System and method for compressor performance monitoring and surge detection

Abstract

The invention provides a system and method for compressor performance monitoring and surge detection. The invention relates to a method and a system for determining the operating state of a dynamic compressor mechanically connected to at least one electric machine, equipped with a rotor exchanging energy with a known fluid by rotating at a speed (N) and with a drive mechanical torque (Tor), and in particular for determining an operating point of the compressor and a distance of said operating point from a surge limit curve defining a stable operating region from an unstable operating region of the compressor.

Description

Technical Field

[0001] The present invention relates to an innovative method for monitoring the performance of a compressor and detecting unstable behaviors such as compressor stall or surge.

[0002] The present invention provides significant advantages in the field of compressors mechanically connected to an electrical machine, preferably a dynamic compressor of the radial or axial type, which are widely used in refrigeration systems, heat pump systems, process industries, oil and gas industries, technical gas management including natural gas and hydrogen compression, and generally in all those applications for managing compressible vapor and gas flows. Background Art

[0003] Compressors are widely used in industrial and distributed applications to handle gas flows such as natural gas, refrigerants, and industrial gases. When low volumetric flow rates are required, positive displacement compressors are typically employed, while radial and axial types of dynamic compressors are the typical technologies of choice when large volumetric flow rates need to be managed. A key characteristic of compressors in most applications, including the refrigerant industry, is their operational flexibility, i.e., the ability to supply a boosted pressure under all required operating conditions. In the case of dynamic compressors, the need to increase their performance and operational flexibility requires accurate prediction and monitoring of their stability limits. In particular, compressor surge must be avoided as it causes abnormal fluctuations in mass flow rate and pressure, leading to abnormal noise and vibration responses, control problems, and possible damage to the compressor structure. The characteristics of the fluid-dynamic circuit incorporating a dynamic compressor significantly affect the onset of surge: in particular, the volumes attached upstream or downstream of the compressor significantly affect the mass flow rate at which instability begins to manifest. This experimental evidence is fundamental when compressors are employed in complex systems such as refrigeration cycles or pressurized fuel cell systems, where complex dynamic mechanisms related to the interaction between the inserted boosted volumes and the compressor may occur. For this reason, surge margin monitoring and early surge detection in axial and centrifugal compressors coupled with large volumes and heat exchangers have been a key aspect that has been widely investigated in recent years.

[0004] Systems and methods for controlling the operation of a compressor to avoid stall, surge, or chugging are described in documents US 8,087,870B2, EP 2414748 B1, US 10,961,919 B2.

[0005] Based on the non - dimensional analysis method applied to compressors, also known as similarity theory, the monitoring of surge margin is typically performed by using pseudo - dimensional parameters such as reduced mass flow rate and reduced speed related to pressure ratio and efficiency, such that compressor performance characteristics are uniformly determined. In fact, appropriate non - dimensional parameters should include geometric dimensions such as diameter or length; however, in turbine performance mapping representation or performance monitoring, since the geometry is fixed, the geometric parameters drop out of the equations and the parameters become "pseudo - dimensional". For example, in [A], the following functional relationships among the pseudo - dimensional parameters are obtained for a dynamic compressor, assuming that the Reynolds number effect is negligible (in such a reference, it is also assumed that Z = 1):

[0006]

[0007] The need to further reduce this general relationship to only two independent parameters has led to alternative formulations aimed at including the k - effect in the mass - flow parameter, such as [B].

[0008]

[0009]

[0010] The first and second independent variables of the functional relationship, the second member of equation (2), are commonly referred to as the reduced mass flow rate and the reduced rotational speed .

[0011] Once the compressor map is known - which includes the stable line - i.e., the line that intercepts each isentropic curve at the point of minimum stable mass flow rate (or maximum stable pressure ratio), the surge margin can be defined. The surge margin typically aims to quantify the distance between the compressor operating point and the stable line. For example, a typical definition of the surge margin is:

[0012]

[0013] However, for experimental evaluation of the surge margin, several on-line measurements are required around the compressor, such as: rotational speed, inlet temperature, inlet pressure, outlet pressure, mass flow rate. The last measurement, the mass flow rate, is very important, slow to respond and usually invasive, and it usually suffers from significant errors (in the range of 2 - 5% in industrial applications). Nevertheless, accurate surge margin estimation requires mass flow rate measurement. In fact, especially for radial compressors, the mass flow rate is the most sensitive parameter close to the surge margin because the pressure ratio tends to be a stable constant and thus does not provide sufficient sensitivity for surge margin estimation. However, in the technical literature, little attention has been paid to formulating a stable line mapping or a surge margin indicator that shows good linearity with respect to at least one main independent variable: such linearity would be extremely beneficial for control purposes.

