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Axial Thrust in High Pressure Centrifugal Compressors: Description of a Calculation Model Validated by Experimental Data from Full Load Test

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Axial Thrust in High Pressure Centrifugal Compressors: Description of a Calculation Model Validated by Experimental Data from Full Load Test Axial Thrust in High Pressure Centrifugal Compressors: Description of a Calculation Model Validated by Experimental Data from Full Load Test Leonardo Baldassarre Michele Fontana Engineering Executive for Compressors and Auxiliary Systems Engineering Manager for Centrifugal Compressors General Electric Oil & Gas Company General Electric Oil & Gas Florence, Italy Florence, Italy Andrea Bernocchi Francesco Maiuolo Senior Engineering Manager for Centrifugal Compressors Lead Design Engineer for Heat Transfer & Secondary Flows General Electric Oil & Gas Company General Electric Oil & Gas Florence, Italy Florence, Italy Emanuele Rizzo Senior Design Engineer for Centrifugal Compressors General Electric Oil & Gas Florence, Italy Leonardo Baldassarre is currently Michele Fontana is currently Engineering Engineering Executive Manager for Manager for Centrifugal Compressor Compressors and Auxiliary Systems with Upstream, Pipeline and Integrally Geared GE Oil & Gas, in Florence, Italy. He is Applications at GE Oil&Gas, in Florence, responsible for requisition and Italy. He supervises the calculation standardization activities and for the activities related to centrifugal compressor design of new products for compressors, design and testing, and has specialized in turboexpanders and auxiliary systems. the areas of rotordynamic design and Dr. Baldassarre began his career with GE vibration data analysis. in 1997. He worked as Design Engineer, R&D Team Leader, Mr. Fontana graduated in Mechanical Engineering at Product Leader for centrifugal and axial compressors and University of Genova in 2001. He joined GE in 2004 as Requisition Manager for centrifugal compressors. Centrifugal Compressor Design Engineer, after an experience Dr. Baldassarre received a B.S. degree (Mechanical as Noise and Vibration Specialist in the automotive sector. Engineering, 1993) and Ph.D. degree (Mechanical Engineering He authored or co-authored eight technical papers about / Turbomachinery Fluid Dynamics, 1998) from the University rotordynamic analysis and vibration monitoring, and holds two of Florence. He authored or coauthored 20+ technical papers, patents in this same field. mostly in the area of fluid dynamic design, rotating stall and rotordynamics. He presently holds five patents. Emanuele Rizzo is currently Senior Design Engineer in New Product Introduction for Andrea Bernocchi is an engineering Centrifugal Compressors with GE manager at GE Oil&Gas. He joined GE in Oil&Gas, Florence, Italy. His current 1996 as Centrifugal Compressor Design duties are mainly focused on structural Engineer after an experience in plastic design, material selection and new machinery industry. He has 18 years of applications of centrifugal compressors. experience in design development, Dr. Rizzo holds an MSc degree (Aerospace production and operation of centrifugal Engineering, 2003) and a Ph.D. degree compressor. He covered the role of LNG (Aerospace Engineering, Conceptual Aircraft Design and compressor design manager for 6 years Structural Design, 2007) from the University of Pisa (Italy). He with responsibility in design of LNG compressors, testing and joined GE Oil&Gas in 2008 as Lead Design Engineer in the supporting plant startup. He’s currently leading the requisition centrifugal compressors requisition team, working mainly on team for centrifugal and axial compressor design. high pressure compressors operating in sour environment. Mr. Bernocchi received a B.S. degree in Mechanical He has authored and coauthored several papers on aircraft Engineering from University of Florence in 1994. He holds 4 design and optimization. He is co-inventor in two patents. patents in compressor field. Copyright© 2015 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station Francesco Maiuolo is currently Lead Design requirements, OEMs have developed internal design criteria Engineer in the Advanced Technology and defined safety margins based on their own experience. Organization of GE Oil&Gas, Florence, Axial thrust prediction is directly related to the calculation Italy. Dr. Maiuolo received his MSc degree of the gas pressure acting on the surfaces of the rotor, that is in Energy Engineering in 2009 and the particularly challenging for the external surfaces of the Ph.D. degree (Energy Engineering impellers. Here the pressure distribution is heavily affected by Department, 2013) from the University of aerodynamic effects related to the gas flowing in the rotor- Florence (Italy). He joined GE Oil&Gas in stator cavities, that are function of a large set of geometric and early 2013 in the Aero-thermal design team, mainly focusing on thermodynamic data, as summarized in the next section. A heat transfer, secondary flows and sealing systems of rotating software tool was developed to solve this physical model and to machinery as centrifugal compressors and gas turbines. calculate the resulting axial force