A Butterfly valve is a type of flow control device, used to make a fluid start or stop flowing through a section of pipe. The valve is similar in operation to a ball valve. A flat circular plate is positioned in the center of the pipe. The plate has a rod through it connected to a handle on the outside of the valve. Rotating the handle turns the plate either parallel or perpendicular to the flow of water, shutting off the flow. It is a very robust and reliable design. However, unlike the ball valve, the plate does not rotate out of the flow of water, so that a pressure drop is induced in the flow.
There are three types of butterfly valve:
Resilient butterfly valve which has a flexible rubber seat. Working pressure up to 1.6 Mpa
High performance butterfly valve which is usually double eccentric in design . Working pressure up to 5.0 Mpa
Tricentric butterfly valve which is usually with metal seated design. Working pressure up to 10.0 Mpa
Butterfly valves are also commonly utilised in conjunction with carburetors to control the flow of air through the intake manifold and hence the flow of fuel and air into an internal combustion engine. The butterfly valve in this circumstance called a throttle as it is 'throttling' the engines aspiration. It is controlled via a cable or electronics by the furthest right pedal in the drivers footwell (although adaptions for hand control do exist). This is why the accelerator pedal in some countries is called a throttle pedal.