[0014] In the case of a compressor mechanically connected to an electric motor, an alternative method to avoid using a mass flow sensor to monitor the compressor margin is shown in [C]. Such a patent proposes using a compressor mapping based on electrical parameters such as power and speed pairs or torque and speed pairs. The inlet pressure and inlet temperature are also recorded. The electric motor can be used as an actuator to avoid surge by modifying its speed. However, this method has several limitations: an adjustable speed drive is required, such that constant speed operation is not covered and thus the compressor mapping uses dimensional parameters that lack generality as a result. In fact, the general representation of compressor performance and the stable line should be invariant with respect to compressor inlet conditions and fluid properties: the performance mapping is only valid for one set of inlet conditions (e.g., reference pressure, temperature and composition) unless dimensionless or pseudo - dimensional parameters are used. Moreover, this method is not applicable to compressors equipped with inlet guide vanes (IGV). Finally, the patent does not consider the possible interference between the control clock of the adjustable speed drive and the hydrodynamic phenomena occurring in the compressor: if they have comparable time scales, the resulting electrical signals may only partially represent the compressor behavior and may be affected by electrical control logic distortions.

[0015] In addition to surge margin monitoring, which aims to keep the compressor in a safe and stable operating point, vibration-acoustic techniques are commonly adopted to detect any incipient instability behavior of the compressor and provide alarm or control signals when the compressor enters surge operation (active surge control). In the literature, there are many theoretical and experimental studies, the general aim of which is to extend the compressor operation to low flow ranges. For example, in [D], experimental activities have been developed at operating conditions close to the stability line to investigate an aero-turboshaft gas turbine axial-centrifugal compressor. The test field data have been investigated, which show the performance and vibration-acoustic response variations towards and during surge. In [E], a thorough investigation of compressor stall and surge perception has been carried out by means of an acoustic two-port model. It has been found that the flow instability due to rotating stall generates notable sound pressures that significantly affect the overall system acoustic response. In [F], an innovative tool for real-time surge prevention in advanced gas turbine cycles has been introduced, which is able to successfully predict surge anomalies in all the operating conditions tested. In [G], the investigation of unstable phenomena with particular attention to surge and rotating stall in the case of a turbocharger port shroud compressor has been discussed. In [H], an experimental investigation of surge in a radial compressor cooler cycle has been described.

[0016] In [I], it has been shown that the system vibration-acoustic response exhibits the main spectral content in the sub-synchronous frequency range close to the initial surge condition that may occur due to rotating stall inception. In [J], cyclic stationary analysis has been applied to the system vibration-acoustic response in order to deepen the knowledge of the hydrodynamic instability phenomena occurring at low mass flow rates. This signal processing technique has proven its reliability in rotating stall cell detection and early surge identification. Recently, the "cepstrum" signal processing technique has been shown to provide an effective insight into incipient surge detection [K]. However, all the previous methods require the installation of vibration-acoustic sensors and high-frequency acquisitions, which increase the system complexity and affect the reliability.

[0017] In the case of an electrically driven compressor, the technical literature shows that electrical signals can be used to detect the occurrence of surge. For example, in [L], the current rate of change signal is used to detect the surge applied to a refrigeration compressor. However, this method has several limitations: the signal processing is complex because the current change signal requirements are coupled with the pressure change signal, the maximum pressure rise signal, and the maximum current signal, there is no measurement of the compressor inlet temperature (required by non-dimensional analysis, otherwise lacking generality), and there is no use of non-dimensional or pseudo-dimensional parameters.

[0018] In [M], a similar method is disclosed. The speed can be changed to avoid surge. Optionally, the evaporator and condenser pressures are obtained. In addition, there are several limitations to this method: it is only limited to variable-speed motors, the compressor inlet temperature is not measured (required by non-dimensional analysis, thus lacking generality), no non-dimensional or pseudo-dimensional parameters are mentioned, and the microprocessor needs to be adjusted online to continuously adapt to the surge map and related thresholds.

[0019] The following cited technical documents are listed:

[0020] [A] S.L. Dixon, Fluid Mechanics and Thermodynamics of Turbomachinery, 5th edition, 1998, Elsevier.

