dannybagpipes,
What you have identified in your original post as being "The two scenarios" is really only partial descriptions of the operating conditions of your tank. Generally speaking, a "scenario" would be in reference to an event that causes the pressure in your system to increase and if the event can exceed the maximum allowable pressure of any component within the system you are trying to protect, then it becomes an overpressure scenario for which you need to provide overpressure protection.
Determining the relief load requires you to have a good understanding of the processes involved that are affecting the system pressure. For example, if your tank is provided with a nitrogen purge system then failure or mis-operation of the nitrogen controls to the tank can cause the tank to overpressure. If so, then you would need to provide overpressure protection equal to the rate of nitrogen that was causing the overpessure. Overfilling the tank by pumping material into the tank is another common case to consider as is external fire resulting in heat input that could cause liquid vapor pressure to increase and eventually boil generating vapor that must be relieved.
This are to name only a few of the possible sources of overpressure. Since you are talking about a rupture disc set at 15 psig, I assume you have a pressure vessel so you should consider the code requirements for overpressure protection of the system (see this thread for additional info thread1203-157530).
The Crosby Handbook is a very good reference and it does include some discussion about how to determine a fire relief load. For more thorough coverage about how to determine your required relief rate, I would suggest you get a copy of API RP-521.
Also, here's a link to a good online resource discussing considerations for rupture discs...
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