Multistage Centrifugal Compressor Research Articles

Numerical Investigation on Labyrinth Seal Leakage Flow and Its Effects on Aerodynamic Performance for a Multistage Centrifugal Compressor

Labyrinth seals are widely used in industrial centrifugal compressors to reduce leakage. However, no work has been conducted to numerically investigate the detailed seal leakage flow and its effects in an environment of multistage centrifugal compressor. To clarify the flow mechanism of leakage flow and the interaction mechanism between leakage and mainstream flow in multistage centrifugal compressors, the flow of the last two stages from a four-stage centrifugal compressor is studied using computational fluid dynamics (CFD) model with and without considerations of labyrinth seal leakage paths, i.e., two shroud seals, one interstage seal, and one balance piston seal. The results show that the leakage flow in shroud and hub cavities can be described as a Batchelor-type flow. The Ekman number of the cavity Batchelor flow is small and corresponds to thin boundary layers while the Rossby number is at unity order implying the importance of rotating effects. The leakage flow through the shroud, interstage, and balance piston labyrinth seals is decreased by the combined effects of throttling and diffusion flow, and has distinctive flow structures associated with the type of labyrinth seal. The influence of leakage flow on the mainstream flow can be described by suction or injection mode. The suction mode is beneficial to the improvement of mainstream flow quality while the injection mode is harmful. This work is of scientific significance to enrich the knowledge of internal fluid mechanics and of potential application value to control and design the leakage flow in real configurations of multistage centrifugal compressors.

Surge prevention in gas turbines: an overview over historical solutions and perspectives about the future

The aim of the present work is to retrace experimental, analytical and numerical analyses which deal with compressor instability phenomena, such as rotating stall and surge. While the first affects only the machine itself, the second involves the whole energy system. Surge onset is characterized by strong pressure and mass flow rate fluctuations which can even lead to reverse flow through the compressor. Experimental studies on prevention of axial compressor fluid dynamic instabilities, which can be propagated and eventually damage the solid structure, have been carried out by many authors. The first important studies on this topic tried to underline the main aspects of the complex detailed mechanism of surge, by replacing the compression system with an equivalent conceptual lumped parameter model. This is specially meant to capture the unsteady behaviour and the transient response of the compression system itself, particularly its dependence on variations in the volume of discharge downstream and in the settings of the throttle valve at its outlet (which simulates the actual load coupled to the compressor). Greitzer’s model is still regarded as the milestone for new investigations about active control and stabilization of surge and, more generally, about active suppression of aerodynamic instabilities in turbomachinery. During the last years, a lot of simulations and experimental studies about surge have been conducted on multistage centrifugal compressors with different architectures (e.g. equipped with vaneless or vaned diffusers). Moreover, further kinds of analysis try to extend the stable working zone of compressors, identifying stall and surge precursors extractable from information contained in the vibro-acoustical and rotodynamic response of the system.

Loss Mechanism of Static Interstage Components of Multistage Centrifugal Compressors for Integrated Blade Design

Although centrifugal compressors are widely used in construction, they consume a large amount of energy; in existing multistage centrifugal compressors, there is a serious pressure loss of ~15.13% when gas flows through the diffuser, bend, and return channel. In this study, we analyze the loss mechanisms of these stages in detail, using computational fluid dynamics. Based on this analysis, we present a new type of integrated blade, connecting the diffuser, bend, and return channels, which can eliminate the airflow stall phenomenon. Through effective control of the airflow spreading process, we minimized losses in the component, which improved its efficiency by 4.39% and increased the pressure ratio by 2.86% relative to a compressor without the newly-designed integrated blade. The concepts used in the creation of this component can provide a reference for the future design of blades for flow through parts of multistage compressors.

Compression performance optimization considering variable charge pressure in an adiabatic compressed air energy storage system

In an adiabatic compressed air energy storage system (A-CAES), the storage pressure persistently increases during the energy storage process causing deteriorate of the charge performance under off-design operating conditions. The compression performance with variable backpressure is essential for the energy storage efficiency and density of A-CAES. A thermodynamic model with energy loss analysis is built up to investigate the performance of a multi-stage centrifugal compressor used in an A-CAES (about 6.0 MW). By energy loss analysis, the blade inlet angle is first focused on to improve the disadvantages of narrow operating conditions and low efficiency at large mass flow rate of the original compressor. Then, the variable rotating speed study is applied on to increase the energy charge efficiency with a wide storage pressure range. Using the thermodynamic model, the available rotating speed is determined to be 0.85–1.05 of design value. Furthermore, the rotating speed is optimized to achieve the best efficiency under variable storage pressures and the corresponding correlation is fitted. Using the optimal rotating speed, the compression efficiency can be kept above 80% while the exergy efficiency is above 82% during the whole energy storage process with a wider storage pressure.

