Separation of oil, water and gas.

Gathering Station

 When oil and water are mixed with some intensity and then allowed to settle, a layer of relatively clean free water will appear at the bottom. The growth of this water layer will follow a curve. After a period of time, ranging anywhere from three minutes to twenty minutes, the change in the water height will be negligible. The water fraction obtained from gravity settling is called "free water." It is common practice to separate the free water before attempting to individually treat the remaining oil and emulsion layers. Three-phase separators, sometimes called free-water knockouts, are used to separate and remove any free-water phase that may be present. Because the flow stream enters a three-phase separator either directly from a producing well, or from another separator operating at a higher pressure, the vessel must be designed to separate any gas that flashes from the liquid as well as the oil and water, hence the name three-phase. The basic design aspects of three-phase separators are identical to those of twophase separators. The only differences are that more space must be provided for oilwater settling and some means of removing the free water must be added. Threephase separators are designed as either horizontal or vertical pressure vessels.

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The fluid enters the separator and hits an inlet diverter which produces a sudden change in the fluid's velocity and direction. The initial gross separation of liquid and vapor occurs at this point. The force of gravity causes the heavier liquid droplets to fall out of the gas stream to the bottom of the vessel, where the liquid is collected. This liquid collection section holds the liquid during the appropriate retention time required to let dissolved gas evolve out of the oil and rise to the vapor space. This section also provides a surge volume, if necessary, to handle intermittent slugs of liquid. The separated liquid then leaves the vessel through the liquid dump valve, which is regulated by a level controller. The level controller senses changes in liquid level and controls the dump valve accordingly. The separated gas flows over the inlet diverter and then horizontally through the gravity-settling section above the liquid. As the gas flows through this section, small drops of liquid that were entrained in the gas and not separated by the inlet diverter are separated out by gravity and fall to the gas-liquid interface. Some of the drops are of such small diameter that they are not easily separated in the gravity-settling section. However, before the gas leaves the vessel it passes through a coalescing section or mist eliminator. In this section, metal vanes, wire

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