Three-phase separators are pressure vessel that are designed to separate and remove the free water from a mixture of crude oil and water. The term of “three-phase separator” is normally used when there is a large amount of gas to be separated from liquid, therefore the size of the vessel is determined by gas capacity equation.
In addition of three-phase separator, there is term called “free-water knockout”. “Free-water knockout” is generally used when the quantity of gas is relatively small compared to the quantity of oil and water. Therefore, the dimension of vessel is determined by oil-water separation equation.
Types of Three-Phase Separator
In term of orientation, three-phase separator can be categorized into horizontal three-phase separator and vertical three-phase separator. Free-water knockout is also classified into horizontal and vertical type.
Figure below shows a typical schematic of horizontal three-phase separator. At first, fluid enters the separator and hits an inlet diverter. This sudden change in momentum does the initial gross separation of liquid and vapor.
The liquid collecting section of the vessel provides sufficient time so that oil and emulsion form a layer or “oil pad” on top of free water. The free water settles to the bottom.
Horizontal Three-Phase Separator Completed with Interface Level Controller and Weir
Horizontal three-phase separator itself has several types. Those are:
- Horizontal three-phase separator completed with interface level controller and weir
- Horizontal three-phase separator “bucket weir” type
- Horizontal three-phase separator with boot type
In a typical horizontal three-phase separator equipped with interface level controller and weir, the weir maintains the oil level, and the level controller maintains the water level. The oil is skimmed over the weir. The level of oil downstream of the weir is controlled by a level controller that operates the oil dump valve.
Produced water flows from a nozzle in the vessel located upstream of oil weir. An interface level controller (ILC) senses the height of the oil-water interface. The controller sends a signal to the water dump valve, therefore allowing the correct amount of water to leave the vessel so that the oil-water interface is maintained at the design height.
The level of the gas-oil interface can vary from 50% to 70% of the diameter. The most common configuration is half-full.
Horizontal Three-Phase Separator with a Bucket and Weir
Figure below show a configuration of horizontal three-phase separator with bucket and weir design. This design eliminates the need of interface level controller. It is because oil and water flow over weirs where level control is accomplished by simple displacer float.
Oil overflows the oil weir into an oil bucket where its level is controlled by a level controller that operates the oil dump valve. The water flows under the oil bucket and then over a water weir. The level downstream of this weir is controlled by a level controller that operates the water dump valve. The back part of oil bucket is higher than the front of the bucket. This differential height will assure that the oil will not flow over the back of the bucket and out with the water.
In the operation, it is critical to ensure that water weir height is sufficiently below the oil weir height so that the oil pad thickness provides sufficient retention time. If the water weir is too low and the difference in specific gravity is not as great as anticipated, the oil pad could grow in thickness to a point where oil will be swept under the oil box and out the water outlet.
Three-phase separators with a bucket and weir design are most effective with high water-to-oil flowrates and/or small density differences. Interface control design has flexibility that it can be adjusted to handle unexpected changes in oil and water specific gravity or flow rates. However, in heavy oil applications or where large amounts of emulsion are anticipated, it may be difficult to sense interface level. In such a case, bucket and weir control is recommended.
Horizontal Three-Phase Separator with Boot Design
Figure below shows a horizontal three-phase separator with a water “boot” on the bottom of the vessel barrel. The boot collects small amounts of water that settle out in the liquid collection section and travel to the outlet end of the vessel. In this design, the flow rate of both oil and water can provide enough retention time to separate oil and water and there is no need to use the main body of separator to provide oil retention time.
Free-Water Knockout (FWKO)
The term FWKO is referred to a vessel that processes an inlet liquid stream with little entrained gas and makes no attempt to separate the gas from the oil.
Figure below illustrates a horizontal FWKO. The main difference between a conventional three-phase separator and FWKO is that in FWKO there are only two fluid outlets. One for oil and very small amounts of gas and the second for the water. Water outlet is usually controlled with an interface level control.
Operation principle of FWKO is the same as common three-phase separator. In sizing, since there is a very little gas, the liquid capacity constraint always dictates the size.
Figure below shows a typical configuration for a vertical three-phase separator.
There are important parts of vertical separator:
- Inlet diverter, to separate the bulk of the gas
- Down comer, to route the liquid through the oil-gas interface so as not to disturb the oil skimming action
- Chimney, to equalize gas pressure between lower section and the gas section
- Spreader, to water wash the incoming stream
Arnold, Ken, and Maurice Stewart, “Surface Production Operations – Design of Oil Handling and Facilities” Vol 1 3rd Edition”, Elsevier, 2008.