MSS SP-51-2000
This standard practice covers a series of cast austenitic type stainless steel flanged fittings and cast and forged type austenitic stainless steel flanges of reduced weight with mating dimensions for assembly with ISO 7005-1, PN20, or ASME B16.5, Class 150 flanges, but idenified as Class 150 LW (Light Weight) and limited to use at reduced pressure-temperature ratings.