Valve Flow Characteristics

The relationship between valve capacity and valve travel. It is usually expressed graphically in the form of a curve. Control valves have two types of characteristics inherent and installed.

The inherent characteristic is derived from testing the valve with water as the fluid and a constant pressure drop across the valve.

When valves are installed into a system with pumps, pipes and fittings, the pressure dropped across the valve will vary with the travel. When the actual flow in a system is plotted against valve opening, the curve is known as the installed flow characteristic.

Valves can be characterised by shaping the plugs, orifices, or cages to produce a particular curve.

Valves are characterised in order to try to alter the valve gain. Valve gain is the flow change divided by the control signal change. This is done in an effort to compensate for non-linearities in the control loop.

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The flow characteristics only apply when:

  • the pressure drop remains constant with the flow
  • there is no onset of flashing, cavitation or sonic flow
  • the valve is not installed between reducers
  • the flow curves normally have a tolerance band of +/-5%
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