Diaphragm Valve

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Introduction

This article takes an in-depth look at Diaphragm Valve.

Read further and learn more about topics such as:

• What are diaphragm valves
• Components of diaphragm valves
• Types of diaphragm valves
• Materials of construction of diaphragm valves
• Considerations in diaphragm valve selection and operation
• Advantages and disadvantages of diaphragm valves
• And much more…

Chapter 1: What are Diaphragm Valves?

A diaphragm valve, or membrane valve, is a valve with an elastomeric diaphragm and a seat upon which the diaphragm rests when it is closed. The flexible diaphragm obstructs, controls, or isolates the flow of fluids and acts as a flow control device.

In a diaphragm valve, the diaphragm element flexes up or down to increase or decrease the fluid flow rate. The valve is sealed when the diaphragm is pressed firmly against its solid seat. Diaphragm valves are linear motion valves that monitor and control linear movement of fluids.

Diaphragm valves are named for the flexible disc that blocks flow when it makes contact with the seat of the valve. The diaphragm is a pressure responsive component that is highly flexible and produces sufficient force to open, close, or control the function of the valve. Diaphragm valves are like pinch valves that use a liner as part of the valve body instead of a diaphragm.

Diaphragm valves can handle liquids, gaseous fluids, and semi-solid media such as slurries, colloids, sludges, and brackish water well. They are ideal for handling liquids with solid particulate matter.

Compared to other valves, diaphragm valves have a simple construction. Due to the minimal contact between their internal components, the build-up of sediments and biofilms in diaphragm valves is very limited and does not interfere with their performance. It is for this reason that they are widely used in food and pharmaceutical manufacturing, water treatment, sewage pipelines and treatment plants, electronics manufacturing, and pulp and paper production.

Chapter 2: Components of Diaphragm Valves

Diaphragm valves have a stem, bonnet, compressor, diaphragm, and actuator that are made from plastics, wood, brass, and steel. The choice of materials is dependent on the purpose and function of the valve since more durable materials are required for stressful and demanding applications.

The key component, the diaphragm, is made of flexible plastics or rubber with ethylene propylene diene monomer (EPDM) backed polypropylene plastics being the most common material and most resilient.

Bonnet

The bonnet covers the top of a diaphragm valve, is a non-wetted portion of the valve like the hand wheel and compressor, and is bolted to the body of the valve. Bonnets are quick opening and lever operated. They are interchangeable with the standard bonnets on conventional weir type valve bodies. Diaphragm valves with bonnets that are up to 10 cm are used for vacuum services. Evacuated and sealed bonnets are used for larger applications.

Bolted to the top of a diaphragm valve, the bonnet protects the compressor, stem, diaphragm, and non-wetted components.

Sealed bonnets are used with a sealing bushing for non-indicating diaphragm valves while a seal bushing and O ring are used with indicating types. The sealed type of bonnet is a necessary part of diaphragm valves that handle dangerous liquids and gasses. If there is diaphragm valve failure, hazardous substances will be sealed in the valve and not released.

Valve Body

The valve body is the component directly connected to the pipeline wherein the fluid passes through. The flow area inside the valve body depends on the type of diaphragm valve.

Both the valve body and bonnet are constructed from strong, rigid, and corrosive resistant materials.

Diaphragm

The diaphragm is made from a highly elastic polymeric disc that moves down to touch the bottom of the valve body to limit or obstruct the passage of fluid. The diaphragm lifts if the fluid flow rate is to be increased or the valve is to be fully opened. The fluid flows underneath the diaphragm. However, this component limits the valve’s operating temperature and pressure due to the diaphragm’s material and structure. It must also be replaced periodically since its mechanical properties degrade during usage.

The diaphragm isolates the non-wetted components (compressor, stem, and actuator) from the flowing media. Therefore, solids and viscous fluids are less likely to interfere with the operating mechanism of the diaphragm valve. This also protects the non-wetted components from corrosion. Conversely, the fluid in the pipeline will not be contaminated with the lubricant used in operating the valve.

