A secondary non-mechanical pressure relief device, like a frangible/rupture/burst disk, is advisable for any pressure tank. These are typically mounted to a tri-clamp ferrule or other fitting on the tank dome and are rated to rupture at a fixed pressure should the primary PRV fail to operate properly. A rupture disk is designed to be a one-time-use failsafe protection and will rupture should tank pressure or vacuum exceed the rating of the disk. It is much better for the disk to rupture than your tank. Non-ASME rated tanks need a PRV set at a pressure not greater than 15 psi.