Control Valve

What is a diaphragm valve and what does it do

What is a diaphragm valve and what does it do

A diaphragm valve is different from other valves, diaphragm valve doesn’t have a valve disc. This valve uses a diaphragm for its opening and closing purpose. This valve is a linear motion valve and it can be used to start to regulate and stop the fluid flow. The diaphragm is connected to the compressor with the help of stud, which is molded into the diaphragm. With the help of the valve stem the compressor is moved up and down, so if the compressor is raised the diaphragm will be lifted.

What are the parts of the diaphragm valve

  • Valve body
  • Bonnet
  • Weir-seat
  • Diaphragm-disc
  • Stud
  • Plunger
  • Stem
  • Actuator

How diaphragm valves are constructed

The diaphragm valves are manufactured in a wide variety of materials to suit the particular media. The valve body is constructed according to the service requirements. Mostly corrosion resistive materials are used for valve body construction. Mostly bonnets are made up of cast iron and epoxy coated inside and outside, diaphragms are mostly made up of elastomers and plastics. The elastomeric mechanical properties will be lowered at high temperatures.

How diaphragm valves work

The flexible diaphragm is connected to a support member and that is the compressor, which represents the closure member. If the valve is opened then the diaphragm is lifted out of the flow path and the fluid will flow streamlined. When the valve is closed then the diaphragm is tightly seated against the seat in the body known as the weir. The valve will be closed completely when the diaphragm is forced downward. There is no need for stem packing in this valve because there is no valve stem in the flow path. While compared to other conventional valves these valves require less maintenance. The material which is used to construct the diaphragm is a mostly rubberized compound so these valves can be used where highly corrosive materials are present.

In the diaphragm control valve, opening air from the pilot acts on the valve diaphragm. The structure in which the diaphragm is contained could be direct-acting in some valves or it can also be reverse acting in others. If the structure is direct-acting then the operating air pressure from the control pilot is applied to the top of the valve diaphragm. If the structure is reverse acting then the air pressure will be applied to the underside of the valve diaphragm. This can be simply described as the diaphragm control valve structure could be reverse acting or direct-acting and the diaphragm control valve may be upward seating or downward seating. These factors, as well as the purpose of the installation, determine how the diaphragm control valve and its air-operated control pilot are installed in relation to each other.

What are the types of diaphragm valves

The diaphragm valves are of two types they are weir type and straight through diaphragm valves

Weir type diaphragm valve

Weir in the flow passage is designed to reduce the flexing of the diaphragm to a minimum level, but it still provides a smooth and streamlined flow passage. So that the flexing stress in the diaphragm is minimal and this would increase the diaphragm life. In weir type diaphragm with the rising stem, the handwheels carry a shroud that covers the stem threads while sliding over a lift indicator sleeve. Shroud is to protect the external stem thread from dust and outside corrosive influences.

Straight through diaphragm valves

This type of diaphragm valves requires a more flexible diaphragm than the weir type. Due to this, the construction material for this type of valves is restricted to elastomers. This diaphragm has high flexibility and large area. Straight through diaphragm valves are available with full bore and reduced bore flow package.

What are the advantages of diaphragm valves

  • This valve is the most corrosion or abrasion resistance valve that we could get
  • Leakage of steam is eliminated
  • These valves can be used for hazardous chemicals and radioactive fluids
  • These valves won’t allow the contamination of the flow medium and because of that they are used in food processing, pharmaceutical applications
  • The construction is simple and they are easy to operate

What are the disadvantages of diaphragm valves

  • Full drainage is prevented by weir type
  • Because of the diaphragm material, the working temperature and pressure is limited, so they cannot be used for high temperature and pressure
  • Hydrostatic pressure is limited by the diaphragm

What are the applications of diaphragm valves

  • These valves can be used for liquids which is corrosive at low temperature and pressure
  • Diaphragm valves are suitable for potable water, air, sludge, chemicals, and low-velocity slurries and effluent and are suitable for installation close to pumps and bends and in vertical pipelines.

To know more about control valves check the following links


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