Comprehensive analysis of hydrogen compression and pipeline transportation from thermodynamics and safety aspects

Hydrogen delivery is a critical contributor to costs, emissions, and energy consumption associated with hydrogen pathways involving central plant production. Pipeline transmission appears to be the most economical means of transporting large quantities of hydrogen over great distances. The aim of this study is to analyze H2 compression and pipeline transportation processes with safety issues related to water electrolysis and H2 production for different values of the hydrogen mass flow rate: 0.2, 0.5, 1.0, 2.0, and 2.8 kg/s. Pipelines are operated at an initial pressure of 10 MPa at 300 K. Depending on the hydrogen mass flow rate, various types of compressors have been proposed, including reciprocating, conventional multistage centrifugal compressors, and an eight-stage integrally geared centrifugal compressor based on the concept of advanced centrifugal stages. Another problem analyzed in this study is H2 pipeline transport from the compressor outlet site to salt caverns under heat-transfer conditions. In order to avoid the choking condition, recompression of H2 is necessary at appropriately selected transportation distances. Simulations are conducted to determine the maximum safe distance of the pipeline to subsequent booster stations depending on the mass flow rate, pipeline diameter, ambient temperature, thermal insulation thickness, and ground-level heat transfer conditions. This analysis makes it possible to select pipeline diameters of 0.065 m, 0.1 m, 0.15 m, and 025 m, depending on the pre-determined hydrogen mass flow rate, at the transportation distance of 50 km. If the H2 pipeline gets damaged and an uncontrollable release of hydrogen occurs, hydrogen pipeline transport poses a potential hazard to humans and the surroundings. In the case of a hydrogen jet fire, zones with a fatal effect on humans are presented.

Design, dynamic modeling, and control of a multistage CO2 compression system

This paper presents a comprehensive model of a post-combustion CO2 compression system that uses a multistage centrifugal compressor train to take the CO2-rich gas from the regenerator of a CO2 capture system and compress it to the supercritical conditions required for pipeline transport. The compression system model includes inter-stage coolers, flash vessels, glycol tower for water removal, and the CO2 inventory of the associated system. Both inline and integral gear compressors are designed using dimensionless numbers and appropriate design constraints. The steady-state integral gear compressor model is validated against industrial compressor performance data provided by a commercial CO2 compressor manufacturer. Its performance is also compared to the performance of an inline compressor model using a typical feed stream from a post-combustion CO2 capture process. Performance curves using dimensionless exit flow coefficient and isentropic head coefficient are used for simulating off-design performance of the integral gear compressor. To avoid surge during operation, a controller has been designed. Transients of key process variables in response to various disturbances have been studied.

Design Features of Multistage Centrifugal Compressor of Vapor Refrigerating Machine with Complete Working Fluid Intercooling

The work sets out the distinctive features of the method of gas-dynamic calculation of multistage centrifugal compressors for refrigerating machines running on a cycle of multi-stage compression of working fluid with its complete intercooling between the stages. The parameters of width of specific sections of the impellers, diffusers, and reverse guide vanes as well as the operating parameters during polytropic compression and parameters of the inner power of the compressor were corrected.The research outcomes can be used in the design of centrifugal compressors for refrigerating machines operating at the specified cycle.

Influences of flow loss and inlet distortions from radial inlets on the performances of centrifugal compressor stages

Radial inlets are typical upstream components of multistage centrifugal compressors. Unlike axial inlets, radial inlets generate additional flow loss and introduce flow distortions at impeller inlets. Such distortions negatively affect the aerodynamic performance of compressor stages. In this study, industrial centrifugal compressor stages with different radial inlets are investigated via numerical simulations. Two reference models were built, simulated, and compared with each original compressor stage to analyze the respective and coupling influences of flow loss and inlet distortions caused by radial inlets on the performances of the compressor stage and downstream components. Flow loss and inlet distortions are validated as the main factors through which radial inlets negatively affect compressor performance. Results indicate that flow loss inside radial inlets decreases the performance of the whole compressor stage but exerts minimal effect on downstream components. By contrast, inlet distortions induced by radial inlets negatively influence the performance of the whole compressor stage and exert significant effects on downstream components. Therefore, when optimizing radial inlets, the reduction of inlet distortions might be more effective than the reduction of flow loss. This research provides references and suggestions for the design and improvement of radial inlets.

