Diaphragm valves get their name from a flexible disc which comes into contact with a seat at the top of the valve body to form a seal. A diaphragm is a flexible, pressure responsive element that transmits force to open, close or control a valve. Diaphragm valves are related to pinch valves, but use an elastomeric diaphragm, instead of an elastomeric liner in the valve body, to separate the flow stream from the closure element.

Selection Tip: There is also an automated valve with a diaphragm type actuator with a similar name. Take care to verify what type of valve to which the manufacturer is referring.

Classification

A diaphragm valve is a linear motion valve that is used to start/stop and control fluid flow.

Method Of Control

Diaphragm valves use a flexible diaphragm connected to a compressor by a stud which is molded into the diaphragm. Instead of pinching the liner closed to provide shut-off, the diaphragm is pushed into contact with the bottom of the valve body to provide shut-off. Manual diaphragm valves are ideal for flow control by offering a variable and precise opening for controlling pressure drop through the valve. The handwheel is turned until the desired amount of media is flowing through the system. For start and stop applications, the handwheel is turned until the compressor either pushes the diaphragm against the bottom of the valve body to stop flow or lifts off the bottom until flow is able to pass through.

Video Credit: CTE Skills.com / CC BY-SA 4.0

Diaphragm Valve Function

The diaphragm is connected to a compressor by a stud molded into the diaphragm. To start or increase flow the compressor is moved up by the valve stem. To stop or slow flow, the compressor is lowered and the diaphragm is pressed against the bottom of the valve. Diaphragm valves are excellent for controlling the flow of fluids containing suspended solids and offer the flexibility of being installed in any position. Weir-type diaphragm valves are better at throttling than straight though diaphragm valves because the design's large shutoff area along the seat gives it the characteristics of a quick-opening valve. The diaphragm acts as the gasket of the valve to seal against leaks between the body and bonnet cap.

Diaphragm valve function. Image Credit: PDHEngineering

Types

Diaphragm valves are available in two basic forms: weir type and straight through types. The basic construction of both valves is similar except for the body and diaphragm.

The weir-type design is the most popular type of diaphragm valve and it is best for general use applications or for tough corrosive and abrasive services. They are best used to control small flows. The body of the weir-type has a raised lip that the diaphragm comes into contact with. Weir-type valves use a smaller diaphragm because the material does not have to stretch as far. The material can be heavier so the valve can be used for high-pressure and vacuum services. The weir design is composed of a two-piece compressor component. To create a relatively small opening through the center of the valve, the first increment of stem travel raises an inner compressor component that causes only the central part of the diaphragm to lift instead of the entire diaphragm lifting off the weir when the valve is opened. Once the inner compressor is opened, the outer compressor piece is lifted along with the inner compressor and the additional throttling is similar to the throttling function in other valves. Weir-type bodies have bonnet assemblies recommended for handling dangerous liquid or gas because if the diaphragm fails the hazardous materials will not be released into the surrounding system. They are also recommended for food-processing applications because the body is self-draining.

Weir-type diaphragm valve. Image Credit: TLV.

The straight through type can be used in situations where the flow direction changes within the system. The body of this design has a flat bottom that is parallel to the flow stream. This allows the flow to move uninhibited through the valve with no major obstructions. A flexible material is required for the diaphragm so that the mechanism can reach the bottom of the valve body; this can shorten the life span of the diaphragm. They are excellent for use with sludge, slurries and other viscous fluids but they are not well suited for high temperature fluids.

Straight-through diaphragm valve. Image Credit:Spiraxsarco

Both styles provide a streamlined path for fluid flow that yields minimal pressure drop across the valve.

Media

The flow transfer and control category covers a wide spectrum of products designed to facilitate, control, maintain, meter, or read the flow of material through hoses, pipes or tubing. The material in question can be liquid, gaseous, or semi-solid (colloids and slurries). The following families fall within this category: valves, valve actuators and positioners, dispensing valves, pumps, flow sensing, level sensing, density and specific gravity sensing, viscosity sensing, and miscellaneous related products.

Diaphragm valves are especially suited for handling corrosive fluids, fibrous slurries, radioactive fluids, or other fluids that must remain free from contamination. Since the diaphragm does not come into contact with the media the valve can be used with sticky or viscous fluids, which can get stuck or clog other types of valve mechanisms.

Valve Components

Diaphragm valves have a very basic body construction.

Body

The stem of a diaphragm valve does not rotate.

• The stem of this valve can be non-indicating, which means that the handwheel rotates a stem bushing to engage the stem threads. This moves the stem and attached compressor up and down. The diaphragm is attached to the compressor. Non-indicating bodies can use sealed bonnets with a seal bushing.
• The stem can also be indicating. This design operates the same as the non-indicating design but the stem is longer and extends up through the handwheel. Indicating bodies can use a sealed bonnet with a seal bushing and O-ring.

