This is a theoretical question regarding how dilution purging with nitrogen gas would be setup for use on a tank. The purging is for the purposes of inertion before handling flamable liquids.
Would you always need a back pressure regulator on the vent line to ensure you are getting approrpriate mixing within the tank before the nitrogen vents out? My thought is an unrestircted vent line will simply cause the nitrogen gas to “short circuit” across the interior of the vessel head and vent out and the rest of the tank atmosphere relies on diffusion to get oxygen out instead of convection.
In practice, I’ve only seen and designed systems that pressure purge vessels prior to introducing flammables. Sometimes a small sweep flow rate is used during operation, but post inertion of the vessel.