Below is a simple chart used in determining tank emissions based on flow, pressure differential, and API gravity of the oil. The conventional wisdom is to size the vru at 1.5 – 2 times the estimated volume required to allow for unknowns, such as temperature fluctuations and future oil production which may be added to the battery.

api-chart

SCF = Standard Cubic Feet
BBL = Barrel of oil (42 US Gallons)
GOR = Gas Oil Ratio

Example:
If API Gravity = 38 º API
Separator Pressure = 40 PSIG
Throughput = 1,000 BBL/Day

From the graph above, GOR = 43 SCF/BBL
Therefore, emission volume = 43 SCF/BBL
* (1,000 BBL/Day) = 43,000 SCF/Day of Flash Gas or 43 MSCFD