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Butterfly Valves
A butterfly valve basically consists of a flat disc that rotates around a shaft. The shaft is connected to an external actuator. The disc is installed in a housing which is connected to a pipe.
From the figure provided here you can see that the actuator can rotate the disc to block the pipe. If the disc is rotated a quarter of a turn, the disc is positioned along the fluid flowpath – thereby allowing the fluid to flow through.
Notice that it only takes a quarter of a turn to move the valve from full closed to full open position.
In butterfly valves, the valve disc remains in the fluid flowpath - whether the valve is open or close. Contrast this with the operation of a gate valve where the valve disc is totally out of the way when the valve is in full open position.