The principle of a diaphragm valve is simple. It consists only of three main components: valve body, diaphragm and actuator or manual bonnet. Usually the body is a 2/2 way body independent of the flow direction, and at its centre is a weir. The diaphragm is pushed onto the weir by a compressor which is connected to the actuator or bonnet via the valve spindle, thus closing the valve.
Valve body as well as compressor and actuator/bonnet are manufactured from cast iron, SG iron 40.3 or various stainless steels depending on the application. To increase the valve service life in applications with corrosive chemicals and abrasive media bodies are lined with natural rubber and synthetic rubbers or chemically-resistant plastic materials such as PP, PTFE, ECTFE, PFA as well as glass. Valves with thermoplastic bodies are used in special applications involving eg corrosive or high purity media. These bodies are mostly manufactured from PP, PVC, PVDF or PFA. Diaphragm materials are EPDM, Butyl, Neoprene, Hypalon, Perbunan as well as fluoroplastics such as PTFE and FPM. When designing diaphragm valves, the materials of valve body and diaphragm must match. Only if the thermic, mechanical and chemical resistance of these materials is similar can a long service life be ensured.
As well as manual bonnets, pneumatic, hydraulic or motorised actuators are available. In particular the last two impart diaphragm valves true regulatory properties for liquids, which are sufficient for several applications although they do not measure up to the globe valves and the special control valves available.
The diaphragm can be replaced with the valve in-line. This is a great advantage particularly with welded-in valves and large nominal sizes. The diaphragm change only takes a few minutes. No other seals have to be exchanged. Thus the maintenance requirements are low. In small nominal sizes the initial cost and maintenance are more cost effective than other valve types. This is particularly true for high-grade lined versions. The valves can be used at working temperatures ranging from -10 to 160°C depending on valve body, lining and diaphragm material. DN 4 to DN 350 are standard nominal sizes.
These advantages, however, have a few limitations. Diaphragm valves are suitable for use with working pressures up to 10 bar, dependent on their nominal size and in rare cases up to 16 bar. With larger diameters the permissible working pressure may only be 3 bar.
Special measures enable considerable improvement of the properties of some of the valve types, however this applies almost exclusively to niche products which are not subject to the normal pricing structure.
Thomas Schulz is with GEMÜ Gebr Müller Apparatebau GmbH & Co KG, Ingelfingen Germany. www.gemue.de