GLOSSARY
OF TERMS

A rigid spacer used in the packing, with packing above and below it. The lantern ring is used to allow lubrication to the packing or allow access to a leak off connection. On some new fugitive emission packing systems, it also acts as a stem guide.

A term that describes a procedure for reducing the leakage rate on metal-to-metal seated valves. The plug and seat are lapped together with the aid of an abrasive compound in an effort to establish a better seating surface than would normally be achieved by means of machining.

A term used to describe certain standardised testing procedures for control valves with a flow coefficient greater then 0. 1 (Cv). These procedures are outlined in ANSI Standard B16.104-1976, which gives specific tests and tolerances for six seat leakage classifications. It should be remembered that these tests are used to establish uniform acceptance standards for manufacturing quality and are not meant to be used to estimate leakage under actual working conditions. Nor should anyone expect these leakage rates to be maintained after a valve is placed in service.

A characteristic where flow capacity or (Cv) increases linearly with valve travel. Flow is directly proportional to valve travel. This is the preferred valve characteristic for a control valve that is being used with a distributive control system (DCS) or programmable logic controller (PLC).

Another name for a globe valve. It refers to the linear or straight-line movement of the plug and stem.

Normally associated with valve packing. It designates the packing is spring loaded to maintain constant tension on the packing rings.

The pressure used to position a pneumatic actuator. It is the pressure that is actually applied to the actuator diaphragm or piston. It can be the instrument pressure if a valve positioner is not used or is bypassed.

A special type of regulator that is installed between the valve positioner and the valve actuator, where it senses the supply air pressure. If that pressure falls below a certain level, it locks or traps the air loaded into the actuator causing the valve to fail-in-place.

A valve design that dissipates a considerable amount of flow stream energy due to turbulence created by the contours of the flow path. Consequently, pressure downstream of the valve vena contracta recovers to a lesser percentage of its inlet value than a valve with a more streamlined flow path. The conventional globe style control valve is in this category.

Low shear conventional and control valves help to avoid mixing and emulsification of petrochemcials by enhancing oil and water separation.