Butterfly valves consist of a solid disc mounted within the valve body, which is connected to the internal end of the valve stem. A hand-lever actuator is connected to the external end of the valve stem for manual operation. When the valve stem is turned such that the disc is parallel to the fluid flow, the valve is in the open position.
When the valve stem is turned such that the disc is perpendicular to the fluid flow, the valve is in the closed position. The disc closes against a resilient seat mounted within the valve body, providing bubble-tight shutoff. Because the hand-lever actuator is installed on the valve stem parallel to the disc, the actuator provides an indication of the valve’s position. Butterfly valves have a straight-through body configuration and are suitable for throttling and bubble-tight shutoff.
Butterfly valves are available in lug-style (or single-flange) and wafer-style bodies. Lug-style bodies have tapped lugs that align with the bolt holes in ASTM pipe flanges, allowing them to be bolted to a single flange on one side of the valve. This capability makes lug-style butterfly valves suitable for dead-end service; that is, they can be mounted on a flange at the open end of a pipe and provide bubble-tight shutoff. For this reason, lug-style butterfly valves can be used as shutoff valves for HVAC equipment, enabling the removal of the equipment and piping up to the valve.
Wafer-style butterfly valves are designed to be installed between two pipe flanges. They do not have tapped lugs that align with the bolt holes in the pipe flanges. Rather, they are held in place by means of compression; that is, the bolts join the two flanges on both sides of the valve. Wafer-style butterfly valves are not suitable for deadend service and should not be used as shutoff valves for HVAC equipment because the flange on the equipment side of the valve cannot be removed independent of the valve.
Butterfly valves have a slightly higher fluid pressure drop than gate valves but a
lower pressure drop than globe valves.