Pneumatic Diaphragm Valve

Pneumatic diaphragm valve manufacturers make the valve in a way that makes it able to handle gaseous fluids, liquids, and semisolids media such as colloids, sludge, and brackish water well, and slurries. Pneumatic diaphragm valves are suitable for controlling liquids with some solid particles. Relative to other valves, these valves employ a simple construction design. In pneumatic diaphragm valves, it is rare to find a buildup of biofilms and sediments due to minimum contact on the internal components of the valve. As such, these valves are used in many applications such as pharmaceuticals, food processing, sewage pipelines, water treatment, pulp, and paper production, electronics manufacturing among others. 

pneumatic diaphragm valve

What is a pneumatic diaphragm valve? 

A pneumatic diaphragm valve is a type of diaphragm valve that uses a pneumatic actuator. This type of valve uses a flexible diaphragm to control, obstruct, or isolate fluid flow. Pneumatic diaphragm valve manufacturers design the valve such that the diaphragm is used as the element controlling the flow that flexes down or upwards to decrease or increase the rate of fluid flow respectively. The sealing action of a pneumatic diaphragm valve is generated when a diaphragm is pressed on a solid dam in the valve body. Pneumatic diaphragm valves are categorized as linear valves that require linear motion of the elements controlling the flow. The name pneumatic diaphragm valve means that the type of actuator used in this type of valve is a pneumatic actuator. Most diaphragm valves manufactured use pneumatic valves, especially where valve speed is necessary. A pneumatic actuator uses air pressure which is applied via a pilot valve into an actuator that raises the diaphragm to open the valve. 

Components of a pneumatic diaphragm valve

The figures below help to illustrate the components of a pneumatic diaphragm valve. 

Figure: Components of pneumatic diaphragm valve.

Bonnet 

This is the part of a pneumatic diaphragm valve that serves as the top cover. It is usually bolted on the valve body. This part helps to protect other parts of the valve such as the stem, compressor, diaphragm, and other components of the pneumatic diaphragm valve. 

Valve body 

This is the part of a pneumatic valve used to directly connect the pipeline where the fluid passes through. The area of the flow depends on the type of the pneumatic diaphragm valve. Pneumatic diaphragm valve manufacturers make the bonnet and valve body from strong materials that are resistant to corrosion and rigid. 

Diaphragm 

This is the part of a pneumatic diaphragm valve that moves to touch the valve body’s bottom part to obstruct or limit fluid flow. The diaphragm is made of a highly polymeric disc. The diaphragm part lifts when the flow rate needs to be increased or when the valve needs to be opened fully. The fluid then flows under the diaphragm. The diaphragm reduces the operating temperature and pressure of the valve because of its structure and material. As such, the diaphragm needs to be replaced after a certain period since its mechanical properties weaken during usage. This component isolates non-wetted parts that are actuator, stem, and compressor from the flowing fluid. As such, viscous fluids and solids are not likely to interfere with the working mechanism of a pneumatic diaphragm valve. This technology also helps to protect non-wetted parts from getting corroded. Also, the fluid being delivered will not be contaminated with lubrication used in operating the pneumatic diaphragm valve. 

Compressor 

This is the disc that lies between the diaphragm and valve stem. This compressor is used to support the pneumatic diaphragm valve and distribute power from the stem during linear movement. Pneumatic diaphragm valve manufacturers design the compressor in a way to optimizes flow control and throttling. 

Stem 

This component is a vertical shaft that is connected to the compressor to exert linear motion to help in moving the diaphragm and compressor, hence operating the pneumatic diaphragm valve. The stem is used to transmit power exerted by the pneumatic actuator in a pneumatic diaphragm valve. The pneumatic diaphragm valve manufacturers make the valve to have a threaded stem or piston stem. As the name suggests, the piston-type valve is moved by piston assembly in the bonnet with the stem as the piston rod. This design employs power exerted by fluid pressure. For the threaded stem, the valve employs a matching stem nut. The threaded stem needs torque to move the stem linearly and also for lubrication to enhance smooth running. 

Actuator 

The actuator is used to move the compressor, diaphragm, and stem altogether. The actuator is used to provide linear force or torque needed by the pneumatic diaphragm valve to quickly control the flow rate of the fluid. The design of the actuator depends on valve stem construction. For a pneumatic diaphragm valve, the type of actuator used is a pneumatic actuator. A pneumatic actuator uses air pressure to power the piston in the valve bonnet. The piston rod of this actuator is connected to the valve compressor. The air pressure is provided on the piston chamber on either side. When the air is moved into the piston upper chamber, it forces the piston rod to move downwards and reduce the flow rate of fluid or close the valve. If air pressure is supplied into the lower chamber, it forces the piston rod upwards increasing the flow rate of the fluid. In the piston and piston rod, there are O-rings to prevent air leakage across the chambers. Pneumatic actuators offer fast control in throttle pneumatic diaphragm valves and enhance on and off applications. 

