Double Flange Butterfly Valves AWWA C 504 is a valve that is most often used in piping systems. Its function is to open and close the flow (on-off), but not for a large set of small flow (throttling).
The surplus lack of obstruction / resistance when the valve is fully opened, so the flow bisamaksimal.
These valves valve controls the flow through the valve body is tubular, with a slab or a vertical wedge (see figure below) that can slide up and down as the handle is rotated. This product is designed for the fully open position, or fully closed. If the valve is in a state of half-open, it will cause erosion of the valve body, and turbulence may lead to vibration resulting in a rattling sound.