多段遠心圧縮機用戻り流路の空力形状最適化

多段遠心圧縮機用戻り流路の空力形状最適化

A new method for reliable performance prediction of multi-stage industrial centrifugal compressors based on stage stacking technique: Part I – existing models evaluation

The growing acceptability of the natural gas as an energy source yields to place a higher research concentration on improving the centrifugal compressor design and performance prediction techniques. This is the first part of a two-part study that aims to introduce a new integrated method for reliable performance prediction of multi-stage industrial centrifugal compressor based on stage stacking technique. In this part, two most common existing models are adapted to be valid for industrial centrifugal compressors and the predicted characteristics are evaluated against measured data. Based on the conducted evaluation, a new model will be derived in the second part which incorporates the advantages of both approaches with more accurate prediction. Lüdtke method predicts the compressor design efficiency based on the correction of the derived experimental set of data. This approach has been applied for a single stage centrifugal compressor at a single speed and flow values. Recently, Casey–Robinson method uses a new set of algebraic equations to obtain the compressor curve with less dependency on the geometrical features. This model has been tested with a single stage turbocharger compressor where the performance curve can be derived without considering the mechanical stages. This approximation will introduce a more significant impact when this method is used for large industrial compressors. Therefore, these models will be improved to be suitable for multi-industrial centrifugal compressors and their prediction capability to estimate the performance map will be examined in this paper.

Fluid-structure interaction analysis and lifetime estimation of a natural gas pipeline centrifugal compressor under near-choke and near-surge conditions

Up to present, there have been no studies concerning the application of fluid-structure interaction (FSI) analysis to the lifetime estimation of multi-stage centrifugal compressors under dangerous unsteady aerodynamic excitations. In this paper, computational fluid dynamics (CFD) simulations of a three-stage natural gas pipeline centrifugal compressor are performed under near-choke and near-surge conditions, and the unsteady aerodynamic pressure acting on impeller blades are obtained. Then computational structural dynamics (CSD) analysis is conducted through a one-way coupling FSI model to predict alternating stresses in impeller blades. Finally, the compressor lifetime is estimated using the nominal stress approach. The FSI results show that the impellers of latter stages suffer larger fluctuation stresses but smaller mean stresses than those at preceding stages under near-choke and near-surge conditions. The most dangerous position in the compressor is found to be located near the leading edge of the last-stage impeller blade. Compressor lifetime estimation shows that the investigated compressor can run up to 102.7 h under the near-choke condition and 200.2 h under the near-surge condition. This study is expected to provide a scientific guidance for the operation safety of natural gas pipeline centrifugal compressors.

Optimal Operation and Maintenance of Gas Compressor Stations: An Integrated Framework Applied to a Large-Scale Industrial Case

This paper presents a framework which integrates maintenance and optimal operation of multiple compressors. The outcome of this framework is a multiperiod plan which provides the schedule of the operation of compressors: the schedule gives the best decisions to be taken, for example, when to carry out maintenance, which compressors to use online and how much to load them. These decisions result in the minimization of the total operational costs of the compressors while at the same time the demand of the plant is met. The suggested framework is applied to an industrial gas compressor station which encompasses large multistage centrifugal compressors operating in parallel. The optimization model of the framework consists of three main parts: the models of compressor maps, the operational aspects of compressors, and a maintenance model. The results illustrate the optimal schedule for 90 days and an example of the optimal distribution of the load of the compressors for 5 days. Finally, the results show the economical benefits from the integration of maintenance and optimization.

An Iterative Method to Derive the Equivalent Centrifugal Compressor Performance at Various Operating Conditions: Part II: Modeling of Gas Properties Impact

This is the second part of a study conducted to model the aerothermodynamic impact of suction parameters and gas properties on a multi-stage centrifugal compressor’s performance. A new iterative method has been developed in the first part to derive the equivalent performance at various operating conditions. This approach has been validated to predict the compressor map at different suction pressures and temperatures using the design characteristics as reference values. A further case is included in this paper in order to emphasize the validity of the developed approach to obtain the performance characteristics at various gas compositions. The provided example shows that the performance parameters at different gas mixtures can be predicted to within ±1.34%. Furthermore, the conducted optimization in this paper reveals that the proposed method can be applied for the compressor design evaluation corresponding to the expected variation in suction conditions. Moreover, the examined case study demonstrates the effect of gas properties’ variation on the operating point and aerodynamic stability of the entire compression system. In order to achieve that, a simple approach has been established to assess the contribution of gas properties’ variation to the inefficient and unstable compressor performance based on the available operational data.

