The following is quoted from the "Valve selection handbook" by R.W. Zappe (Gulf Publishing):
"Wedge gate valves differ from parallel gate valves in that the closure member is wedge shaped instead of parallel. The purpose of the wedge shape is to introduce a high supplementary seating load that enables metal-seated wedge gate valves to seal not only against high, but also low, fluid pressures. The degree of seat tightness that can be achieved with metal-seated wedge gate valves is therefore potentially higher than with conventional metal-seated parallel gate valves. However, the upstream seating load is not normally high enough to permit block-and-bleed operation of metal-to-metal seated wedge gate valves."
Disadvantages:
"Wedge gate valves cannot accomodate a follower conduit as conveniently as parallel gate valves can.
As the disc approaches the valve seat, there is some possibilityy of the seatings trapping solids carried by the fluid. However, rubber seated wedge gate valves are capable of sealing around small trapped solids.
An electrical drive for wedge gate valves is more complicated than for parallel gate valves in that the drive must be torque-limited instead of travel-limited. The operating torque of the drive must thereby be high enough to effect the wedging of the wedge into the seats while the valve is being closed against the full differential line pressure. If the valve is closed against zero differential pressure, the wedging of the wedge into the seats becomes accordingly higher. To permit the valve to be opened again the full differential pressure, and to allow also for a possible increase of the operating effort due to thermal movements of the valve parts, the operator must be generously sized."
