ASME 16.5 is for regular pipe flanges and ASME 16.36 is for Orifice Flanges.
ASME B16.36 defines the taps, ports, jacking bolts etc. that are required for Orifice Flanges. B16.36 references B16.5 but is limited to the flange sizes that are suitable for taping. The flange thickness may be thicker on some of the B16.36 flanges than a B16.5 flange of the same size and ratings.
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