An Iterative Method to Derive the Equivalent Centrifugal Compressor Performance at Various Operating Conditions: Part I: Modelling of Suction Parameters Impact

This paper introduces a new iterative method to predict the equivalent centrifugal compressor performance at various operating conditions. The presented theoretical analysis and empirical correlations provide a novel approach to derive the entire compressor map corresponding to various suction conditions without a prior knowledge of the detailed geometry. The efficiency model was derived to reflect the impact of physical gas properties, Mach number, and flow and work coefficients. One of the main features of the developed technique is the fact that it considers the variation in the gas properties and stage efficiency which makes it appropriate with hydrocarbons. This method has been tested to predict the performance of two multistage centrifugal compressors and the estimated characteristics are compared with the measured data. The carried comparison revealed a good matching with the actual values, including the stable operation region limits. Furthermore, an optimization study was conducted to investigate the influences of suction conditions on the stage efficiency and surge margin. Moreover, a new sort of presentation has been generated to obtain the equivalent performance characteristics for a constant discharge pressure operation at variable suction pressure and temperature working conditions. A further validation is included in part two of this study in order to evaluate the prediction capability of the derived model at various gas compositions.

A new method for reliable performance prediction of multi-stage industrial centrifugal compressors based on stage stacking technique: Part II–New integrated model verification

This is the second part of a conducted study to develop a new integrated model for reliable performance prediction of multi-stage industrial centrifugal compressors. The conducted evaluation in part 1 revealed a high deviation between the predicted surge flows using Casey–Robinson approach and measured values especially at low rotational speeds. Besides, a significant difference between the estimated and the experimental characteristics was observed at choke and surge conditions and this deviation is growing as the Mach number increases. One of the main disadvantages of this model is the dependency on the peak and design efficiencies and associated flow coefficients at both normal and low Mach number operations which are obviously not available at early preliminary stage. In contrast, Lüdtke method fails to detect the instable flow regions and with lower degree of accuracy in the predicted efficiencies. However, the predicted design efficiency was found very close to the experimental value. Therefore, this paper introduces a new approach to estimate the performance map of multistage industrial centrifugal compressors based on stage staking principle and by incorporating the advantages of both methods. The new model has been tested at both low and high flow coefficients applications and for vaned and vaneless diffusers. Comparing with the existing models, the developed method was proven to generate more accurate estimation for performance characteristics of multistage centrifugal compressors with less dependency on the geometrical features. One of the unique advantages of the new method is the fact that it does not require a prior knowledge of the peak and design efficiencies and associated flow coefficients.

Physical modelling of industrial multistage centrifugal compressors for monitoring and simulation

Currently, industrial applications to monitoring, simulation and optimization of compressors employ empirical models that are either data-driven or based on the manufacturer performance maps. This paper proposes the use of one-dimensional aerodynamic models for industrial applications such as simulation and monitoring. The physical model establishes causality relationships among input and output variables that are tuned to match the real compressor by using operation data. The application of the method is shown using data from an industrial multistage centrifugal compressor with interstage coolers and variable inlet guide vanes. This is a more complex but more relevant case study for process industry, as opposed to the single-stage variable speed compressors, which is the common example in the literature.

Optimization of a network of compressors in parallel: Real Time Optimization (RTO) of compressors in chemical plants – An industrial case study

Abstract The aim of this paper is to present a methodology for optimizing the operation of compressors in parallel in process industries. Compressors in parallel can be found in many applications for example in compressor stations conveying gas through long pipelines and in chemical plants in which compressors supply raw or processed materials to downstream processes. The current work presents an optimization framework for compressor stations which describe integration of a short term and a long term optimization approach. The short-term part of the framework suggests the best distribution of the load of the compressors (where the time scale is minutes) and the long-term optimization provides the scheduling of the compressors for large time periods (where the time scale is days). The paper focuses on the short-term optimization and presents a Real Time Optimization (RTO) framework which exploits process data in steady-state operation to develop regression models of compressors. An optimization model employs the updated steady-state models to estimate the best distribution of the load of the compressors to reduce power consumption and therefore operational costs. The paper demonstrates the application of the RTO to a network of parallel industrial multi-stage centrifugal compressors, part of a chemical process in BASF SE, Germany. The results from the RTO application showed a reduction in power consumption compared to operation with equal load split strategy.

