Diaphragm Valves

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 Diaphragm Valve:

  • A diaphragm valve uses a rubber, plastic or elastomer diaphragm to seal the stem.
  • The diaphragm not only seals the stem but forms the closure member.
  • There are two styles of diaphragm valves, one having a body with weir and the other having a straight-through body.
  • On the weir type, the stem is connected to a finger arrangement, which in turn presses the diaphragm down onto a weir. this creates an extremely tight seal that will seal even on some solids.
  • Since the diaphragm is not metallic and forms the closure, the valve is severely limited in pressure and temperature.
  • To enable draining of horizontal pipelines, the weir valve must be mounted 15° from the horizontal plane (because of the weir). This complicates installation, especially with air actuators.

Working of Diaphragm Valve:

A resilient, flexible diaphragm is connected to a compressor by a stud molded into the diaphragm. In valve diaphragm ,the compressor is moved up and down by the valve stem. Hence, the diaphragm lifts when the compressor is raised. As the compressor is lowered, the diaphragm is pressed against the contoured bottom in the straight through valve or the body weir in the weir-type. A straight through type diaphragm valve is shown below.


The weir-type diaphragm valve is the better throttling valve but has a limited range. It uses a two-piece compressor component. Instead of the entire diaphragm lifting off the weir when the valve is opened, the first increments of stem travel raise an inner compressor component that causes only the central part of the diaphragm to lift. This creates a relatively small opening through the center of the valve. After the inner compressor is completely open, the outer compressor component is raised along with the inner compressor and the remainder of the throttling is similar to the throttling that takes place in a conventional valve. A weir type diaphragm valve is shown below. Diaphragm type valve has stem that does not rotate.
Some diaphragm valves use a quick-opening bonnet and lever operator. A 90° turn of the lever moves the diaphragm from full open to full close position of valve. Diaphragm valves may also be equipped with chain wheel operators, extended stems, bevel gear operators, air operators, and hydraulic operators.

Advantages:

1. Can be used as on-off and throttling service valves.
2. Offer good chemical resistance due to variety of linings available.
3. Stem leakage is eliminated.
4. Provides bubble-tight service.
5. Does not have pockets to trap solids, slurries, and other impurities. It is suitable for slurries and viscous fluids.
6. These valves are particularly suitable for hazardous chemicals and radioactive fluids.
7. These valves do not permit contamination of flow medium, thus they are used extensively in food processing, pharmaceutical, brewing, and other applications which cannot tolerate any contamination.

Disadvantages:

1. The weir may prevent full drainage of piping.
2. Working temperatures and pressures are limited by the diaphragm material. Generally the pressures are limited to 200 psi (1380 kPa) and temperatures up to 400 F (204 C).
3. The diaphragm may also limit the hydrostatic pressure.
4. The diaphragm may experience erosion when used extensively in severe throttling service containing impurities.
5. Diaphragm valves are available in limited sizes, usually NPS ¹⁄₂ to 12 (DN 15 to 300).

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