Valves are mechanical or electro-mechanical devices that are used to control the movement of liquids, gases, powders, etc. through pipes or tubes, or from tanks or other containers. In most instances, valves rely on some form of mechanical barrier—a plate, a ball, a diaphragm, for example—that can be inserted and removed from the flow stream of the material passing by. Some valves are designed as on-off varieties, while others allow very fine control of the passage of media.
Types of Valves and Their Applications
Ball Valves are quarter-turn valves incorporating ported spheres that swivel in the pipe stream to either block, or allow, flow. Special designs are available which enable a degree of flow regulation. Key specifications include the number of ports, port configuration, port connections, valve size, and the materials that make up the valve body, its seat, seal, and stem packing. Ball valves are used practically anywhere a fluid flow must be shut off, from a compressed-air line to a high-pressure, hydraulic system. Ball valves can provide low head-loss characteristics as the port can exactly match the pipe diameter. Ball valves also tend to seal better than butterfly valves, but they can be costlier to purchase and maintain. Typically they are actuated with a lever which provides a visual indication of the valve status.
Check Valves permit fluid to flow through them in one direction only. Lift-type check valves are similarly constructed as globe valves and use a ball or piston, often backed by a spring that opens under a specified pressure but closes as the pressure decreases, thus preventing backflow. These valves are often suited for high-pressure applications. A variant is the stop check valve which doubles as a shut-off valve.
Gate Valves are used mainly for blocking fluid flow and are less likely to be employed for flow regulation. A gate valve uses a plate-like barrier that can be lowered into the flow stream to stop the flow. Its operation is similar to that of a globe valve except the gate provides less flow restriction than with a globe-valve plug when the valve is in the fully opened position.