Control Valve Fundamentals
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Transcript of Control Valve Fundamentals
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SAMSONControl valves Fundamentals
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IEC 534 Part 1 defines a control valve as
" A power-operated device forming a final element inan industrial process control system. It consists of abody subassembly containing internal means forchanging the flow rate of the process fluid. The bodysubassembly is linked to one or more actuators whichrespond to a signal transmitted from a controlling
element. "
Automatic operationThe control valve is operated by means of ananalog or digital controller or a PC-based processcontrol system.
Manual operation
The control valve is operated using a remoteadjuster or a handwheel
Control valves are operated continuously and, ifneed be, opened or closed by a higher-level safetycontrol system.
Control valves
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Control valves
The control valve ...
is part of the automation system. is a component of the processing plant. is subject to design and operation regulations
for pipelines and pressure vessels. must show low-maintenance features since the
points of installation are distributed all over theprocessing plant and often difficult to access.
The valve is used in all kinds of processingplants and, hence, comes into contact with alltypes of matter, such as aggressive gases,vapors and liquids under varying operating
conditions.
Environmental considerations as well as thecustomers demand low noise emission andvery often tight shut-off of the process medium.
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Control valves
The control valves used for fluids are
linear valves as well as valves with rotatingclosure members, such as butterfly, ball androtary plug valves.
The control valves used in processing plantsare mostly linear valves.
Control valves and shut-off valves haveuniform installation dimensions. This allowsvalves from different vendors and in differentversions to be exchanged easily. The generaldesignations comply with international standards.
Control valves largely comply with DIN 2401and DIN 2402, hence, with the basic pipelinestandards.
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Valve styles
Globe valves
Three-way valvesas mixing valve or
diverting valve
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Valve styles
Butterfly valves
Rotary plug valves
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Direction of flow
Inlinear valves,the direction of flowalways opposes theclosing direction ofthe plug
If thedirection of flowis the same as theclosing direction ofthe valve, instabilitiesare caused whencontrolling around the
closing point.
A suction forms at theplug trying to pull itinto the seat.
Closing direction
Direction of flow
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Direction of flow
In three-way valves,the direction of flowalways opposes theclosing direction of
the plug.
It must be taken intoaccount whether thethree-way valve isused for mixing or
diverting service.
Closing direction
Direction of flow
Diverting valve
Closing direction
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Technical data
Apart from the valve style, the following technical
data are important. They always have to beindicated in the order:
Nominal size DN and type of connection(valves with screwed ends, valves with flanges).
Nominal pressure PN resulting from theoperating pressure and the operatingtemperature.
Permissible differential pressure Dp
Flow coefficient: Kvs
Control valve characteristics(abbreviations:GN = equal percentageLN = linear)
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Nominal size [DN]
Thenominal size DN(DN 15, 20, 25, 32,40, 50, 65, 80...) isusually identical
with the size of thepipe.
One nominal size isassigned to severalKvs values.
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Nominal pressure [PN]
Nominal pressuresPN16, PN25, PN40,PN65, PN100 ...are stated in gauge
pressure (bar or pa)1 bar = 100 000 pa
The nominal pressureof cast ironcorresponds to theprevailing operating
pressure fortemperatures up to120 C.
Material: cast iron
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Nominal pressure [PN]
Note:Different materialsproduce differentcharacteristics at
operatingtemperature!
Material: spheroidal graphite iron
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Pressure ratios within the system
An easy example
The heating system inwhich the pressure
prevails includes:
Heat generator Pump Valve Heat consumer
Pump
Heat generator
Valve
Consumer
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Pressure development in apipeline
PipelineThree different valve positions
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Reductions in cross section
Cross-sectional reductionslead to a higher velocityof the medium ...
across pipe reductions
across throttling areas
w1, w2velocities
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Stream profiles
Small cross section=
Large difference in
velocity
Large cross section=
Small difference in
velocity
Flow ...
across large crosssections causes low
internal friction.
across small crosssections causes highinternal friction.
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Flow rate depending on Dp
Tank filled with a liquidthat can flow outthrough a hole:
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Flow equation
In practice, the measurable flow across a control
valve turns out to be slightly smallerthan thetheoretical value calculated by means of apreviously derived equation.This is caused by the following:
Style of the control valve
Ratio of opening between the control valve inletand outlet (with reducers and increasers)
Viscosity of the medium(-> Reynolds number)
Amount of pressure recovery
Expansion of compressible media
Flow limitation of liquids
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To be able to compare different types of control
valves, a uniform procedure for measuring the flowrate (IEC 534, Part 4) has been agreed upon.
The Kv (Kvs) value of a control valve considers theinfluence factors mentioned above and is determinedby:
Water as medium Density of 1000 kg/m3 Temperature of 5 to 50 C Differential pressure of 1 bar
The Kvs value is a value which results from theoperating conditions stated above when the valvehas reached the rated travel (=100%).
Definition of Kv value
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Pressure development in thevalve
P1 Upstream pressurep2 Downstream pressurepvc Lowest static pressure
in the valvepv Saturated steam
pressure of themedium
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Types of flow
Laminar flow
No exchange betweenthe molecules of aliquid in the flow
Turbulent flow
Immediate exchangebetween themolecules of aliquid in the flow
laminar
turbulent
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Working equations
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Details from a specification sheet
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Valve characteristics
Equal percentageOften logarithmicrepresentation
Linear
Plottedrangeability:
Smallest flow rateto be controlled
:Largest flow rate tobe controlled
50 : 1