Valve Terminology - I

Actuator:

A device that operates a valve by utilizing electricity, pneumatics, hydraulics, or a combination of one or more of these energies. Sometimes actuators are referred to as operators. In this chapter, the word operator will be used for a person who operates any equipment, machine, plant, or system. Ambient conditions: The pressure and temperature of the environment surrounding a valve. Backflow: The flow that occurs in the opposite direction of the normal or expected fluid flow. Back pressure: The static pressure existing at the outlet of a pressure-relief device due to pressure in the discharge system.

Backseat:

A seat on the bonnet or bonnet bushing which contacts a corresponding seating surface of the stem or disc holder when the stem is fully retracted. It provides a seal between the stem and the inside of the bonnet. It prevents leakage of flow medium and allows replacement of valve packing while the valve is open and under pressure.

Block-and-bleed valve:

A valve with two seating surfaces which provide simultaneous blockage of flow from both valve ends, and means for draining or venting the cavity between the seating surfaces. When the valve is closed (blocked) the drain is opened, allowing the trapped fluid between the seats to drain (bleed).

Block valve:

A valve that is used to start or stop the flow. It is also referred to as an on-off valve. Blowdown: The difference between the set pressure and the disc-reseating pressure of a pressure-relief valve, which is expressed as a percentage of the set pressure.

Blowdown valve:

A valve used to release the pressurized contents of a pressure vessel or piping.

Bonnet:

A valve body closure component that contains an opening for the stem.

Bore:

The diameter of the smallest opening through a valve. It is also called port.

Bubble tight:

A valve is termed bubble tight when the upstream side of the valve is pressurized with air and the downstream side is filled with water and no air bubbles are detected on the downstream side with the valve in fully closed position.

Bypass:

A piping loop provided to permit flow around the flow control element (disc, plug, etc.) of a valve in its closed position. A stop valve installed in the bypass loop is called the bypass valve.