Valve Categories

Stop (Isolation) Valves As the name implies, stop valves are used to stop flow or isolate a portion of the system until it is desirable to achieve flow downstream of the valve. The basic design requirement of stop valves is to offer minimum resistance to flow in the fully open position and to exhibit tight shut-off characteristics when fully closed. Gate, globe, ball, butterfly, plug, and diaphragm valves satisfy the above requirements in varying degrees and, therefore, are widely used in shut-off service. The actual type of valve selected is dictated by several parameters, including:

● Pressure drop

● Seat leakage

● Fluid properties

● System leakage

● Actuation requirements

● Initial cost

● Maintenance

Regulating Valves

Regulating valves are used extensively in piping systems to regulate the flow of fluid. Whether the desired effect is to control flow, pressure, or temperature, the task is accomplished by increasing or decreasing the flow through the valve in response to a signal from a pressure, flow, or temperature controller. The primary requirement of a flow-control valve is to predictably regulate the flow with respect to its open position and impart the required pressure drop without sustaining damage. Specially designed globe, needle, butterfly, ball, plug, and diaphragm valves are capable of satisfying these requirements in varying degrees. The manufacturer’s literature should be consulted for the limitations placed on a particular valve.

Backflow Prevention

Valves are generally used for the prevention of backflow. The valves are selfactuating and the valve disc is kept open by the forward flow of fluid. The valve disc is quickly closed by reverse flow. In certain applications, pneumatic actuators may be used to assist in the rapid closure of the valves on reversal of flow.

Pressure-Relief Devices

Pressure-relief devices are used to protect piping and equipment from being subjected to pressures that exceed their design pressures. Generally, the seating of relief valves is accomplished by a compressed spring, which exerts a force on the valve disc, pressing it against the valve seat. When the force exerted by the fluid on the valve disc exceeds the spring force, the valve automatically opens to release the excess pressure. Other designs incorporate a pilot valve, which uses system pressure to control the movement of the disc. Another type of pressure-relieving device, although not a valve, is a rupture disc. See Fig. relief device. A10.4. The rupture disc is designed to burst open at a predetermined pressure. A rupture disc cannot be reseated and, therefore, must be replaced once it has performed its relieving function. Rupture discs have the advantage of being leak tight up to the rupture pressure and of being capable of relieving large rates of flow. The set pressure of rupture discs cannot be adjusted.