IMHO designating it as an occasional load is entirely appropriate. Even with the 1.33 Sh allowed by B31.3 your still likely to be below SMYS at temperature because the Sh value at temperature is based upon 2/3 SMYS at temp for systems below the creep range. (ref B31.3 2006 Para 302.3.2 (4))

But in doing so one should also use a DLF that is apropriate do a search on DLF or Dynamic Load Factor (Also read B31.1 Non Mandatory section on relief piping and also articles on this web site as well)