There are different kinds of butterfly valves, suitable for different pressures and different usage. The resilient butterfly valve, which uses the flexibility of rubber, has the lowest pressure rating. The high performance butterfly valve, which is used in slightly higher pressure system, has a slight offset in the disc is placed, increasing the valve's sealing ability and reducing its tendency to wear. The valve best suited for high pressure is the triple offset butterfly valve, which uses a metal seat, and is thus able to withstand a greater amount of pressure.
A butterfly valve is a valve which can be used to isolate or regulate the flow. The closing mechanism takes the form of a disc. The operation is similar to a ball valve which allows for rapid shutdown. Butterfly valves are generally favored because they are lower in cost to other valve designs, and are lighter in weight, meaning less support is required. The disc is placed in the center of the tube which passes through the disc, a rod connected to an actuator on the outside of the valve. Rotating the actuator turns the disc either parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present in the flow, because a pressure loss is always induced in the flow, regardless of the valve position.
A butterfly valve is from a family of valves called quarter turn valves. The "butterfly" is a metal disc mounted on a rod. When the valve is closed, the disc is turned so that it completely blocks off the passageway. When the valve is fully open, the disc is rotated a quarter turn so that it allows an almost unrestricted passage of the liquid. The valve can also be opened incrementally to throttle flow.