Valve Terminology - II

Cavitation:

This occurs when the fluid pressure at the vena contracta falls below the vapor pressure, followed by pressure recovery above the vapor pressure. The pressure below vapor pressure causes vapor bubbles to form, which then collapse as the pressure recovers. Collapsing bubbles can cause erosion of valve and downstream pipe metal surfaces. The phenomena of fluid pressure falling below and recovering above vapor pressure, combined with forming and collapsing of bubbles, is termed cavitation.

Chatter:

Abnormal rapid reciprocating motion of the movable parts of a pressure relief valve in which the disc contacts the seat.

Choked flow:

When the flow rate cannot be increased even if the downstream pressure is decreased. In liquid applications, it is caused by vapor bubbles, formed by cavitation or flashing, choking the flow passage. In the case of gases, choked flow is caused when the flow velocity reaches sonic proportions and a reduction in downstream pressure cannot increase the gas flow.

Closing pressure:

The pressure equal to the valve of decreasing inlet static pressure at which the valve disc reestablishes contact with the seat or at which lift becomes zero.

Coefficient of discharge:

The ratio of the measured relieving capacity to the theoretical relieving capacity of a pressure-relief valve.

Coefficient of flow:

The flow rate that passes through the fully open valve at unit pressure differential. It is measured in gal (3.8 liters) per minute of 60F (16C) water with 1 psi (6.9 kPa) pressure differential. It is also referred to as flow coefficient or valve coefficient.

Cold working pressure (CWP):

This is the maximum flow-medium pressure at the ambient temperature to which the valve may be subjected during normal service. This is also referred to as water-oil-gas (WOG) rating.

Control valve:

A valve serving as a control element in a system, providing means for varying the rate of flow of the fluid passing through the valve.

Cracking pressure:

The upstream fluid pressure at which a closed check valve starts to open and allow the flow through the valve.

Cryogenic valve:

A valve used in applications with fluid temperatures below 50⁰F (45⁰C). A cryogenic valve is provided with an extended stem contained in an extension tube so that the valve packing and operator remain at ambient temperature when the cryogenic fluid is in the valve body. The valve is oriented so that the packing is at a higher elevation than the valve body. It allows a vapor-gas pocket to form inside the extension tube, thereby making the extension tube more effective at insulating the packing from cryogenic fluid cold temperature.

Disc:

The part of the valve which is positioned in the flow stream to permit or to obstruct flow, depending on closure position. In specific designs, it may also be called a wedge, plug, ball, gate, or other functionally similar expression. In international standards it is referred to as obturator.

Double-disc:

A two-piece disc or two separate discs that mate with two seating surfaces. Double discs are used in some designs of gate valves.

Double-seated valve:

A valve with two separate seating surfaces that come in contact with two separate seating surfaces of a disc or a double disc.

Flow characteristic:

Defines the relationship between the Flow Coefficient and the valve stroke.

Flow coefficient:

See Coefficient of flow.

Flow control element:

The part of the valve that allows, stops, obstructs, and controls the fluid flow through the valve.

Fugitive emission:

The amount of leakage of hazardous and toxic flow medium (fluids) from a valve to the environment.

Full bore:

When valve bore (port) is approximately of the same size as the inside diameter of the connecting pipe, it is called full bore (full port).

Fully stellited:

A valve is termed fully stellited when seating surfaces of the valve seat(s) and the disc(s) are hard faced with wear and corrosion-resistant material (Stellite or other such material).