Abstract: A method for dehydrating and desalting an oil-in-water emulsion includes the steps of: injecting a demulsifier into the flow to destabilize the emulsion; injecting water to promote desalting; and passing the flow through a mixing pipeline having a length (L) and inside diameter (D) determined as follows: ##EQU1## wherein: f--is a Fanning Friction factor.rho..sub.e --is a density of the emulsion (gm/cc);Q.sub.e --is a flow rate of the emulsion (bbl/day);.mu..sub.e --is a viscosity of the emulsion (poise);K--is an instrument constant of a dynamic coalescer;F--is a rotational speed of the dynamic coalescer (rpm);t--is a mixing time (hr);D--is the inside diameter of mixing pipeline (inches); andL--is the length of the mixing pipeline (km).