Diaphragm Valves

Diaphragm valves get their name from a flexible disc which comes into contact with a seat at the top of the valve body to form a seal. A diaphragm is a flexible, pressure responsive element that transmits force to open, close or control a valve. Diaphragm valves are related to pinch valves, but use an elastomeric diaphragm, instead of an elastomeric liner in the valve body, to separate the flow stream from the closure element.

A diaphragm valve is a linear motion valve that is used to start, regulate, and stop fluid flow. The name is derived from its flexible disk, which mates with a seat located in the open area at the top of the valve body to form a seal.

A diaphragm valve is illustrated in below Figure.

Diaphragm valves are, in effect, simple “pinch clamp” valves. A resilient, flexible diaphragm is connected to a compressor by a stud molded into the 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 illustrated in above Figure or the body weir in the weir-type valve illustrated in below Figure.

Diaphragm valves can also be used for throttling service. The weir-type is the better throttling valve but has a limited range. Its throttling characteristics are essentially those of a quick opening valve because of the large shutoff area along the seat. A weir-type diaphragm valve is available to control small flows. 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. Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination.

Diaphragm Valve Function

The diaphragm is connected to a compressor by a stud molded into the diaphragm. To start or increase flow the compressor is moved up by the valve stem. To stop or slow flow, the compressor is lowered and the diaphragm is pressed against the bottom of the valve. Diaphragm valves are excellent for controlling the flow of fluids containing suspended solids and offer the flexibility of being installed in any position. Weir-type diaphragm valves are better at throttling than straight though diaphragm valves because the design's large shutoff area along the seat gives it the characteristics of a quick-opening valve. The diaphragm acts as the gasket of the valve to seal against leaks between the body and bonnet cap.

Diaphragm Valves Construction

The operating mechanism of a diaphragm valve is not exposed to the media within the pipeline. Sticky or viscous fluids cannot get into the bonnet to interfere with the operating mechanism. Many fluids that would clog, corrode, or gum up the working parts of most other types of valves will pass through a diaphragm valve without causing problems. Conversely, lubricants used for the operating mechanism cannot be allowed to contaminate the fluid being handled. There are no packing glands to maintain and no possibility of stem leakage. There is a wide choice of available diaphragm materials. Diaphragm life depends upon the nature of the material handled, temperature, pressure, and frequency of operation.

Some elastomeric diaphragm materials may be unique in their excellent resistance to certain chemicals at high temperatures.

However, the mechanical properties of any elastomeric material will be lowered at the higher temperature with possible destruction of the diaphragm at high pressure. Consequently, the manufacturer should be consulted when they are used in elevated temperature applications.

All elastomeric materials operate best below 150°F. Some will function at higher temperatures. Viton, for example, is noted for its excellent chemical resistance and stability at high temperatures.

However, when fabricated into a diaphragm, Viton is subject to lowered tensile strength just as any other elastomeric material would be at elevated temperatures. Fabric bonding strength is also lowered at elevated temperatures, and in the case of Viton, temperatures may be reached where the bond strength could become critical. Fluid concentrations is also a consideration for diaphragm selection. Many of the diaphragm materials exhibit satisfactory corrosion resistance to certain corrodents up to a specific concentration and/or temperature.

The elastomer may also have a maximum temperature limitation based on mechanical properties which could be in excess of the allowable operating temperature depending upon its corrosion resistance.

Diaphragm Valve Stem Assemblies

Diaphragm valves have stems that do not rotate. The valves are available with indicating and non-indicating stems. The indicating stem valve is identical to the non-indicating stem valve except that a longer stem is provided to extend up through the handwheel.

For the non-indicating stem design, the handwheel rotates a stem bushing that engages the stem threads and moves the stem up and down. As the stem moves, so does the compressor that is pinned to the stem. The diaphragm, in turn, is secured to the compressor.

Types

Diaphragm valves are available in two basic forms

Weir types
Straight through types

The basic construction of both valves is similar except for the body and diaphragm.

