Valve and Glossary

Valves are an essential part of any piping system that conveys liquids, gases, vapors, slurries and mixtures of liquid, and gaseous phases of various flow media. Starting with primitive means for stopping, allowing, or diverting water-flow from a source through shallow or deep channels, such as wooden or stone wedges stuck between the edges of a water passage, man has developed several different types of simple and sophisticated valve designs. Different types of valves include: gate, globe, check, ball, plug, butterfly, diaphragm, pinch, pressure relief, and control valves. Each of these types has several categories and designs, each offering different features and functional capabilities. Some valves are self-actuated while others are manually operated or have actuators that are powered with electric motors, are pneumatic or hydraulic, or a combination to operate the valve. Valves are manufactured with metals and nonmetals. Valves are used in industrial piping systems, transportation and distribution pipelines, building services piping, civic facilities, and irrigation piping systems. The piping systems of industrial, commercial, residential, and other civic facilities carry the lifeblood of modern civilization, like arteries and veins. And the valves in those piping systems serve the functions of allowing, stopping, regulating, and controlling the flow, to fulfill the intended objectives of the system.

When fluid pressure builds up beyond a set limit, the valves relieve the overpressure to safeguard the integrity of the piping system or a component. Valves help in maintaining or breaking a vacuum. Valves also assist in maintaining the pressure or temperature of the flow medium within the desired range or limit. In order to select a suitable valve for a particular application, the user must evaluate the valve characteristics, including the design features, materials of construction, and performance, in light of specific application requirements: flow medium, process design requirements, piping design criteria, and economic factors. This chapter provides a brief discussion of topics considered in the selection and application of commonly used valves.

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.


Valve Handbook (McGraw-Hill Handbooks)