Development of a Twin-Screw Pump for Multiphase Duties

Abstract

Summary This paper discusses the reasons for developing a pump capable of pumping gas/liquid mixtures. Preliminary trials that led to the design, construction, and testing of a full-specification pump are described. Introduction All oil wells produce a mixture of hydrocarbon fluids that, when reduced to atmospheric pressure, partially vaporize to give a mixture of gas and liquid. Until now, it has been common for the gas to be flared and only the liquid to be retained for further treatment. Finite energy resources, restrictions on flaring, and the increasing value of the gas frequently make such procedures unacceptable. The separation and treatment of gas/liquid mixtures close to the wellhead may be costly and operationally inconvenient. Transport in a multiphase pipeline to a central treatment station is possible but may require higher pressure than is available at the wellhead. In this case, it is currently necessary to separate the liquid and gas, pump the liquid, compress the gas separately, and recombine the streams. A multiphase pump offers an alternative that likely has a lower capital cost. Outline Requirements of the Pump A number of case studies suggested the following broad requirements. The pump should be capable of pumping up to 40,000B/D [265-m3/h] total suction volume. Its differential pressure should be up to 500 psi [3.45 MPa], and it should have an ANSI Class 900 casing-pressure rating. The pump should be able to withstand up to 260°F [127°C] pumping temperature. The pump should have a proven corrosion resistance against hot salt water with the presence of H2S and CO2 and a proven erosion resistance against small quantities of sand (up to 751bm/l,000 bbl [214 g/m3]). The pump should handle a gas volume fraction, defined as the volume of gas at suction conditions divided by the volume of gas plus liquid at suction conditions, of up to approximately 90%. The pump should withstand severe slugging. Selection of Pumping Principle The high gas fraction immediately rules out the use of any kind of rotodynamic pump. In principle, any positive-displacement pump could handle gas/liquid mixtures, but the service conditions eliminate most pump types. Thus, for the required capacity, piston and plunger pumps would be very large and would need to be run slowly to avoid shock. The presence of sand rules out pumps that depend on a sliding motion in contact with the pumped fluid - e.g., vane pumps and screw pumps without timing gears-and the combination of capacity and differential pressure rules out gear and lobe pumps. The presence of hydrocarbons renders the suitability of progressing-cavity pumps with elastomeric stators doubtful. It is possible to use metallic stators for these pumps, but the necessary clearances involve a loss in volumetric efficiency. The required capacity would necessitate the building of a pump larger than any built to date. The twin-screw pump with timing gears had already been built for such capacities and differential pressures - e.g., for fuel oil in a U.K. power station. The casing-pressure rating was thought to be only a matter of designing a stronger casing than had been required previously. The use of external timing gears should minimize the wear of components by abrasives. Assessment of susceptibility to wear is difficult, but experience suggests that this type of pump has a reasonably long life in mildly abrasive environments. The use of external timing gears also gives it the ability to run dry. Thus, the twin-screw pump was felt to offer a suitable combination of properties. Although considerable development would be required, it would be less than for any other pump type. A further factor in the selection of the twin-screw pump was the existence of experience with gas/liquid mixtures.1,2 Confirmation of Pumping Principle To verify that twin-screw pumps would be capable of pumping mixtures with such a high gas fraction, a simple test rig was set up with a standard Size 95 (scroll diameter in millimeters) twinscrew pump, water as the test liquid, and air from the works compressed-air system as the gas. Trials supported the theoretically based expectations.Volumetric efficiency was similar to that on water alone at the same differential pressure. The volumetric efficiency of a positive-displacement machine is defined as the volume pumped measured at suction conditions divided by pump swept volume.Power consumption was not changed significantly by the presence of gas.Both of these statements were true up to the highest gas fractions tested (approximately 90%). Formation of Joint Venture At this stage, potential business needs had been identified and the feasibility of pumping gas/liquid mixtures with a twin-screw pump had been demonstrated. A joint venture was therefore formed to develop the concept. Partners in the joint venture were BP Petroleum Development Ltd., Stothert and Pitt plc, Mobil North Sea Ltd., and Shell (U.K.) E&P. Financial support was also provided by the Dept. of Trade and Industry.