Estimation of the Aerodynamic Force Induced by Vaneless Diffuser Rotating Stall in Centrifugal Compressor Stages

Abstract Rotating stall in centrifugal compressors not only adversely affects the performance before surge, but also can generate high subsynchronous vibrations, marking the minimum flow limit of a machine. Recent works presented an experimental approach to estimate the stall force induced by the unbalanced pressure field in a vaneless diffuser using dynamic pressure measurements. In this study, the results of a 3D-unsteady simulation of a radial stage model were used to estimate the stall force and to compare it with the approximation obtained with an "experimental-like" approach. Results showed that: a) the experimental approach, using an ensemble average approach for transposing data between time and space domains provides sufficiently accurate results; b) the momentum contribution, neglected in experiments, gives negligible contribution to the final intensity of the stall force

STALL INDUCED AERODYNAMIC FORCING AND ROTOR VIBRATIONS IN A MULTISTAGE CENTRIFUGAL COMPRESSOR

LectureThe Oil & Gas industry is looking with increased interest at solutions for improving operating flexibility of centrifugal compressors. The stable operation of a compressor stage or machine is generally limited at the left of operating range by the occurrence of a local aerodynamic unsteady phenomenon, the rotating stall, which usually precedes the surge. Rotating stall could cause, depending on the actual operating conditions, severe sub-synchronous vibrations to the rotor which may compromise rotordynamic behavior, preventing the machine from operating at very low flow rates. An accurate characterization of rotating stall phenomena, and their impact on rotordynamic stability, may represent an important step forward in centrifugal compressor design and performance predictability, insofar as it allows to correctly predict the real operating range of the machine. In recently published works the authors presented a procedure which allows reconstructing the pressure unbalance due to the diffuser rotating stall, to estimate the rotating force acting on the shaft and, by means of a rotordynamic model, the vibration at the bearings. In addition to this, a criterion to scale the rotating force coming from model test conditions up to fullscale machine conditions has been developed and successfully validated. In this framework a thorough work has been performed to apply the aforementioned procedure to an LNG multistage compressor. Firstly, the stages which equip the machine were tested as single scaled-down stages in a model test rig, in order to fully characterize their dynamic behavior while approaching the left limit and operating in stall condition. Then, the full scale machine has been equipped with dynamic pressure probes in different locations along the gas flow path, and has been tested according to ASME PTC-10 standard; this allowed to capture the stall inception and its evolution and finally to get the rotating pressure pattern acting on the rotor. A noticeable agreement was obtained between the force resulting from the pressure field integration and the one obtained through a proper scaling of the test data. Finally, the calculated stall force has been used as an input in a rotordynamic model of the whole compressor: the predicted Subsynchronous Vibration (SSV) estimated at the displacement probe location has been compared with the measured value showing some differences which can be related to the proximity of the first rotor mode

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

Feasibility study of a geothermal power plant with a double-pipe heat exchanger

The technologies currently in use for the power production in the geothermal field are sometimes affected by two crucial problems: the environmental impact and the drilling costs. The first issue is related to the techniques adopted to increase the heat exchange (as artificial subsoil fracturing) or to the use of aquifers which contain pollutants that are not properly disposed or confined. The drilling costs are high because two wells are generally necessary to operate properly the power plant. An interesting solution could be the adoption of a double-pipe heat exchanger that consists in two concentric pipes placed in a single well. This setup allows the operation of the plant in a closed loop configuration with no mass exchange between the subsoil and the surface. A coupled approach based onalD model for the pipe and a 2D axisymmetric model for the surrounding rocks was developed in this study to investigate the actual capability of a double-pipe system in different operating conditions

Experimental analysis of the pressure field inside a vaneless diffuser from rotating stall inception to surge

An accurate estimation of rotating stall is one of the key points 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 vaneless diffuser rotating stall, the most common practice in industrial machines is to make use of a limited number of dynamic pressure probes to reconstruct the stall characteristics after an ensemble averaging approach. In this study, a 1:1 model of an industrial compressor stage was tested in a dedicated test rig and equipped with 24 pressure probes properly distributed along the diffuser circumference with the scope of providing a real-time visualization of the spatial pressure distribution within the diffuser. The results allowed the assessment of some important characteristics of the stall cells that were historically supposed based on averaged data, e.g. the cells rigidity. Moreover, the present study confirmed the existence of a stall pattern with two almost axisymmetric lobes. Finally, the transient analysis of both the stall inception and the surge onset was carried out, highlighting the flow field evolution in the diffuser under these conditions

An AI-Based Fast Design Method for New Centrifugal Compressor Families

Limiting global warming’s effects requires a sudden reduction of greenhouse gas emissions to pursue a net-zero carbon growth in the next decades. Along with this energy transition, drastic and rapid changes in demand are expected in many sectors, including the one for centrifugal compressors. In this context, new aerodynamic design processes exploiting the know-how of existing impeller families to generate novel centrifugal compressors could quickly react to demand variations and ensure companies’ success. Modifying the characteristics of existing compressors using a 1D single-zone model is a fast way to exploit this know-how. Besides, artificial intelligence could be useful to highlight relationships between geometrical parameters and performance, thus facilitating the achievement of optimized machines for new applications. Although the scientific literature shows several studies on mono-dimensional approaches, the joint use of a 1D single-zone model with an artificial neural network for designing new impellers from pre-engineered ones remains understudied. Such a model was provided in this paper. An application to the case study of an expander–compressor impeller family derived from other existing natural gas liquefaction one was presented. Results proved that the proposed model enabled developing a new family from an existing one, improving the performance while containing design time and computational efforts

Detection of vaneless diffuser rotating stall by means of dynamic pressure sensors and acoustic measurements

An accurate estimation of rotating stall is one of the key technologies for high-pressure centrifugal compressors. Several techniques have been proposed to detect the stall onset; inter alia, few dynamic pressure probes have been shown to not only properly detect the phenomenon, but also reconstruct the stall characteristics after an ensemble averaging approach. The massive use of this technique in the field is, however, not a common practice yet. In the present study, the use of dynamic pressure probes has been combined with that of an environmental microphone to evaluate the prospects of this latter for a possible stall onset detection. To this end, experimental tests have been carried out in the experimental test rig of the Department of Industrial Engineering (DIEF) of Università degli Studi di Firenze. Results show that the microphone was able to distinguish the onset of rotating stall accurately and promptly, even though – differently from dynamic pressure sensors - it does not provide sufficient information to determine the characteristics of the stall pattern. On this basis, the use of acoustic measurements could find room for automatic detection of rotating stall onset

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

TL11 - Numerical and Experimental Analysis of Flow-induced Subsynchronous Vibrations in Integrally Geared Turboexpanders

LectureTurboexpanders are widely used as energy recovery systems, in plants where a stream of pressurized gas is available as output of the main process. The train is typically composed of a single or multi-stage radial inflow turbine, coupled with an electrical generator by means of a gearbox. When a turboexpander is operated in off-design conditions at low load, the tangential velocity of the gas at expander discharge is high with respect to its axial velocity. In this condition the decelerated, swirled flow may generate a helical vortex in the diffuser, resulting in a pressure gradient of the core flow rotating around the diffuser axis. This rotating pressure gradient is a potential source of radial vibrations for the expander rotor. The present study starts with an analytical description of the phenomenon and a review of dedicated numerical simulations and experimental tests results. The characterization of this flow-induced vibration is followed by the description of mitigating actions that can reduce the risk of high vibrations. Finally, the analysis of three case studies provides evidence of how the vibration can be reduced through such mitigating actions, and allows to draw some general conclusions and recommendations for the design and operation of turboexpanders