GLOSSARY
OF TERMS

Voltage to pneumatic converter.

For a diaphragm actuator, the effective area is that part of the diaphragm area that is effective in producing a stem force. Usually the effective area will change as the valve is stroked – being at a maximum at the start and a minimum at the end of the travel range. Flat sheet diaphragms are most affected by this; while moulded diaphragms will improve the actuator performance, and a rolling diaphragm will provide a constant stem force throughout the entire stroke of the valve.

Also known as an Electro-Mechanical Actuator uses an electrically operated motor-driven gear train or screw to position the actuator stem. The actuator may respond to either a digital or analogue electrical signal.

A valve that uses energy which is stored in the actuator to close rapidly in an emergency.

The configuration provided to make a pressure-tight joint to the pipe carrying the fluid being controlled. The most common of these connections are threaded, flanged, or welded.

A term used to describe a type of valve flow characteristic where for equal increments of valve plug travel the change in flow rate with respect to travel may be expressed as a constant percent of the flow rate at the time of the change. The change in flow rate observed with respect to travel will be relatively small when the valve plug is near its seat and relatively high when the valve plug is nearly wide open.

A bonnet with a packing box that is extended above the body-to-bonnet connection so as to maintain the temperature of the packing above (cryogenic service) or below (high-temp service) the temperature of the process fluid.