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.
The lug version of the butterfly valve’s design is similar to a 3-piece ball valve in that one end of the line can be taken off without having an effect on the opposing side. This can be executed by using threaded inserts, flanges, along with two sets of lugs (bolts) that don’t utilize nuts since each flange features its own bolts.
It’s also important to note that you don’t need to shut down the entire system in order to clean, inspect, repair, or replace a lug butterfly valve (you would need to with a wafer butter valve).