The specific gravity difference between the dispersed water droplets and the oil should result in the water “sinking” to the bottom of the treatment vessel.

Since the oil continuous phase is flowing vertically upward in both vertical and horizontal treaters previously described, the downward velocity of the water droplet must be sufficient to overcome the velocity of the oil traveling upward through the treater. By setting the settling velocity equal to the oil velocity the following general sizing equations can be derived:

Note that the height of the coalescing section for a vertical treater does not enter the settling equation. The cross sectional area of flow for the upward velocity of the oil is a function of the diameter of the vessel alone. In a horizontal vessel the cross sectional area for flow for the upward velocity of the oil is a function of the diameter times the length of the coalescing section.

The sizing equation for gunbarrels includes a short-circuiting factor (F). This factor accounts for imperfect liquid distribution across the entire cross-section of the treating vessel or tank and is a function of the flow conditions in the vessel. The larger the retention time, the larger the short-circuiting factor. It may be necessary to apply a short-circuiting factor for large vertical treaters as well.