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.
• Diaphragm valves are suitable for controlling corrosive liquids at low temperatures and pressures. They are particularly effective for handling potable water, air, sludge, chemicals, low-velocity slurries, and effluents.
• Additionally, these valves are well-suited for installation close to pumps, bends, and in vertical pipelines.

Relevant Standards for Diaphragm Valves

These below standards provide guidelines for the design, manufacturing, and testing of diaphragm valves, ensuring they meet the necessary safety and performance criteria for various industrial applications.

ASME B16.42 – Ductile Iron Pipe Flanges and Flanged Fittings:

• This standard covers the design and manufacturing requirements for ductile iron pipe flanges and flanged fittings, which may include components used in diaphragm valves.

ISO 16138:2015 – Industrial Valves: Diaphragm Valves Made of Metallic Materials:

• This international standard specifies the requirements for the design, manufacture, and testing of metallic diaphragm valves used in industrial applications.

MSS SP-88 – Diaphragm Valves:

• This standard practice provides guidelines for the design, material selection, dimensions, and performance of diaphragm valves.

BS EN 13397:2002 – Industrial Valves: Diaphragm Valves:

• A British standard that details the requirements and testing procedures for diaphragm valves used in industrial applications.

API 609 – Butterfly Valves: Double Flanged, Lug- and Wafer-Type:

• Although this standard is primarily for butterfly valves, it includes some relevant information that can be applied to diaphragm valve design and installation, especially in terms of compatibility and standards for similar applications.

ASTM F2373 – Standard Specification for Diaphragm Valves for Chemical Process Industries:

• This ASTM standard specifies requirements for diaphragm valves used in the chemical process industry, focusing on materials, construction, and performance criteria.

What materials are commonly used in the construction of diaphragm valves?

Diaphragm: 

The diaphragm in a valve is typically made from flexible, elastomeric materials. These materials are versatile but have limitations regarding temperature and pressure. High temperatures and pressures can reduce their effectiveness. Therefore, selecting the right diaphragm material depends on factors such as operating temperature, pressure, the nature of the fluid being handled, and how often the valve is operated.

• EPDM (Ethylene Propylene Diene Monomer): EPDM is a synthetic, general-purpose elastomer known for its good corrosion resistance. It is well-suited for handling acids, alkalis, alcohols, and is resistant to ozone. However, it is incompatible with oil and petroleum products. EPDM is also effective for steam sterilization and operates within a temperature range of -20°F to 230°F.
• PTFE (Polytetrafluoroethylene): PTFE, commonly known as Teflon, is a synthetic fluoropolymer renowned for its exceptional resistance to corrosion and chemicals. It is ideal for handling strong acids, alkalis, and solvents. Although its rigidity provides a strong sealing force, it requires more effort to operate. PTFE diaphragms can function in a temperature range from -300°F to 3000°F. For improved strength, wear resistance, and pressure tolerance, PTFE is often reinforced with glass fibers.
• Neoprene: Neoprene is a synthetic rubber commonly used as a diaphragm material in wastewater pipelines. It offers good resistance to corrosion and abrasion and can handle fluids with entrained oils, acids, alkalis, petroleum, explosives, and fertilizers. Neoprene diaphragms operate within a temperature range of -20°F to 200°F.
• Butyl Rubber: Butyl rubber is known for its low vapor and gas permeability, making it ideal for handling gaseous media. It is also resistant to steam sterilization and various acids and alkalis. Butyl rubber diaphragms function effectively between -4°F and 248°F.
• Nitrile Rubber: Nitrile rubber is a versatile material known for its high strength and abrasion resistance. It handles gases, fuels, fats, oils, alcohols, and petroleum products well, but is unsuitable for acetones, ketones, ozone, and some modified hydrocarbons. Nitrile rubber diaphragms operate within a temperature range of -14°F to 134°F.
• Natural Rubber: Natural rubber provides good abrasion resistance and can manage moderate acids and alkalis. It is commonly used in applications involving abrasives, dilute mineral acids, and brewing processes. Natural rubber diaphragms operate between -40°F and 134°F.
• Viton: Viton, a fluorocarbon elastomer, offers excellent resistance to most chemicals, solvents, and oils, even at high temperatures. However, it is not suitable for steam sterilization or handling ammonia and polar solvents. Viton diaphragms function within a temperature range of -20°F to 300°F.

Valve Body: 

The valve body and bonnet are constructed from durable materials to protect the internal components of the diaphragm valve. The bonnet, which is isolated from the wetted parts of the valve, can be made from materials with slightly lower corrosion resistance. The valve body, however, often features a smooth lining to prevent clogging and buildup from sticky or viscous fluids.

Like the diaphragm material, the valve body must be corrosion-resistant and capable of withstanding sterilization processes. For applications requiring enhanced sanitation, materials with antimicrobial properties, such as brass and bronze, may be chosen. Additionally, the valve body can be lined with antimicrobial materials to further improve hygiene.