acting on the rotor. He authored or co-authored ten technical papers on heat The present work provides a description of the tool and of transfer and secondary flows studies. the model adopted to simulate the physical system and governing laws. The validation of the tool is then addressed, by ABSTRACT comparing its predictions to the experimental results collected on model tests of single stages and on full load tests of The residual axial thrust acting on the rotor of a centrifugal complete centrifugal compressors. The experimental data are compressor is the result of the non-uniform pressure further analyzed, providing insights of other features that can distribution on the surfaces in contact with the process gas, plus be identified and explained basing on the knowledge of the the differential pressure acting on the faces of the balance physical model and its governing laws. piston(s) and the contribution due to the momentum variation Conclusions derived from this study provide some of the process gas. During the design phase the axial load shall recommendations on centrifugal compressor axial thrust be verified to remain safely lower than the thrust bearing evaluation and on the physical model . capacity, under all possible operating conditions; this requires a high degree of accuracy in the calculation model used to THRUST CALCULATION evaluate each thrust component. Errors in this calculation may lead to high bearing pad temperature during operation, to early During operation the rotor of a centrifugal compressor is wearing of the pad surfaces and ultimately to the damage or subject to an axial thrust T resulting from the sum of several failure of the thrust bearing (Moll and Postill, 2011), thus components: jeopardizing the integrity of the whole compressor. Tm due to variation of gas momentum The main difficulty of axial thrust calculation lies in the T due to differential pressure across the impellers correct prediction of the static pressure distribution over the a T due to differential pressure across the balance piston external surface of the impeller hub and shroud. This b T due to coupling pre-stretch distribution depends on a large set of parameters, including c rotor geometry, operating conditions, properties of the process 푇 = 푇 + 푇 + 푇 + 푇 (1) gas, leakages flows across the rotor-stator seals. A detailed 푚 푎 푏 푐 fluid-dynamic model of the gas in the cavities between impeller The present analysis is focused on the evaluation of and diaphragm was developed and applied first to stage model aerodynamic effects and therefore does not address the tests and then to high-pressure centrifugal compressors, and its coupling pre-stretch contribution, which is generally predictability was assessed by direct comparison with compensated by the thermal deformation of the shaft and has experimental data. The compressors were tested in full load very limited impact during normal compressor operation. The conditions, with thrust bearing pads equipped with load cells, other effects are described in detail below. and the thrust values were recorded for several points across the operating envelope. Axial thrust due to momentum variation INTRODUCTION An axial force is generated on the rotor as a result of the momentum variation of the gas flow, and specifically by the The accurate prediction of axial thrust is a key factor for difference of gas axial speed between impeller inlet and outlet the correct design of a centrifugal compressor. The correct (see Figure 1): selection of the thrust bearing and the sizing of the balance drum(s) are assessed by evaluating the residual axial thrust 푇 = 푚̇ 푣 − 푚̇ 푣 (2) across the operating envelope and the consequent bearing pad 푚 푎푥 푂푈푇 푎푥 퐼푁 load and temperature. with axial thrust considered positive in the direction of Standard requirements for the selection of the thrust bearing impose limits on the maximum allowable load and impeller suction. In case of radial gas exit the term 푣푎푥 푂푈푇 is temperature (API617, 2014). In order to comply with these equal to zero and Equation (2) becomes: Copyright© 2015 by Turbomachinery Laboratory, Texas A&M Engineering Experiment Station 푇푚 = −푚̇ 퐼푁푣푎푥 퐼푁 (3) The axial force due to gas pressure can be calculated by integrating the axial component of the pressure distribution This force is directed towards compressor discharge. over the rotor surfaces. With the same sign convention of Figure 1: 푇푎 = ∫ 푝 푑퐴 − ∫ 푝 푑퐴 − ∫ 푝 푑퐴 (5) 퐴퐻 퐴푆 퐴퐼푁 where 퐴퐼푁, 퐴퐻, 퐴푆 are the inlet, hub and shroud areas respectively, and are defined below: 휋 퐴 = (퐷2 − 퐷2 ) (6) 퐻 4 푡푖푝 푓표표푡 휋 퐴 = (퐷2 − 퐷2 ) (7) 푆 4 푡푖푝 푒푦푒 휋 퐴 = (퐷2 − 퐷2 ) (8) 퐼푁 4 푒푦푒 푓표표푡 Figure 1. Variation of gas momentum The control volume for the momentum balance shall include the part of the rotor in front of the impeller (in Figure 1, the shaft portion and the triangular sleeve section on the left side). This leads to a reduction of Tm, as a function of the 훼 angle as shown in Figure 2, since: 푣푎푥 퐼푁 = 푣퐼푁cos 훼 (4) Figure 3. Pressure distribution on a compressor stage. The pressure can be assumed constant over the impeller inlet area AIN and therefore the last integral of Equation (5) is simply equal to p1AIN, while this assumption is not valid for AH and AS.
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