De-rate internal design pressure of pipes

[SVJAIN]

Hello

SHELL DEP pipe class have the pre defined Internal design pressures and also the pipe wall thickness.
In high pressure rating ( class 900 and above),sour service, the pipe class mentions the design limits is as per ASME B 16.5 and is limited by the allowable stress value of the bolt material.
Further the Pressure at corresponding temperature has been de rated as against the ASME B 16.5 values

Question 1 : Why do we have to limit the pressure temperature design based on the bolting material. ( The bolt material being used is A193 B7M)
Question 2 : How the de-rating the design limits helps

Appreciate the guidance.


Rgds
SVJain

 

[jassin]

The design limits specified in the piping classes have been derived from the pressure/temperature (P/T) ratings for flanges given in ASME B16.5 for sizes u/i DN 600 (NPS 24) and B16.47 for DN 750-1200 (NPS 30-48) unless otherwise stated in the piping class notes. There are no piping classes for pressures higher than those allowed by the ASME B16.5 rating class 2500 for the specified design temperature and material. For materials listed, the relevant values listed in table 2 of ASME B16.5/B16.47 apply. For comparable materials not listed, the P/T rating is established in accordance with annex B of ASME B16.5.
Where specified by ASME B31.3, bolting calculations have been performed to verify the ability to seat the selected gasket and to maintain a sealed joint under the given pressure/temperature (P/T) rating; ASME VIII, Division 1, Appendix 2 has been followed for this.
Allowable internal pressures for pipe and fittings have been verified in accordance with ASME B31.3 paragraph 304.1.2 equation (3a) in order to meet the design limits of piping classes. For sizes DN 15 (NPS ½) to DN 600 (NPS 24) a manufacturing tolerance of 12.5 % has been applied. For sizes larger than DN 600 (NPS 24) all components have been specified so that a negative manufacturing tolerance of 0.3 mm (0.01 in) is used as the base for the mechanical design. For manufacturing tolerances of fittings larger than DN 600 (NPS 24) reference is made to MSS-SP-75 chapter 13 (instead of chapter 11 of ASME B16.9).
The lowest pressure at coinciding temperature selected from the above P/T ratings is governing for the piping class P/T rating. The piping class notes state which factor(s) is/are governing.