basic control valve and sizing and selection

7/23/2019 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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DeZurikOperation

October 2005

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

October 2005

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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DeZurikOperation

October 2005

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

October 2005

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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DeZurikOperation

October 2005

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

October 2005

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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DeZurikOperation

October 2005

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

October 2005

lobeValvesB Way conigurations or

Ci-ing and diverting!

"emperature control

#lending


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DeZurikOperation

October 2005

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

October 2005

6otar/ ControlValves


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DeZurikOperation

October 2005

6otar/ControlValves

4esign +onstruction

8@SI IS8 %ace to %ace


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DeZurikOperation

October 2005

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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DeZurikOperation

October 2005

>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

basic control valve and sizing and selection

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