GLOSSARY
OF TERMS

An abbreviation for current-to-pneumatic signal conversion. This term is commonly used to describe a type of transducer that converts an electric (4-20m.a) input signal to a pneumatic (3-15 psig.) output signal.

A term used to describe the early stages of cavitation. At this point the bubbles are small and the noise is more of a hiss, like the sound of frying bacon. There is normally no mechanical damage associated with incipient cavitation although it could have an effect on the corrosive properties of some fluids.

A fluid such as water, which has no significant change in volume and density as the pressure increases.

The high and low values of pressure applied to the diaphragm to produce rated valve plug travel with atmospheric pressure in the valve body. 

The relationship between valve capacity and valve travel – usually expressed graphically. It is derived from testing a valve with water as the fluid and with a constant pressure drop across the valve. The most common types of inherent flow characteristics are linear, equal percentage, modified parabolic, and quick opening.

The flow characteristic when the pressure drop across the valve varies with flow and related conditions in the system in which the valve is installed. The purpose of characterising a control valve is to help compensate for nonlinearities in the control loop.

A valve body whose flange connection is an integral or cast part of the body. Valves with integral flanges were traditionally known to have the ANSI short face-to-face dimension ANSI/ISA S75.03. However, many manufacturers now produce valve bodies with both integral and separable flanges that will meet both the ANSI short and long face-to-face dimensions.

The flow control orifice and seat that is an integral part of the valve body or cage. The seat is machined directly out of the valve body and is normally not replaceable without replacing the body itself – although some can be repaired by welding and re-machining.