Control Valves Characteristics

7/23/2019 Control Valves Characteristics

1/24

Control Valves and FlowControl Valves and FlowCharacteristicsCharacteristics

T-ICB INSTRUMENTATIONTT--ICB INSTRUMENTATIONICB INSTRUMENTATION

The relationship between controlThe relationship between controlvalve capacity and stem travelingvalve capacity and stem traveling


2/24

The relationship between control valvecapacity and valve stem travel is known

as

THE FLOW CHARACTERISTICTHE FLOW CHARACTERISTIC

OF THEOF THECONTROL VALVECONTROL VALVE



3/24

Trim design of the valve affects how the controlvalve capacity changes as the valve moves throughits complete travel. Because of the variation in trimdesign, many valves are not linear in nature.

Valve trims are instead designed, or characterized,in order to meet the large variety of controlapplication needs.

Many control loops have inherent non linearity's,which may be possible to compensate selecting theControl Valve trim.

Trim design of the valve affects how the controlvalve capacity changes as the valve moves throughits complete travel. Because of the variation in trimdesign, many valves are not linear in nature.

Valve trims are instead designed, or characterized,in order to meet the large variety of controlapplication needs.

Many control loops have inherent non linearity's,which may be possible to compensate selecting theControl Valve trim.



4/24



5/24


Inherent Control ValveInherent Control ValveFlow CharacteristicsFlow Characteristics


6/24

The most common characteristics areshown in the figure above. The percent offlow through the valve is plotted against

valve stem position.

The curves shown are typical of thoseavailable from valve manufacturers. Thesecurves are based on constant pressuredrop across the valve and are calledInherent Flow Characteristics



7/24

Linear flow capacity increases linearlywith valve travel.

Linear flow capacity increases linearlywith valve travel.

Equal percentage

Flow capacity increases exponentially with valve trimtravel. Equal increments of valve travel produce equalpercentage changes in the existing CV.

A modified parabolic characteristic is approximatelymidway between linear and equal-percentage

characteristics. It provides fine throttling at low flowcapacity and approximately linear characteristics athigher flow capacity.



8/24

Quick opening provides large changes in flowfor very small changes in lift. It usually has toohigh a valve gain for use in modulating control.

So it is limited to on-off service, such assequential operation in either batch or semi-continuous processes.

Quick openingQuick opening provides large changes in flowprovides large changes in flowfor very small changes in lift. It usually has toofor very small changes in lift. It usually has toohigh a valve gain for use in modulating control.high a valve gain for use in modulating control.

So it is limited to onSo it is limited to on--off service, such asoff service, such assequential operation in either batch or semisequential operation in either batch or semi--continuous processes.continuous processes.



9/24

The majority of control applications are

valves with linear, equal-percentage, or

modified-flow characteristics

The majority of control applications areThe majority of control applications are

valves with linear, equalvalves with linear, equal--percentage, orpercentage, or

modifiedmodified--flow characteristicsflow characteristics



10/24

Installed Control Valve Flow CharacteristicsInstalled Control Valve Flow Characteristics

When valves are installed with pumps, piping and fittings, andother process equipment, the pressure drop across the valve will

vary as the plug moves through its travel.

When the actual flow in a system is plotted against valve opening,the curve is called the INSTALLED FLOW CHARACTERISTIC

In most applications, when the valve opens, and the resistance due tofluids flow decreases the pressure drop across the valve. This moves

the INHERENT CHARACTERISTIC:A linear inherent curve will in general resemble a quick openingcharacteristic.An equal percentage curve will in general resemble a linear curve



11/24

Control Valves and CavitationControl Valves andControl Valves and CavitationCavitation



12/24

When a fluid passes a valve the fluid velocity

increases and the pressure drops according the

Bernoulli Equation

CAVITATION:CAVITATION:

If the speed through the VALVE is high enough, the pressure in the liquid

may drop to a level where the fluid may start bubble or flash. Thepressure recovers sufficiently and the bubbles collapse upon themselves.



13/24



14/24



15/24



16/24



17/24



18/24



19/24



20/24



21/24



22/24



23/24



24/24


Documents

Control Valves Characteristics