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INSTRUMENTATION DEPARTMENT
FINAL CONTROL ELEMENTS
Final control elements are generally in-line deviceswhich receive the Controller O/P and accordinglyregulate the process flow or pressure to control theinput variable.
Types:
Con trol Valvesmost common type, available in
various types, sizes and materials to cover almost
all applicationsRegulatorsused only for application where very
precise control is not required.
Dampersused only in large sized ducts.
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INSTRUMENTATION DEPARTMENT
Control Valves
Numerous types of control valves some of general design ,others designed for specific application are available. Some ofthe General type of Control valves categorized based on theirtype of construction are
* Globe type
including Angle, Three way, Ytype etc.
* Ball type
* Butterfly type
* Plug type
Each of the above have features which make them suitable forfor one particular application or the other in addition differentvariants are available in each for specific applications
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INSTRUMENTATION DEPARTMENT
Control Valve Construction
Irrespective of the type the control valve assembly can be divided into
the following sub-parts* Body
* Trim or Internals
* Bonnet
* Actuator
* Accessories
Body: Body is defined as the valve housing which is mounted in lineand can be with Flanged, welded, screwed or wafer type endconnections. Generally for low to moderately high pressure ratingsFlanged end connection are used for extremely high pressure ratings
welded end connection is used. For low pressure utility servicesscrewed end connection may also be used whereas butterfly valves aregenerally available in wafer type body which is mounted sandwitchedbetween the piping flanges.
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INSTRUMENTATION DEPARTMENT
Trim or Internals
Valve trim is the heart of any control valve design, itnot only characterizes the valve behavior but alsodefines the no and type of ports, valve capacity,Pressure drop, flow profile, leakage, noise, and
other valve parameters. A valve trim comprises offollowing parts -
* Plug
* stem
* Seat
* seat ring
* Guiding
* Bushing
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INSTRUMENTATION DEPARTMENT
Bonnet
Valve bonnet, houses the plug stem and providesrigidity to valve trim and actuator connection andsupports actuator housing. Bonnet also houses thevarious packing, seals etc for stopping stemleakage.
Bonnets can be:
Extended type for cryogenic services below
0 deg C
Finned type for high temperature services
Bellow seal type for toxic services
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INSTRUMENTATION DEPARTMENT
Actuators
Actuators provide the force required for valve movement andholding the valve in fully open or fully closed state against thefluid pressure.
Types : Based on the medium of operation actuators can beclassified in following categories
* Pneumatic
* hydraulic
* Electric
Most of the instrumentation application in hydrocarbon industryemploy Pneumatic actuators for control applications. Hydraulicactuators are generally used for large size valves operating atvery high differential pressure. Electrical actuators are used inMotor operated valves for mainly On/Off service.
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Pneumatic Actuators
Pneumatic actuators can be
Linear type Spring diaphragm , Piston
operated- Globe valves
Rotary type Cylinder with scotch yoke,
Rack and pinion, vane, Dual cylinder
operated
Ball,Butterfly,Quarter turnvalves
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INSTRUMENTATION DEPARTMENT
Failure position
Failure position is defined as the final resting
place of valve plug.Accordingly valves can
be:
FO- Fail open(Air to close)
FC- Fail close (Air to open)
FL- Fail lock (FLDC,FLDO)
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INSTRUMENTATION DEPARTMENT
NOISEAllowable noise limit in a control valve
90 Dba
Major sources of noise in control valves
are:
-Hydrodynamic noise
-Aerodynamic noise
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INSTRUMENTATION DEPARTMENT
Hydrodynamic noise is associated with cavitation
and flashing in liquids .
Aerodynamic noise is due to high velocity of gasesand vapors passing through valve orifice.
Control valve noise treatment includes-Treatment of the noise source(revising
the valve)
Treatment of noise path (providing
insulation or silencers)
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INSTRUMENTATION DEPARTMENT
Control valve accessories
Various accessories available for control valves are
Air accumulators
Air filter regulators
Hand wheels
I/P converters
Limit stops
Limit switches
Positioner
Relays ( Booster, lock-up, reversing etc.) Solenoid valves
Stem position transmitters.
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INSTRUMENTATION DEPARTMENT
Valve Positioners
Valve positioners are high gain proportional controllers whichmeasure the actual valve stem position ( through mechanicallinkage, compares the same with the pneumatic signal I/P fromI/P converter and produce a O/P to reduce this error.
Thus a positioner acts as a slave controller under the mainprocess controller which ensures that the variations due toaging factors of the valve and actuator ( change in packingfriction due to dirt or lack of lubricant, change in dynamicbehavior of valve and actuator, wearing out of linkages etc) arecorrected.
Positioner also allow for split ranging of control valves, and it isalways recommended to use positioner if the control valve isprovided with non spring return actuators.
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INSTRUMENTATION DEPARTMENT
Control Valve characteristics
Control valve characteristics are governed
by the type of trim used. Various type of
trims used are
* Linear trim
* Quick opening trim.
* Equal percentage trim
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INSTRUMENTATION DEPARTMENT
CONTROL VALVE SIZING & SELECTION
Selection of Control valve type :
Control Valve selection depends on the following
parameters :
Type of service
Capacity
Body ratingPressure drop
Operating temperature
Leakage rate required.
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INSTRUMENTATION DEPARTMENT
CONTROL VALVE SIZING & SELECTION
Factors determining control valve size :
* Fluid type
* Flow rate* Pressure Drop
* Fluid properties Sp gravity, Viscosity,Critical pressure, Vapor pressure,
Molecular weight, etc.* Line size and schedule.
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INSTRUMENTATION DEPARTMENT
CONTROL VALVE SIZING & SELECTION
Control valve sizing consists of determining thevalve type & size for the given process condition,checking for valve noise, inlet outlet velocities,cavitation etc.
The earlier sizing program had sizing in two steps,step 1 involves calculation of approx. CVs (Valvedischarge coefficients). Based on these Cv value thevalve port size and corresponding coefficients are
selected. This is done because for a given port sizeThe valve capacities are different for different makesof valves.
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INSTRUMENTATION DEPARTMENT
OVERVIEW OF NEW SIZING PROGRAM
The program is split into three modules:
Selection of type of control valve
Material selection of the valve
Sizing of the control valve
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INSTRUMENTATION DEPARTMENT
Selection of type of control valve
This is a preliminary selection.The
process data is put into this module of thesoftware which helps in the selection of
type of valve.
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INSTRUMENTATION DEPARTMENT
Material selection
We move into the second module of the
software where material selection of body,
bonnet, seating, packing and trim is done
based on data input.
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INSTRUMENTATION DEPARTMENT
Sizing of control valve
In this module, sizing is done based on
type of fluid.Calculations are done
accordingly for each type of fluid.Iterationtakes place within the module itself.
The selection of the valve is also
dependent on the sizing module, in caseof conditions where we cannot go for the
selected valve because the size of valve or
actuator is not suitable practically
.
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INSTRUMENTATION DEPARTMENT
THANK YOU
