Common Design Problems of Sump Tanks

Abstract

E&P Exchange When gravity settling is used for water treating, the buoyancy of oil droplets makes them rise at a velocity proportional to the difference between the specific gravities of water and oil (Stokes' law). If the flow rate in a vertical vessel or skim pile is such that the average water velocity from the water inlet to the vessel outlet is greater than the droplet velocity as it rises owing to buoyancy, the droplet will be carried out with the water. The maximum flow rate for a vertical or horizontal sump is proportional to the difference in specific gravities. If a vessel is sized for produced water and oil but also is being used for rain water from deck runoff, the allowable flow rates for the two cases are different because of differences in the specific gravities of water and oil. Rain water normally has a lower specific gravity than produced water, and oil washed off the deck into the sump by rain is weathered oil with a specific gravity greater than that of the fresh oil. This is particularly true with very light oil or gas condensate, both of which contain more light components removed by weathering. As an example, assuming the weathering process leaves 30° API oil on the deck, the difference in specific gravities between rain water and weathered oil is 0.13. For process fluids, assuming a light oil (60°API) and produced water (specific gravity= 1.07), the difference in specific gravity could be 0.33. For a sump with the same diameter, the allowable flow rate for rain is 0.13/0.33 = 0.39 times the allowable flow rate for process fluids. A sump that is adequate for normal conditions and wash down may be undersized to handle runoff from a rainstorm because of a significantly smaller difference in specific gravities of the fluids involved. A second problem that shows up during rainstorms is not related to vessel diameter but to the configuration of the internals. For the vertical sump shown in Fig. 1, the static head of the water standing in the water leg is equal to the head of the water plus the oil pad inside the sump. If pure water flows into the sump, the level of the water inside the sump is equal to the height of the water leg outlet (neglecting a small additional height to overcome drag caused by the flow through the pipe).