Why there is 1000 F temperature limit on ASME B16.34 for flanged end-connections, but ASME B16.5 does not have 1000F temperature limitation. (Please see foot note on the material tables. Example material group 1.16) The rationale used by the B16.34 committee, but creep relief of the yiled stresses built into the threaded bolts that hold the flange together will tend to allow loosening of the flange. Thermal cycling of the valve will tend to loosen the nuts after the yield stresses are relieved . In the temperature range 1000F-1200F standard steel based materials can be applied, but as the temperature increases the use of specialty alloys or the ASTM A193, Class 1 bolting and the de-rating of ball valves becomes more common. The de-rating of valve flanges and bolting is very similar to that required by NACE for flanges subject to the H2S environment.

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