Centrifugal Pumps for Co

Abstract

There is a renewed interest in pumping CO2 in the liquid and supercritical states for many enhanced oil recovery (EOR), and new carbon capture and sequestration (CCS) projects. Referenced centrifugal technology currently available in the industry can cover discharge pressures up to 25 MPa (3600 psi). GE Oil & Gas has now developed and successfully tested pumps that are capable of providing solutions up to 60 MPa (8700 psi). This paper reports the activities performed for this development and the results of the tests. INTRODUCTION GE Oil&Gas has a 50-year history in manufacturing a wide range of API 610/ISO13709 compliant centrifugal pumps for hydrocarbon processing, refineries, water injection and pipeline services. A large fleet of these multistage pumps is currently operating with liquefied gases. The centrifugal pumps used for liquefied gases already include most of the technology necessary for CO2 pumping because of similarities in viscosity, density, and compressibility. Therefore, this reference fleet has been the starting point for the development of the high pressure CO2 injection pumps. We then utilized experience from our very high pressure gas re-injection centrifugal compressors that discharge up to 80MPa (11000 psi). PUMPS FOR LIQUID CO2 The BB5 type multistage barrel pump has been selected for the very high pressure CO2 applications. The opposed impeller back-to-back rotor configuration provides good overall efficiency compared to the inline rotor configuration. This is because the central balancing bushing has a smaller diameter than a balancing drum, thus reducing the internal leakages that become a significant source of power losses when operating with low viscosity fluids. In addition, for pumps with such high DeltaP and long rotors, it also provides improved rotor-dynamic damping, even in worn conditions. The main differences between pumps suitable for CO2 pumping and conventional centrifugal pumps for hydrocarbon or water applications are described in the following paragraphs. The low density of warm supercritical CO2 forces the pump to make high differential heads required to reach high discharge pressures. With limited number of stages available due to shaft span, high speed is required to achieve reasonable specific speeds and efficiencies. Rotordynamic assessment is the most critical part of the analysis and design process. In this application, the use of swirl brakes is often necessary, and magnetic Proceedings of the Twenty-Seventh International Pump User Symposium September 12-15, 2011, Houston, Texas 2 bearings for dense phase acid gas pumping at high rotating speeds is also possible. The high compressibility of supercritical CO2 must always be taken into account during the selection of the pump as in the case of centrifugal compressors. Therefore, the correct parameter expressing the work per unit of mass provided by the pump is the polytropic head instead of the differential head. Also, the definition of the efficiency is not unique. The polytropic efficiency is preferred because it is the parameter used for the compressors. It should also be noted that the traditional equations to calculate polytropic head and efficiency, based on perfect gas assumptions, give high errors for supercritical CO2 in the region near the critical point (Pc=7.4MPa (1029psi), Tc=31°C). In this region the specific heat and the isentropic exponent cannot be used as average values between inlet and outlet because they do not vary monotonically. Another effect of compressibility relates to the stall characteristics of the impellers and the overall system pressure pulsations. For high-speed pumps, the formation of potentially dangerous standing waves must be considered because the speed of sound is much lower than in liquid hydrocarbons or water. The equations of state used for the thermodynamic calculations of the flow internal to the pump must have an adequate level of precision. In the cases where the CO2 is not pure, they may not be accurate enough, so several models must be used and a sensitivity analysis performed. An experimental program has been conducted by GE to increase the accuracy in those cases where contaminants significantly alter the physical behavior of the gas mixture. Another requirement, peculiar to CO2 pumps, is the optimization of the overall train comprising the compressor and pump, including the selection of the intermediate pressure between the last compression stage and the pump inlet. The objective function to be minimized can be the total absorbed power, the total life cycle cost, or the cost per unit mass of CO2 re-injected underground. The constraints to be considered in the optimization, can be the available cooling power, the total footprint, the cooling temperature when heat recovery is possible, and the operating range and different duty points that may apply in the future. For enhanced oil recovery applications with very high suction and discharge pressures, the structural design and material selection criteria have been derived from proven experience with CO2 compressors. Three sealing technologies are available for selection depending on the type of process. When operating with pure CO2, single face quasi-gas seals are the simplest solution. When the pump suction pressure must be increased significantly above the critical pressure and the process fluid is contaminated, dual or triple mechanical seals are preferred. Recent developments on wet seals (provided with light oil as buffer fluid) made them very robust to process upset conditions but have a higher power consumption and larger footprint for the auxiliaries compared with dry gas seals. Dry gas seal systems, specifically tuned for this application, provide the same reliability and MTBM of those installed on the compressors upstream of the pump. With the objective of maximizing MTBF and availability, a monitoring and smart diagnostic system has been developed that includes algorithms specific for CO2 applications. Figure 1. Operating Envelope of CO2 pumps. In Figure 1 the pressure vs. enthalpy (Mollier) diagram of pure CO2 is shown. The envelope in which CO2 pumps usually operate is also shown on the diagram. The envelope is bounded by the melting line on the left (about –50°C), the saturated liquid limit curve on the bottom and the minimum operating density on the right. The value of the minimum operating density, between 500 and 400 kg/m, depends on rotor-dynamics of the pump and the pressure ratio between discharge and suction pressure. As the ratio is increased the variation of the density can decrease the efficiency of the pump and the operating range unless differentiation of the stages is adopted. There is also a small segment on the upper right corner due to the max design operating temperature. The discharge pressure upper limit is not a physical limit but rather, is based only on available references. To define the minimum suction pressure a margin is added to the saturated liquid limit curve. This margin depends on the NPSH requirements of the pump considered, the accuracy of the control of the temperature of flow stream at pump inlet and on the expected variations in the composition of the gas. Within the envelope, there is one example of the polytropic transformation obtained during the tests. Proceedings of the Twenty-Seventh International Pump User Symposium September 12-15, 2011, Houston, Texas 3 PUMP FOR TUPI PILOT FPSO This project is the first of its kind. It is a very high pressure gas reinjection train for enhanced oil recovery. The pump, shown in Figure 2, has a suction pressure of 30 MPa (4350 psi) and a rated discharge pressure of 54 MPa (7830 psi). The design pressure of the barrel casing is 67 MPa (API 10000 rating class). Because of the very high suction pressure, the mechanical seals are the most critical component. A triple seal arrangement was necessary to split the total pressure differential into smaller steps. Figure 2. Cross section of the CO2 injection pump for the TUPI pilot project. The main development activities conducted for this project have been: Qualification of the prototype mechanical seals capable to operate up to 48 MPa of pressure difference. Modification of the impellers to improve the stall characteristics with the compressible mixture (see Figure 3) and avoid any surge phenomena. A detailed rotordynamic assessment using the experience, the tools and additional criteria developed for the centrifugal compressors. An experimental campaign to validate the thermodynamic properties (density, specific heats and speed of sound) since the gas mixture that this pump operates with contains, in addition to CO2, up to 23% molar percentage of hydrocarbons. Figure 3. CFD’s of Standard and New Impeller. The graph in Figure 4 is part of the rotordynamic stability analysis. The damping factors achieved are high enough to guaranty stable operation even at the critical speeds thanks to the introduction of swirl brakes and additional damping devices. This was also confirmed by the tests. Backward 2nd mode Forward 2nd mode

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Cite this paper

@inproceedings{Bergamini2011CentrifugalPF, title={Centrifugal Pumps for Co}, author={Lorenzo Bergamini and Carlo del Vescovo and Fabrizio Milone}, year={2011} }