Introduction to Pneumatic Control Valves.ppt

Introduction to Pneumatic Control Valve

1. Lecture: 38
PNEUMATIC CONTROL VALVE

2. WhatIsAControlValve?
Processplantsconsistofhundreds,oreven
thousands,ofcontrolloopsallnetworkedtogether
toproduceaproducttobeofferedforsale.
Eachofthesecontrolloopsisdesignedtokeep
someimportantprocessvariablesuchaspressure,
flow,level,temperature,etc.withinarequired
operatingrangetoensurethequalityoftheend
product.
Eachoftheseloopsreceivesandinternallycreates
disturbancesthatdetrimentallyaffecttheprocess
variable,andinteractionfromotherloopsinthe
networkprovidesdisturbancesthatinfluencethe
processvariable.
BASICS OF CONTROL VALVE

3. Toreducetheeffectoftheseloaddisturbances,
sensorsandtransmitterscollectinformationabout
theprocessvariableanditsrelationshiptosome
desiredsetpoint.
Acontrollerthenprocessesthisinformationand
decideswhatmustbedonetogettheprocess
variablebacktowhereitshouldbeafteraload
disturbanceoccurs.
Whenallthemeasuring,comparing,andcalculating
aredone,sometypeoffinalcontrolelementmust
implementthestrategyselectedbythecontroller.
BASICS OF CONTROL VALVE

4. Themostcommon finalcontrolelementinthe
processcontrolindustriesisthecontrolvalve.
Thecontrolvalvemanipulatesaflowingfluid,such
asgas,steam,water,orchemicalcompounds,to
compensatefortheloaddisturbanceandkeepthe
regulatedprocessvariableascloseaspossibleto
thedesiredsetpoint.
BASICS OF CONTROL VALVE

5. Manypeoplewhotalkaboutcontrolvalvesorvalves
arereallyreferringtoacontrolvalveassembly.
Thecontrolvalveassemblytypicallyconsistsof
thevalvebody,
theinternaltrimparts,
anactuatortoprovidethemotivepowertooperate
thevalve,
andavarietyofadditionalvalveaccessories,which
caninclude
positioners,transducers,supplypressureregulators,
manualoperators,snubbers,orlimitswitches.
BASICS OF CONTROL VALVE

6. Accessory:Adevicethatismountedontheactuatorto
complement theactuator’sfunctionandmakeita
completeoperatingunit.Examplesincludepositioners,
supplypressureregulators,solenoids,andlimitswitches.
Actuator:Apneumatic,hydraulic,orelectrically
powereddevicethatsuppliesforceandmotiontoopenor
closeavalve.
ActuatorAssembly:Anactuator,includingallthe
pertinentaccessoriesthatmakeitacompleteoperating
unit.
Capacity:Therateofflowthroughavalveunderstated
conditions.
DeadTime:Thetimeinterval(Td)inwhichnoresponse
ofthesystemisdetectedfollowingasmall(usually
0.25%-5%)stepinput.
TERMINOLOGY

7. Disk:Avalvetrimelementusedtomodulatetheflow
ratewitheitherlinearorrotarymotion.Canalsobe
referredtoasavalveplugorclosuremember.
Hysteresis:Themaximumdifferenceinoutputvaluefor
anysingleinputvalueduringacalibrationcycle,
excludingerrorsduetodeadband.
I/P:Shorthandforcurrent-to-pressure(I-to-P).Typically
appliedtoinputtransducermodules.
Linearity:Theclosenesstowhichacurverelatingtotwo
variablesapproximatesastraightline.
Positioner:Apositioncontroller(servomechanism)that
ismechanicallyconnectedtoamovingpartofafinal
controlelementoritsactuatorandthatautomatically
adjustsitsoutputtotheactuatortomaintainadesired
positioninproportiontotheinputsignal.
TERMINOLOGY

8. Relay:Adevicethatactsasapoweramplifier.Ittakes
anelectrical,pneumatic,ormechanicalinputsignaland
producesanoutputofalargevolumeflowofairor
hydraulicfluidtotheactuator.Therelaycanbean
internalcomponentofthepositioneroraseparatevalve
accessory.
Sensor:Adevicethatsensesthevalueoftheprocess
variableandprovidesacorrespondingoutputsignaltoa
transmitter.Thesensorcanbeanintegralpartofthe
transmitter,oritmaybeaseparatecomponent.
Sizing(Valve):Asystematicproceduredesignedto
ensurethecorrectvalvecapacityforasetofspecified
processconditions.
For further refer
FISHER’S CONTROL VALVE HANDBOOK (4
TH
ED)
TERMINOLOGY

9. MAJOR PARTS OF SLIDING -STEM
CONTROL VALVE ASSEMBLIES

10. MAJOR PARTS OF SLIDING -STEM
CONTROL VALVE ASSEMBLIES

11. MAJOR PARTS OF SLIDING -STEM
CONTROL VALVE ASSEMBLIES

12. TYPICAL REVERSE-ACTING
DIAPHRAGM ACTUATOR

13. TYPICAL DOUBLE-ACTING PISTON
ACTUATOR WITH BIAS SPRING

14. CHARACTERIZED CAGES FOR
GLOBE-STYLE VALVE BODIES
Cage:Apartofavalvetrimthatsurroundsthe
closurememberandcanprovideflowcharacterization
and/oraseatingsurface.Italsoprovidesstability,
guiding,balance,andalignment,andfacilitates
assemblyofotherpartsofthevalvetrim.Thewallsof
thecagecontainopeningsthatusuallydeterminethe
flowcharacteristicofthecontrolvalve.

