API 594 Check Valves: Flanged, Lug, Wafer and Butt-welding
API 594 covers design, material, face-to-face dimensions, pressure-temperature ratings, and examination, inspection, and test requirements for two types of check valves.

Type ‘A’ check valves are short face-to-face and can be: wafer, lug, or double flanged; single plate or dual plate; gray iron, ductile iron, steel, nickel alloy or other alloy designed for installation between Class 125 and 250 cast iron flanges as specified in ASME B16.1, between Class 150 and 300 ductile iron flanges as specified in ASME B16.42, between Class 150 –2500 steel flanges as specified in ASME B16.5, and between Class 150 – 600 steel pipeline flanges as specified in MSS SP-44 or carbon steel flanges as specified in ASME B16.47.

Type ‘B’ check valves are long face-to-face and can be: flanged or butt-welding; steel, nickel alloy or other alloy designed for installation between Class 150 – 2500 steel flanges as specified in ASME B16.5 or for butt-welding into industry accepted piping systems.

This standard covers the following nominal valve size ranges:

Type ‘A’ valves:
a. Classes 125 and 250, 2 ≤ NPS ≤ 48 (excluding NPS 3 1/2).
b. Classes 150 and 300, 2 ≤ NPS ≤ 48*
c. Class 600, 2 ≤ NPS ≤ 42*
d. Classes 900 and 1500, 2 ≤ NPS ≤ 24*
e. Class 2500, 2 ≤ NPS ≤ 12*

Type ‘B’ valves:
f. Classes 150 through 1500, 2 ≤ NPS ≤ 24*
g. Class 2500, 2 ≤ NPS ≤ 12*

Note: *Valve sizes NPS 3 1/2 and 5 are non-preferred sizes whose usage is discouraged.