Singular spectrum analysis as a tool for early detection of centrifugal compressor flow instability

Centrifugal compressor machinery is subject to a potentially damaging phenomenon called surge at low mass flow rates. This effect may be preceded by a phenomena known as inlet recirculation — a flow reversal upstream of the impeller. A methodology to isolate inlet recirculation as a characteristic feature for monitoring of centrifugal compressor instability is presented in this study. The methodology is based on a nonparametric time series analysis technique called as singular spectrum analysis (SSA). SSA decomposes a signal into a number of Reconstructive Components (RCs), from which data trends and oscillatory components may be extracted. The frequency spectra of each RC and their relative contributions to the reconstruction of the original signal were examined and comparisons were made with spectral maps in existing literature. Individual and independent RCs were chosen to construct a compressor's instability monitoring system. Additionally, the performance of SSA was determined by the Window length parameter. The effect of modification of this parameter was also studied, and the various viable choices of component for the basis of inlet recirculation diagnosis were considered. The methodology was implemented in pressure dynamical signals measured in an experimental centrifugal compressor rig. High frequency pressure measurements were taken at a number of flow conditions and locations within the compressor. The results demonstrated the potential of a methodology based on SSA to identify and extract oscillatory components with information about the local effect of inlet recirculation and eventually successfully monitor centrifugal compressor's instability

Flow Patterns Causing Saddle Instability in the Performance Curve of a Centrifugal Pump with Vaned Diffuser

In the present study, a reduced scale model of a low-pressure stage of a two-stage pump turbine was investigated when it operates in pump mode, which is as a centrifugal pump with vaned diffuser. An instability region was found in its Q-H curve. This instability region, centered into the flow rate interval between 0.45 to 0.7 QDes, restricts the stable operating range of the test pump. The object of this study was to analyze the origins and development of the unsteadiness inner pump during this saddle instability region, and to find the causes of this saddle, which could guide the design, and optimization of the test pump. Experimental and numerical approaches have been adopted to contribute the understanding on the highly complex flow interactions inside the test pump. The analyses pointed out that the instability of Q-H curve during this flow rate interval was mainly affected by the flow field unsteadiness into the diffuser. Severe flow separation occurred into the diffuser, affecting and/or affected by the unsteadiness from the impeller and return channel remarkably. Both at full and part load, due to the U shape stay vanes in the return system, a 3-dimentional complex flow was found on the suction side of stay vane near the U turn corner. This flow caused a periodic unforced pressure disturbance with frequency St=0.6625 in the diffuser. A rotating stall with 5 cells into the diffuser was also highlighted by the experimental analyses and the numerical results validated that this disturbance was mainly due to the unsteady flow patterns in the return channel. With the reduction of flow rate, the location of this unsteadiness in return channel move closer to the outlet of diffuser, and the unsteadiness made the flow separation near the stay vane leading edge on the suction side more severe and it impacted the flow in diffuser flow field in a more severe way. At the same time, two types of unsteadiness were detected in the impeller in the saddle instability region. They were the flow separation with the frequency St=0.6625 at the impeller blade suction side and the wake-jet near the trailing edge of impeller blade with the frequency St=0.335, respectively. The wake-jet caused a periodic disturbance with a frequency of St=0.335 in diffuser as well. The experimental results indicated that it propagate circumferentially with 2 cells. At the beginning of this saddle instability, in diffuser, the intensity of the fluctuations at St=0.335 and St=0.6625 both increase with the falling flow rate. With the development of these unsteady patterns, they mixed and caused a blockage in the gap between impeller and diffuser. This blockage caused the increase of the amplitude at BPF, which has been detected both by experimental and numerical results. At the same time, the further development of the blockage weakened the intensity of the two disturbances with frequency of St=0.335 and St=0.6625 in the blockage region. The blockage fully developed near the critical flow rate of 0.6 QDes. At this condition, the intensity of rotor and stator interaction increased and energy loss also increased which caused the drop of head and mean pressure at the exit of impeller. Furthermore, the disturbance in diffuser which caused by wake-jet of impeller lost the periodic character with frequency of St=0.335 at this condition. This is the reason why the non-linear component St=0.335 disappeared in diffuser around 0.6 QDes

Characterising Vibro-Acoustic Signals of a Reciprocating Compressor for Condition Monitoring

