A separation tank for crude oil. Fluid enters an inlet section of a center column of the tank via an offset inlet pipe so the fluid enters swirling. Solids that settle in the inlet section are removed by a center column drain and a solids removal system. Free gas rises and exits from the top of the tank. Liquid flows out of the center column via a diffuser that spirals the fluid evenly toward the wall of the tank where oil coalesces and wicks upward. Liquid flows downward around two flow diverting baffles where more oil coalesces and wicks upward via an oil conduit into the oil layer. The water flows under the lower flow diverting baffle and exits the tank through the outlet section. A large circular oil collector weir uniformly removes oil from the oil layer. Interface draw offs located below the oil-water interface remove excess BS&W.