First you need to understand what the "standard conditions" are in this 510 SCFM number for the compressor. Back in high school and college "standard conditions" was 1 atmosphere and 273.15 K. In the oil and gas industry it is 1 atm and 15 C a lot of the time. I've seen a lot of references to 14.7 psig and 60 F in the chemical industry. Standard conditions are not so standard! Different industries use different standard conditions.
Once you know Ts and Ps, you can use the ideal gas law and the molecular weight of air to calculate the mass flow rate of air.
PV = nRT --> n = PV/RT
m = n x MW
Then you can use the mass flow rate *at* the conditions of your scenario, not at STP, to size the relief. Once you know what size relief you need, it will flow a certain amount of air at the set pressure and at the scenario conditions. *This* maximum flow rate can be converted to standard conditions and that will be the SCFM of that PSV at that set pressure in that scenario.
Is the 2100 SCFM the flowrate seen at the orifice of the SRV? I have no idea. Is the 2100 SCFM the *true* flowrate seen at the orifice of the SRV? Absolutely not! The inlet of the orifice is going to be close to the set pressure, not the standard pressure.
Good luck,
Latexman