Figure 5-3 shows an alternate configuration known as a “bucket and weir” design. This design eliminates the need for a liquid interface controller. Both the oil and water flow over weirs where level control is accomplished by a simple displacer float. The 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 height of the oil weir controls the liquid level in the vessel. The difference in height of the oil and water weirs controls the thickness of the oil pad due to specific gravity differences. It is critical to the operation of the vessel that the water weir height be sufficiently below the oil weir height so that the oil pad thickness provides sufficient oil retention time. If
the water weir is too low and the difference in specific gravity is not as great as anticipated, then the oil pad could grow in thickness to a point where oil will be swept under the oil box and out the water outlet. Normally, either the oil or the water weir is made adjustable so that changes in oil/water specific gravities or flow rates can be accommodated.
To obtain a desired oil pad height, the water weir should be set a distance below the oil weir, which is calculated by:
This equation neglects the height of the oil and water flowing over the weir and presents a view of the levels when there is no inflow. A large inflow of oil will cause the top of the oil pad to rise; the oil pad will thus get thicker, and the oil bucket must be deep enough so that oil does not flow under it. Similarly, a large inflow of water will cause the level of water flowing over the water weir to rise, and there will be a large flow of oil from the oil pad over the oil weir until a new hw is established. These dynamic effects can be minimized by making the weirs as long as possible.
Derivation of Equation 5-1 p is in lb/ft3, h is in inches. Setting the pressures at Point “A” in Figure 5-4 equal,
Interface control has the advantage of being easily adjustable to handle unexpected changes in oil or water specific gravity or flow rates. However, in heavy oil applications or where large amounts of emulsion or paraffin are anticipated it may be difficult to sense interface level. In such a case bucket and weir control is recommended. In some areas of the world, the term “free-water knockout” is reserved for a vessel which processes an inlet liquid stream with little entrained gas and makes no attempt to separate the gas from the oil. Such a vessel has only an oil outlet and a water outlet (no separate gas outlet), as shown in Figure 5-5. It should be clear that the principles of operation of such a vessel are the same as those described above.