TYPES OF VALVES
Diaphragm valves are, in effect, simple "pinch clamp" valves. A resilient, flexible diaphragm is
connected to a compressor by a stud molded into the diaphragm. The compressor is moved up
and down by the valve stem. Hence, the diaphragm lifts when the compressor is raised. As the
compressor is lowered, the diaphragm is pressed against the contoured bottom in the straight
through valve illustrated in Figure 14 or the body weir in the weir-type valve illustrated in
Diaphragm valves can also be used for throttling service. The weir-type is the better throttling
valve but has a limited range. Its throttling characteristics are essentially those of a quick-
opening valve because of the large shutoff area along the seat.
A weir-type diaphragm valve is available to control small flows. It uses a two-piece compressor
component. Instead of the entire diaphragm lifting off the weir when the valve is opened, the
first increments of stem travel raise an inner compressor component that causes only the central
part of the diaphragm to lift. This creates a relatively small opening through the center of the
valve. After the inner compressor is completely open, the outer compressor component is raised
along with the inner compressor and the remainder of the throttling is similar to the throttling that
takes place in a conventional valve.
Diaphragm valves are particularly suited for the handling of corrosive fluids, fibrous slurries,
radioactive fluids, or other fluids that must remain free from contamination.
The operating mechanism of a diaphragm valve is not exposed to the media within the pipeline.
Sticky or viscous fluids cannot get into the bonnet to interfere with the operating mechanism.
Many fluids that would clog, corrode, or gum up the working parts of most other types of valves
will pass through a diaphragm valve without causing problems. Conversely, lubricants used for
the operating mechanism cannot be allowed to contaminate the fluid being handled. There are
no packing glands to maintain and no possibility of stem leakage. There is a wide choice of
available diaphragm materials. Diaphragm life depends upon the nature of the material handled,
temperature, pressure, and frequency of operation.
Some elastomeric diaphragm materials may be unique in their excellent resistance to certain
chemicals at high temperatures. However, the mechanical properties of any elastomeric material
will be lowered at the higher temperature with possible destruction of the diaphragm at high
pressure. Consequently, the manufacturer should be consulted when they are used in elevated