[0021] [B] A. Traverso, "TRANSEO: A New Simulation Tool For Transient Analysis Of Innovative Energy Systems", 2004, Ph.D. Thesis, DiMSET, University of Genoa, Genoa, Italy.

[0022] [C] Compressor surge control system and method, U.S. Patent 9,328,949 (2016).

[0023] [D] Munari, E., Morini, M., Pinelli, M., Spina, P.R., Suman, A., "Experimental Investigation of Stall and Surge in a Multistage compressor", ASME Paper GT2016-57168, Turbo Expo 2016, Seoul, Korea.

[0024] [E]Kabral, R., Åbom, M., "Investigation of turbocharger compressor surge inception by means of an acoustic two-port model". Journal of Sound and Vibration 412 (2018), pp. 270-286.

[0025] [F]Reggio F., Silverstri P., Ferrarri M.L., Massgaro A.F., "Operation extension in gas turbine-based advanced cycles with a surge prevention tool", (2022) Meccanica, 57(8), pp. 2117-2130.

[0026] [G]Guillou, E., Ganceo, M., Gutmark, E., "Experimental Investigation of Flow Instability in a Turbocharger Ported Shroud compressor", Journal of Turbomachinery (2016, Vol.138 / 061002-1).

[0027] [H]Ferrando, M., Reboli, T., Reggio, F., Nicolini Marmont Du Haut Champ, C.A., Silversti, P., Traverso, A., Sishtla, V., Centrifugal compressor surge in innovative heat pump – Part 1: fluid dynamic and vibrational analysis, ASME Paper GT2023-102776, Turbo Expo 2023, Boston, USA.

[0028] [I]Zhenzhong, S., Wangzhi, Z., Xinqian, Z., "Instability detection of centrifugal compressors by means of acoustic measurements", Aerospace Science and Technology 82 - 83 (2018) 628 - 635.

[0029] [J]Silversti, P., Reggio, F., Nicolini Marmont Du Haut Champ, C.A., Ferrarri, M.L. and Masardo, A.F., "Compressor Surge Precursors for a Turbocharger Coupled to a Pressure Vessel", Journal of Engineering for Gas Turine and Power 144 (11) (2022), 111014.

[0030] [K]Silversti, P., Nicolini Marmont Du Haut Champ, C.A., Reggio, F., Ferrari, M.L. and Massardo, A.F., Vibro - acoustic responses and pressure signal analysis for early surge detection in a turbocharger compressor, ASME Paper GT2023 - 101699, TurboExpo 2023, Boston, USA.

[0031] [L]Method and apparatus for detecting surge in centrifugal compressors driven by electrical motors, US Patent 4,581,900 (1986).

[0032] [M]Method and apparatus for detecting surge in a centrifugal compressor, U.S. Patent 5,894,736 (1999). SUMMARY OF THE INVENTION

[0033] The present invention aims to overcome the limitations of the prior art for compressor stall and surge monitoring and detection by providing an innovative and general method applicable to compressors mechanically connected to motors operating at constant or variable speed. In particular, the recent availability of very fast variable speed drives for motors, having an internal control clock < 1 ms, i.e., significantly faster than the typical hydrodynamic phenomena occurring in compressors, allows the use of the motor as a sensor for the hydrodynamic machine. This is proposed for the first time in the present invention. In fact, the presence of such a drive, sometimes called an inverter in the case of alternating current applications, allows the accurate knowledge of the phase angle between current and voltage, and thus allows a very accurate and fast estimation of the torque from the electrical parameters. The compressor can be of any dynamic type, such as axial, centrifugal, mixed flow or axial - centrifugal compressors. In addition, the compressor can include variable angle blades: if such devices are placed at the compressor inlet, they are commonly called IGVs. If present, the angle of the variable blades should be included as a separate parameter in the following formulas. Hereinafter, compressor surge or compressor stall will be used synonymously, indicating an unexpected compressor time - related behavior involving the occurrence of non - stationary fluid dynamic phenomena (such as rotating stall, mild surge, deep surge, etc.). Generally, compressor surge and stall occur at or near the compressor stable line (or compressor surge line) or at a lower mass flow rate, resulting in non - stationary hydrodynamic phenomena, enhanced mechanical vibration, higher acoustic emission and shaft torque variations. Finally, the method proposed here allows the detection of abnormal torque variations. In addition, similar effects can also be driven by mechanical damage or object ingestion and can thus also be detected by the present invention.

