GLOSSARY
OF TERMS

A hollow cylindrical trim element that is sometimes used as a guide to align the movement of a valve plug with a seat ring. It may also act to retain the seat ring in the valve body. On some types of valve, the cage may contain different shaped openings which act to characterise the flow through the valve. The cage may also act as a noise attenuation or anti-cavitation device.

The mass flow rate through a valve under specified conditions.

Occurs only in liquid service. In simple terms, cavitation is the two-stage process of vaporisation and condensation of a liquid. Vaporisation is simply the boiling of a liquid, which is also known as flashing. In a control valve this vaporisation takes place because the pressure of the liquid is lowered, instead of the more common occurrence where the temperature is raised. As fluid passes through a valve just downstream of the orifice area, there is an increase in velocity or kinetic energy that is accompanied by a substantial decrease in pressure or potential energy. This occurs in an area called the vena contracta. If the pressure in this area falls below that of the vapour pressure of the flowing fluid, vaporisation (boiling) occurs. Vapour bubbles then continue downstream where the velocity of the fluid begins to slow and the pressure in the fluid recovers. The vapour bubbles then collapse or implode. Cavitation can cause a choked flow condition to occur and can cause mechanical damage to valves and piping.

Also known as critical flow. This condition exists when at a fixed upstream pressure the flow cannot be further increased by lowering the downstream pressure. This condition can occur in gas, steam or liquid services. Fluids flow through a valve because of a difference in pressure between the inlet (Pl) and outlet (P2) of the valve. This pressure difference (Delta-P) or pressure drop is essential to moving the fluid. Flow is proportional to the square root of the pressure drop, which means that higher pressure drops allow more fluid to be moved through the valve. If the inlet pressure to a valve remains constant, then the differential pressure can only be increased by lowering the outlet pressure. For gases and steam, which are compressible, the maximum velocity of the fluid through the valve is limited by the velocity of the propagation of a pressure wave which travels at the speed of sound in the fluid. If the pressure drop is sufficiently high, the velocity in the flow stream at the vena contracta will reach the velocity of sound. Further decrease in the outlet pressure will not be felt upstream because the pressure wave can only travel at sonic velocity and the signal will never translate upstream. Choked Flow can also occur in liquids but only if the fluid is in a flashing or cavitating condition. The vapour bubbles block or choke the flow and prevent the valve from passing more flow by lowering the outlet pressure to increase the pressure drop. A good rule of thumb for gas and steam services is that if the pressure drop across the valve equals or exceeds one half the absolute inlet pressure, then there is a good chance of a choked flow condition.

The movable part of the valve which is positioned in the flow path to modify the rate of flow through the valve. Some of the different types of closure members are the Ball, Disk, Gate, and Plug.

The maximum pressure that a valve or fitting is designed to withstand at room temperature.

A gaseous fluid such as steam, which has a significant change in volume and density as pressure increases.

Also known as the final control element. A power-operated device used to modify the fluid flow rate in a process control system. It usually consists of a body or valve and an actuator, which responds to a signal from the controlling system and changes the position of a flow controlling element in the valve.

The relationship between valve travel and the flow rate through the valve. It is described by means of a curve on a graph expressed as an installed or inherent characteristic.

A device that tells a control valve what to do. Controllers can be either pneumatic or electronic. There are pressure, temperature, pH, level, differential and flow controllers. The job of the controller is to sense one of the above variables and compare it to a set point that has been established. The controller then outputs a signal, either pneumatic or electronic, to the control valve, which then responds to bring the process variable to the desired set point.

See the definition for choked flow.

A term used to describe valves designed to operate below –40°C.

The valve flow coefficient is the number of US gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1psi across the valve.