Common materials used for diaphragm valve bodies include stainless steel, cast iron, ductile iron, cast steel, brass, bronze, PVC, U-PVC, and CPVC

What factors should be considered when selecting and operating diaphragm valves?

When selecting and operating diaphragm valves, several key factors must be considered to ensure optimal performance and suitability for the application:

Valve Flow Coefficient (Cv)

The valve flow coefficient, or Cv, is a critical metric that indicates the valve’s capacity to permit fluid flow. It is defined as the volume of water (in US gallons) at 60°F that can flow through the valve per minute with a pressure drop of 1 psi across it. Understanding the Cv is essential for selecting the correct valve size to achieve the desired flow rate. The Cv is calculated using the following formula:

Where:

• Cv = Valve flow coefficient
• Q = Flow rate in gallons per minute (GPM)
• SG = Specific gravity of the fluid
• ΔP = Pressure drop across the valve in psi

The Cv value increases with greater valve opening and stem travel, indicating the valve’s capacity to handle higher flow rates.

Pressure Drop

Pressure drop refers to the reduction in pressure from the valve’s inlet to its outlet. It is an important consideration because a significant pressure drop across the valve can impact system efficiency. If the pressure drop is minimal compared to the overall system pressure drop, the difference in flow rate will be negligible until the valve is nearly closed. In such situations, a diaphragm valve with a quick or fast-acting mechanism is recommended to maintain control over the flow rate.

Rangeability

Rangeability measures a valve’s ability to manage different flow rates and is expressed as the ratio of the maximum to the minimum controllable flow rates. It depends on factors such as the precision and size of the actuator, as well as the design of the valve body, diaphragm, and compressor. A valve with high rangeability is better suited to effectively control a wide range of flow rates, making it more versatile for various applications.

Valve Sizing

Accurate valve sizing is essential, especially for diaphragm valves used in throttling applications. Proper sizing involves calculating the volume of fluid that will pass through the valve, considering factors such as flow rate, inlet and outlet pressures, temperatures, specific gravity, and fluid viscosity. Correct sizing ensures that the valve meets the necessary capacity and pressure drop requirements. Techniques like using the pipe geometry factor are often applied in the sizing process to ensure that the valve operates efficiently within the desired parameters.

FAQ on Diaphragm Valves

What are diaphragm valves used for?

Diaphragm valves are used to control the flow of fluids in a wide range of industrial applications. They are particularly effective for handling corrosive, abrasive, or viscous fluids, as well as slurries and powders. Common applications include:

• Chemical processing
• Water and wastewater treatment
• Food and beverage processing
• Pharmaceutical manufacturing
• Mining and mineral processing
• Pulp and paper industry

Diaphragm valves are favored in these applications because they provide reliable, leak-free operation and can be used with highly corrosive or hazardous fluids.

What is the difference between a ball valve and a diaphragm valve?

The primary difference between a ball valve and a diaphragm valve lies in their construction and operation:

Ball Valve:

• Has a spherical ball with a hole through its center.
• The ball rotates within the valve body to either block or allow flow.
• Commonly used for quick shut-off applications.
• Provides minimal flow resistance when fully open.

Diaphragm Valve:

• Uses a flexible diaphragm that is pressed against a weir or seat to control flow.
• Suitable for throttling and can handle abrasive or corrosive fluids.
• No rotating parts; instead, the diaphragm is lifted or pressed down.
• Often used in applications requiring clean, sterile, or contaminant-free flow.

Is a diaphragm valve a check valve?

No, a diaphragm valve is not a check valve.

Diaphragm Valve:

• A diaphragm valve is designed to control the flow of fluids by manually or automatically adjusting the position of a diaphragm to open, close, or throttle the flow.

Check Valve:

• A check valve is a one-way valve that allows fluid to flow in only one direction, preventing backflow. It operates automatically based on the pressure of the fluid and typically does not require manual operation.

While both types of valves can be used to control flow, their purposes and operating mechanisms are different.

What is a diaphragm relief valve?

A diaphragm relief valve is a type of pressure relief valve that uses a flexible diaphragm to control the release of excess pressure from a system.

• It is designed to protect equipment and piping from overpressure conditions by automatically opening when the pressure exceeds a predetermined level, allowing the excess fluid or gas to escape, and then re-closing once the pressure returns to a safe level.
• Diaphragm relief valves are commonly used in applications where the process media is corrosive, viscous, or contains particulates, as the diaphragm provides a barrier that prevents the media from contacting the internal components of the valve.

These valves are essential for maintaining system safety and preventing damage to equipment due to excessive pressure.

To know more about control valves check the following links

• Globe valve
• Needle valve
• Control valve installation
• Parts of control valves
• Control valve and it’s working