To assure that the liquid and gas reach equilibrium at separator pressure a certain liquid storage is required. This is defined as “retention time” or the average time a molecule of liquid is retained in the vessel assuming plug flow. The retention time is thus the volume of the liquid storage in the vessel divided by the liquid flow rate.
For most applications retention times of between 30 seconds and 3 minutes have been found to be sufficient. Where foaming crude is present retention times up to four times this amount may be needed.