Control valve characteristic guidelines

09/01/1999



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




Comments?
E-mail dharrold@cahners.com





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