Centrifugal separators work on the principle of differing densities of fluids. Of the three components, oil, water and gas, gas is the most difficult to separate, due to the fact that gas becomes entrained in the liquid constituents of the fluid mixture, and it takes time for the gas to be released. For this reason many centrifugal separators restrict their operation to two phase separation; permitting the gas to exit the centrifugal separator in the water or oil stream.
Centrifugal separators are susceptible to pressure fluctuations. A fluid mixture with a significant amount of entrained gases can cause a number of problems. One problem disclosed by Kartinen in U.S. Pat. No. 3,810,347 is that of the gas releasing in the flow paths of the separator and building up a back pressure which offers resistance to the flow of fluids through the separator. Another problem disclosed by Brown and Erickson in U.S. Pat. No. 4,044,943 is that of a fluctuation in the relative amounts of the oil, water, and gas phases, leading to an intermixing of the separation phases as evidenced by gas exiting through the outlet intended for water or oil.
The inability of the centrifugal separators to operate under pressure creates practical problems. In order to connect a centrifugal separator to an operating pipeline some line pressure is required. As the line pressure adversely affects the operation of existing centrifugal separators they cannot be connected to pipelines.