AbstractWe present the results of theoretical studies of the pressure fluid flow fundamental characteristics in tube coalescers used in the treatment processes for oily wastewater. It is shown that three different regions of flow (wall sublayer, transition sublayer, and flow kernel), having their own hydrodynamic characteristics, are formed in a cross section of the tube coalescer. In the viscous wall sublayer (of thickness δ*), viscous frictional forces exceed inertial forces (Reδ* < 1), and “creeping flow” is observed. This region borders on the transition sublayer (of thickness δ** = δ*), in which inertial forces exceed viscous frictional forces (Reδ** > 1). For both laminar and turbulent flow, distribution laws of local velocities and velocity gradients along the pipe radius are obtained in each of the three regions. In the flow kernel, the linear distribution law of velocity gradients gives a square distribution law of velocities for both types of flow, but for the turbulent flow, the correction coefficients A = β and B = 2β − 1 must be introduced.