For FRP vessels exposed to fire, relief devices can't provide any meaningful protection. Thus, sizing for fire exposure is a waste of time and money. Worse, it actively misleads people into thinking that the relief device is actually providing fire protection.
When choosing to use an FRP vessel, and choosing the vessel's location, one should consider the fire exposure risks. If it's intolerable for this vessel to fail during a fire, then one must choose a different design, or a different location for installing the vessel.
Since you're a petroleum guy, I'm guessing this is an API 12P tank. If you look in the API 12P document, you find cautionary guidance regarding the use of those tanks in a fire zone, for the reasons I've stated above.