I have a new vessel that will be installed that will be liquid full and I need some help with a relief valve.
We are not considering fire case for the relief valve as the fluid has a high boiling point and a relief valve will provide little protection. The cases we identified were blocked outlet and thermal expansion. Blocked outlet load far exceeds thermal expansion as can be expected.
The tank will be far away from any existing knock out pots and flare header so it was suggested that the relief valve could relieve to the downstream side of the block valves that could be closed creating the relief scenario. A simplified drawing of this idea is attached. I know you would have to run calculations to make sure back pressure would not be too high and then potentially use a bellows or pilot valve. From what I can read in API 520 and API 521 this seems to be an ok thing to do, but it rubs me the wrong way. Maybe I've just never seen it done before. Does anyone else have any opinions on the matter? If you believe this may be an acceptable solution, is there anything I need to make sure and check? If you don't like this idea, please tell me why.
Thanks so much for your help.