GET DIAPHRAGM VALVE DIAGRAM INFORMATION

What is a Diaphragm Valve?

Diaphragm valves get their name from a flexible disc that 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.

How do Diaphragm Valves Work?

Diaphragm valves are simple in construction and operation. A valve actuator is in contact with the inner membrane (or “diaphragm”) of the valve in its open position. When the user desires the valve to be shut, the actuator is pressed and/or turned and the membrane is pushed into the edge of the solid damn, closing the valve. Certain diaphragm valves can also have half open/restricted positions where the membrane is partially closed, allowing a throttled flow through the valve. In the next section, we will examine the broad categories of diaphragm valves and where they are most useful.

Types of Diaphragm Valves

Weir-Type Diaphragm Valves

The weir-type diaphragm valve is one of the most common designs available and is what is shown in Figure 2, above. The raised lip/saddle design is best suited for small flow control and is kept leak-proof thanks to the bonnet over the diaphragm and actuator. This design is optimal for dangerous or corrosive liquid and gas, as any failure with the membrane will be kept contained by this bonnet. Also, the sloped body makes this valve inherently self-draining, but this draining can happen in both valve directions which may not be optimal for some applications. The weir-type valve is mostly specified for clean, homogenous fluids, as viscous slurries and sediments may build up on either side of the saddle. They are most often found in processes such as food/chemical, gas production, corrosive, and water applications.

Straight Through Type Diaphragm Valves

The straight-through diaphragm valve is structurally similar to weir-types; however, it does not contain the characteristic saddle and instead has a completely straight pathway. Their membranes are typically more flexible and allow for farther travel distance, as it must touch the very bottom of the valve. These valves must often be serviced/replaced, as their lifespan is typically less than weir-type designs due to their more flexible membranes. Straight-through diaphragm valves are used for slurries, viscous fluids, and other applications where blocking must be reduced. They are also useful for bi-directional flow regimes, as there is no saddle to impede swift changing from inlet to outlet.

Diaphragm Valve Parts

The components of diaphragm valves are the following:

Diaphragm Valve Bonnet

The bonnet serves as the top cover and is bolted to the valve body. It protects the compressor, stem, diaphragm, and other non-wetted components of the diaphragm valve.

Diaphragm Valve Body

The valve body is the component directly connected to the pipeline wherein the fluid passes. 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 Valve 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.

Material of Diaphragm Valve Diaphragm

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

Diaphragm Valve Compressor

The compressor is a disc that lies between the valve stem and the diaphragm. It supports the diaphragm valve and distributes the forces from the stem during linear movement. The compressor design is optimized to provide better flow throttling and control.

Diaphragm Valve 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.

Advantages and Disadvantages of Diaphragm Valves

Advantages	Disadvantages
Extremely clean	Can only be used at moderate temperatures (-60 to 450ºF)
Leak proof seal	Can only be used in moderate pressures (approximately 300psi)
Tight shut-off	Cannot be used in multi-turn operations
Easy maintenance	No industry standard face-to-face dimensions
Repairs can be made without interrupting the pipeline	The body must be made of corrosive resistant material
Reduce leakage to the environment