Machine monitoring in industries such as chemical process plants, petroleum refineries and pulp and paper industries has significantly increased over the years, mainly because of the economic impact associated with the breakdown of a piece of equipment. With downtime sometimes costing up to 100,000 USD a day (Wachel, N.D), industrial organisations have made it mandatory to put in place systems for monitoring the condition of critical machines used for production purposes to prevent unforeseen machine breakdown. Reciprocating compressors are one of the widely used compressor types in diverse fields of application particularly in the oil and gas industry or chemical industry. In these industries, reciprocating compressors are mainly used to deliver high-pressure gas from one location to another. Due to the importance of these machines in delivering high-pressured air and sometimes toxic gases safely, their reliability has gained widespread interest over the years. To improve reciprocating compressor operational performance and reliability, this research focuses on investigating the characteristics of vibro-acoustic signals from a reciprocating compressor based on a comprehensive analysis of non-intrusive vibration measurement and discharge gas oscillations (pulsations). This study will provide more knowledge on using two techniques (vibration and gas pulsations) for online monitoring and diagnosing of reciprocating compressor faults. Other monitoring techniques such as in-cylinder pressure, instantaneous angular speed (IAS), airborne acoustic as well as vibration are previously reported in literature, however, it is believed that no information for condition monitoring of discharge gas pulsation of a reciprocating compressor has been explored. To fulfil this study, in-depth modelling and an extensive experimental evaluation for different and combined faults common to reciprocating compressor systems are explored for a wide discharge pressure range to better understand the vibro-acoustic sources. Three common faults including discharge valve leakage, intercooler leakage, discharge pipeline leakage and two combined faults: discharge valve leakage and intercooler leakage, discharge valve leakage and discharge pipeline leakage under various discharge pressures are studied in this thesis. The simulation of compressor performance with and without faults for several discharge pressures were in good agreements with the corresponding experimental evaluations, and was used to understand the compressor dynamics. Furthermore, a preliminary study on the effectiveness of conventional methods such as time-domain and frequency-domain analysis of both vibration and gas pulsation measurements were investigated. Results show that, these traditional methods were insufficient in revealing fault characteristics in the vibration signal due to the usual noise contamination and nonstationary nature of the signal. Although, with the gas pulsation signal, waveform patterns and resonant frequencies varied with faults at several discharge pressures, nevertheless, effective band pass filtering needed to identify the best frequency band that can represent the characteristic behaviour of gas pulsation signals proofed difficult and time consuming. Amongst several advanced signal-processing approaches reviewed such as wavelet transform, time synchronous average, Hilbert transform, and empirical mode decomposition; wavelet packet transform is regarded as the most powerful tool to describe gas pulsation and vibration fault signals in different frequency bands. A combination of wavelet packet transform (WPT) and Hilbert transform (envelope analysis) is proposed to achieve optimal and effective band pass filtering for resonance band identification in gas pulsation signals, and WPTs de-noising property, which can effectively reduce excessive noise revealing key transient features in vibration signals. Optimal band selection for vibration signal was achieved using entropy computation. The band with the highest entropy was used to reconstruct the signal and the envelope of the new vibration signal was used for classification. The fundamental frequency and its harmonics were used as a tool for fault classification. All fault conditions were clearly separated using the fundamental frequency and its third (3X) harmonic. Regarding gas pulsation signals, the optimal band was selected by computing the root mean square (RMS) values of all eight enveloped band signals for several discharge pressures and faults. The band with the best RMS separation trend was selected for further classification using two main diagnostic features: the kurtosis and entropy of optimal band. The plot of kurtosis against entropy as a diagnostic tool showed good valve fault classification across a wide discharge pressure range. Although the analysis of vibration signal using the proposed methods gave more reliable results for reciprocating compressor fault detection and diagnosis compared to the gas pulsation results, analysis of gas pulsation signals gave a better result on the optimal frequency band selection that can represent the behaviour of reciprocating compressor (RC) valve fault. Therefore, it can be deduced that analysis of the RC vibration signal together with the gas pulsation signal has a promising potential to be used for condition monitoring and fault diagnostics of reciprocating compressors online

Aeronautical engineering: A continuing bibliography with indexes (supplement 307)

This bibliography lists 338 reports, articles, and other documents introduced into the NASA scientific and technical information system in Aug. 1994. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Aeronautical engineering: A continuing bibliography with indexes (supplement 292)

This bibliography lists 675 reports, articles, and other documents recently introduced into the NASA scientific and technical information system database. Subject coverage includes the following: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

The 1992 Research/Technology report

The 1992 Research & Technology report is organized so that a broad cross section of the community can readily use it. A short introductory paragraph begins each article and will prove to be an invaluable reference tool for the layperson. The approximately 200 articles summarize the progress made during the year in various technical areas and portray the technical and administrative support associated with Lewis technology programs

Aeronautical engineering: A continuing bibliography with indexes (supplement 293)

This bibliography lists 476 reports, articles, and other documents introduced into the NASA scientific and technical information system in July, 1992. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment, and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics

Aeronautical engineering: A cumulative index to a continuing bibliography (supplement 274)

This publication is a cumulative index to the abstracts contained in supplements 262 through 273 of Aeronautical Engineering: A Continuing Bibliography. The bibliographic series is compiled through the cooperative efforts of the American Institute of Aeronautics and Astronautics (AIAA) and the National Aeronautics and Space Administration (NASA). Seven indexes are included: subject, personal author, corporate source, foreign technology, contract number, report number, and accession number