The function of stainless steel orifice flanges is to provide access to a line for meeting off gases or liquids. Stainless steel orifice flanges is clamped between a pair of flanges when installed in a line and the whole assembly is referred to as an orifice flange union. Jack-screws within the assembly facilitate removal of the orifice plate. The orifice plate, the metering device, consists of a thin plate with a concentric, square edge, circular hole in the center. Two pressure tap-holes are drilled in each orifice flange to measure pressure differences through the orifice.