Hydrocarbon streams as produced at the wellhead are composed of a mixture of gas, liquid hydrocarbons, and sometimes free water. In most cases it is desirable to separate these phases as soon as possible after bringing them to the surface and handle or transport the two or three phases separately. This separation of the liquids from the gas phase is accomplished by passing the wellstream through an oil-gas or oil-gas-water separator.

Different design criteria must be used in sizing and selecting a separator for a hydrocarbon stream based on the composition of the fluid mixture. In the case of low pressure oil wells, the liquid phase will be large in volume as compared to the gas phase. In the case of high pressure gas-distillate wells the gas volume will be higher as compared to the liquid volume. The liquid produced with high pressure gas is generally a high API gravity hydrocarbon, usually referred to as distillate or condensate. However, both 1ow pressure oil we11s or high pressure gas-distillate wells may contain free water.

Separators are used in many other locations other than at wellhead production batteries, such as gasoline plants, upstream and downstream of compressors, and liquid traps in gas transmission lines. They are also found on inlets to dehydration units, gas sweetening units, et cetera. At some of these locations separators are referred to as knockouts, free liquid knockouts, and traps. Sometimes these vessels are cal1ed scrubbers. Caution should be used when referring to a vessel required for gas/ liquid separation as a scrubber. Within the gas industry there is another type of vessel often ca11ed a scrubber. This is one that is designed to handle a gas stream with only trace amounts of free liquid present in the gas. They are not designed using the same criteria as is used for gas/ liquid separation where appreciable amounts of liquid are present or where liquid slugging may be encountered.

However, all of the vessels mentioned above that are designed to separate gas and free liquids serve the same primary purpose. This technical paper is concerned primarily with the use of separators in field installations. The theory and basic design criteria will be the same no matter where they are located or their basic use.

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