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Discrete Control Elements

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). In the case of valves, this means a valve designed to operate either in “open” mode or “closed” mode, not in-between.

On/off valves

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:



Ball Valve Butterfly Valve
Gate Valve Diaphragm Valve


1A 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.

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Fitzgerald, A.E. and Higginbotham, David E., Basic Electrical Engineering, Second Edition, McGraw-Hill Book Company, New York, NY, 1957.

“General Service Solenoid Valves – 3/2 Series 8300/8315”, document 8300R1, ASCO Valve Inc.

“ASCO Nuclear Catalog – Nuclear Products Qualified to IEEE Specifications”, ASCO Valve Inc.

Croft, Terrell and Summers, Wilford I., American Electrician’s Handbook, Eleventh Edition, McGraw-Hill Book Company, New York, NY, 1987.


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written by MISHRA, January 05, 2013
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