We have learned that valves are used to either shut-off flow, change the flow path or throttle flow, and that valves are characterized by how their flow coefficient varies with relation to closure member position, but valves are typed according to the motion of their closure member. The two types of control valves are linear motion and rotary motion.
Linear motion valves have a closure member that moves with a linear motion to modify the rate of flow through the valve. Linear motion valves are generally named for the shape of their closure member. Common linear motion valves include globe, gate, diaphragm and pinch valves.
Globe valves are so named for their globular shaped cavity around the valve seat area. The closure member of a globe valve is a plug with a flat or convex bottom that is lowered onto a matching horizontal seat located in the center of the valve. Raising the plug opens the valve, allowing fluid flow. Globe valves have good throttling characteristics but because the flow path is not linear they have a relatively high pressure drop across the valve. Globe valves are used in throttling and shut-off applications where this pressure drop is acceptable. The three primary body designs for globe valves are Z body, Y body and angle.
The closure member of a gate valve is a flat face, vertical disc, or gate that slides down through the valve to block
the flow. Gate valves are designed to operate in their fully open or fully closed position and therefore are found only in
flow shut-off applications. When fully open the disc is removed completely from the flow stream. This offers
virtually no resistance to flow when the valve is fully open, therefore gate valves operate with little pressure drop across the valve. Gate valves have very poor flow throttling characteristics and are not used for throttling purposes.
The closure member of a diaphragm valve is a flexible surface (the diaphragm) that is deformed. The main advantage of a diaphragm valve is that the stem seal is eliminated. Diaphragm valves are used mostly for shut-off service of slurries, corrosive or viscous fluids but may also be used in flow throttling applications as well.
Diaphragm valves may be used in pumping applications with a set constant pressure on the diaphragm. This allows flow to be stopped in the absence of a motive force (pump), but when a sufficient pressure is generated in the pipe to overcome the force on the diaphragm flow is allowed.
A pinch valve is similar to a diaphragm valve, however in a pinch valve the entire valve body is flexible and the closure member pinches the valve shut closing off flow. As a pinch valve has no internal obstructions it has a very low pressure drop and is well suited for applications of slurries or liquids with large amounts of suspended solids
Rotary valves have a closure member that moves with a rotary motion to modify the rate of flow through the valve. Like linear motion valves, rotary motion valves are generally named for the shape of their closure member. Common rotary motion valves include ball, butterfly and plug valves.
The closure member of a ball valve is shaped like a ball with a port for fluid flow. A ball valve allows straight-through flow in the open position and shuts off flow when the ball is rotated 90 degrees. Because of their quarter turn
actuation and low pressure drop ball valves are commonly found in flow shut-off applications. Depending on the particular flow port configuration of the ball they may be used in flow throttling applications as well.
The closure member of a butterfly valve is a circular disc or vane with its pivot axis at right angles to the direction of flow in the pipe. Like ball valves, a butterfly valve allows straight-through flow in the open position and shuts off flow when the ball is rotated 90 degrees. Because of their quarter turn actuation and low pressure drop butterfly valves are commonly found in flow shut-off applications. Unlike ball valves, butterfly valves are generally not used for flow
throttling applications. The advantage of a butterfly valve over a ball valve is its relative compactness
The closure member of a plug valve is a cylindrical or tapered cylindrical shaped plug with a flow port.
Like a ball valve, a plug valve allows straight-through flow in the open position and shuts off flow when the ball is rotated 90 degrees. Like ball valves, plug valves are found mostly in flow shut-off applications. However plug valves are available in much larger sizes that ball valves.