Control Valve Fundamentals

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SAMSONControl valves Fundamentals


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2Control valves FundamentalsL301E

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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

Documents

Control Valve Fundamentals