The bonnet of a valve is the cover of the top of the valve; it contains the nonwetted portion of the valve, the compressor, and the handwheel mechanism. The bonnet is then bolted to the valve body. Diaphragm valve bonnets are quick opening and lever operated. This type of bonnet is interchangeable with the standard bonnet on conventional weir-type bodies. The lever opens and closes the diaphragm with a 90° turn. Diaphragm valves can also be used in vacuum services using a bonnet construction up to 4 inches in size. For larger valves, a sealed, evacuated bonnet should be used.

A compressor is required to operate the diaphragm in the valve. The compressor is located above the diaphragm and below the handwheel stem. The compressor is rounded and shaped much like the body's flow passage.

Actuator

The valve actuator operates the stem and disc to open and close the valve. There are several types of actuators to consider depending on the needs of the system such as the torque necessary to operate the valve, speed and the need for automatic actuation. Actuated varieties can offer features as adjustable opening, positioners for precise flow control, and electric relay of valve position.

Manual/hand operated actuators use a hand-wheel or crank to open or close the valve. They are not automatic but offer the user the ability to position the valve as needed. Manual actuators are used in remote systems that may not have access to power, however they are not practical for applications that involve large valves. Gearheads can be added for additional mechanical advantage and open/close speed.

Electric motor actuators permit manual, semi-automatic, and automatic operation of the valve. The high speed motor is usually reversible and used for open and close functions. The actuator is connected through a gear train to reduce the motor speed and thereby increase the torque. The actuator is operated either by the position of the valve or by the torque of the motor. A limit switch can be included to automatically stop the motor at fully open and fully closed.

Pneumatic operated valves can be automatic or semi-automatic. They function by translating an air signal into valve stem motion by air pressure acting on a diaphragm or piston connected to the stem. These actuators also use a spring and can be programed in a variety of manners, such as using the air pressure to open the valve and spring pressure to close the valve or vice versa. Pneumatic actuators are fast-acting for use in throttle valves and for open-close positioning.

Hydraulic actuators provide for semi-automatic or automatic positioning of the valve. They are used when a large force is required to open the valve, such as a main steam valve. With no fluid pressure, the spring force holds the valve in the closed position. Fluid enters the chamber, changing the pressure. When the force of the hydraulic fluid is greater than the spring force, the piston moves upward and valve opens. To close the valve, hydraulic fluid (such as water or oil) is fed to either side of the piston while the other side is drained or bled. Hydraulic actuators are available in a wide range of sizes and are economical to use in a valve system as well as with a single valve.

Thermally actuated valves are activated by a change in media temperature. A temperature and pressure are preset and the valve will open or close to adjust to the desired specifications.

When selecting an actuator it's important to consider the output power, which is used to overcome resistance and ensure high degree sealing and valve opening.

Material Of Construction

Diaphragms can be made from a variety of materials. The material choice is based on the nature of the material handled, temperature, pressure, and frequency of operation. Elastomeric diaphragm materials are highly resistant to chemicals at high temperatures. However, the mechanical properties of elastomeric materials will be weakened at a higher temperature (above 150°F). High pressure can also destroy the diaphragm. Another factor that can affect the diaphragm function is the concentration of the media it will handle. The material may exhibit satisfactory corrosion resistance to certain corrodents up to a specific concentration and/or temperature. Examples of diaphragm material and their specifications can be seen in the chart below.

Diaphragm materials. Chart Credit: Engineeringtoolbox.com

Selection Tip: If the diaphragm valve will be used in a high pressure, high temperature system, be sure to consult the manufacturers to ensure safe valve design.

Diaphragm valve bodies can be made of materials are plastic lined, rubber lined, glass lined, various solid metals and alloys, and solid plastic. Diaphragm valves are less expensive because only the body and diaphragm need to be chemically compatible with the system media since the rest of the valve components are sealed from the system.

Connection Type

Diaphragm valves are connected into a variety of piping systems. They type of connection should be chose based on pre-existing system requirements and the type of seal desired. Several types of connections are available, such as:

• Threaded- Threaded valve ends have internal or external threads so that a pipe can be screwed into or over the valve end. This is a simple, popular design for many piping systems.
• Compression fitting- Compression fittings seal a pipe connection without soldering or threading. The seal if formed as one of the nuts is tightened, compressing a washer around the second pipe to form a watertight closure.
• Bolt flange- Bolt flanges are connections that can be used at the inlet or outlet of the valve
• Clamp flange- Clamp flanges are a spring hinge flanges that wrap around a pipe to make a connection.
• Tube fitting- Tube fittings allow for direct connection from a tube to a valve.
• Butt weld- A butt weld holds a butt joint, which is a joint between two plates, bars, or sections. The two pieces are butted together and do not overlap or interlock.
• Socket weld/Solder- Socket weld connections between two pieces are soldered together making them difficult to undo.
• Metal face seal- Metal face seals use a metal gasket sandwiched between two fittings parts. The gasket forms a face seal on each side of the fitting.

