Spherical tank replacement
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I would like to ask the experts here: we store propylene in spherical tanks, and these tanks are scheduled for maintenance next year. We plan to purge the tanks with nitrogen, and there are two options: (1) Introduce nitrogen from the top of the tank and vent it through the bottom via a flare. (2) Standard practice: introduce nitrogen at the bottom and manually discharge the flare line to release the flare. I would like to ask the experts whether these two methods are feasible, and which one is better?
Generally, a dedicated vent line at the top is used for discharge; initially, the gas is released directly due to the pressure inside the tank. Once the pressure drops and self-discharge is no longer possible, nitrogen is used to displace the gas until the propylene concentration in the tank falls to a safe level. After that, the manhole is opened and air is introduced so that the concentration exceeds 18% before maintenance work can be carried out.
That would be a waste; it’s better to use water first, as that allows the propylene to be recovered.