Analysis off Gravity Separation in Freewater Knockouts

Abstract

Summary The selection of a freewater knockout depends on the desired performance. The design criterion developed here is based on the perception that most FWKO's are installed to remove the bulk of the water from a high-water-cut flow stream so that the oil can be dehydrated economically to salable specifications and to discharge water requiring minimal treatment before disposal. Only the gravity separation process is considered; the contribution of coalescing devices is excluded. Vertical FWKO's are discussed briefly, but the bulk of this paper is devoted to horizontal FWKO's, which constitute a majority of vessels. Hydraulic similarity of horizontal FWKO's and API separators is demonstrated, and a design criterion for FWKO's is developed with the basis used in the design development for API separators. This analysis discloses that commonly used criteria overrate the ideal flow capacity of FWKO's by a factor of 4/π. It shows that maximum FWKO capacity occurs when the oil/water interface is maintained at a level of 0.769 diameters in the case of ideal vessels and somewhat higher for nonideal vessels, rather than 0.50 as commonly believed. Published FWKO residence-time distributions (RTD's) indicate nonideal flow. Consequently, a design allowance for short-circuiting and turbulence effects was incorporated into the FWKO design. The effect of vessel proportions on cost per unit capacity is analyzed and an equation is developed for determining optimum proportions.