Hi,
I'm working on emissions control project with Vapor Recovery Units. We are calculating the flash gas generated within the storage tank when the crude oil enters. I'm using Vasquez-Beggs equation to get estimation on vapor flow rate, here's where I have some doubts:
These kind of equations use upstream crude oil separation pressure, the higher the separation pressure the higher the flash gas flow rate that occurs in the tank, but, since the tanks operate at atmospheric pressure, the maximum flash would occur despite the upstream pressure, wouldn't it?
I ask this since a supplier quoted VRU for a given flow considering 300 psig for the crude oil separation pressure, but a coworker noted the actual upstream pressure is 170 psig and stated that the VRU should be sized then for lower capacity. Would that be the case? If I ran a HYSYS simulation with those parameters the flow rate won't change with increasing the differential pressure by means of changing the separation pressure (as long as Storage tank is fixed at atm pressure), so it doesn't matter if upstream separation pressure is 170, 300 or 500 psig, the atm pressure in the tank does the job, doesn't it?