DIAPHRAGM VALVES

All diaphragm valves are bi-directional. They can be used as shut-off and regulating valves. Diaphragm valves offer advantages in some low pressure applications that are not possible with other valve types. Fluid passages are smooth and streamlined to minimize pressure drop. They are suitable for moderate throttling applications and exhibit excellent sealing properties even when conveying liquids containing solids. The fluid flow is isolated from the working parts of the valve, preventing contamination of the fluid and wear of the operating mechanism. Since there are no leakage paths around the valve body, the valve is virtually leak-tight. This feature makes the valve indispensable in situations where leakage into or out of the system cannot be tolerated. The maximum pressure to which these valves can be subjected is a function of the diaphragm material and service temperature. Also, the nominal design life of the valve is affected by service conditions. Furthermore, the system hydrostatic test pressure must not exceed the maximum pressure rating of the diaphragm.

Diaphragm Valve Structure

Diaphragm valves consist of a body placed in the flow path, a flexible diaphragm that forms the upper pressure limit of the valve, a gate used to force the diaphragm against the body, and the bonnet and flywheel that fix the diaphragm to the body and operate the compressor: weld end socket or butt weld; flanged, or threaded. Valve body: straight-through model and angled model. available in two versions Diaphragm valves are available in a wide range of body, diaphragm and liner materials suitable for service with a wide range of chemicals. For severely corrosive applications, diaphragm valves are made of stainless steel or PVC plastic or lined with glass, rubber, lead, plastic, titanium or other materials. Adjustable travel stops are used to prevent over-compression of the diaphragm. The adjustable travel stop can be sealed in a heat-resistant housing. The temperature-resistant housing prevents accidental misuse after the travel length has been set. It requires a special tool to open the housing and make adjustments to the travel length. Where an actuator is required, it must be dimensioned and tested by the valve manufacturer and supplied with the valve as an assembled unit with adjustments in the workshop.

Diaphragm Valve Types

First of all, there are two basic diaphragm valve designs: Weir and straight types. The inside body and end flanges can be lined to make diaphragm valves suitable for corrosive applications. Depending on the application, various lining materials can be used.

Weir Type Diaphragm Valves

A vertical riser is provided as an integral part of the valve body, which acts as the valve seat where the diaphragm is compressed to stop the flow. This type of diaphragm valve is usually manufactured in large sizes. The raised weir reduces the amount of transition of the diaphragm from the fully open to the fully closed position, thus reducing the amount of stress and strain on the diaphragm.

Straight-through Diaphragm Valves

Variations of the Weir diaphragm valve are direct, when the straight-through valve is open, its diaphragm lifts up for full streamline flow in both directions. When the valve is closed, the diaphragm seals tightly for positive shut-off, even with sandy or fibrous materials in the line. The full bore valve type is most commonly used in the beverage industry. It allows cleaning with steam or caustic soda without opening or removing the valve from the line.

Advantages of Diaphragm Valves

1. Can be used as on-off and throttling service valves.
2. Good chemical resistance due to the variety of coating materials available.
3. Shaft leakage is eliminated.
4. Provides Zero Tightness.
5. No pockets to hold solids, slurries and other foreign matter. Suitable for slurries and viscous liquids.
6. These valves are particularly suitable for hazardous chemicals and radioactive liquids.
7. These valves do not allow contamination of the flow medium, so they are widely used in food processing, pharmaceutical, brewing and other applications that cannot tolerate any contamination.

Disadvantages of Diaphragm Valves

1. Weir can prevent pipes from draining completely.
2. Operating temperatures and pressures are limited by the diaphragm material. In general, pressures are limited to 200 psi (1380 kPa) and temperatures to 400 F (204 C).
3. The diaphragm can also limit hydrostatic pressure.
4. The diaphragm may be subject to erosion when used extensively in severe throttling service containing impurities.
5. Diaphragm valves are available in limited sizes, usually NPS (to 12 (DN 15 to 300).

Typical Applications of Diaphragm Valves

1. Clean or polluted water and air service applications
2. Demineralized water systems
3. Abrasive and Corrosive applications
4. Nuclear facilities
5. Vacuum service
6. Food processing, pharmaceutical and brewing systems