Compressor

The compressor of a diaphragm valve operates the diaphragm. It is a disc with one end connected to the stem, while the other end is connected to the diaphragm. The compressor supports a diaphragm valve and distributes the forces from the stem during linear movement. It is designed to improve flow throttling and control.

When the handwheel of a diaphragm valve is turned to move the stem up or down, the motion of the stem is transferred to the compressor. As the compressor moves, the diaphragm moves upward or downward to regulate fluid flow.

Stem

The stem is a vertical shaft connected to the compressor that exerts linear motion to move both compressor and diaphragm, thus operating the diaphragm valve. It transmits the motion exerted by the actuator. Diaphragm valves can either have piston- or threaded-type stems. As the name implies, piston-type valves are moved by a piston assembly inside the bonnet, with the valve stem likely acting as the piston rod. This type requires linear force exerted by fluid pressure. Threaded stem valves have a matching stem nut. This type requires torque to linearly move the stem, as well as lubrication for smooth motion.

The threaded stems of diaphragm valves can have a rising or non-rising mechanism:

• Rising Stem Rising stems or indicating stems extend their length up to the handwheel. As the handwheel is turned, the stem rises or descends to open or close the valve, respectively. Hence, it is easier to determine the extent to which the valve is opened by looking at the amount of stem exposed. However, rising stems take up more space than non-rising stems.
• Non-Rising Stem Non-rising stems or non-indicating stems are rotated to open or close the valve, but it does not cause the stem to move up or down. Non-rising stem valves are used in limited spaces, such as in underground piping systems.

Actuator

The valve actuator is used to move the stem, the compressor, and the diaphragm altogether. It provides the torque or linear force required by the diaphragm valve to rapidly control the fluid flow rate. The actuator is dependent on the construction of the valve stem. The following are the types of actuators used in diaphragm valves.

• Manual actuators use a handwheel or a crank in which an operator applies torque. This torque is necessary to rotate the threaded stem and consequently move linearly to modify the fluid flow rate. However, these actuators have slower control speeds and require more effort to operate. Gearheads can be installed to amplify torques and enhance the opening or closing speeds. Lockability, stroke adjustment, position indication, and electrical feedback switches are the features that can be installed on manual actuators.

  
  

• Electric actuators utilize a motor in modifying the fluid flow rate. The electric motor is connected to the gear train to reduce the speed and increase the torque. These valves can operate reversibly; they can open a diaphragm valve from a closed position and vice versa.

  
  

• Pneumatic actuators utilize air pressure to move the piston inside the valve bonnet, with its piston rod connected to the compressor. Air pressure is supplied in the chamber on either side of the piston. When air is supplied in the upper chamber of the piston, it causes the piston rod to move down and lower the fluid flow rate or close the valve. Otherwise, when air is supplied in the lower chamber, it causes the piston rod to move up and increase the fluid flow rate. O-rings are present in the piston rod and the piston to prevent air leakage across both chambers. Pneumatic actuators provide fast-acting control in throttle diaphragm valves and for on and off applications.

  
  
• Hydraulic actuators utilize hydraulic fluids such as oil or water to exert a large force to open or close a diaphragm valve. These actuators are typically used in lower-speed operations.
• Thermal actuators are activated by a change in temperature in the flowing media. This activation controls the fluid flow rate.

Position Indicators

Position indicators are visual guides installed to identify the position of the diaphragm valve, whether it is in an open or closed position. It can be light, switch, or stem. Piston indicators are installed on some valves to indicate the flow direction. Terms used to describe position indicators are limit switch, beacon, position transmitter, and switch box.

The functions of valve position indicators are:

1. A visual indication of valve position that makes it possible to quickly and easily determine a valve’s position and is at the top of the visual position indicator enclosure.
2. Electrical feedback from internal switches, sensors, transmitters and other devices provide valve position to a PLC by sending electrical signals with the position encoded in the feedback signals.
3. Local junction boxes are used to protect position switches and provide a platform to mount solenoid valves and termination points for wiring the appropriate signal wire to the enclosure.