Weir type

The weir-type design is the most popular type of diaphragm valve and it is best for general use applications or for tough corrosive and abrasive services. They are best used to control small flows. The body of the weir-type has a raised lip that the diaphragm comes into contact with. Weir-type valves use a smaller diaphragm because the material does not have to stretch as far. The material can be heavier so the valve can be used for high-pressure and vacuum services. The weir design is composed of a two-piece compressor component. To create a relatively small opening through the center of the valve, the first increment of stem travel raises an inner compressor component that causes only the central part of the diaphragm to lift instead of the entire diaphragm lifting off the weir when the valve is opened. Once the inner compressor is opened, the outer compressor piece is lifted along with the inner compressor and the additional throttling is similar to the throttling function in other valves. Weir-type bodies have bonnet assemblies recommended for handling dangerous liquid or gas because if the diaphragm fails the hazardous materials will not be released into the surrounding system. They are also recommended for food-processing applications because the body is self-draining.

Straight through type

The straight through type can be used in situations where the flow direction changes within the system. The body of this design has a flat bottom that is parallel to the flow stream. This allows the flow to move uninhibited through the valve with no major obstructions. A flexible material is required for the diaphragm so that the mechanism can reach the bottom of the valve body; this can shorten the life span of the diaphragm. They are excellent for use with sludge, slurries and other viscous fluids but they are not well suited for high temperature fluids.

Both styles provide a streamlined path for fluid flow that yields minimal pressure drop across the valve.

Diaphragm Valve Bonnet Assemblies

Some diaphragm valves use a quick-opening bonnet and lever operator. This bonnet is interchangeable with the standard bonnet on conventional weir-type bodies. A 90° turn of the lever moves the diaphragm from full open to full closed. Diaphragm valves may also be equipped with chain wheel operators, extended stems, bevel gear operators, air operators, and hydraulic operators. Many diaphragm valves are used in vacuum service. Standard bonnet construction can be employed in vacuum service through 4 inches in size. On valves 4 inches and larger, a sealed, evacuated, bonnet should be employed. This is recommended to guard against premature diaphragm failure.

Sealed bonnets are supplied with a seal bushing on the non-indicating types and a seal bushing plus O-ring on the indicating types. Construction of the bonnet assembly of a diaphragm valve is illustrated in above Figure. This design is recommended for valves that are handling dangerous liquids and gases. In the event of a diaphragm failure, the hazardous materials will not be released to the atmosphere. If the materials being handled are extremely hazardous, it is recommended that a means be provided to permit a safe disposal of the corrodents from the bonnet.

Advantages of Diaphragm Valves

Diaphragm valves can also be used for throttling service.
Its throttling characteristics are essentially those of a quick opening valve because of the large shutoff area along the seat.
A weir-type diaphragm valve is available to control small flows.
Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination.
The operating mechanism of a diaphragm valve is not exposed to the media within the pipeline. Sticky or viscous fluids cannot get into the bonnet to interfere with the operating mechanism.
Many fluids that would clog , corrode, or gum up the working parts of most other types of valves will pass through a diaphragm valve without causing problems. Conversely, lubricants used for the operating mechanism cannot be allowed to contaminate the fluid being handled.
There are no packing glands to maintain and no possibility of stem leakage in valves.

Disadvantages of Diaphragm Valves

Valve transition has sets, preventing the pipeline from complete discharge.
The operation temperature and pressure have limits depending on the diaphragm material. Typically, they are used under 200 psi (14bar) pressure and 204ºC temperature limits.
The hydrostatic pressure that can be applied to the diaphragm is limited.
Dimensions of diaphragm valves are limited. They are produced in DN15-DN300 dimensions.

Applications

Since diaphragm valves have minimal contact surfaces and are considered the cleanest valve (least likely to cause contamination) they have found widespread use in the

Pharmaceutical,
Food processing,
Water treatment industries,
Electronics industry,
Pulp and paper industry,
Power industry,
High-purity water systems.

Diaphragm Valves construction types and applications