Wafer check valves, also known as dual-plate check valves, have two spring loaded half-moon shaped discs that rotate about their shaft. The disc plates are kept shut by a torsion spring. When fluid flows in a forward direction, the pressure of the fluid causes the disc plates to swing open, allowing flow. The check valve is closed by the spring as soon as flow ceases, before any reverse flow can occur.

Normative reference
     API 6D Pipeline Valves
     API 594 Check Valves: Flanged, Lug, Wafer and Butt-Welding
     ASME B16.34 Valves Flanged, Threaded and Welding Ends
     ASME B16.5 Pipe Flanges and Flanged Fittings: NPS 1/2 through 24
     ASME B16.47 Series A Large Diameter Steel Flanges: NPS 26 Through NPS 60 Metric/Inch 

Standard
     ASME B16.25 Buttwelding Ends
     ASME B16.10 Face to Face and End to End Dimensions of Valves
     MSS SP-25 Standard Marking System for Valves, Fittings, Flanges and Unions

Size available
     NPS 2 to NPS 24 (DN50 to DN600)

Pressure rating available
     Class 150 to Class 900