GLOSSARY
OF TERMS

Is the distance between the face of the inlet opening and the face of the outlet opening of a valve or fitting. These dimensions are governed by ANSI/ISA specifications.
The following uniform face-to-face dimensions apply.
SPECIFICATION VALVE TYPE
ANSI/ISA S75.03 INTEGRAL FLANGED GLOBE STYLE CONTROL VALVES
ANSI/ISA S75.04 FLANGELESS CONTROL VALVES
ANSI/ISA S75.20 SEPARABLE FLANGE GLOBE STYLE CONTROL VALVES

Or normally closed. Another way of describing an air-to-open actuator. Approximately 80% of all spring-return diaphragm operators in the field are of this construction.

A term used to describe the ability of an actuator to stay at the same percent of travel it was in when it lost its air supply. On spring return actuators this is accomplished by means of a lock-up valve. On piston actuators a series of compressed air cylinders must be employed.

Or normally open. Another way of describing an air-to-close actuator.

A term used to describe the desired failure position of a control valve. It could be fail-closed, fail-open, or fail-in-place.

The return signal that results from a measurement of the directly controlled variable. An example would be where a control valve is equipped with a positioner. The return signal is usually a mechanical indication of valve plug stem position which is fed back into the positioner.

See airset.

A valve that does not have integral line flanges. This type of valve is sometimes referred to as a Wafer Style valve. The valve is installed by bolting it between the companion flanges with a set of bolts or studs, called line bolting. Care should be taken that strain-hardened bolts and nuts are used in lieu of all-thread, which can stretch when subjected to temperature cycling.

Is the boiling or vaporising of a liquid. See the definition of cavitation. When the vapour pressure downstream of a control valve is less than the upstream vapour pressure, part of the liquid changes to a vapour and remains as a vapour unless the downstream pressure recovers significantly, in which case cavitation occurs. Flashing will normally cause a choked flow condition to occur. In addition, the vapour bubbles can also cause mechanical damage to the valve and piping system.

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 instaled. 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 instaled flow characteristic. Valves can be characterised by shaping the plugs, orifices or cages to produce a particular curve. Valves are characterised 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 nonlinearities in the control loop.

See the definition for Cv.