MODELING OF PRESSURE DYNAMICS DURING SURGE AND ESD

LectureCentrifugal compressors operability at low flow is normally limited by the onset of surge that occurs when operating near maximum achievable pressure rise. Surge is characterized by large amplitude and periodic pressure oscillations in which the compressor can also experience a series of flow reversal and recovery. Surge may happen when compressors are subjected to rapid transients. Examples are represented by the emergency shutdown (ESD) or a power failure. To prevent this from occurring, compressor stations are equipped with single or dual recycle systems using valves, which are required to quickly open in presence of ESD. For the proper sizing of recycles valves and whole system 3rd Middle East Turbomachinery Symposium (METS III) 15-18 February 2015 | Doha, Qatar | mets.tamu.edu Page 2 layout in general the estimation of surge inception and frequency during transients are key parameters especially for high-pressure centrifugal compressors and usually require the characterization of compressor pressure ratio curve including the reverse flow region. Spool dynamics is important to simulate ESD transients so also the torque law for the compressor needs to be provided and extended in the negative flow region. Usually pressure ratio and torque law are not available from experiments. A quite simple approach based on considerations about velocity triangles is analyzed in this paper and compared with experimental data. The approach is firstly assessed in the analysis of simple systems (one model test and a full scale compressor), both modeled according to the classical plenum-pipe description suggested by Greitzer. Computed pressure and speed trends are compared with experimental data from the dynamic pressure probes during surge and ESD. Kulite probes were installed in addition to the standard instrumentation in the test loops of the model test as also of the full scale string test. The curve extension model based on velocity triangles proved good accuracy in the simulation of dynamic behavior of those systems. The same method was also applied for the analysis of a more complex compression train equipped with four machines and modelled with a commercial tool for dynamic simulation of fluid systems. The results obtained showed good agreement with pressure and speed trends coming from site acquisition devices

Four Quadrant Centrifugal Compressor Performance

LectureLecture 16: The characterization of the compressor behavior in all quadrants of performance map has acquired, in the last years, growing attention. Two dedicated experimental campaigns have been performed to characterize fourth quadrant and second quadrant operation in terms of performance, pressure fluctuations and mechanical vibrations. Experimental campaigns allowed the acquisition of performance curves of a centrifugal compressor in deep choke up to reverse pressure and indicated a large region of safe operating conditions at low peripheral Mach number. In stable reverse flow, the trends of work coefficient and pressure ratio were acquired for a large range of flow rates together with the evolution of vibration and pressure fluctuation along the speed lines. Pressure ratio and absorbed power for the different modes of operation can be very important to simulate dynamic scenarios far from the steady state operation and size accordingly compressor protection equipment and predict accurately compressor startup torque from pressurized condition (Fourth Quadrant Operations). The main frequency content and amplitude of pressure variations within the flow field or the radial/axial vibrations at the bearings are important to estimate blade loading and possible presence of excitation frequencies in the system. In the reported experience the operating points characterized by stable reverse flow rates close to the nominal one in direct flow did not show features much more critical than in standard conditions (Second quadrant operation) . This result may be hopefully used to reduce BOP in early design phase of system layout

Development of a Research Test Rig for Advanced Analyses in Centrifugal Compressors

Abstract In this study, the design process of a new research test rig for centrifugal compressor stages is presented. The rig has been specifically conceived for advanced analyses, with particular focus on rotating stall and in general on the operating conditions close to the minimum flow limit, which represent the research frontier in view of an extension of the stages rangeability. The new rig will be able to test industrial impellers at peripheral Mach numbers up to 0.7, operating in open-loop with ambient inlet conditions. A modular design will allow to test different stage configurations and then to carry out systematic optimization campaigns on a single specific component. The conceptual design of the rig is here described and explained, including the selection of the best architecture and layout, the drivetrain assessment and the rotordynamic analysis

Effects due to the temperature measurement section on the performance estimation of a centrifugal compressor stage

A wide-ranging analysis was performed by GE Oil & Gas and the University of Florence to investigate the effects on the estimation of centrifugal compressor performance induced by a different choice of the total temperature measurement section. With this goal in mind, the study focused on the analysis of a commonly found discrepancy between the measurements at the impeller outlet section and at the stage exit section. Based on the experimental data collected on a centrifugal impeller, three main physical phenomena were analyzed and discussed in further detail. First, the effect of the heat exchange was examined, and its influence on the total temperature variation throughout the machine was extrapolated. Next, the influence of the heat-exchange phenomena affecting the temperature sensors was evaluated by means of numerical models and physical assumptions. Finally, the effects on the temperature measurement of the flow structure at the impeller outlet were investigated. In particular, a corrective model to account for the thermal inertia of the thermocouples normally applied in this section was applied to the experimental data. The corrected temperatures at the investigated measurement sections were then compared, and their influence on the correct stage performance estimation is discussed in this study

Some Guidelines for the Experimental Characterization of Vaneless Diffuser Rotating Stall in Stages of Industrial Centrifugal Compressors

