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

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

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

Aero-Damping Measurements And Computation In A Full-Scale Multistage Centrifugal Compressor

LectureGreat interest is being paid by Oil & Gas industry to the design of high speed and high pressure ratio centrifugal compressors. Static and dynamic stresses are becoming higher and, under critical conditions, aerodynamic forcing may lead to impeller high cycle fatigue failures. Thorough aeromechanics knowledge is hence necessary for the advanced design of centrifugal compressors, to ensure reliability and life-time. Open impellers, which are commonly used for high speed applications, may be prone to relatively high vibration levels, due to aerodynamic excitations present in the flow field. Indeed, resonant crossings with forcing functions are in some cases unavoidable in variable speed machines since a wide speed range results in a wide frequency range of rotor-stator interactions, as well as flow-field circumferential distortions, occurring at multiples of the impeller excitations. Impeller forcing amplitudes and aeromechanical damping both contribute to vibratory response levels and must be properly predicted. This paper is focused on the aerodynamic damping prediction. A comprehensive study was undertaken to develop and validate aerodynamic damping prediction capability for open impellers for the operating conditions and mode shapes of interest. An extensive experimental campaign was conducted on a full scale multistage compressor equipped with transonic unshrouded impellers instrumented with strain gauges connected to a telemetry system. The strain gauge data are compared to the CFD (Computational Fluid Dynamics) predicted damping for blade and disk modes and show good agreement. The present paper describes the challenges related to aerodynamic damping prediction in a real machine environment taking into account complex flow features, for example flow distortions, off-design conditions and different pressure levels. The level of agreement between test data and CFD predictions achieved in this work represents a significant step forward towards building and validating a fully physics based predictive capability for open impeller forced response. In fact, such validated aerodynamic damping prediction capability is required for aeromechanic risk mitigation and accurate impeller high cycle fatigue analysis in early stages of the design process

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

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

Development of a criterion for a robust identification of diffuser rotating stall onset in industrial centrifugal compressors

Recent studies showed that a prompt detection of the stall inception, connected with a specific model to predict its associated aerodynamic force, could provide room for an extension of the left margin of the operating curve of high-pressure centrifugal compressors. In industrial machines working in the field, however, robust procedures to detect and identify the phenomenon are still missing, i.e. the operating curve is almost ever cut preliminary by the manufacturer by a proper safety margin; moreover, no agreement is found in the literature about a well-defined threshold to define the onset of the stall. In particular, in some cases the intensity of the arising subsynchronous frequency is compared to the revolution frequency, while in many other ones it is compared to the blade passage frequency. A large experience in experimental stall analyses collected by the authors revealed that in some cases unexpected spikes could make this direct comparison not reliable for a robust automatic detection. To this end, a new criterion was developed based on an integral analysis of the area subtended to the entire subsynchronous spectrum of the dynamic pressure signal of probes positioned just outside the impeller exit. A dimensionless parameter was then defined to account for the spectrum area increase in proximity to stall inception. This new parameter enabled the definition of a reference threshold to highlight the arising of stall conditions, whose validity and increased robustness was here verified based on a set of experimental analyses of different types of full-stage test cases of industrial centrifugal compressors at the test rig