There exists a common misconception in the minds of end-users of valves, with regard to the flange thickness of 150# and 300# valves. This is due to the fact that the ASME B16.5 standard treats these two pressure class valve flanges differently when compared to the other higher pressure class valve flanges. The flange thicknesses in the case of 150# and 300# valve flanges are required to be in accordance with the flanged fittings table and are lesser than the flange thickness for the corresponding class pipeline flanges. Sometimes, end users are confused as to whether they should accept valves with thinner flanges than the corresponding pipeline flange thicknesses.
The reason for the confusion stems from the fact that the explanation for this anomaly is given in section 6.2.2 of ASME B16.34 standard and not in ASME B16.5 standard. Section 6.2.2 of ASME B16.34 states that “Flanged ends shall be prepared with flange facing, nut bearing surface, outside diameter, thickness, and drilling in accordance with ASME B16.5 requirements for
(a) flanged fittings for Class 150 and 300 valves
(b) flanges for Class 600 and higher valves”
In light of the above, it is important to understand that Class 150# and 300# valve flanges shall follow the flanged fittings table and shall have lesser flange thicknesses compared to the corresponding pipeline flanges.