Connection

Diaphragm valves have several different types of connections due to the different kinds of piping systems and are chosen in accordance with the existing system and the necessary seal. Connections that are available include welding of end sockets known as butt welding, flanged, screwed, threaded, clamped, grooved, and solvent cemented.

Threaded

Threaded valve ends have internal or external threads and are screwed in or over the valve end. It is the most common form of connection and is the most secure type of seal.

Compression Fitting

Compression fittings seal tightly without the use of threads or soldering. The seal is formed by tightening a screw that compresses a washer against the connecting pipe.

Bolt Flange

A bolt flange connection is similar to a compression fitting. They create compressive force on a flange using tension across their length. Stud bolts or machine bolts can be used for bolt flange connections.

Clamp Flange

Clamp flanges wrap around a pipe using a spring hinged flange to make the connection.

Tube Fitting

Tube fittings are a direct straight connection between the valve and the pipe.

Butt Weld

There are several varieties of butt welds with each one designed to serve a particular purpose. The main types of butt welds are single and double sided with partial or full penetration. The valve and pipe are joined together without overlapping.

Socket Weld

With a socket weld, the pipe is inserted into a recessed area of the valve and fillet welded. Socket welds are leak proof and are used on high pressure pipelines. Prior to welding, the pipe ends should be cleaned of debris to ensure a tight secure weld.

Metal Face Seal

Metal face seals, also known as duo cone seals, have two metal rings and two large O rings. An O ring and metal ring are installed in the valve housing while the other set of O ring and metal ring are brought together to create an axial load between the metal seal rings. The dynamic movement is between the metal rings to form the seal. The O rings form a face seal on each side of the fitting.

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Chapter 3: Types of Diaphragm Valves

The two main types of diaphragm valves are the straight and weir through diaphragm valves, which only differ on the valve body and diaphragm design.

Weir Diaphragm Valves

The weir-type design is the most popular diaphragm valve design.

Weir diaphragm valves have a raised lip or saddle into which the diaphragm presses to create a sealing action. Because of the raised lip, the amount of the diaphragm travel decreases from fully opened to a completely closed position. Hence, the stress induced in the diaphragm from closing the valve is reduced, and the diaphragm requires only a small amount of material. The diaphragm is usually made from a heavier material to make it suitable for vacuum and high-pressure applications.

Weir diaphragm valves are good in fluid flow control, even in small flow rates, and are widely used in throttling applications. They may utilize a two-piece compressor system to create a relatively small opening at the center of the valve. In this system, the inner compressor lifts the central portion of the diaphragm during the first increments of stem travel instead of the entire diaphragm. Once the inner compressor has fully opened, the outer compressor is lifted along with the inner compressor; this provides additional throttling to the flow.

Bonnet assemblies are recommended in handling dangerous fluids. The bonnet caps ensure that no fluid will be released to the surroundings in case the diaphragm fails. Diaphragm valves are also self-draining; hence it is recommended in food processing applications.

Weir diaphragm valves are commonly used in handling gases and clean and homogeneous liquids. This is because sediments can build up, and viscous liquids can gum up on either side of the saddle. They can also convey corrosive, hazardous, and abrasive media.

Straight Through Diaphragm Valve

Straight through diaphragm valves have a flat bottom valve body to reduce the impediment of the fluid flow. This makes the fluid flow in a straight pathway. To put the valve in a fully closed position, the diaphragm must touch the bottommost portion of the valve body. Therefore, straight-through diaphragm valves require a more flexible material for the diaphragm. Their diaphragm also needs more frequent replacement as they have a shorter service life.

Straight through diaphragm valves are suitable in handling semi-solid media such as slurries, sludges, and viscous fluids, which require only minimal obstruction on the flow path. These valves can also be used in a bi-directional flow since there will be no saddles blocking the flow path when the flow directions are switched.

