A diaphragm valve is a manual or control type block valve which uses a diaphragm as the closing device. The diaphragm valve is usually used to isolate or block flow. Because the material of the membrane can chemically degrade, diaphragm valves are used under the conditions of low pressure and a limited temperature operating range.
Diaphragm valves may be used for regulation of most gases and liquids. There are two types of diaphragm valves available, weir and straight through flow. These two configurations have many types of diaphragms and linings that are incorporated into them. Since diaphragms are subject to wear and tear, maintenance is frequently needed for valves that are used on a regular basis. When operating with a diaphragm valve, one must be aware of the operating pressures and temperatures of the process because the valve is restricted by the physical limitations of the diaphragm membrane.
Valve selection is very important to biochemical processes. Diaphragm valves should be used for processes where the absence of dead space inside the valve is essential. Diaphragm valves are generally selected to provide low resistance to flow by means of a straight through flow configuration. Both weir valves and straight-through valves will be discussed in the following sections.
Diaphragm valves are normally operated by manual intervention or by powered actuators. When manually operated, a handwheel type closing device is used. Diaphragm valves may also be operated remotely by using actuators. These actuators use fluid pressure (pneumatic) or an electrical signal to open or close the valve.
The weir type valve provides tight shut-off with comparatively low operating force. The weir valve also produces only a short diaphragm movement which, in the long run will have an effect on the length of the diaphragm's life. By doing this it reduces the need for maintenance. Also, the weir valve is better at throttling flow than the straight through configuration. However, a disadvantage of the weir configuration is that at very low flowrates, the flow control is poor.
The straight-through configuration is better than the weir configuration at handling viscous fluids, thick slurries, and fluids containing solids. The straight through flow also facilitates line clearing. However, one serious flaw in the straight-through type valve is it's has long diaphragm movements. These longer movements decrease the diaphragm's life and increase the need for maintenance. Furthermore, since a more flexible diaphragm is needed, the diaphragm material choice is limited to elastomers.
A diaphragm check valve is not as commonly used as the valves mentioned above, but it is a worth consideration. Some of the characteristics which make it a reasonable choice are free unrestricted movement, full closure, fast closure response, and the abilty to handle viscous or abrasive fluids and slurries more reliably than other types of check valves. The small sizes are conical, and larger sizes are typically nozzle type. The design of a diaphragm check valve provides stable operation with pressure variations, pulsing flow, and frequent flow reversals. One of the drawbacks of the diaphragm check valve is that a lot of care is required in the selection of suitably durable materials for the diaphragm.