A valve is a mechanical device which regulate either the flow or the pressure of the fluid. Its function can be stopping or starting the flow, controlling flow rate, diverting flow, preventing back flow, controlling pressure, or relieving pressure.

Basically, the valve is an assembly of a body with connection to the pipe and some elements with a sealing functionality that are operated by an actuator. The valve can be also complemented whit several devices such as positioners, transductors, pressure regulators, etc.