Skim Tanks and Skim Vessels

The simplest form of primary treating equipment is a skim (clarifier) tank or vessel; refer to Figure 9-6. These items are normally designed to provide long residence times during which coalescence and gravity separation can occur. Skim tanks can be used as atmospheric tanks, pressure vessels, and surge tanks ahead of other produced water treating equipment.

Figure 9-6: Schematic of a skimmer tank.

The terminology used to describe the different equipment often is a source of great confusion. A "skim (clarifier) tank" is the terminology used to describe a tank that is used to remove dispersed oil. "Settling tanks," however, is the terminology used to describe tanks whose primary purpose is to remove entrained solids. On the other hand, "wash tanks" function as a free-water knockout or gunbarrel and are used when the incoming stream contains 10 to 90% oil. They are designed to make only a rough separation of the oil and water. The water from wash tanks is generally sent to a skim (clarifier) tank or another unit to remove the remaining oil.

If the desired outlet oil concentration is known, the theoretical dimensions of the vessel can be determined. Unlike the case of separation, with skim vessels one cannot ignore the effects of vibration, turbulence, short-circuiting, etc. American Petroleum Institute (API) Publication 421, Management of Water Discharges: Design and Operation of Oil-Water Separators, uses short-circuit factors as high as 1.75 and is the basis upon which many...