Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online.A butterfly valve is a valve used for flow regulation in which the disc takes the form of disk. Operation is similar to that of a ball valve. A plate or disc is positioned in the center of the pipe. The disc has a rod passing through it that is 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. 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.