[0034] In particular, the present invention proposes two main innovations: (i) a new compressor map representation based on electrical - related quantities obtained from the motor or sensed by dedicated sensors, and (ii) a method for identifying the compressor operating point on such a new map and detecting unstable compressor operation. The new theoretical method of compressor map representation uses the principle of similarity through pseudo - dimensional parameters based on electrical signals obtained from the motor used as a sensor for the hydrodynamic machine or sensed by dedicated sensors: this allows the use of electrical signals for compressor surge margin estimation without loss of generality and with increased accuracy. The present invention can achieve a reliability level equivalent to or better than traditional methods in monitoring the compressor surge margin, with the advantage of avoiding any precision or expensive sensors, such as mass flow meters, and bringing reduced complexity, lower cost and enhanced robustness or redundancy.

[0035] In addition, the present invention uses the same electrical signal to quantify the compressor surge margin, i.e., the distance from the compressor stable line, and to detect the onset of compressor surge and / or surge occurrence, without the need for additional vibration acoustic sensors, as is widely done in conventional methods. As a preferred embodiment, such electrical signals are processed by an electronic analog operator to obtain a surge detection signal that can be used for alarm or control actions. In one embodiment, the acquired electrical signal can provide the power required by itself to obtain a surge signal; in another embodiment, a low-voltage external power supply can be included to amplify the surge detection signal, thereby increasing the sensitivity. In both of the previous embodiments, analog / digital conversion and high-speed microprocessors can be avoided, with the significant advantages of system simplicity, reliability, and durability.

[0036] In another preferred embodiment, the electrical signal can be converted into a digital signal and processed by a microprocessor to improve data analysis and obtain the same or additional information about compressor performance. In both the analog and digital cases, the proposed method is generic and does not require on-line tuning to a specific compressor, e.g., for determining a threshold for detecting surge. In fact, the threshold for surge detection is just above the signal noise, since during normal operation, the signal used for surge detection is ideally zero. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 – Components of the electrical system, compressor, and instrumentation;

[0038] Figure 2 – Conventional compressor map, with reduced mass flow (x-axis) and pressure ratio (y-axis), reduced speed as a parameter;

[0039] Figure 3 – New compressor map, with reduced torque speed ratio (x-axis), speed at reduced exponent (y-axis), reduced speed as a parameter;

[0040] Figure 4 – Compressor map along the stable line: reduced torque speed ratio versus reduced mass flow;

[0041] Figure 5 – Compressor map along the stable line: speed at reduced exponent versus pressure ratio;

[0042] Figure 6 – Algorithm for compressor surge detection by coil signal processing via analog or digital electronics;

[0043] Figure 7 – Compressor surge detection by coil signal processing via analog electronics with signal amplification (active): circuit and results;

[0044] Figure 8 – Compressor surge detection by processing the coil signal via analog electronic devices without signal amplification (passive): circuit and results;

[0045] Figure 9 – Compressor surge detection by processing the coil signal via digital electronic devices: logical steps. DETAILED DESCRIPTION

[0046] The present invention proposes a new method for monitoring a compressor mechanically connected to at least one electric motor, such that the electrical signal from the electric motor can be used to identify the compressor operating point as a new “electrical” base map, overcoming the limitations of the prior art in detecting compressor surge and its precursors.

[0047] REFERENCE Figure 1 , a fluid is processed from a low pressure stage (1) to a high pressure stage (2) by a compressor (3). This compressor is powered by an electrical system (4), which generally consists of an electric motor (5) connected to a driver (6). Such a driver can consist of an inverter for variable speed applications or can consist of the power grid for constant speed applications: such a driver is characterized by a clock time much faster (e.g., <1 ms) than the compressor hydrodynamic stall and surge phenomena (e.g., >1 ms). The connection from the driver to the electric motor includes at least one electrical phase through which the AC current passes. The current exchange between the driver and the electric motor allows the compressor to be moved at a given rotational speed N. This current value can be sensed by different methods known in the art, including but not limited to a coil (7) installed around the electrical phase conductor: such a coil requires at least one full turn of wire around the conductor. At the extremes of the coil, an AC voltage signal Vin occurs due to the AC current. The root mean square (RMS) value of this AC voltage signal is proportional to the RMS value of the AC current and thus represents the measurement of the current itself. By knowing the configuration and characteristics of the electrical system (4) and measuring the coil AC voltage Vin, the mechanical torque provided by the electric motor (5) to the compressor (3) can be obtained. For a DC-operated compressor, an alternative can be a Hall effect current meter.

