2012-08-08
DIAPHRAGM VALVES
Whenever high purity or very corrosive media or media
containing solids in suspension have to be conveyed and dosed in
chemical processes, diaphragm valves are the first choice. The greatest
advantage of this valve design is that only the valve body and diaphragm
come into contact with the medium. Any particles are safely held in
place by the soft diaphragm and carried off when the valve is opened.
Whilst observing a rubber hose, a mining engineer working in a coal mine
in South Africa at the end of the twenties had an idea: If you bend the
hose, the flow of the medium is completely shut off, if you release it,
it returns to its original state without being damaged. Based on this
observation the engineer developed the diaphragm valve. 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.
The valve body as well as the compressor and actuator/bonnet are
manufactured from cast iron, S.G. 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
corrosive or high purity media for example. 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 lined or
unlined valve bodies and the diaphragm must match. Only if the thermic,
mechanical and chemical resistance of these materials is similar can a
long service life be ensured
