Valve Terminology

ANSI Class

ANSI or American National Standards Institute coordinates standards development, publishes standards and operates a voluntary certification program. The ANSI class is the temperature and pressure rating of a valve.

Characterized Port

A valve port that modifies the flow characteristic for modulation; usually an equal percentage characteristic is produced.

Close-off Pressure

The maximum differential pressure that an actuator will hold a valve closed against allowing only the publish leakage rate to pass through. 


Flow coefficient. Flow rate in gallons of 60°F water that passes through a valve in one minute at a one pound pressure drop.

Differential Pressure

Differential Pressure is the difference between the pressures immediately upstream and downstream of a valve.

Direct Acting

Direct Acting is a valve that increases flow with an increase in signal.

Equal Percentage

In valves, this usually refers to a deep logarithmic response curve. Globe valves and characterized ball valves have equal percentage curves. Standard ball valves and butterfly valves have shallow logarithmic curves, not normally considered to be equal percentage.

Linear Characteristic

A valve response curve is linear if an equal amount of lift, rotation, or signal change always produces the same change in flow quantity.

Normally Closed (NC)

A valve that is closed when the signal to the actuator is low or when power is lost.

Normally Open (NO)

A valve that is open when the signal to the actuator is low or when power is lost.

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