I am doing analysis of a line with 700 degree Celsius temperature. Pressure is 10.9 barg. The line material is A312 tp 304H. Questions that I need to ask from you all are:

1. Is it ok to take SIF as 0.75i for sustained case? In my opinion, as sustained stresses cause the failure in creep, so I should not do it but need your expert opinion on this.

2. In some cases the line goes at higher temperatures, which are the worst case for this line. The duration of those cases is satisfying the conditions of event being less than 20 hrs and maximum of 500 hrs per year. Should this relaxation of 20% addition in stresses (ASME B31.3) be given for this high temperature (783 degree Celsius) for which normal operating temperature is 700 Celsius?

3. Is this 20% relaxation applicable for the thickness calculations as well generally?