Single-Eccentric Butterfly Valve
A single offset butterfly valve will have the shaft placed behind the centerline of the sealing area. In addition on a single eccentric butterfly valve, the shaft is placed centered to the valve centerline.
In a single-offset butterfly valve, the stem is located behind the disc. There are few, if any, valves of this type left on the market today due to the development of the double-offset, or high-performance, valve.
The single-offset of the stem causes the disc to contact the seat with three to four degrees left to travel with the idea that less seat contact will enable the valve to last longer.