The following are the types of diaphragm valves based on the application:

Sanitary Diaphragm Valves

Sanitary diaphragm valves are popular in industries that require a high degree of fluid purity and cleanliness, such as winemaking, dairy, beverages, food, and pharmaceutical processing industries. These valves create an aseptic environment for the flowing media, wherein no bacteria, fungi, and viruses can thrive. Several valve designs are available to handle liquid, gaseous, and semi-solid media.

Biotech Valves

Biotech valves are diaphragm valves constructed to handle fluids containing microorganisms, cells, and other biological matter. These valves are typically installed in pipelines involving bioreactors, fermenters, filtration and chromatography skids, and freeze-thaw equipment. Biotech valves are used in the biotechnology fields such as medicine, agriculture, pharmaceutical industry, and food science.

Hygienic Valves

Hygienic valves are diaphragm valves designed such that there are fewer areas where the fluid can stagnate. These valves are also used in handling products intended for human consumption. They are a vital part of CIP or COP processes. Hygienic valves are commonly used in food and beverage processing lines.

Process Valves

Process valves are used in regulating the flow rates of liquid and gaseous fluids. Diaphragm valves are primarily used as process valves, either as a throttle, shut-off, or isolation valve.

Zero Static Valves

Zero static valves are one of the diaphragm valves essential to the pharmaceutical industry. Multi-port valves that allow process fluids to be transferred, drained, sampled, or diverted without inducing a major impact on critical systems such as Water for Injection (WFI) or Purified Water. The bonnet of zero static valves allows the weir to be positioned directly on the inner diameter of the pipeline. This eliminates dead legs and prevents fluid contamination and stagnation.

Chapter 4: Materials of Construction of Diaphragm Valves

Diaphragm

The diaphragm material is typically made from a flexible, elastomeric material. However, these materials limit the temperature and pressure rating of the diaphragm valve, as they weaken at high temperatures and pressures. Diaphragm materials must be chosen based on the service temperature and pressure, nature of the material being handled, and frequency of operation.

EPDM

Ethylene Propylene Diene Monomer (EPDM) is a synthetic, general-purpose elastomer. It has good corrosion resistance and is suitable for handling acids, alkalis, and alcohols. It is also resistant to ozone. However, it is not compatible with oil and petroleum products. EPDM is also suitable for steam sterilization. EPDM diaphragms operate between -20°F to 230°F.

PTFE

Polytetrafluoroethylene (PTFE, or commonly known as Teflon) is a synthetic fluoropolymer. It has excellent corrosion and chemical resistance which are great in handling strong acids, alkalis, and solvents. Its stiffness creates a high sealing force but requires a larger force to operate. PTFE diaphragms operate between -300F to 3000F. To increase the compressive strength, wear and abrasion resistance, and pressure rating, PTFE is reinforced with glass fibers.

Neoprene

Neoprene is a synthetic rubber commonly used as a diaphragm material in wastewater pipelines. It has good corrosion and abrasion resistance. It can handle fluids with entrained oils, as wells as acids, alkalis, petroleum, explosives, and fertilizers. Neoprene diaphragms operate between -20°F to 200°F.

Butyl Rubber

Butyl rubber has low vapor and gas permeability, making them ideal for gaseous media. It is also suitable for steam sterilization and can handle a variety of acids and alkalis. Butyl rubber diaphragms operate between -4°F to 248°F.

Nitrile Rubber

Nitrile rubber is a multi-purpose rubber that has high strength and abrasion resistance. It can handle gases, fuels, fats, oils, alcohols, and petroleum but are not compatible with acetones, ketones, ozone, and some modified hydrocarbons. Nitrile rubber diaphragms operate between -14°F to 134°F.

Natural Rubber

Natural rubber possesses good abrasion resistance and can handle moderate acids and alkalis. Natural rubber diaphragms are used in abrasives, dilute mineral acids, and brewing. These diaphragms operate between -40°F to 134°F.

Viton

Viton is a fluorocarbon elastomer. It has excellent resistance and compatibility with most chemicals, solvents, and oils, even at elevated temperatures. However, it is not recommended for steam sterilization and in handling ammonia and polar solvents. Viton diaphragms operate between -20°F to 300°F.

