Valve Types and Applications–III
Diaphragm valves use flexible membranes to close the fluid in the pipeline. Like pinch valves, the diaphragm completely seals the drive device from the process fluid, which is beneficial for valves in sanitary facilities. The main specifications include port configuration, port connection, valve size, medium and sealing material. Diaphragm valves are mainly used in the pharmaceutical, cosmetic, food and semiconductor industries. Sometimes, a control valve driven by a pneumatic diaphragm is erroneously called a “diaphragm valve.” Readers should pay attention to this distinction.
Gate valves are mainly used to prevent fluid flow and are unlikely to be used for flow regulation. The gate valve uses a plate-shaped barrier that can be lowered into the water flow to stop the water flow. Its operation is similar to that of a stop valve, except that when the valve is in the fully open position, the flow limit provided by the gate is less than the flow limit of the stop valve plug. The main specifications include port configuration, port connection, valve size, and the materials that make up the valve body, valve seat, seal, lining and stem packing.
Gate valves can use wedge plugs or parallel plates. The valve plug usually seals the upstream and downstream sides of the valve, while the valve plate usually seals only the upstream surface. The wedge can adopt a variety of design schemes to reduce or adapt to the wear of the sealing surface. Although gate valves have the advantage of having less head loss when opened compared to shut-off valves, they are not suitable for throttling and may not produce the positive closing provided by the shut-off valves. Gate valves are used in wastewater treatment plants, power plants and process plants for shutting down and isolation.
Gate valves are usually designated as rising and rising stem designs. The advantage of rising stem valves is that they can easily observe whether the valve is open or closed. The advantage of non-rising stem or NRS valves is that the stem is protected from corrosion or other environmental conditions by the bonnet. Both designs have little impact on the actual valve function.