Produced well fluids typically contain a varied amount of oil, produced water, natural gas and sediment. The first step in processing these fluids is to split them up into individual components. This generally takes place at some types of separator. A three phase separator uses gravity to separate produced well fluids into gas, oil, and water phases.

Three phase separator design types

A typical horizontal three phase separator.
A typical vertical three phase separator.

Three phase separators may be designed in either vertical or horizontal configurations.

Horizontal three phase separator

In a horizontal separator, fluid enters the vessel and hits an inlet diverter. The resulting sudden change in momentum provides the initial gross separator of liquid and vapor. The liquid collection section of the vessel must provide sufficient for the oil and emulsion to form a layer or pad above the water.

A weir maintains the oil level while an interface controller maintains the water level. The oil spills over the top of the weir, and then a level controller which operates the oil dump valve controls its level. An interface level controller senses the height of oil/water interface. This controller signals the water dump valve to release as much water from the vessel as it is needed to maintain interface at designed height. The gas flows horizontally and exits through a mist extractor to a pressure control valve which maintains constant vessel pressure.

Vertical three phase separator

In a vertical three phase separator, flow enters the vessel through the side. As in the horizontal separator, the inlet diverter separates the bulk of the gas. A downcomer is used to transmit the liquid through the oil/gas interface to keep from disturbing the oil skimming action. A chimney equalizes gas pressure between the lower section and the gas section. The spreader, or the downcomer outlet, is located at the oil/water interface. From this point, as the oil rises, any free water separates out from the oil phase. The water droplets flow counter current to the oil. Similarly, the water flows downward and the oil droplets trapped in the water phase rise counter current to the water flow.

Separator type selection

Selection of a separator type is based primarily on gas handling requirements and space availability.

A horizontal separator is normally more efficient at handling large volumes of gas and because of its large interfacial areas, it has better phase separation capability. It does not handle solids as well as a vertical separator and it requires more space.

A vertical separator on the other hand has good solid handling capability. requires far less space than a horizontal separator, and has much better liquid surge capacity, but it is more difficult to service.

Common components in a separator vessel

Regardless of type, all three phase separators have in common certain internal vessel components.

Inlet diverter

Inlet diverters provide the initial gross separation by changing the flow direction as the flow enters the vessel.

  • The deflector baffle is one type of commonly used diverter.
  • Another is the cyclone inlet.
Inlet diverter.

Wave breaker

Wave breakers limit the wave propagation that might otherwise occur in large horizontal vessels.

Wave breaker in a three phase separator.

Defoaming plates

Defoaming plates reduce foaming at the gas/liquid interface, which tends to occur when gas bubbles are liberated from the liquid.

Defoaming plate.

Vortex breaker

A vortex breaker keeps vortexes from developing when the liquid control valve is open, thus preventing gas from being drawn out of the vapor phase and re-entrained in the liquid outlet.

Vortex breaker.

Mist extractor

Mist extractors coalesce and collect small liquid drops from separated gas before the gas leaves the vessel.

Mist extractor.

Potential operating problems

Potential operating problems in three phase separators include foaming crude, paraffin build up, sand accumulation, liquid carry over, gas blowby, and formation of emulsions.

Separators must be sized properly in order to avoid such problems. Design procedures require a thorough understanding of the separator operating principles and relation to such variables as settling of the oil droplets, retention time, and droplet size.

See also