Numerical Investigations on the Leakage and Rotordynamic Characteristics of Pocket Damper Seals—Part II: Effects of Partition Wall Type, Partition Wall Number, and Cavity Depth

Effects of partition wall type, partition wall number and cavity depth on the leakage and rotordynamic characteristics of the pocket damper seal (PDS) were numerically investigated using a presented 3D transient computational fluid dynamics (CFD) method based on the multifrequency elliptical whirling orbit model. The accuracy and availability of this transient CFD method and the multifrequency elliptical whirling orbit model were demonstrated with the experimental data of the experimental eight-bladed fully partitioned pocket damper seal (FPDS). The leakage flow rates and frequency-dependent rotordynamic coefficients of PDS were computed for two types of partition wall (namely conventional PDS and fully partitioned PDS), four partition wall numbers including the labyrinth seal (no partition wall) and six cavity depths including the plain smooth seal (zero cavity depth) at operational conditions with or without inlet preswirl and 15,000 rpm rotational speed. The numerical results show that the FPDS has the similar leakage performance and more superior stability capacity than the conventional PDS. The FPDS possesses slightly larger leakage flow rate (∼2.6–4.0% larger) compared to the labyrinth seal. Eight is a preferable value for the partition wall number to gain the best leakage performance of the FPDS with the least manufacturing cost. The FPDS possesses significantly larger stiffness and damping than the labyrinth seal. Increasing partition wall number results in a significant increase in the direct stiffness but limited desirable effect on the effective damping. The FPDS possesses the lowest leakage flow rate when the cavity depth is about 2.0 mm. Compared to the plain smooth seal, the FPDS possesses larger positive direct stiffness and significantly less direct damping and effective damping. Increasing cavity depth results in a significant decrease in the stabilizing direct damping and the magnitude of the destabilizing cross-coupling stiffness. H= 3.175 mm is a preferable value of the cavity depth for which the effective damping of the FPDS is largest, especially for the concerned frequencies (80–120 Hz) where most multistage high-pressure centrifugal compressors have stability problem.

On the Predicted Performance of Oil Lubricated Thrust Collars in Integrally Geared Compressors

Integrally geared compressors (IGCs) comprise single stage impellers installed on the ends of pinion shafts, all driven by a main bull gear (BG) and shaft system. When compared to single shaft multistage centrifugal compressors, the benefits of IGCs include better thermal efficiency, reduced footprint and simple foundation, dispensing with a high speed coupling, as well as better access for maintenance and overhauls. In IGCs, the compression of the process gas induces axial loads on the pinion shafts that are transmitted via thrust collars (TCs) to the main drive shaft and balanced by a single thrust bearing. The TCs, located on either side of pinion gears, slightly overlap with the BG outer diameter to form lentil-shaped lubricant-wetted regions. Archival literature on the design and optimization of TCs is scant, in spite of their widespread usage as they are comprised of simple geometry mechanical elements. This paper presents an analysis of the hydrodynamic film pressure generated in a lubricated TC due to the rotation of both TC and BGs and specified taper angles for both bodies. The model solves the Reynolds equation of hydrodynamic lubrication to predict the operating film thickness that generates a pressure field reacting to impellers' thrust loads, these forces being a function of the pinion speed and the process gas physical properties. The model also predicts performance parameters, such as power loss and axial stiffness, and damping force coefficients. A parametric study brings out the taper angles in the TC and BG that balance the transmitted load with a lesser friction factor and peak pressure, along with large axial stiffness and damping.

Performance analysis and improvement of a high flow coefficient centrifugal compressor

Very high flow coefficient centrifugal compressor stages are mostly applied as the first stages of multistage compressors. The performance of this type of stages is critical to the entire centrifugal compressors, but surprisingly little related information is available in the open literature. A centrifugal compressor with high inlet flow coefficient of 0.2, presenting a narrow operating range and unstable running situation even at design speed during the test, is investigated here. To reveal flow details in this centrifugal compressor, numerical simulations have been carried out and indicate that excessive impeller flow diffusion results in the poor performance of this centrifugal compressor. With the same inlet flow coefficient, six redesign cases coming from an in-house one-dimensional analysis program are proposed together with impeller trimming and equal flow area design method for corresponding vaneless diffuser. Performance comparison among these redesign centrifugal compressors is presented and the most suitable one is recommended for test in the future. In addition, three redesign cases with lower inlet flow coefficient developed by means of flow trimming are shown in the end to satisfy potential application areas. Finally, the results in this study can provide valuable reference information for multistage centrifugal compressor design.