Valve Body

The valve body and the bonnet are made from a rigid and strong material to protect the diaphragm valve components. The bonnet can be made from a slightly less corrosion-resistant material since it is isolated from the wetted portion of the valve. To prevent clogging and gumming of sticky and viscous fluids, the inner diameter of the valve body may be smoothly-lined.

Like the diaphragm material, the valve body material must be corrosion resistant and can withstand sterilization requirements. To make the valve more sanitary, materials with antimicrobial properties (e.g., brass and bronze) may be chosen. The valve body can also be lined with materials with antimicrobial properties.

The common valve body materials for diaphragm valves include stainless steel, cast iron, ductile iron, cast steel, brass, bronze, PVC, U-PVC, and CPVC.

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Additional reading:
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Chapter 5: Considerations in Diaphragm Valve Selection and Operation

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The following are the considerations in the selection and operation of diaphragm valves:

Valve Flow Coefficient

The valve flow coefficient measures the capacity of the valve to allow fluids to flow through it. It is defined as the "volume of water at 600F (in US gallons) that will flow through a valve per minute with a pressure drop of 1 psi across the valve". It is a critical parameter in determining the size of the diaphragm valve that will allow a fluid at the desired flow rate to pass through. Mathematically, it is represented by the equation:

Cv = Q√SG/ΔP

Where Cv is the valve flow coefficient, Q is the flow rate in gallons per minute, SG is the specific gravity of the fluid, and ΔP is the pressure drop.

The valve flow coefficient increases with the valve opening and stem travel.

Pressure Drop

Pressure drop refers to the decrease in pressure from the valve inlet to the discharge side. When the pressure drop ratio across the valve to the total system pressure drop is small, the difference in flow rate will be very small until the valve fully closes. Hence, a fast-acting or a quick opening diaphragm valve will be appropriate.

Rangeability

Rangeability is a property of valves which is the ratio of the maximum to the minimum controllable flow rate. It is affected by the size and accuracy of the actuator and geometries of the valve body, diaphragm, and compressor. As the rangeability increases, the diaphragm valve controls a wider range of flow rates.

Valve Sizing

Valve sizing is an important consideration for diaphragm valves intended for throttling applications. The desired volume that will be passing through the valve must be determined, which is affected by flow rate, inlet and outlet temperatures and pressures, specific gravity, and viscosity of the fluid. After obtaining those properties, the valve capacity and pressure drop the diaphragm valve must close against is determined. Several Methods are used in valve sizing; using the pipe geometry factor is one of the techniques used in sizing diaphragm valves.

Chapter 6: Advantages and Disadvantages of diaphragm valves

The advantages of diaphragm valves are the following:

• Diaphragm valves are useful in throttling applications.
• Diaphragm valves are hygienic and extremely clean since the areas or pockets that trap sediments or biofilms are reduced.
• Diaphragm valves are excellent in handling highly viscous, sticky, and particle-containing media.
• There is a low probability of stem leakage to the environment will occur with diaphragm valves.
• The operating mechanism of the diaphragm valve is isolated from the flowing media. Therefore, contamination is less likely to happen. The fluid will not interfere with the operating mechanism. Maintenance and servicing can be done without interrupting the pipeline.

The disadvantages of diaphragm valves are the following:

• Diaphragm valves are used in moderate pipeline temperatures and pressures.
• Diaphragms limit the high hydrostatic pressures.
• The diaphragm may erode when extensively used in severe throttling applications.
• The weir may prevent full drainage of the piping.

