Anindya,
Thank you for providing input, I've been thinking about how to reply...
I agree that relief valve load is occasional load, with primary stresses.
I also envision a relief valve event as one in which many hundreds of load cycles can occur before the system is stabilized, and such events can occur dozens of times in the life a system. So, for that reason, there seems to also be a fatigue issue.
Perhaps in evaluating an actual system, in order to meet the occasional load allowable stress of 1.33Sh, the system will be under the endurance limit, and fatigue is not normally a factor.
I will be looking at a relief system again soon and will look at the stresses with this discussion in mind.

Regards,