15. OTHER STYLE VALVE BODIES

16. OTHER STYLE VALVE BODIES

17. ROTARY-SHAFT CONTROL VALVES

18. ACTUATOR
Pneumaticallyoperatedcontrolvalveactuatorsarethe
mostpopulartypeinuse,butelectric,hydraulic,and
manualactuatorsarealsowidelyused.
Thespring-and-diaphragmpneumaticactuatorismost
commonly specifiedduetoitsdependabilityand
simplicityofdesign.
Pneumaticallyoperatedpistonactuatorsprovidehigh
stemforceoutputfordemandingserviceconditions.
Electricandelectro-hydraulicactuatorsaremore
complex andmore expensivethanpneumatic
actuators.
Theyofferadvantageswherenoairsupplysourceis
available,wherelowambienttemperaturescould
freezecondensedwaterinpneumaticsupplylines,or
whereunusuallylargestemforcesareneeded.

19. ACTUATOR TYPES
Diaphragm Actuator

20. ACTUATOR TYPES
Diaphragm Actuator
Pneumaticallyoperateddiaphragmactuatorsuseair
supplyfromcontroller,positioner,orothersource.
Variousstylesinclude:
directacting(increasingairpressurepushesdown
diaphragmandextendsactuatorstem);
reverse-acting(increasingairpressurepushesup
diaphragmandretractsactuatorstem);
reversible(actuatorsthatcanbeassembledforeither
directorreverseaction);
direct-actingunitforrotaryvalves(increasingair
pressurepushesdownondiaphragm,whichmay
eitheropenorclosethevalve,dependingon
orientationoftheactuatorleveronthevalveshaft,).

21. ACTUATOR TYPES
Diaphragm Actuator Characteristics
Netoutputthrustisthedifferencebetween
diaphragmforceandopposingspringforce.
Molded diaphragms provide linear
performanceandincreasedtravels.
Outputthrustrequiredandsupplyair
pressureavailabledictatesize.
Diaphragm actuators are simple,
dependable,andeconomical.

22. ACTUATOR TYPES
Piston Actuator
oPistonactuatorsarepneumaticallyoperatedusinghigh-
pressureplantairto150psig,ofteneliminatingthe
needforsupplypressureregulator.
oPistonactuatorsfurnishmaximum thrustoutputand
faststrokingspeeds.
oPistonactuatorsaredoubleactingtogivemaximum
forceinbothdirections,orspringreturntoprovidefail-
openorfail-closedoperation(figure3-35).
oVariousaccessoriescanbeincorporatedtopositiona
double-actingpistonintheeventofsupplypressure
failure.Theseincludepneumatictripvalvesandlock-up
systems.
oAlsoavailablearehydraulicsnubbers,handwheels,and
unitswithoutyokes,whichcanbeusedtooperate
butterflyvalves,louvers,andsimilarindustrial
equipment.

23. ACTUATOR TYPES
Piston Actuator

24. ACTUATOR TYPES
Electrohydraulic Actuators
Electrohydraulicactuatorsrequireonlyelectricalpower
tothemotorandanelectricalinputsignalfromthe
controller(figure3-36).
Electrohydraulicactuatorsareidealforisolated
locationswherepneumaticsupplypressureisnot
availablebutwhereprecisecontrolofvalveplug
positionisneeded.
Unitsarenormallyreversiblebymakingminor
adjustmentsandmightbeself-contained,including
motor,pump,anddouble-actinghydraulicallyoperated
pistonwithinaweatherprooforexplosion-proofcasing.

25. ACTUATOR TYPES
Electrohydraulic Actuators

26. ACTUATOR TYPES
Manual Actuators
Manualactuatorsareusefulwhereautomaticcontrolis
notrequired,butwhereeaseofoperationandgood
manualcontrolisstillnecessary(figure3-37).
Theyareoftenusedtoactuatethebypassvalveina
three-valvebypasslooparoundcontrolvalvesfor
manualcontroloftheprocessduringmaintenanceor
shutdownoftheautomaticsystem.
Manualactuatorsareavailableinvarioussizesforboth
globe-stylevalvesandrotary-shaftvalves.
Dial-indicatingdevicesareavailableforsomemodelsto
permitaccuraterepositioningofthevalveplugordisk.
Manualactuatorsaremuchlessexpensivethan
automaticactuators.

27. ACTUATOR TYPES
Manual Actuators

28. ACTUATOR TYPES
Rack and Pinion Actuators
Rackandpiniondesigns
provideacompactand
economicalsolutionfor
rotaryshaftvalves(figure
3-38).
Becauseofbacklash,they
aretypicallyusedforon-
offapplicationsorwhere
processvariabilityisnota
concern.

29. ACTUATOR TYPES
Electric Actuators
Traditionalelectricactuatordesignsuseanelectric
motorandsomeformofgearreductiontomovethe
valve.
Throughadaptation,thesemechanisms havebeen
usedforcontinuouscontrolwithvaryingdegreesof
success.
Todate,electricactuatorshavebeenmuchmore
expensivethanpneumaticforthesameperformance
levels.
Thisisanareaofrapidtechnologicalchange,and
futuredesignsmaycauseashifttowardsgreateruse
ofelectricactuators.