Summary

• Diaphragm valves utilize a flexible diaphragm to obstruct, control, or isolate the flow of fluids. The diaphragm moves up or down to increase or decrease the fluid flow rate, respectively.
• The major components of diaphragm valves are valve body, diaphragm, compressor, and valve stem. Other components include the actuator, position indicator, and valve connection.
• Valve stems can either be a piston- or a threaded-type stem. Threaded stems may be rising or non-rising.
• The two main types of diaphragm valves are weir diaphragm valves and straight-through diaphragm valves.
• The types of diaphragm valves based on the application include sanitary diaphragm valves, biotech valves, hygienic valves, process valves, and zero static valves.
• The diaphragm must be made from a flexible, elastomeric material to stretch to the flow pathway. The material for the diaphragm limits the temperature and pressure rating of the diaphragm valve.
• The valve body and bonnet must be made from a rigid and strong material to protect the internal components. To make the valve more hygienic, it must be made from materials with high corrosion resistance, has antimicrobial properties, and can withstand sterilization requirements.
• The considerations in the selection and operation of diaphragm valves are valve flow coefficient, pressure drop, rangeability, and sizing.

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Flow control device

Rubber Lined Diaphragm Valve

Diaphragm valves (or membrane valves) consists of a valve body with two or more ports, a flexible diaphragm, and a "weir or saddle" or seat upon which the diaphragm closes the valve. The valve body may be constructed from plastic, metal, wood or other materials depending on the intended use.

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There are two main categories of diaphragm valves: one type seals over a "weir" (saddle) and the other (sometimes called a "full bore or straight-through" valve) seals over a seat. In general, straight-through diaphragm valves are used in on-off applications and weir-type diaphragm valves are used for control or throttling applications. While diaphragm valves usually come in two-port forms (2/2-way diaphragm valve), they can also come with three ports (3/2-way diaphragm valves also called T-valves) and more (so called block-valves). When more than three ports are included, they generally require more than one diaphragm seat; however, special dual actuators can handle more ports with one membrane.

Diaphragm valves can be manual or automated. Automated diaphragm valves may use pneumatic, hydraulic or electric actuators along with accessories such as solenoid valves, limit switches and positioners.

In addition to the well known, two way shut off or throttling diaphragm valve, other types include: Three way zero deadleg valve, sterile access port, block and bleed, valbow and tank bottom valve.

Valve body

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Many diaphragm valve body dimensions follow the Manufacturers Standardization Society MSS SP-88[1] However, most non-diaphragm valves used in industrial applications are built to the ANSI/ASME B16.10 standard.[2] standard. The different standards makes it difficult to use diaphragm valves as an alternative to most other industrial valves. Some manufacturers offer diaphragm valves that conform to ANSI B16.10 standards thereby making these diaphragm valves interchangeable with most solid wedge, double disc, and resilient wedge gate valves as well as short pattern plug and ball valves.

Actuators

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Diaphragm valves can be controlled by various types of actuators e.g. manual, pneumatic, hydraulic, electric etc. The most common diaphragm valves use pneumatic actuators; in this type of valve, air pressure is applied through a pilot valve into the actuator which in turn raises the diaphragm and opens the valve. This type of valve is one of the more common valves used in operations where valve speed is a necessity.

Hydraulic diaphragm valves also exist for higher pressure and lower speed operations. Many diaphragm valves are also controlled manually.

Body materials

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Body lining materials

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Depending on temperature, pressure and chemical resistance, one of the following is used:

Diaphragm materials

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• Unlined or Rubber Lined Type:
  • NR/Natural Rubber
  • NBR/Nitrile/Buna-N
  • EPDM
  • FKM/Viton
  • BUNA-N
  • SI/Silicone rubber
  • Leather
• Fluorine Plastic Type:
  • FEP, with EPDM backing
  • PTFE, with EPDM backing
  • PFA, with EPDM backing

Applications

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Diaphragm Valves are ideally suited for:

• Corrosive applications, where the body and diaphragm materials can be chosen for chemical compatibility. (E.G. Acids, Bases etc.)
• Abrasive applications, where the body lining can be designed to withstand abrasion and the diaphragm can be easily replaced once worn out
• Solids entrained liquids, since the diaphragm can seal around any entrained solids and provide positive seal
• Slurries, since the diaphragm can seal around entrained solids and provide positive seal

Markets

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Diaphragm valves have many applications in the following markets:

See also

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References

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If you are looking for more details, kindly visit diaphragm valve straight through type.