It was concluded in a previous study that for an electric submersible pump to operate effectively and efficiently while pumping gaseous fluids, the amount of free gas ingested into a centrifugal pump must be maintained at low values. This can be accomplished either by increasing the submergence pressure on the pump intake or by mechanically separating the free gas from the fluid to be pumped. This study presents data for the performance of a reverse-flow and a centrifugal type gas separator attached to a centrifugal submersible pump. The results of two independent test programs are presented. One program used water and air as test fluids. and the other test results were obtained using diesel fuel and CO.
The reverse-flow separator depends on buoyancy and surface tension effects and a sharp flow reversal to accomplish its free gas separation. Cross sections of both a reverse-flow and a centrifugal separator are shown in Fig. 1. The reverse-flow type consists of housing intake ports located above the inner intake ports. This configuration causes the inlet flow to make a sharp 180 deg. bend, thus allowing some free gas separation at the housing inlet ports. The upturned impeller will produce some head and assist in allowing the pump to recharge itself with liquid if it becomes gas-locked. If the pump gas locks because of either an accumulation of free gas or a large gas slug, the fluid will fall back through the upturned impeller, thereby allowing this impeller to recharge the first regular pump stage. This reduces the number of electrical shutdowns of the unit by the underload protection device.
The centrifugal gas separator as shown in Fig. 1 consists of an inducer and a high-capacity, highly mixed-flow pump stage followed by a separator chamber. The inducer and pump stage are incorporated to provide sonic means to overcome the resistance of the internal flow and vent passages. Both inducer and pump stage underwent separate testing and showed the capability of generating a small amount of head without surging with fluids of high free-gas content. The separator chamber consists of a rotating unit with radial vanes and an integral outer shell, The rotating outer shell of the chamber is a significant feature of this separator. It provides a radially closed chamber where flow-shearing or turbulence are minimized because the fluid rotates with the chamber as a solid body. The cross-sectional area of the chamber is kept as large as possible to keep the axial flow velocity minimal, thereby allowing the maximum residence time in the chamber, where it is acted on by intense centrifugal acceleration forces. The heavier Hubs accumulate near the outer wall. and the free gas accumulates near the shaft because of these forces. These fluids then are separated physically in the top section of the separator chamber and are ducted to the first pump stage. The gases are vented to the casing annulus by use of a crossover diffuser.
The performance tests for Centrilift-Hughes Inc. were conducted by the R.C. Ingersoll Research Center. A schematic of the test facility is shown in Fig. 2. A 20-ft section of 6-in. annulus casing was installed above the pump discharge. At the top of this casing a vent and gas metering section were added. The pump and separator were installed in the 8-in. housing as shown. The unit was tested on a mixture of diesel fuel and CO. The CO was injected into the inlet flow line to the test loop housing and was vented at the top of the annulus casing.