The primary function of a diaphragm valve is for isolation. Diaphragm valves are susceptible to erosion / flow damage and should therefore not be used for flow control or throttling applications.
The operation of a diaphragm valve is generally by a hand wheel device screwing down a stem with a saddle on the end, which in turn forces an elastomeric diaphragm on to a polished surface inside the valve body.
Diaphragm valves can be either “weir” type or “full bore” type configurations. Energisation of the seal is purely by mechanical force, applied by the hand wheel through the screwed stem and saddle.