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[thinman56]

I'm working on some pressure relief systems for jackets that are ASME BPVC Sec VIII stamped at 55 psig, 300F, but the tanks themselves are not code pressure vessels. I'm looking at three scenarios - closed outlet valves, open pool fire and pressure regulator failure. Failed pressure regulator seems straightforward, evaluate the cooling water supply piping and see if friction losses keep the flow rate below the flow capacity of the SRV at the relieving pressure. Regulators all have a max Cv to use in this calc. Closed outlet valve is also easy, the pressure regulator will still limit pressure downstream with no flow. I don't believe double jeopardy applies as a failed pressure regulator would be unlikely to be considered a 'latent failure', I think it's the kind of thing you'd notice in operation for tanks on the operating floor in a system that is batch-run on a daily basis. 

The open pool fire is easy if I can demonstrate that cooling water will continue to flow through the jacket at a rate that absorbs the heat without getting to the boiling point. However, if there's more heat than the flow can absorb, I'm dealing with two-phase flow in downstream piping, probably slug flow, trying to predict if the resulting backpressure will exceed the SRV setpoint, if two-phase flow needs to be considered for the lifted safety, etc., and I'm not sure how to deal with this. The tanks are vertical SS, less than 1000 gallons, typical contents are isopropyl alcohol (IPA) at 80% with water. I assumed the tank was empty of product for worst case. 

Anyone been through this with jackets? API 521 has no specific detail on jacketed tanks. Thanks for any feedback.