Control valve characteristic guidelines

Liquid Level Control System

Control valve pressure drop
Best inherent characteristic

Constant delta pressure
Linear

Decreasing delta pressure with increasing load, delta pressure at maximum load > 20% of minimum load delta pressure.
Linear

Decreasing delta pressure with increasing load, delta pressure at maximum load & 20% of minimum load delta pressure.
Equal percentage

Increasing delta pressure with increasing load, delta pressure at maximum load & 200% of minimum load delta pressure.
Linear

Increasing delta pressure with increasing load, delta pressure at maximum load > 200% of minimum load delta pressure.
Quick opening

Source: Control Engineering with data from Fisher Controls

Flow Control Processes

Best inherent characteristic

Flow measurement signal to controller is:
Location of control valve in relation to measuring element is:
Wide range of flow setpoint
Small range of flow but large delta pressure change at valve with increasing load

Proportional to flow
In series
Linear
Equal percentage

In bypass *
Linear
Equal percentage

Proportional to flow squared
In series
Linear
Equal percentage

In bypass *
Equal percentage
Equal percentage

* When control valve closes, flow rate increases in measuring element.Source: Control Engineering with data from Fisher Controls

Pressure Control System

Application
Best inherent characteristic

Liquid process
Equal percentage

Gas process, small volume, less than 10 ft (3 m) of pipe between control valve and load.
Equal percentage

Gas process, large volume [process has a receiver, distribution system, or transmission line exceeding 100 ft (30.5 m) of nominal pipe volume], decreasing delta pressure with increasing load, delta pressure at maximum load >20% of minimum load delta pressure.
Linear

Gas process, large volume, decreasing delta pressure with increasing load, delta pressure at maximum load &20% of minimum load delta pressure.
Equal percentage

Source: Control Engineering with data from Fisher Controls

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