Butterfly valves operation is similar to the ball valve, which allows it to close quickly. Butterfly valves are generally preferred because they cost less than other valve designs and are lighter and therefore require less support. The disc is in the middle of the pipe. Axial through the disc to the actuator on the outside of the valve. Rotating the actuator causes the disc to be parallel or perpendicular to the flow. Unlike ball valves, the ball valve is always in the flow, thus causing the pressure to drop even when open.