
Control Valves
anuacturers $ata an$ gui$es
1l 8olton
>or4ast Controls
alGallagashvalvecom
;#H5H"A5;IHA
mailto:[email protected]:[email protected] -
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DeZurikOperation
October 2005
6otar/ Control
Valves