This paper reports the results of two independent laboratory investigations on the effect of free gas on centrifugal submersible pump performance. One investigation used air and water as the working fluids, with the free gas allowed to escape out an annulus. A separate investigation was conducted using diesel fuel and CO as the working fluids with intake pressures up to 400 psi. The results of the two investigations agreed in general at low pressures, where gas volumes exceeding 10% by total volume began to cause serious reduction in pump performance curves. Better performance at higher pressures with the same gas volumes present was found to occur at higher pump intake pressures.
The presence of free gas has been recognized as having, in general, a detrimental effect on the performance of a submersible centrifugal pump. This paper describes two independent test programs undertaken to define the effect of the gas on the performance of this type pump. The effects of the free gas show up as a deterioration of the head-capacity curve, such as areas of unstable head production, and effects similar to cavitation at higher flow rates. Depending on the amount of free gas through the pump, these effects may vary from slight interference to gas locking. Gas interference is indicated on the surface amp chart by rapid variation of the motor loading. Gas locking occurs when the pump ingests too much gas and actually stops pumping because its head (or pressure) production is drastically decreased. This causes the motor to unload and to shut down because of excessive underload (control protection). When designing an electric submersible pump for a gassy application, it is very desirable to know the amount of free gas the pump can tolerate and to compare this to the downhole gas conditions.
Test facilities, methods of data collection and reduction, and results are presented for two independent test programs. Tests conducted by Amoco Production Research Co. involved using water and air. A separate program was conducted for Centrilift Hughes Inc. at the R.C. Ingersoll Research Center using diesel fuel and CO. In both programs, the purpose of the tests, was to define the performance of typical submersible centrifugal pump stages when free gas volumes were introduced at the pump suction under various flow and pressure conditions. If this information is available and can be shown to be extended to field use in the petroleum industry, improved pump design or selection of gas separation equipment can be made for the many cases where pumps are needed to handle gassy fluids. The two test facilities and programs are described, and results from each are presented and discussed.
The test facility at Amoco was an aboveground installation, in which the flow patterns and distribution of air volumes could be viewed. For simplicity, water and air were chosen as working fluids. A schematic of the test loop is shown in Fig. 1. The pump housing, containing five Centrilift I-42 pump stages, was installed in an 8-in.-OD, 7-in.-ID plexiglass tube.