A butterfly valve is a valve which can be used for isolating or regulating flow.
The closing mechanism takes the form of a disk.
Operation is similar to that of a ball valve, which allows for quick shut off.
Butterfly valves are generally favored because they are lower in cost to other valve designs
as well as being lighter in weight, meaning less support is required.
The disc is positioned in the center of the pipe, passing through the disc is a rod connected
to an actuator on the outside of the valve.
Rotating the actuator turns the disc either parallel or perpendicular to the flow.
Unlike a ball valve, the disc is always present within the flow, therefore a pressure drop
is always induced in the flow, regardless of valve position.
A butterfly valve is from a family of valves called quarter-turn valves.
In operation, the valve is fully open or closed when the disc is rotated a quarter turn.
The "butterfly" is a metal disc mounted on a rod.
When the valve is closed, the disc is turned so that it completely blocks off the passageway.
When the valve is fully open, the disc is rotated a quarter turn so that it allows an
almost unrestricted passage of the fluid.
The valve may also be opened incrementally to throttle flow.
There are different kinds of butterfly valves, each adapted for different pressures and different
usage.
The resilient butterfly valve, which uses the flexibility of rubber, has the lowest
pressure rating.
The high performance butterfly valve, used in slightly higher-pressure systems, features
a slight offset in the way the disc is positioned, which increases the valve's sealing ability
and decreases its tendency to wear.
The valve best suited for high-pressure systems is the triple offset butterfly valve, which
makes use of a metal seat, and is therefore able to withstand a greater amount of pressure.
Types Concentric butterfly valves - This type of
valve has a resilient rubber seat with a metal disc.
Doubly-eccentric butterfly valves - Different type of materials is used for seat and disc.
Triply-eccentric butterfly valves - The seats are either laminated or solid metal seat design.
Triple-eccentric design Triple eccentric Design prevents galling and
scratches between the metal seat and the metal disc due to its unique design.
The only time where 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 and gas, LNG/NPG terminal and tanks, chemical factories, and shipbuilding.
They are also used for dirty/heavy oil to prevent extrusion.
Wafer-style butterfly valves The wafer style butterfly valve is designed
to maintain a seal against bi-directional pressure differential to prevent any 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 butterfly valve Lug-style valves have threaded inserts at
both sides of the valve body.
This allows them to be installed into 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.
Images
See also Ball valve
Control valve Gate valve
Globe valve Plastic pressure pipe systems
Needle valve References