The helium tank has 3000 psig, and a PRV located downstream is set at 70 psig. A relief valve is located further downstream of the PRV. I was wondering what are the factors to determine the set pressure for the Relief Valve? In this case, it looks like I could pick a set pressure from 75 -240 psig. 1. There IS a Safety and Relief valve engineering forum in the Chemical engineering section. You might wish to have the forum manager move this thread there. 2. You are allowed, by code, to set the SRVs at up to the MAWP of the system. 3. Rated flow of the SRV is at set pressure +10% 4. Section VIII SRVs close at 10% below set pressure, so if your max setting of the regulator is 70 psi, then you don't want the SRV set at less than 70/.9 or 77.78 psi. 5. If it's not going to hurt any component in the system ands the system has ASME class 150 flanges and the temperature is 100F or less, you can set the SRVs at up to 240 psi, the B16.5/B16.34 limit. 6. If you set the SRV at the higher pressure limit, you will almost undoubtedly be able to relieve the required flow with a smaller SRV.

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