A weir valve is a type of diaphragm valve in which the flow passage includes a weir, or a notch with a horizontal edge through which a substance flows. In the open position, the diaphragm lifts up and allows a substance to flow freely through the valve. Once in the closed position, the diaphragm closes down on the weir, shutting off flow partially or completely.
A weir valve provides a higher rate of flow than many other types of diaphragm valves, and may be chosen for applications in which this higher flow is desirable. The weir valve can leave some opportunity for fluid to become entrapped in the valve. The diaphragm in a weir valve is designed to seal on a sealing bead outside the weir area. In the open position, the diaphragm lifts up and exposes the valve body along the perimeter of the bowl. As the valve closes, the diaphragm closes back toward the body of the valve, allowing small amounts of fluid to become trapped.
Weir valves may be operated manually, or they may be pneumatic or motorized. They can be constructed from several materials, including cast iron, cast steel, brass or plastics. Depending on the material from which they are constructed, they may be used to control the flow of materials at various pressures and temperatures. Weir valves may have different body configurations.
A straight-through body has one entrance port and one exit port. A t-valve body style allows flow to enter or exit from two ports, and multi-port weir valves allow flow to enter or exit the valve from three or more ports.