Valves handle a wide variety of functions related to liquids and gases — turning flow on or off, controlling flow rate, and preventing reverse or back flow, as well as regulating and relieving pressure. Valves fall into two general classifications — linear (such as a gate valve) or rotary (such as a ball valve) — based on the action of the closure member. They also are categorized by their closure member’s shape (e.g., gate, globe, butterfly, ball, plug and diaphragm). Selection primarily depends upon the application and the pressure and temperature conditions. In addition, certain services, such as those handling flammable materials, may require valves to be fire-safe or approved for fire protection use.
Some valves are actuated manually. Valves that are located remotely in a machinery package, require frequent operation or must automatically respond to equipment needs (such as to prevent surge in turbo-compressors), or control system demands (such as to adjust flow rate) use a powered actuator.