Solving Oil Water Separator Problems

Coalescing Plate Separators – API Separators

Coalescing Plate Separators & API Separators
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API Type & Coalescing Plate Separators Design & Performance

Coalescing plate separators and a modern-day solution to an age-old problem. For centuries, humanity has known that oil and water do not mix – at least not very well. However, what ancient man did not realize is that oil and water will mix under certain conditions and emulsions will form that can be very difficult to separate.

19th Century

Petroleum products came into general use in the late 19th century due to the replacement of well oil with kerosene. Kerosene, originally called coal oil, was manufactured by the destructive distillation of coal. Patented in 1854 by Dr. Gessner of Williamsburgh, NY (Scientific American 1854), a distillation of crude oil allows for bulk production.

Advent of Automobiles

After automobiles became common in the early part of the 20th century, oil refining grew from a cottage industry to a major industrial part of the US economy; subsequently, the quantity of effluent water entering the rivers and streams increased.  Much water of the contained various hydrocarbons because most of the water in the effluent was the result of hydrocarbon processing. After World War II, the quantity of water exiting the refinery and therefore the number of hydrocarbons became a substantial nuisance and in addition, it became obvious that a great deal of money in the form of hydrocarbons was being wasted.

Development of Coalescing Plate Separators

The American Petroleum Institute (API), an industry society, decided it was necessary to have a method of removing the oil from the effluent water. This alleviates the nuisance while capturing a lot of hydrocarbons for recycling through the refinery.

A 1947 API commissioned study from the University of Wisconsin determined the coalescing plate separator design method used for removing oil from water in refinery effluent water. The design methods focused primarily on resource recovery and mitigating the nuisance effect of oil exiting the refineries and entering streams and lakes.

This design method has been revised several times, notably in 1979 and 1990.

Accepted Practices

A design method is provided in the API Manual on Disposal of Refinery Wastes, Chapters 5 and 6 Oil-Water Separator Process Design and Construction Details (API publication 1630, 1979). API separators, gravity type separators, come usually equipped with oil removal facilities. Refineries and chemical processing facilities use API separators in oil where waters containing relatively large amounts of oil. After processing they then meet the requirements of NPDES permits.

Types of Systems

API type separators have the same general baffle arrangement as a regular API separator. They are noncompliant to the design criteria established by the API. Separators which have a lesser residence time are smaller and less expensive than rigorously designed API separators. Because of this, they are noncompliant for the API design criteria; therefore, they fall short of the API’s modest effluent expectations.


API separators and API type separators advantages include simplicity of design, low cost, low maintenance, and they are also resistance to plugging with solids. The primary disadvantage of these simple gravity separators is the poor quality of separation that they provide as discussed below.

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