The oil must be held at temperature for a specific period of time to enable de-emulsifying the water-in-oil emulsion. This information is best determined in the laboratory but, in the absence of such data, 20 to 30 minutes is a good starting point.
Depending on the specific properties of the stream to be treated, geometry required to provide a certain retention time may be larger or smaller than the geometry required to satisfy the settling equation. The geometry of the vessel is determined by the larger of the two criteria. The equations for retention time are as follows:
Equations 6-10 and 6-12 are derived in the same manner as the retention time equation for horizontal separators.