Diaphragm Valve: Detailed Overview and Selection Guide
Figure 1: An electrically actuated diaphragm valve
Diaphragm valves are control valves that can allow, block, and throttle media flow. Their design allows them to handle fluids with solid particles (e.g., slurries), and they also work well in applications with high sanitation requirements (e.g., food and beverage and pharmaceutical). This article provides a complete overview of diaphragm valves to assist in selecting them for an application.
Table of contents
- What is a diaphragm valve?
- Operating principle
- Selection guide
- Advantages and disadvantages
What is a diaphragm valve?
A diaphragm valve controls the media flow using a flexible diaphragm that moves up and down to allow or block flow. Diaphragm valves can also throttle fluids, controlling their flow rates. These valves can be actuated manually, electronically, or pneumatically. A diaphragm valve's relatively simple design prevents a significant build-up of particles (e.g., silt) from the fluid, which means diaphragm valves do not easily get clogged. Therefore, diaphragm valves are especially suitable for slurry media.
As seen in Figure 2, a diaphragm valve has the following primary components:
- Actuator (A): A manual diaphragm valve has a handwheel for actuation. Pneumatic and electronic actuators can be used in place of the handwheel.
- Stem (B): The stem connects the actuator to the compressor. The actuator moves the stem up and down, which moves the compressor up and down.
- Compressor (C): The compressor is disc-shaped. It connects the stem to the diaphragm and distributes the stem's force to improve throttling and control.
- Diaphragm (D): The diaphragm is a rubber disc that rises to allow flow and lowers to the valve seat to block flow.
- Bonnet (E): The bonnet bolts onto the valve body. It protects the valve stem and packing from the environment. Bonnets in properly functioning diaphragm valves do not interact with the media.
- Valve body (F): Media flowing through the valve goes through the valve body. The volume of the valve body determines the valve's flow rate when it is fully open.
Figure 2: The components of a diaphragm valve: handwheel/manual actuator (A), stem (B), compressor (C), diaphragm (D), bonnet (E), and valve body (F).
A diaphragm valve uses an elastomeric or plastic diaphragm to control flow through the valve. Diaphragm valves are multi-turn valves, meaning the stem must rotate greater than 360° for the valve to open or close fully. When actuated, the diaphragm moves towards or away from the valve seat. The amount the diaphragm is lifted away from the seat controls the flow rate. When the diaphragm closes, its edges press against the valve seat's edges, and the rest of the diaphragm deforms. The diaphragm's shape, when it deforms, fits perfectly with the bottom of the valve seat and creates a tight seal.
Diaphragm valves come in two main types: weir diaphragm valves and straight/full port diaphragm valves.
Weir diaphragm valves
Weir diaphragm valves have a raised ridge (weir) in the center of the valve body. The diaphragm presses against the ridge to form a seal. Due to this ridge, there is less travel distance for the valve to close fully, which puts less stress on the diaphragm. This diaphragm valve is suitable for controlling low flow rates (vacuum) and high flow rates. A disadvantage is an increased pressure drop across the valve.
Full port diaphragm valve
With full port diaphragm valves, the diaphragm seals to the bottom of the valve seat. No weir obstructs flow. Therefore, there is a lower pressure drop across full port diaphragm valves. These valves are better for high-flow rate applications. A disadvantage is more wear and tear on the diaphragm from the sealing process.
Diaphragm valves come in various materials, making them suitable for multiple applications. Since the valve body and the diaphragm material come into contact with the media, it is important to learn more about material resistance, which is in our chemical resistance of materials guide.
Valve body materials
Typical materials for the valve body are PVC, stainless steel, brass, bronze, and cast iron. The material should withstand the environment and media flowing through it. For example, stainless steel is commonly chosen for applications requiring sterile conditions (e.g., food and beverage).
The diaphragm's material is flexible so that the valve can operate correctly. The materials available for the diaphragm limit the max temperature that the valve can operate at.
- EPDM: EPDM is resistant to corrosion and is compatible with alkalis and alcohols. It is not suitable for petroleum products. This material operates between -6 °C and 110 °C (-20 °F to 230 °F).
- PTFE: PTFE diaphragms are also highly resistant to corrosion and have a much wider temperature range: -184 °C to 1650 °C (-300 °F to 3000 °F).
- Butyl rubber: Butyl rubber diaphragms are not as permeable to gas. That makes these more suitable for gas media applications. Their temperature range is -20 °C to 120 °C (-4 °F to 248 °F).
- Nitrile rubber: Nitrile rubber diaphragms are suitable for petroleum-based media applications. Their temperature range is -25 °C to 56 °C (-40 °F to 134 °F).
Figure 3: A diaphragm valve used in a hot water pumping system.
To ensure that the diaphragm valve has optimal performance, reliability, and longevity, pay attention to the following criteria:
- Media: Understand the media's corrosiveness to determine which material to choose for the valve body and diaphragm.
- Temperature: Ensure the diaphragm's material can withstand the application's minimum and maximum temperatures.
- Flow rate: Select a valve size and style that can provide the flow rate desired for the application.
- Full bore: Low pressure drop and can handle slurries and viscous fluids better.
- Weir: More precise flow control.
- Manual: Cost-effective and direct control of the diaphragm's position.
- Pneumatic: Requires compressed air and provides fast response time.
- Electric: Requires electricity and provides very high precision.
Advantages and disadvantages
Diaphragm valves have several advantages and disadvantages.
- Leak free: The diaphragm protects much of the valve from the media, so there is no need for packing and glands on the stem. Unless the diaphragm breaks, leaking will only occur at connection points and not through the stem.
- Contamination control: Media flowing through the valve only interacts with the valve body and diaphragm, so there is more control over material selection to prevent contamination.
- Flow control: Diaphragm valves have excellent flow control properties over a wide range of flow rates.
- Aggressive media: Diaphragm valves are especially suitable for corrosive, viscous, and abrasive media. They work well with slurries.
- Simple maintenance: The diaphragm can be removed from the valve body without disconnecting the valve from the system. This is advantageous when the diaphragm needs to be replaced.
- Pressure and temperature limitations: Most diaphragm materials have limited pressure and temperature ranges, which control the overall ratings of the valve.
- Diaphragm replacement: The valve's diaphragm is relatively vulnerable to wear and tear, requiring more frequent valve maintenance than with other valve types.
- Slower actuation: Because they are multi-turn valves, diaphragm valves have slower response times than other valve types, such as ball valves and butterfly valves.
Where are diaphragm valves used?
Diaphragm valves can handle liquids with solid particulates. Also, they are used in the food and pharmaceutical industries. There is little interaction between the media and valve components.
How does a diaphragm valve work?
Actuating the valve raises or lowers the diaphragm. The amount raised determines the flow rate. The diaphragm forms a tight seal on the valve seat to close the valve.