[0048] To measure the compressor speed, in the case of constant speed applications, its rotational speed is known and / or determined by the power grid. In the case of variable speed applications, the motor speed can be obtained from the driver itself or measured based on the RMS voltage between the electrical phase conductor and the neutral conductor (the combined information of the current RMS value and the voltage RMS value, as well as the stator resistance of the electric motor, allows an accurate estimation of the electric motor rotational speed). Thus, the rotational speed is known.

[0049] Once the mechanical torque (Tor) and rotational speed (N) are known from the electrical signals, the original similarity theory applied to a dynamic compressor (Equation 2) can be reformulated in terms of pseudo-dimensional parameters as follows:

[0050]

[0051] Significantly, in the above formula, the reduced torque term includes the specific heat ratio k and the inlet total pressure in the denominator: the formula includes a reduced torque which can preferably be measured directly from the motor (without using any complex mechanical torque meter), the formula allows to overcome the lack of generality of existing methods, and allows to accurately represent the compressor performance map, duly incorporating the effects of the inlet composition and the inlet pressure. Given the recent availability of very fast variable speed drives, significantly faster than the hydrodynamic effects in the compressor, the torque measured from the electrical signals can be considered to fully represent the hydrodynamic phenomena, also under time-dependent conditions, without distortion, aliasing or bias due to the drive control logic. On the other hand, according to conventional methods, the inlet total temperature is already included in the reduced mass flow and reduced velocity formulas.

[0052] By swapping the reduced mass flow with the reduced torque, the above equation becomes:

[0053]

[0054] Therefore, it is evident that once the reduced torque and the reduced velocity are known, the operating point of the compressor is unambiguously determined without any direct measurement of the mass flow. The only missing information is the inlet pressure and the inlet temperature, which can typically be measured at the compressor inlet by a probe (8) and a probe (9) respectively.

[0055] Finally, in order to obtain a new representation of the compressor map in terms of motor-derived parameters, but retaining a direct interpretation in terms of more conventional hydrodynamic parameters, from considerations of the similarity amplitude theory, it can be concluded that the reduced mass flow is well represented by the ratio of the reduced torque and the reduced velocity, while the pressure ratio is well represented by the velocity at a reduced exponent greater than 1 (the best-fit exponent for obtaining a linear relationship mainly depends on the k value of the working fluid; for a relatively constant value of k across the compressor operating range, the best-fit exponent value depends only on the k value; typical exponent values are in the range [1; 10], and good results are observed in the range [2; 10]). Thus, the previous functional relationships are finally formulated as follows:

[0056]

[0057] In this way, several advantages are obtained: the compressor performance map has a general validity invariant with respect to the inlet conditions and the fluid composition; it is based on electrical parameters derived from the motor and is employed as a compressor sensor, thus avoiding the use of expensive and intrusive probes such as mass flow meters; the independent pseudo-dimensional variables (the second member of equation (7)) show a direct interpretation in terms of physical parameters; due to the very fast variable speed drive connected to the motor, it also maintains high precision in the time-dependent domain.

[0058] One can infer the goodness of such a new compressor map from Figure 2 and Figure 3 : the former presents the performance map of a typical radial compressor with conventional parameters and deals with fluids with real gas behavior, while the latter shows the same performance on this new "electrical" compressor map. In fact, the stable lines even show a more linear behavior, ensuring good predictability and controllability.

[0059] The following Figure 4 and Figure 5 show the excellent physical interpretation of the proposed "electrical" parameters, namely the reduced torque-speed ratio as an indicator of reduced mass flow rate, and the speed at the reduced exponent as an indicator of pressure ratio. In fact, in both cases, a very precise linear regression is possible, as demonstrated by the regression parameter R 2 very close to 1 (the experimental points considered are along the constant speed lines - i.e., along the Figure 2 last stable operating points of the stable lines). Therefore, the reduced torque-speed ratio presents a sensitivity to compressor surge similar to that of the reduced mass flow rate, while the speed at the reduced exponent presents a sensitivity to compressor surge similar to that of the pressure ratio. Thus, any linear combination of them can be used to define a compressor surge margin estimator with very good linearity with respect to physical parameters, allowing for a direct interpretation and effectiveness in control actions.

