There are many types of plastic valves. Ball valves provide tight shut-offs, but are not suitable for sanitary applications. Butterfly valves permit flow in only one direction. Check valves are self-actuating and prevent the reversal of process flow. Diaphragm valves separate the flow of gases from the closure element. Directional valves steer flow through selected passages. Diverter valves redirect process flow. Drain valves reduce surplus media. Float valves open or close automatically as the level of a fluid changes. Foot valves are check valves with a built-in strainer. Gate or knife valves are linear motion valves in which a closure element slides into the flow to shut off the stream. Globe and pinch valves are other types of linear motion devices. Needle valves have a slender, tapered point at the end of a valve stem. Poppet valves open and close ports with a sealing device and spring. Plug or stop-cock valves are designed for both on/off and throttling functions. Other types of plastic valves include sanitary or hygienic valves, sampling or dispensing valves, shut off valves, solenoid valves, and toggle valves.