ON/OFF Valves

The word “discrete” means individual or distinct. In engineering, a “discrete” variable or
measurement refers to a true-or-false condition. Thus, a discrete control element is one that has but
a limited number of states (usually two: on and off).
An on/off valve is the fluid equivalent of an electrical switch: a device that either allows unimpeded
flow or acts to prevent flow altogether. These valves are often used for routing process fluid to
different locations, starting and stopping batch processes, and engaging automated safety (shutdown)
functions.
Valve styles commonly used for on/off service include ball, plug, butterfly (or disk), gate, and
globe. Large on/off valves are generally of such a design that the full-open position provides a
nearly unimpeded path for fluid to travel through. Ball, plug1, and gate valves provide just this
characteristic:

*A plug valve is very much like a ball valve, the difference being the shape of the rotating element. Rather than a spherical ball, the plug valve uses a truncated cone as the rotary element, a slot cut through the cone serving as the passageway for fluid. The conical shape of a plug valve’s rotating element allows it to wedge tightly into the “closed” (shut) position for exceptional sealing.