[0060] The present invention for surge margin estimation provides improved surge monitoring and control over traditional stand-alone surge controllers. This alternative does not use four fluid sensors (inlet temperature, inlet pressure, outlet pressure, mass flow rate) or three fluid sensors such as in [C] (inlet temperature, inlet pressure, outlet pressure), but instead it uses only two physical sensors (inlet temperature, inlet pressure) plus electrical signals, ensuring higher reliability and lower cost. Moreover, for those compressors mechanically connected to a motor already equipped with a conventional anti-surge device, the present invention can be installed as a cost-effective redundancy, thus contributing to system reliability.

[0061] Using the aforementioned coil, the voltage signal Vin at its extreme values can be used to evaluate the compressor surge margin according to the new "electrical" compressor mapping method explained previously, and it can also be employed to detect incipient compressor instability behaviors such as rotating stall and surge cycles. However, as mentioned, the coil is only one of the possible ways to detect the signal representing the current intensity driving the compressor, and a person skilled in the art can use other alternative devices for detecting such a signal.

[0062] The incipient compressor instability behaviors can be detected precisely, especially when using a variable speed drive operating with an internal control clock much faster than the hydrodynamic phenomena occurring in the compressor: in this way, both the steady-state and time-dependent electrical signals only represent the hydrodynamic phenomena because of the separation of the frequency ranges between the signals related to the hydrodynamic phenomena and the signals related to the electrical / electronic phenomena of the compressor drive; preferably, the drive should be selected such that the clock speed (Hz) is one order of magnitude greater than the frequency of the physical phenomenon signals observed.

[0063] As is well known, when a dynamic compressor enters surge, the blade profile stalls and thus the fluid can no longer be pushed from the low-pressure inlet (1) to the high-pressure outlet (2): in some cases, the flow even reverses (negative flow direction) until the outlet pressure decreases and the compressor can again push the fluid in the correct direction (positive flow direction). Surge occurs in the case of such recursive cycles establishing high-low positive flow or even positive-negative flow, and is accompanied by strong variations in mechanical torque. As discussed previously, the measured voltage Vin at the extreme of the coil (7) is a direct function of the current exchanged between the electric motor (5) and the drive (6), which is also a direct function of the mechanical torque. Thus, it is obvious that rapid changes in torque will result in rapid changes in Vin. Based on this principle, a process can be designed to properly process the Vin signal until an effective and robust signal of the surge precursor or occurrence, hereinafter "surge alarm", is obtained. Figure 6 The logic path for obtaining such a robust surge alarm is described:

[0064] – (i) Filter the original coil signal using a band-pass filter in order to retain the most significant content, especially around the rotational frequency;

[0065] – (ii) The filtered signal needs to be demodulated in order to clearly identify any variations in the current RMS value;

[0066] – (iii) Then filter the demodulated signal using a low-pass filter to remove the DC component and a high-pass filter to remove the noise;

[0067] – (iv) Compare the resulting signal with a threshold value to provide a surge alarm in case the threshold value is overcome, e.g., by lighting an LED.

[0068] It should be emphasized that the threshold value in (iv) can be null or very small because during normal operation, regardless of the rotational speed or current value, the signal from step (iii) will be a zero-voltage signal, except for background noise. This is because surge detection is done by monitoring the energy variation of the dynamic component in a response signal whose energy is negligible during normal operation. Thus, the threshold value only needs to be higher than the remaining background noise since it is not linked to any physical parameter: in other words, it does not need to be tuned on a specific compressor.

[0069] The above signal processing can be carried out by analog electronics ( Figure 7 active electronics with amplifiers and Figure 8 for passive electronics) or by digital electronics ( Figure 9 ).

[0070] In Figure 7 the case of the active electronics, the analog circuit relies on standard electronic components such as resistors (21), capacitors (22), operational amplifiers (23), ground connections (24), and diodes (25). The electrical characteristics of the aforementioned components must be defined so as to act correctly on the input raw signal according to the desired analog operation for compressor surge diagnosis. In turn, the first circuit is an active band-pass filter that allows a significant energy level to be maintained in a sufficiently wide frequency band around the nominal synchronization frequency. The second circuit allows the extraction of the envelope of the filtered signal, which represents the energy trend in the frequency band of interest of the filtered signal. The third circuit is a high-pass filter that allows a zero-mean signal to be obtained. The fourth circuit is a low-pass filter necessary to remove unwanted high-frequency noise. Operational amplifiers are used to maintain a well-distinguishable signal even in field applications where background noise can cover the signal of interest.

