Before oil can be shipped from an offshore platform for sale or for production impurities must be removed from the stream. In most cases, these impurities are water, but there is usually a ratio of gas that will also be in the stream. To remove impurities and entrained gases, the stream is normally sent through a series of separators. The separated oil stream is sent for export and the gas stream can be compressed and used for injection or for export. Figure 1 shows a flow diagram of a typical process.

Figure 1: Process Flow Diagram of Typical Platform Separation Process

There are many different types of separators used in this process, but all must achieve results similar to the following:

§ The liquids must be separated from the gas in a primary separating section

§ Gas velocities must be lowered to allow liquids to drop out

§ Water and oil must be diverted to a turbulence-free section of the vessel

§ Liquids must be retained in the vessel long enough to allow separation

§ The water-oil interface must be maintained

§ Water and oil must be removed from the vessel at their respective outlets

To control the level of oil and water in the vessel and the separation process, level control valves must be used. Figure 2 shows a cross section of a separator with the oil and water level control valves.

Figure 2: Cross Section of a Spherical Separator

The level control valve in this application can be exposed to the effects of severe flashing, outgassing, and associated erosion and vibration. Because of the many constituents in the oil, it is possible that the valve outlet pressure will be less than the respective vapor pressures of certain constituents. This will cause these constituents to either flash or outgas and thus change to the vapor phase. The same type of effect can occur in the water level control valve if the outlet pressure falls below the vapor pressure. Because of the high velocities associated with the flashing vapor, erosion of the valve internal components can occur. Erosion can also be caused by sand that is entrained in the oil as it moves through the valves.

Proper analysis of the two-phase fluid during valve sizing and selection is the first step to solving this application challenge. The Fisher solution will be selected on the liquid / vapor volume ratio; there are several geometries available each suited for a particular volume ratio. For example, when pressure staging is required the DST-G or SS-250 could be applied to stage the liquid pressure reduction and accommodate the vapor expansion, both of these minimize erosion due to liquid droplets or entrained particulate.