GLOSSARY
OF TERMS

Used to designate Inlet Pressure.

Used to designate Outlet Pressure.

A sealing system that normally consists of a deformable material such as PTFE, graphite, etc. It is usually in the form of solid or split rings contained in a packing box that are compressed so as to provide an effective pressure seal.

The chamber located in the bonnet that surrounds the stem and contains the packing and other stem-sealing components.

A part that transfers a mechanical load to the packing from the packing flange or nut.

A fluid-powered, normally pneumatic device in which the fluid acts upon a movable cylindrical member, the piston, to provide linear motion to the actuator stem. These units are spring or air opposed and operate at higher supply pressures than a spring return actuator.

See closure member.

A valve plug or cage where the flow control orifice is in the shape of a slot or port.

A switch that is normally fitted on the actuator to detect extremes of valve travel. The switch is normally electric.

A device that is mechanically connected to the valve stem and will generate and transmit either a pneumatic or electric signal that represents the valve stem position.

A device used to position a valve with regard to a signal. The positioner compares the input signal with a mechanical feedback link from the actuator. It then produces the force necessary to move the actuator output until the mechanical output position feedback corresponds with the pneumatic signal value. Positioners can also be used to modify the action of the valve (reverse-acting positioner), alter the stroke or controller input signal (split-range positioner), increase the pressure to the valve actuator (amplifying positioner) or alter the control valve flow characteristic (characterised positioner).

Normally associated with top and bottom-guided valves where the upper and lower sections of the plug, the post, are guided in the valve by a guide bush.

A type of bonnet design where the fluid pressure is used to produce the seal between the body and bonnet.

Polytetrafluoroethlene – a soft polymer that is compatible with almost any substance.

A term used to describe a linear or globe style valve that uses a direct acting plug and stem arrangement. The plug is located above the seat ring. When the plug is pushed down, the plug contacts the seat and the valve closes. Note: Most control valves are of this type.

A term used to describe a linear or globe style valve that uses a reverse acting plug and stem arrangement. The plug is located below the seat ring. When the plug is pushed down, it moves away from the seat and the valve opens.