[0071] The passive electronics solution ( Figure 8 ) is similar to the active electronics but without an amplification stage. The logic scheme is the same as the active scheme (the operation sequence remains the same: BP filter, envelope, HP / LP filter), and the difference is mainly related to the absence of an operating element that, in the active solution, allows the signal to be amplified after each operation in order to maintain a good signal / noise ratio throughout the process. The advantage of the passive solution is its independence from any external power source (e.g., battery or DC voltage source), its robustness and resilience to industrial applications. Such advantages are balanced by the lower voltage in the resulting signal, which can be more sensitive to external interference and thus has a higher risk of false positive alarms.

[0072] Figure 9 Based on the equivalent digital method, it relies on digital signal processing algorithms. In this case, an analog-to-digital converter DAC (26) is required to be able to apply digital signal processing algorithms (such as FFT routines). This method has the advantage of being applicable to any type of coil input signal without the need to modify the computing hardware and only requires different parameter settings. However, it requires sufficient computing resources to be installed on-site to perform the required signal processing, which may hinder its application in some specific cases.

[0073] Compared with fluid sensors, the surge detection technology explained only uses electrical signals that are easily obtained with high precision and fast response, and for any compressor mechanically connected to the motor, there is no need to install additional fast-response accelerometers and related signal acquisition systems. In fact, the use of the electric drive current ensures an almost instantaneous response to changes in motor torque, and in the case of variable-speed drives, it usually executes its control algorithm at the synchronous frequency.

[0074] For those compressors that have already adopted traditional surge control, the present invention provides cost-effective redundancy that contributes to system reliability.

[0075] From the above description, it is clear that the systems and methods according to the present invention fully achieve the intended purpose.

[0076] The objectives of the present invention are susceptible to many modifications and variations, all of which fall within the scope of protection defined by the claims. Without departing from the scope of the present invention, all details can be replaced by other technically equivalent elements according to requirements.

[0077] Even if the object has been described with reference to the drawings for improving the understanding of the present invention, this does not constitute any limitation to the scope of protection claimed.

[0078] Nomenclature

[0079] c p Specific heat at constant pressure [J / kg K]

[0080] c v Specific heat at constant volume [J / kg K]

[0081] D Diameter [m]

[0082] h Enthalpy [J / kg] [W / m 2 K]

[0083] k Specific heat ratio (c p / c v )

[0084] Kp Surge Margin

[0085] Mass flow rate [kg / s]

[0086] N Rotational speed [rpm]

[0087] p Pressure [Pa]

[0088] Pow Power [W]

[0089] R Specific gas constant [J / kg K]

[0090] T Temperature [K]

[0091] Tor Torque [N m]

[0092] Z Compressibility factor [-]

[0093] Greek symbols

[0094] Pressure ratio

[0095] Efficiency

[0096] Angular velocity [rad / s]

[0097] Subscripts

[0098] crit Critical

[0099] el Electrical

[0100] eq Equivalent

[0101] in Inlet

[0102] is Isentropic

[0103] red Reduced

[0104] 0 Total

Claims

1. A method for determining an operating state of a dynamic compressor, the dynamic compressor being equipped with a rotor that exchanges energy with a known fluid by rotating at a certain speed (N) and with a certain driving mechanical torque (Tor), and in particular for determining an operating point of the compressor and a distance of the operating point from a surge limit curve, the surge limit curve delimiting a stable operating zone and an unstable operating zone of the compressor, the method comprising the following steps: - defining a compressor map, which is intended as a two-dimensional locus of possible operating points of the compressor and is expressed as a reduced mechanical torque Tor of the rotor red and the reduced number of revolutions N of the rotor red The function of and ; - determining the surge limit curve as a set of points within the compressor map as defined above; - According to the mechanical torque of the rotor (Tor), the number of revolutions of the rotor (N), the pressure of the fluid entering the compressor (p 0in ) and the temperature of the fluid entering the compressor (T 0in ), periodically detecting instantaneous operating parameters of the compressor during operation; – According to the reduced mechanical torque (Tor red ) value and the reduced number of revolutions of the rotor (N red ) determine instantaneous operating conditions; - comparing the detected transient operating condition with the surge limit curve within the compressor map; - Based on the comparison with the previous step, generating one or more notifications about the status of the compressor and / or control signals for said compressor.

