Valve characteristics selection guidelines

By Dave Harrold, CONTROL ENGINEERING February 1, 1999

C ontrol valves are called to handle all kinds of fluids at temperatures from the cryogenic range to well over 1,000F (538selecting a control valve to ensure satisfactory operation without undue initial expense.

Reputable control valve manufacturers are dedicated to helping customers select the control valve best suited for the existing service conditions. Frequently several correct choices may be available, thus it is important for customers to provide control valve manufacturers the following information.

  • Type of fluid to be controlled.

  • Temperature range of fluid.

  • Viscosity range of fluid.

  • Specific gravity range of fluid.

  • Minimum and maximum flow required.

  • Minimum and maximum inlet pressure at the control valve.

  • Minimum and maximum outlet pressure at the control valve.

  • Pressure drop across the valve expected during normal flowing conditions.

  • Pressure drop across the valve at zero flow.

  • Maximum permissible noise level, if pertinent, and the measurement reference point.

  • Degrees of superheat or existence of flashing across the valve, if known.

  • Inlet and outlet pipeline size and schedule of pipe.

When conducting an audit of existing processes, the control valve is already installed. The following guidelines can be useful in determining if the control valve installed, is suitable for the application.

Flow control processes
Flow measurement signal to controller Location of control valve in relation to measuring element Wide range of flow setpoint
Proportional to flow In series Linear
In bypass (see note) Linear
Proportional to flow squared In series Linear
In bypass (see note) Equal percentage
Small flow range with large changes in pressure drop across the valve In series
In bypass (see note)
Equal percentage characteristics should be used for applications with a small range of flow setpoint, large delta pressure at the valve, and increasing loads.
Note: When control valve closes, the flow rate as measured by the sensing element increases.

Pressure control processes
Liquid process Equal percentage
Gas process with small volume and less than 10 ft (3 m) of pipe between control valve and load valve. Equal percentage
Gas process with large volume (process has a receiver, distribution system, or transmission line exceeding 100 ft (30.5 m) of pipe). As load increases, pressure drop across the valve decreases; and the pressure drop across the valve at maximum load is >20% of the minimum load pressure difference. Linear
Gas process with large volume. As load increases, pressure drop across the valve decreases; and the pressure drop across the valve at maximum load is &20% of the minimum load pressure difference. Equal percentage

Level control processes
Pressure drop across valve increases with load. Linear
Pressure drop across valve increases greater than 2:1 with load. Quick opening
Pressure drop across valve decreases with increasing load. Linear
Pressure drop across valve decreases with increasing load and full load pressure drop is &20% of no-load drop. Equal percentage

The above information was collected from numerous sources including, Control Engineering, Fisher Controls Control Valve Handbook, and Techmation Applications Guide.

Comments?
E-mail dharrold@cahners.com