A butterfly valve operates on the same basic principle as other quarter-turn valves like ball valves, meaning it requires only a 90-degree turn to go from fully closed to fully open.

Here's how it works:

1. Opening the Valve:

When the valve handle or actuator is rotated, the disk (also called a butterfly) inside the valve body rotates around a central shaft.

As the disk rotates, it moves from a perpendicular position (blocking the flow) to a parallel position (allowing the flow of fluid through the valve).

2. Closing the Valve:

When you rotate the valve handle in the opposite direction, the disk turns back to a perpendicular position, blocking the flow path completely and stopping the flow of fluid.

3. Sealing:

The butterfly valve's seal (usually made of rubber, PTFE, or elastomeric materials) ensures a tight closure when the valve is in the closed position. The disk presses against the valve seat to create a seal and prevent leaks.

Key Components of a Butterfly Valve:

1. Body: The main outer casing that holds all the internal components. It's usually made of metal, such as cast iron, stainless steel, or aluminum.

2. Disc (Butterfly): The rotating part of the valve, which is mounted on the central shaft. It blocks or allows flow when rotated.

3. Shaft: The component that connects the disc to the actuator or handle. It transmits rotational motion to open or close the valve.

4. Seat:  The sealing surface around the valve body where the disc presses against to prevent flow when the valve is closed.

5. Actuator:  A device (manual handle, electric motor, pneumatic actuator) that provides the force to rotate the shaft and disk. This can be manual, electric, pneumatic, or hydraulic depending on the application.