API Std 594, Check Valves: Flanged, Lug, Wafer, and Butt-welding, is a standard developed and published by the American Petroleum Institute (API) that covers the design, materials, face-to-face dimensions, pressure-temperature ratings, and examination, inspection, and testing requirements for Type "A" and Type "B" check valves. This standard was originally published in 1997, with the most recent 8th Edition being published in July of 2017.
- Type "A" check valves are short face-to-face, and can be wafer, lug, or double- flanged; single-plate or dual-plate; or gray iron, ductile iron, steel, nickel alloy, or other alloy designed for installation between Classes 125 and 250 cast-iron flanges as specified in ASME B16.1, between Classes 150 and 300 ductile-iron flanges as specified in ASME B16.42, between Classes 150 and 2500 steel flanges as specified in ASME B16.5, and between Classes 150 and 600 steel pipeline flanges as specified in MSS SP-44, or steel flanges as specified in ASME B16.47.
- Type "B" bolted-cover swing check valves are long face-to-face, and can be flanged or butt-welding ends of steel, nickel alloy, or other alloy material. End flanges shall be as specified in ASME B16.5, or ends shall be butt-welding as specified in ASME B16.25.
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