Is it acceptable practice to use API 620 code for the design of a storage tank with 5 psig design pressure and 316°F design temperature? I know the code says it is only applicable for up to 250°F, couldn't I use the max allowable stress values found in ASME VIII at the design temperature and plug these into the API 620 equations?
I'm no expert on API 620, so you are refering to 1.2.2:
"The tanks described in this standard are designed for metal temperatures not greater than 250°F and with pressures in their gas or vapor spaces not more than 15 lbf/in? gauge."
and 5.2 "Operating Temperature"
Yes, for the materials defined in the standard exceeding 250°F is a bad idea. I think at around 300°F your stress releiving the tank materials.