16-08-2017, 12:02 PM
A butterfly valve is a valve that isolates or regulates the flow of a fluid. The locking mechanism is a rotating disc. The operation is similar to that of a ball valve, which allows quick closing. Butterfly valves are generally favored because they cost less than the design of other valves, and are lighter so they need less support. The disc is placed in the center of the pipe. A rod passes through the disc to an actuator on the outside of the valve. Turning the actuator rotates the disk parallel or perpendicular to the flow. Unlike a ball valve, the disc is always present inside the flow, so it induces a pressure drop, even when it is open.
A butterfly valve is a family of valves called quarter turn valves. In operation, the valve is fully open or closed when the disc rotates a quarter turn. The "butterfly" is a metal disk mounted on a bar. When the valve is closed, the disc is rotated in a way that completely blocks the passage. When the valve is fully open, the disc is turned a quarter turn so that it allows an almost unrestricted passage of the fluid. The valve can also be opened incrementally to accelerate flow.
There are different types of butterfly valves, each adapted for different pressures and different uses. The zero displacement butterfly valve, which utilizes the flexibility of the rubber, has the lowest pressure rating. The high-performance double-acting butterfly valve used in slightly higher pressure systems is offset from the center line of the disc seat and body seal (offset one) and centerline of the inner diameter (two-shift). This creates a cam action during the operation to lift the seat out of the seal, producing less friction than the one created in the zero displacement design and decreasing its tendency to wear. The most appropriate valve for high pressure systems is the triple compensation butterfly valve. In this valve, the contact axis of the disc seat is displaced, which virtually acts to eliminate the sliding contact between the disc and the seat. In the case of triple compensation valves, the seat is made of metal so that it can be machined in such a way as to achieve a hermetic closure of the bubble when in contact with the disc.