How does a pneumatic diaphragm valve work?

The pneumatic diaphragm valve employs a pinching method to control the flow of fluid. The diaphragm is connected to the compressor. The compressor is also is connected with the stem. To increase the flow rate, the stem moves upwards. While the stem moves upwards, it also moves the compressor upwards. The compressor motion further is transferred to the diaphragm and thus the diaphragm moves upwards. When the diaphragm starts moving upwards, the flow of fluid increases. To reduce or close the flow of fluid, the stem moves downward. The stem transfers the power to the compressor. The compressor then presses on the diaphragm downwards and it then closes or reduces the flow. Some pneumatic diaphragm valves are designed with throttling fluid flow. In this case, they can partially close and partially open.

Figure: Working a pneumatic diaphragm valve.

Types of pneumatic diaphragm valves 

Weir pneumatic diaphragm valve 

This is a very famous pneumatic diaphragm valve. This type of valve is suitable for leak-free and small controls because of the diaphragm and actuator bonnet. The weir pneumatic diaphragm valve manufacturers design the valve with an inclined body to provide the self-draining capability. The self-draining ability occurs in two directions of the valve. As such, this valve is not suitable for some applications. The weir pneumatic diaphragm valve is best suited for hazardous and corrosive liquids and gases because this valve bonnet can control failure of the diaphragm or valve leakage. These valves are mainly used for clean and homogenous liquids since contaminations and viscous sludge can accumulate on the saddle side. These valves are often used in applications such as corrosive, water, food processing, gas production, and chemical applications. 

Straight-way pneumatic diaphragm valve 

The straight-way pneumatic diaphragm valve is also known as an in-line pneumatic diaphragm valve. It is identical in structure to weir type pneumatic diaphragm valve. However, this valve is designed with straight flow path instead of a saddle characteristic. The diaphragms in these valves are flexible. The diaphragm makes contact with the bottom of the valve which helps to increase the travel distance of the diaphragm. The straight-way pneumatic diaphragm valve life span is short relative to the weir type valve due to its flexibility. Also, this valve needs replacement and repairs frequently. This pneumatic diaphragm valve are used for applications such as sludge, water, viscous liquids, and other applications that need to reduce clogging. 

Process pneumatic diaphragm valve 

These are pneumatic diaphragm valves commonly used to regulate, start or stop fluid flow. 

Sanitary pneumatic diaphragm valve

This is a pneumatic diaphragm valve used to prevent fluid contamination. Germ-free materials are used to manufacture these pneumatic diaphragm valves. These valves can promote a germ-free atmosphere. 

Zero static pneumatic diaphragm valve

These are valves are most suitable for clean applications since it is capable of removing bacterial growth and flow stagnation. 

Shut-off pneumatic diaphragm valve

This is a type of pneumatic diaphragm valve that employs a positive closure to prevent fluid flow. 

Pneumatic diaphragm solenoid valve

This is a pneumatic diaphragm valve used to control flow. This valve is made with a rubber diaphragm in its valve body. This valve can act directly or indirectly. It closes and opens against the hard seat. 

Pneumatic diaphragm valve selection criteria 

Valve size 

Pneumatic diaphragm valves are often used in throttling applications. As such, the size of the valve largely affects their working abilities. To ascertain the size of the valve required for your application, find out the required fluid volume through the system. This is the case where you need to find out details about your fluid flow and its specific viscosity or gravity and the needed flow rate. Next is to find the maximum outlet/inlet pressure and temperature besides the required valve capacity and forces required to block the fluid flow. Such details can then be used to find out the correct ratio of outer and inner valve diameters. 

Pressure range and pressure drop 

The value of pressure drop across the valve is very important. As such, you need to specify this value. However, if not sure about this value, you can select a percentage large enough to create a fluid flow difference. Also, determine the pressure ranges that the pneumatic diaphragm valve can experience when the system operates. This helps to select a safety valve that can operate within the entire range. 

Type of pneumatic diaphragm valve and actuator

Using the above details for valve selection parameters, estimate the diaphragm to use for your process fluid flow. Next, determine if the stem and the body of the pneumatic diaphragm valve can be exposed to media or if needs to be separated. 

