Butterfly valves are used for isolating or regulating flow. They are a quarter turn valve similar to a ball valve except the closing mechanism is a disk instead of a ball. Butterfly valves are typically lower in cost to other valve types as they are lighter in weight meaning less support is required to have them in line. The disc of the valve is operated by either a lever, gear or actuator. When the valve is closed the disc blocks the flow- when open the disk turns to allow flow to pass through; however the disk will always be present within the flow resulting in a pressure drop regardless of valve position. Butterfly valves can also be open incrementally to throttle flow.
Butterfly valves come typically in wafer or lug style; however it is not uncommon to see a flanged butterfly valve in high performance applications.
The wafer style butterfly valve is designed to maintain a seal against bi-directional pressure differential to prevent backflow in systems designed for unidirectional flow. It accomplishes this with a tightly fitting seal, i.e., gasket, o-ring, precision machined, and a flat valve face on the upstream and downstream sides of the valve.
Lug-style valves have threaded inserts at both sides of the valve body. This allows them to be installed in a system using two sets of bolts and no nuts. The valve is installed between two flanges using a separate set of bolts for each flange. This setup permits either side of the piping system to be disconnected without disturbing the other side.
A lug-style butterfly valve used in dead end service generally has a reduced pressure rating. For example, a lug-style butterfly valve mounted between two flanges has a 150 psi pressure rating. The same valve mounted with one flange, in dead end service, has a 75 psi rating.
- 'Concentric Butterfly Valve' - This type of valve has a resilient rubber seat with a metal disc.
- 'Double Eccentric Butterfly Valve' - This type of valve is also referred to as a 'High Performance Butterfly Valves' or 'Double Offset Butterfly Valve'. Different type of materials are used for the seat and disc.
- 'Triple Eccentric Butterfly Valve' - This type of valve is also commonly called the 'Triple Offset Butterfly Valve'. The seats are either laminated or a solid metal seat design. Triple eccentric(offset) This design prevents gulling and scratches between the metal seat and the metal disc due to its unique construction. The only time the seal comes into contact with the seat is at the point of complete closure. Triple offset valves are generally used in applications which require bi-directional tight shut-off in Oil & Gas, LNG/NPG terminal and tanks, Chemical Factories, Shipbuilding. Widely use for dirty/ heavy oil to prevent extrusion.
Contact - The Valve Pipeline
In really low pressure applications sometimes a trunnion ball is better than a floater given that the spring will help seal the flow instead of the weak flow in the line.