Please refer ASME B 31.3 -302.3.6(Limits of Calculated Stresses due to Occasional Load)-
The code compliance requires Checking the "Sum of Longitudinal Stresses due to Pressure,weight,other Sustained loads SL AND of the stresses produced by Occasional loads such as Wind,Earth Quake may be as much as 1.33 times Sh.

Also refer section 302.2.4 (Allowances for Pressure-Temp Variations)-Para f(2) which states-
When the variation is self limiting(eg . Due to pressure relieving event)and lasts no more than 50Hr at any one time and not more than 500hr in a year. it is permissible to
--exceed the pressure rating or
--the allowable stress for pressure design at the temp of the increased condition by not more than 20%.

As a stress analyst during the analysis for a Relief Valve Pop Up Case, (which is normally considered occasional),the usual practice is to check for the 1.33 Sh criterion.
Is the 20% criterion per 302.2.4 also to be checked for this Popup case?

Request the community to throw some light on the Basis of these allowances(33 % for Wind,Earthquake per 302.3.6 and 20% per 302.2.4)