Diaphragm valves are closed by means of diaphragm which squeezes off the flow of the media through the valve. The diaphragm is held inside a bonnet and acts as the top portion of the pipe for a certain length. Above the diaphragm is a plate which is lowered by some means, forcing the diaphragm down until is makes contact with the bottom of the pipe shutting off the flow. There are a variety of means of actuating the plate: with a gear operator; using a pneumatic or hydraulic piston; an electric motor operating a screw mechanism.

The biggest drawback of diaphragm valves is that the diaphragms crack and break because of the stress of stretching them down to the bottom of the pipe. Another drawback is that the diaphragm materials used do not tolerate temperatures above 100 Celsius very well.

One of the advantages of diaphragm valves is that there are no spaces for media to get trapped which is a consideration in the food and drug processing industry. Also, the interior of the valve casting can be lined or coated with a variety of elastomers or ceramics. Coupled with the correct selection of diaphragm material, this allows the valves to be used for corrosive applications or situations requiring the minimization of contamination like Ultra Pure Water.

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