Performance Specifications

There are several key terms and characteristics for diaphragm valves that effect their operation and performance. For more information on valve performance specifications please refer to the Industrial Valves page on IEEE Engineering360.

"Non-wetted" and "Wetted"

Non-wetted and wetted are terms used to describe the body and stem design.

• Non-wetted valves have the stem and body isolated from the media in the system. Therefore, the stem and body do not need to be made of a corrosive resistant material.
• Wetted valves leave the stem and body exposed to the media in the line. In a diaphragm valve the body and diaphragm are the only wetted pieces of the valve, allowing for chemically compatible materials to be selected for almost any process media.

Flow

Generally, diaphragm valves have an inherent, equal-percentage characteristic. This tends to move toward linear when the valve is installed.

Flow Coefficient

The valve flow coefficient is the number of U.S. gallons per minute of 60°F water that will flow through a valve at a specified opening with a pressure drop of 1 psi across the valve. The coefficient is used to determine the size that will best allow the valve to pass the desired flow rate, while providing stable control of the process fluid. For a control valve, the flow rate is related to the opening of the valve. There are two relationships available to determine flow rate.

Linear- The flow rate is directly proportional to the amount the disc travels. If the disc is open 50%, the flow rate is at 50% of maximum flow.

Equal percentage- The flow rate is related to the percent the valve opening changed in an incremental manner. For example, if the valve changed from 20% open to 30% open and produced a 70% change in flow rate, changing the valve from 30% to 40% open would increase the flow rate another 70%.

Image Credit: Enggcyclopedia.com

Pressure Drop

Pressure drop is the change in pressure that occurs between the inlet and outlet of the valve. It's an important specification to understand when selecting the size of the diaphragm valve needed. If the pressure drop across the fully opened valve is not a large enough percentage of the total system drop, there will be little change in the fluid flow until the valve closes. In this case, a fast-acting valve would be appropriate.

Rangeability

Rangeability is a very important factor when selecting a valve type. It is defined as the maximum to minimum flow rate that can be controlled by a given valve type. The characteristic is affected by three factors: the geometry of the valve, the seat leakage, and the actuator's accuracy or stiffness at near closure of the valve. Geometry is inherent due to the design of the seat and closure and excessive seat leakage can cause instability in the valve as it lifts off the seat.

Rangeability is easily calculated based on the geometry and actuator of the valve. If the valve is not accurate at 5 percent of stroke, then the rangeability is 20:1 (100 percent divided by 5 percent). As the rangeability increases, a wider range of flow rates can be controlled by the valve. It is not imperative that the valve has the highest rangeability because most systems do not have that wide of a flow rate range. V-notched ball valves have the highest rangeability at 200:1, while globe valves have a high rangeability of 100:1. Higher rangeability usually indicates the sensitivity is lower when the closing element is near closed and increases as the valve opens.

Valve Sizing

Sizing is very important when selecting a diaphragm valve as a throttle device. Since there is no pressure drop across an open/close system, the inlet and outlet ports are generally the same size. In this case, the size of the valve is determined by the volume of media going through the system and the flow coefficient. There are several variables to consider when determining sizing for a valve. First is what type of media the valve will be controlling. The specific gravity and viscosity of the media will affect flow rate. Second is the maximum inlet pressure and temperature, along with the outlet pressure (pressure drop) at maximum load. Third is the maximum capacity and last is the maximum pressure drop the valve must close against. The pipe geometry factor can be adjusted for using the chart below.

Piping- geometry factor for valves with reducers and increasers on both ends. Table Credit: Valtek International

Features

Diaphragm valve manufacturer have diaphragm valves available with several different features to fit an application.

· Position indicators- Piston indicators are placed on the top of the ball to indicate the direction of flow.

Applications

Since diaphragm valves have minimal contact surfaces and are considered the cleanest valve (least likely to cause contamination) they have found widespread use in the pharmaceutical, food processing, and water treatment industries. Diaphragm valves are also used in the electronics industry, pulp and paper industry, power industry, and in high-purity water systems.

Standards

Manufactures Standardization Society for the Valves and Fittings Industry: Bubble-tight shut off for zero leakage.

For more information on valve standards, please visit the Industrial Valve Selection Guide.

Resources

Types of Manual Valves

Isolation Valves-Linear Movement

PDHEngineering- Valve Fundamentals

Skousen, Philip L. Valve Handbook. New York: McGraw-Hill, 1998. Print.

Image Credits:

GIE Inc. | Hayward Flow Control