Focus on the control valve/regulator, you are wanting to determine the flow across the fully open regulator with a specified pressure drop across the regulator (upstream pressure - downstream pressure).
What is the 100% open Cv value for the regulator? (check datasheet or use model number and search online)
The upstream pressure is the maximum possible upstream pressure (e.g. 2210 kPag or 1500 kPag if you want to credit the upstream regulator is still operating)
The downstream pressure is the relieving pressure of the PSV (700 kPag + accumulation)
Get out your control valve formulas and work out the flow rate based on the above information.
Compare this to the rated capacity of the relief device?
The calculation above will give you a conservative result which could be within the capacity of the relief device.
Though, as this is a control valve failure, then you could also credit the normal flow to the downstream users, so you could subtract this normal flow from the relief rate as we are saying there is a second open/available path for the gas to go.
Edited by Fletch, 18 February 2019 - 12:48 AM.