wich API standard shall apply to a storage tank working under 15 PSIG @ 300F?
Note : API 650 has maximum temperature of 260F (Appendix M) & API 620 has maximum at 250F
Neither strictly apply. API-650 only goes up to 2.5 PSIG pressure but 500 degrees F in App. M. API-620 goes up to 15 PSIG, but only 250 degrees F. You could use API-620 and modify similar to API-650 App. M, or switch to ASME Section VIII or some other standard. It would partly depend on the size and application as to which route I would persue.