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In oil and gas industry, separator is a pressure vessel used for separating the well stream from oil and gas wells into gaseous and liquid components. It is normally the initial processing vessel in the production facility. Its design and performance are essential for downstream equipment which cannot handle gas-liquid mixtures very well, such as pumps, compressors, and flow meters.

Oil and gas separators can be classified according to application as test separator, production separator, low temperature separator, metering separator, etc. By function, they are classified as “two-phase” if they separate gas from the total liquid stream and “three-phase” if they also separate the liquid stream into its crude oil and water components. They can also be classified according to configurations as horizontal, vertical, spherical, and a variety of other configurations, such as centrifugal separators, venturi separators, double-barrel horizontal separators, filter separators, scrubbers, and slug catchers.

How does a three-phase separator work?

The following figures depict the schematics of typical horizontal and vertical three-phase separators. For a three-phase horizontal separator, the fluid enters the separator and hits the inlet diverter. The inlet diverters rapidly change the momentum of fluid, providing the initial gross separation of liquid and vapor. Inlet diverter has many types, such as baffle plates (shown in the figure), centrifugal diverters, and elbows. In most designs, the inlet diverter contains a down-comer that directs the liquid flow below the oil-water interface. The oil and water are separated in the liquid collecting section at the bottom of the vessel, which provides sufficient time so that the oil and emulsion form a layer or “oil pad” on top of the free water while the free water settles to the bottom. The weir maintains the oil level, while a level controller controls the water level. The gas flows horizontally in the gravity settling sections where small liquid droplets are separated out by gravity and fall to the gas-liquid interface. Before the gas leaves the vessel, it passes through a mist extractor that separates tiny liquid droplets which are not separated in the gravity settling section.

Schematic of a three-phase horizontal separator
Schematic of a three-phase vertical separator

In a three-phase vertical separator, a down-comer is used to route the liquid through the oil-gas interface into the spreader located just below the oil-water interface. The water flows downward and oil droplets trapped in the water phase tend to rise countercurrent to the water flow.

Each separator type has advantages and disadvantages. For example, horizontal separators occupy more space than vertical vessels, which is of importance at a land location where space is limited. Horizontal separators do not handle solids production as well as vertical separators, and have less liquid surge capacities. But they are more efficient in gravity separation, and have greater interface areas that enhance phase equilibrium, and foam and emulsion separation.