valve selection handbook zappe

Valves are the components in a fluid flow or pressure system that regulate
either the flow or the pressure of the fluid. This duty may involve
stopping and starting flow, controlling flow rate, diverting flow, preventing
back flow, controlling pressure, or relieving pressure.
These duties are performed by adjusting the position of the closure
member in the valve. This may be done either manually or automatically.
Manual operation also includes the operation of the valve by means of a
manually controlled power operator. The valves discussed here are manually
operated valves for stopping and starting flow, controlling flow
rate, and diverting flow; and automatically operated valves for preventing
back flow and relieving pressure. The manually operated valves are
referred to as manual valves, while valves for the prevention of back
flow and the relief of pressure are referred to as check valves and pressure
relief valves, respectively.
Rupture discs are non-reclosing pressure-relieving devices which fulfill
a duty similar to pressure relief valves.