Key Advantages of the Progressing Cavity Pump in Multiphase Transfer Applications

Abstract

Abstract Robbins & Myers has developed a system for transporting all multiphase fluids produced from the well, simultaneously and without separation or emulsification, through one pipeline to a central processing station. This system utilizes a specially designed progressing cavity pump, and efficiently handles a wide range of flow regimes. Of special interest is its ability to handle very high gas fractions. Introduction In recent years, there has been considerable interest in pumping multiphase fluids, and several pump concepts have enjoyed varying degrees of success in handling these fluids. The ideal pump system would be capable of handling combinations of oil, water, gas and, in many cases, small amounts of wax and sand at high gas volume fractions. The perfect system would be able to handle these fluids over a wide range of flows and pressures, be reasonably priced, be relatively maintenance-free, and have low operating costs. Unfortunately, we don't live in a perfect world and the perfect pump for all multiphase pumping applications doesn't exist. Fortunately, there are a number of multiphase pump systems available which, when properly selected and applied, can perform reliably in multiphase applications and provide substantial benefit to their users. One such pump system has been developed by Robbins & Myers. This system uses a specially designed progressing cavity pump and has proven to be capable of handling multiphase fluids with sand cuts of 7 or 8% and gas volume fractions to 99%. This paper reviews progressing Cavity pump technology, the advantages of this technology, how some of these advantages apply to multiphase fluid pumping, and the formal development programs which have verified the suitability of this technology for pumping multiphase fluids. The results of these test programs are also discussed, including observations relevant to successful use of progressing cavity pump systems for pumping high gas fraction multiphase fluids. This paper also highlights the advantages of multiphase pumping in general, as well as the specific advantages derived from utilizing progressing cavity pump systems for pumping multiphase fluids. Overview of Progressing cavity Pumps In 1929, a French scientist named Rene Moineau invented progressing cavity pumps while trying to develop an improved turbocharger for French fighter aircraft. Figure 1 shows a typical progressing cavity pump manufactured by Robbins & Myers. The original patents for this invention state that the concept can be used as a pump or compressor; therefore, adapting this technology for pumping high gas fraction multiphase fluids would appear to be a natural progression. Unfortunately, the materials of construction of most progressing cavity pumps have prevented using this concept as a compressor or high gas fraction multiphase pump in all but a few selected applications. Progressing cavity pumps in their simplest form consist of a single threaded screw (rotor) turning inside a double threaded nut (stator), as shown by Figure 2. This simplest form is called a 1:2 profile because there is one lead on the rotor and two on the stator. Any combination is possible as long as the stator has one more lead than the rotor. The geometry of the parts is such that as the rotor rotates, cavities are formed at the suction end. As the rotor continues to rotate, the cavities progress from the suction to the discharge end of the stator. With the 1:2 profile elements, two cavities are formed in each revolution of the rotor, one cavity opening at the same rate as the second cavity is closing. In most progressing cavity pumps, the stator is made with an elastomeric material that fits on the rotor with a compression fit. The areas of contact between the rotor and stator create seal lines which separate the cavities. The positive separation of the cavities makes progressing cavity pumps a unique type of positive displacement pump with predictable, pulsation-free flow rates. The use of an elastomeric stator also enables progressing cavity pumps to pump abrasive or solids-laden fluids, as well as water-like liquids, or highly viscous fluids. P. 623^