An accurate estimation of rotating stall is one of the key technologies for high-pressure centrifugal compressors, as it is often connected with the onset of detrimental subsynchronous vibrations which can prevent the machine from operating beyond this limit. With particular reference to the vaneless diffuser stall, much research has been directed at investigating the physics of the phenomenon, the influence of the main design parameters and the prediction of the stall inception. Few of them, however, focused attention on the evaluation of the aerodynamic unbalance due to the induced pressure field in the diffuser, which, however, could provide a valuable contribution to both the identification of the actual operating conditions and the enhancement of the compressor operating range by a suitable choice of the control strategy. Although advanced experimental techniques have been successfully applied to the recognition of the stall pattern in a vaneless diffuser, the most suitable solution for a wider application in industrial test-models is based on dynamic pressure measurements by means of a reduced number of probes. Within this context, a procedure to transpose pressure measurements into the spatial pressure distribution was developed and validated on a wide set of industrial test-models. In this work, the main guidelines of the procedure are presented and discussed, with particular reference to signals analysis and manipulation as well as sensors positioning. Moreover, the prospects of using a higher number of sensors is analyzed and compared to standard solutions using a limited probes number

Analysis of the rotordynamic response of a centrifugal compressor subject to aerodynamic loads due to rotating stall

In the current industrial research on centrifugal compressors, manufacturers are showing increasing interest in the extension of the minimum stable flow limit in order to improve the operability of each unit. The aerodynamic performance of a compressor stage is indeed often limited before surge by the occurrence of diffuser rotating stall. This phenomenon generally causes an increase of the radial vibrations, which, however, is not always connected with a remarkable performance detriment. In case the operating curve has been limited by a mechanical criterion, i.e. based on the onset of induced vibrations, an investigation on the evolution of the aerodynamic phenomenon when the flow rate is further reduced can provide some useful information. In particular, the identification of the real thermodynamic limit of the system could allow one to verify if the new load condition could be tolerated by the rotordynamic system in terms of radial vibrations. Within this context, recent works showed that the aerodynamic loads due to a vaneless diffuser rotating stall can be estimated by means of test-rig experimental data of the most critical stage. Moreover, by including these data into a rotordynamic model of the whole machine, the expected vibration levels in real operating conditions can be satisfactorily predicted. To this purpose, a wide-range analysis was carried out on a large industrial database of impellers operating in presence of diffuser rotating stall; the analysis highlighted specific ranges for the resultant rotating force in terms of intensity and excitation frequency. Moving from these results, rotordynamic analyses have been performed on a specific case study to assess the final impact of these aerodynamic excitations

A systematic approach to estimate the impact of the aerodynamic force induced by rotating stall in a vaneless diffuser on the rotordynamic behavior of centrifugal compressors

One of the main challenges of the present industrial research on centrifugal compressors is the need for extending the left margin of the operating range of the machines. As a result, interest is being paid to accurately evaluating the amplitude of the pressure fluctuations caused by rotating stall, which usually occurs prior to surge. The related aerodynamic force acting on the rotor can produce subsynchronous vibrations, which can prevent the machine's further operation, in case their amplitude is too high. These vibrations are often contained due to the stiffness of the oil journals. Centrifugal compressor design is, however, going towards alternative journal solutions having lower stiffness levels (e.g., active magnetic bearings or squeeze film dampers), which will be more sensitive to this kind of excitation: consequently, a more accurate estimation of the expected forces in the presence of dynamic external forces such as those connected to an aerodynamically unstable condition is needed to predict the vibration level and the compressor operability in similar conditions. Within this scenario, experimental tests were carried out on industrial impellers operating at high peripheral Mach numbers. The dedicated test rig was equipped with several dynamic pressure probes that were inserted in the gas flow path; moreover, the rotor vibrations were constantly monitored with typical vibration probes located near the journal bearings. The pressure field induced by the rotating stall in the vaneless diffuser was reconstructed by means of an ensemble average approach, thus defining the amplitude and frequency of the external force acting on the impeller. The calculated force value was then included in the rotordynamic model of the test rig: the predicted vibrations on the bearings were compared with the measurements, showing satisfactory agreement. Moreover, the procedure was applied to two real multistage compressors, showing notable prediction capabilities in the description of rotating stall effects on the machine rotordynamics. Finally, the prospects of the proposed approach are discussed by investigating the response of a real machine in high-pressure functioning when different choices of journal bearings are made. © 2013 by ASME

Analysis of the rotordynamic response of a centrifugal compressor subject to aerodynamic loads due to rotating stall

In the current industrial research on centrifugal compressors, manufacturers are showing increasing interest in the extension of the minimum stable flow limit in order to improve the operability of each unit. The aerodynamic performance of a compressor stage is indeed often limited before surge by the occurrence of diffuser rotating stall. This phenomenon generally causes an increase of the radial vibrations, which, however, is not always connected with a remarkable performance detriment. In case the operating curve has been limited by a mechanical criterion, i.e., based on the onset of induced vibrations, an investigation on the evolution of the aerodynamic phenomenon when the flow rate is further reduced can provide some useful information. In particular, the identification of the real thermodynamic limit of the system could allow one to verify if the new load condition could be tolerated by the rotordynamic system in terms of radial vibrations. Within this context, recent works showed that the aerodynamic loads due to a vaneless diffuser rotating stall can be estimated by means of test-rig experimental data of the most critical stage. Moreover, by including these data into a rotordynamic model of the whole machine, the expected vibration levels in real operating conditions can be satisfactorily predicted. To this purpose, a wide-range analysis was carried out on a large industrial database of impellers operating in presence of diffuser rotating stall; the analysis highlighted specific ranges for the resultant rotating force in terms of intensity and excitation frequency. Moving from these results, rotordynamic analyses have been performed on a specific case study to assess the final impact of these aerodynamic excitations