2. The method according to claim 1, wherein: The compressor map is determined as a two-dimensional map in an xy plane having the reduced mechanical torque (Tor red ) and the reduced number of revolutions (N red ) and having the reduced number of revolutions (N) as the second coordinate red ) for a mathematical power of an exponent (exp), said exponent being greater than 1 and preferably in the range comprised between 2 and 10.

3. The method according to claim 2, wherein: The value of the exponent (exp) is also determined as a function of the specific heat ratio (k) of the fluid exchanging energy with the compressor.

4. The method according to one or more of the preceding claims, wherein: The step of generating one or more notifications about the compressor status and / or control signals for the compressor comprises the further step of defining a surge margin curve, the surge margin curve being intended as a locus of points belonging to the stable operating region of the compressor and being arranged at a known and predefined distance from the surge limit curve.

5. A method according to the preceding claim, wherein: The distance between the surge margin curve and the surge limit curve constitutes a surge margin (Kp), and is such that for the same reduced number of revolutions (N red ), within the stability margin with reduced flow rate There is a linear relationship between them.

6. The method according to one or more of the preceding claims, providing the following further steps: – Provide dynamic compressors equipped with variable guide vanes; - measuring the opening angle of the variable guide vanes; - As the opening angle varies, the compressor map and the stability limit curve are replicated.

7. The method according to one or more of the preceding claims, providing the following further steps: – provide a dynamic compressor connected and driven by an electric drive; - detecting the current absorbed by the electric drive connected to the compressor and the electrical operating voltage of the electric drive compressor; - determining the driving torque (Tor) of the compressor as a function of the current absorbed by the electric drive; - determining the rotational speed (N) of the compressor rotor as a function of the voltage and / or as a function of the operating frequency of the electric drive.

8. The method according to claim 7, wherein: The step of generating one or more notification and / or control signals comprises the following steps: - determining a current signal representative of the instantaneous current of the electric drive during operation; - determining an electrical frequency signal representative of the instantaneous electrical frequency of the supply voltage of said electric drive during operation; - performing a first band-pass filtering of the current signal by means of a band-pass filter centered on the instantaneous frequency of the electric drive supply voltage; - determining the envelope of said signal as previously filtered; - performing a second bandpass filtering of said envelope signal as previously determined; - comparing said signal thus obtained with a reference value or with a range of reference values; - Generate notification and / or control signals based on the comparison in the previous step.

9. The method according to claim 7 or 8, wherein: The dynamic compressor connected to and operated by a motor comprises the following further steps: – Determine the time scale of compressor instability phenomena; - Use of a drive system for driving the electric motor of the compressor, said drive system being of the inverter type and equipped with a control module operating with an internal clock having a frequency equal to or greater than at least one order of magnitude of the maximum frequency of said instability phenomena of the compressor.

10. A system for monitoring an operating state of a dynamic compressor including a rotor and for predicting and determining a surge state of the compressor, the system comprising: - means for detecting the operating state of the compressor according to the mechanical drive torque (Tor) and the rotational speed (N) of the compressor rotor; - used to detect the pressure of the fluid entering the compressor (p 0in ) and temperature (T 0in ) sensors; an electronic control unit comprising at least one processing unit and a memory in which a map of the compressor is loaded, intended as a two-dimensional position of possible operating points of the compressor and expressed as a reduced mechanical torque Tor of the rotor red and the reduced number of revolutions N of the rotor red The function of and ; - a set of software instructions loaded into said memory which, when executed by said processing unit, implement the steps of said method according to one or more of the preceding method claims for predicting and determining said surge state of said compressor; - means for generating at least one notification signal for said operating state of said compressor and / or a control signal for said compressor.

11. The system according to claim 10, wherein: The means for detecting the operating state of the compressor in terms of a mechanical drive torque (Tor) consist of at least one current sensor for detecting the current absorbed by the compressor driver.

12. System according to one or more of the preceding claims, wherein: At least one further module is provided for processing the electrical signals detected by the device and / or sensor to detect the operating state of the compressor in terms of the mechanical drive torque (Tor) and the rotation speed (N) of the compressor rotor and to detect the pressure (p) of the fluid entering the compressor. 0in ) and temperature (T 0in ).

13. The system according to claim 12, wherein: The at least one module for processing the electrical signal detected by the sensor is of electronic type with passive and / or active type components and / or is provided with a digital signal processing submodule.

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