Type of connection, features, and material 

Before purchasing the valve determine how the valve needs to be connected to the pumping system. The connection could be by use of threads, flanges, welded connections, or other procedures specific to the needs of that application. Next is to determine the suitable material properties of your pneumatic diaphragm valve to ensure that the process fluid will not negatively affect the working of the valve. For instance, very corrosive media makes it necessary to have corrosion-resistant materials such as bronze, stainless steel, and other materials that cannot degrade under caustic environments. Next is to determine other features such as safety measures, position indicators, instrumentation, and others that will help the valve to work efficiently and safely. 

Applications of the pneumatic diaphragm valve

• Pharmaceutical manufacturing systems. 
• Water treatment plant. 
• Food and beverage processing. 
• Chemical processing plants. 
• Corrosive applications. 
• Vacuum services. 
• Power generation plants.
• Regulate clean or dirty air and water. 

Advantages of a pneumatic diaphragm valve 

• They can be used on-off flow and throttling applications. 
• Pneumatic diaphragm valves do not have stem leakages. 
• It offers good chemical resistance due to many linings. 
• It provides a bubble-tight service. 
• Do not have pockets that can trap contaminants. 
• They offer the best service in slurry and viscous fluids. 
• Provide a superb choice for use in hazardous and radioactive chemicals. 
• These valves do not allow impurities in the fluid which makes them suitable for use in breweries, pharmaceuticals, food, among applications. 

Disadvantages of a pneumatic diaphragm valve

• Weir of pneumatic diaphragm valve can cause incomplete drainage piping. 
• These valves can restrict hydrostatic pressure. 
• Must have its body made of good corrosion-resistant material. 
• Not appropriate for high-pressure applications such as above 300 psi. 
• These valves can corrode if used in heavy throttling services that have contamination. 
• These valves can only be used for applications of moderate temperatures of around 400 oF.
• Do not work perfectly in multi-turn operations. 

Troubleshooting pneumatic diaphragm valve

Rupture on Diaphragm 

• Fluor plastics, rubber aging the diaphragm. Do regular replacement. 
• Very high operating pressure crushing the diaphragm. Reduce operating force and check on the closing mark. 
• Foreign materials clogged between the valve seat and diaphragm, wear the diaphragm. Do not force operation to close, repeat operation up and down for a while, wash away foreign materials, on-time replacement of the diaphragm, formal closing of the pneumatic diaphragm valve. 
• The height is open a lot breaking the diaphragm. Do not open too high. 

Failure to operate

• Disc and diaphragm falling off. Avoid opening too high. Prevent falling off by doing timely repair or replacing the diaphragm. 
• Disc and valve stem connection pinned off or broken. Check wear and removal methods. When an opening does not open beyond the dead point. Repair falling off. 
• Worn out pneumatic diaphragm valve cover and stem nut and stem joint. To avoid this ensure cleaning regularly, coat active parts with graphite and dry powder molybdenum disulfide for lubrication. 

Fluid flow not turning off

• Check if the diaphragm is blocked. Unscrew bonnet screws and remove bonnet and cap going over the diaphragm. Clean and replace the diaphragm. Try to clean the filter but if necessary replace it. 

Fluid leakage

• This could be due to dirt accumulation in the diaphragm. Flush out the pneumatic diaphragm valve. Remove dirt that could be clogged in the diaphragm. 

Summary:

Pneumatic diaphragm valves are diaphragm valves that employ pneumatic valves to enhance fluid flow control. These valves use diaphragm components to control, obstruct or isolate fluid flow. The diaphragm moves down or upwards to decrease or increase the rate of fluid flow respectively. The main parts of the pneumatic diaphragm valve are the diaphragm, valve body, stem, pneumatic actuator, compressor, and valve connection and position indicator. The valve stems could be either threaded or piston type.

There are several types of pneumatic diaphragm valves such as straight-way pneumatic diaphragm valves, weir type pneumatic diaphragm valves, sanitary pneumatic diaphragm valves, zero static pneumatic diaphragm valve among others. These types of valves depending on the application of the valve. The diaphragm of a pneumatic diaphragm valve needs to be made of elastomeric and flexible material to help stretch to the fluid flow pathway.

The pneumatic diaphragm valves are used in many applications such as corrosive fluid control, power generation, vacuum services, food, and chemical processing, and water treatment plants. These valves have superb characteristics which make them useful in many industrial applications. The advantages of these valves include bubble-tight ability, being used in slurry and viscous fluids, do not allow impurities, high chemical resistance, and can trap contaminants. These valves can work for moderate pressures of 300 psi. However, such disadvantages cannot outweigh the advantages of pneumatic diaphragm valves.