Systems for prevention of surging of centrifugal compressors and methods of rotating stall detection and early surge diagnosis using statistical processing of sensor signals of various parameters, i.e., the method of calculation of the correlation function of the signal and the method of calculation of signal dispersion, are discussed. The results of routine tests of air and high-speed air centrifugal compressors and their detachable flow parts are adduced.The presently available systems for prevention of surging of centrifugal compressors (CC) can be divided roughly into two groups.1. Systems preventing surge, which are often called antisurge control systems. In this case, the surge boundary is plotted on the gas-dynamic curve of the compressor and to its right (10-15% toward the flow rate increase) is drawn the antisurge control line. If the working point of the compressor curve lies to the left of the control line, the bypass (or dump) valve slightly opens up, which prevents surging of the compressor. The surge boundary is determined experimentally in the course of surge tests or by calculation. As an example, in Fig. 1 are shown the surge boundary line for the supercharger N133-21-01 (curve 1) determined in the course of the surge tests and the antisurge control line (curve 2).2. Systems responding to incipient surge (surge signal indicators). In these systems, use is made of the distinguishing features of the behavior of the signals from the sensor, which characterize the compressor operation conditions (flow rate, supercharging pressure, current strength of the main electric drive, etc.), and a discrete signal is generated for forced opening of the bypass valve and/or for stopping of the compressor.A surge signal indicator can be installed in addition to the antisurge control system, and if this system fails either due to variation in the parameters of the gas being compressed or due to inadmissible increase in the thickness of the deposit layer in the flow part, the surge signal indicator prevents operation of the compressor in inadmissible surge conditions. Let us examine the second group of surge prevention systems. Research carried out by developers of surge prevention systems for centrifugal compressors was focused on detection of distinguishing features of incipient surge, which is characterized by fluctuations of the (discharge) pressure and the rate of gas flow through the compressor, the delivery temperature, the current strength of the electric drive, etc. Many patents have been granted on this subject in Russia and abroad. For example, an antisurge device based on fixing of the moment of concurrent drop in the delivery pressure and the rate of gas flow through the compressor has been patented [1].In a series of experiments performed in the 1960s [2, 3], it was found that rotating stall appears in the centrifugal compressor in the region of the working points of the compressor characteristics (curve) near the surge boundary. Theoretical and experimental studies of rotating stall that precedes surg...

Stall tests are performed for superchargers under operating conditions with a reduction in flow rate by 15-20% below the calculated stall limit (confirmed by tests on model gas, and here stall is not recorded). It is established that the position of the line for the safe operating limit of gas dynamic stability depends markedly both on the electrodynamic properties of the magnetic suspension control system and on the magnetic properties of the supercharger rotor (weight, moment of inertia, natural bending vibration frequencies, etc.).In recent years, publications have demonstrated that magnetic, including electromagnetic, rotor suspenders are being introduced extensively into centrifugal compressor building. Many leading domestic and overseas firms, i.e., suppliers of centrifugal compressors, have developed, manufactured and supplied at least test examples of compressors with magnetic suspenders (MS).Kazan'kompressormash together with the V. B. Shnepp NIIturbocompressor have more than ten years of experience in developing and manufacturing compressors with rotor MS and they cooperate with two leading organizations in the field of magnetic suspenders: NPP VNIIÉM (Russia) and S2M (France).However, publications contain extremely sparse experimental material about the operation of a centrifugal compressor (CC) with rotor MS in the field of low gas flow rate approaching an unstable compressor operating regime.In carrying out set-up work for a CC with a rotor electromagnetic suspender, certain complications have been revealed in adjusting the system for antistall protection and the MS control system that are connected (distinct in the appearance of signs of gas dynamic instability of compression for these compressors from CC) with traditional oil bearings.In performing stall tests for gas pumping units (GPU) there are marked difficulties in determining the actual limits of stall for centrifugal compressors with MS. The overwhelming majority of operating superchargers are made with oil bearings (rigid rotor supports), whose behavior is predictable, and there are many years of experience of performing stall tests for them. Use of superchargers with rotor electromagnetic suspenders with unusual behavior of the rotor-support system with low gas flow rates (critical, unstable regimes for superchargers with oil supports) gives rise to difficulties.Signs of stall normal for GPU oil superchargers relate to the following: • flow surge (sharp change in gas flow rate with a change of direction); • simultaneous drop in gas flow rate and discharge pressure; • a frequency of pressure drop oscillation in the reducer of more than 1 Hz; • a frequency of rotor rotation frequency oscillation of more than 1 Hz;

An experimental test unit is created for studying low-consumption stages of centrifugal compressors with a rotor magnetic suspension system (MSS) and a torsional moment transducer. In addition, this unit makes it possible to perform MSS studies in the area of functional construction, analysis and synthesis of dynamic characteristics of the rotor-MSS system. The rotation speed of the model rotor is 1000-25000 rpm. In order to reduce the loss of power in bearings, a rotor magnetic suspension device is used, developed and manufactured in the V. B. Shnepp Turbocompressor Research Institute. The torsional moment transducer, installed in a coupling between the set-up gear and the bearing support, provides the possibility of measuring power in the impeller shaft with compression of the working gas.In view of the expansion of the field of using centrifugal compressors and, specifically, use of compressors with low productivity (Φ 0 < 0.025), a requirement arises in experimental studies for low consumption stages and the development of new procedures for their design and planning. As a rule, creation of new design procedures in centrifugal compressor construction is based on reliable experimental data.Using a thermal balance method in order to determine stage characteristics, it is necessary to consider thermal losses of the capacity, losses in friction of the air impeller and in flow through compaction, and also the error in measuring temperature and pressure. It is impossible to estimate heat losses precisely, and in order to reduce errors in temperature and pressure measurement expensive highly accurate transducers and secondary measuring equipment are required. Therefore, it has been decided in order to determine stage characteristics to use a torsional moment transducer that makes it possible to measure the moment and power in a shaft. However, the power used by a low-consumption stage, commensurate with the power loss in oil bearings, is the same as in magnetic bearings and these losses are small.Taking account of these factors, the problem was set of creating an experimental test unit for studying the low-consumption stages of centrifugal compressors with a magnetic suspension system (MSS) for the rotor and a torsional moment transducer, and also for studying the magnetic suspension system with the aim of functional construction, analysis and synthesis of the dynamic characteristics of the rotor-MSS system.The following conditions were taken into account in developing the system: • the working gas is air;• the circuit is open;

It is noted that one of the most promising trends in the field of detecting turbocompressor stall and revelation of the signs that characterize different forms of the development of gas dynamic instability (stall and rotating surge) is the use of statistical processing methods and signals. A procedure is presented for using statistical functions for analyzing the nonstationary nature of compression in turbocompressors and revelation of signs of stall based on calculating covariation of signals of regime parameters. Results are provided for studies based on analyzing covariation of the signals of regime parameters for centrifugal compressors of different construction that demonstrate the suitability of this parameter for detecting the phenomenon of compression gas dynamic instability. The procedure proposed is used successfully in practice in the form of special software for stall signalers used in centrifugal compressor automation systems.

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