Some problems of vibration-based health monitoring

Some Problems of Vibration-based Health Monitoring Sergey N. Boichenko, Alexey V. Kostyukov, Alexander P. Naumenko Dynamics SPC, 644043, Omsk, Russia, E-mail: [email protected] Integration of the Russian scientific and technological achievements with similar foreign developments caused a conflict of interest and misunderstanding, particularly, in the machinery condition monitoring, diagnostics and health monitoring. The conflict of interest is primarily associated with the commercial interest of certain firms and companies aimed at selling their products and the related intentional and unintentional misconception in the machinery diagnostics. The misunderstanding relates to the lag of foreign countries in some fields of science and technology, specifically, in machinery vibroacoustic diagnostics and health monitoring. The purpose of the paper is to present the basic terminology of diagnostics, monitoring and vibroacoustics and its application to the real engineering solutions. The terminology is suggested by guidelines and regulations, reference, scientific and educational materials, including the researches carried out by the authors. The analysis of the terminology used in Russia and abroad revealed the differing interpretations in solving engineering problems of the machinery condition monitoring, diagnostics, and health monitoring. In particular, the term “condition monitoring” shall be understood as monitoring of state parameters without diagnostics. It is shown that the vibroacoustic signal is a random process, and the vibroacoustic diagnostics requires analysis of its stochastic characteristics. For this reason, it is necessary to measure vibration acceleration, velocity and displacement, since the stochastic characteristics of these vibroacoustic oscillation parameters are statistically independent. The studies performed brought us to the conclusion that the real-time health monitoring systems should provide real-time machinery diagnostics at such time intervals, during which the machinery state of health cannot change from normal to limit. In order to minimize the failure missing risk and ensure reliable diagnostics based-on the vibroacoustic signal parameters, one should measure the vibration acceleration, velocity and displacement with an accuracy of few percent over the frequency range of at least 3000 Hz. Keywords: condition monitoring, real-time health monitoring, failure, failure missing risk, acceleration, velocity, displacement. 1 The current state and achievements of science and technology in various spheres of human life enable solving the problems of industrial safety in a completely new way compared with the engineering solutions of 20-30 years ago, which continue to be used at numerous plants of various industries. One of the main components that gives rise to the systems for health monitoring of the machinery and equipment potentially hazardous to human is the development of methods and tools of non-destructive testing, real-time health diagnostics and monitoring [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Given the introduction of innovative information and communication technologies and solutions, another component is the use of these achievements in the technology-related spheres of human life. First and foremost, this refers to the Internet-technology capabilities and the enormous computing capabilities of the modern technology, including the distributed computing systems. The “diagnostic system” concept is inextricably connected with the definition of the “diagnostics” that is understood to mean “the area of knowledge covering the theory, methods and tools of determining the state of health of assets” (as per GOST 20911-89) and the “technical diagnosis” that means the asset condition determination. The objectives of the technical diagnosis are, first, condition monitoring, second, failure (fault, defect) localization and cause identification, and, third, technical condition prediction. The “technical diagnosis” term is employed where the problems of the technical diagnosis being solved are equivalent or the main problem is to locate and determine the causes of the failure, fault or defect [16, 17]. The “condition monitoring” term is employed where the main problem of the technical diagnosis is to define the condition evaluation [16, 17]. It should be noted that by the condition evaluation is meant, for example, good, operable, defective, nonoperable, limit [16, 17], and, according to GOST 27.002-2015 [17] operable, nonoperable and hazardous condition statuses, etc., depending on the parameter values at a given time [16, 17]. At the same time, GOST 32106 [18] sets forth the following machinery and equipment condition evaluations based on the vibration parameters: ‑ OK (until the vibration parameter reaches the boundary between A and B evaluation zones). This evaluation shall be met by machines during the acceptance tests after installation or turnaround (intermediate maintenance). The evaluation corresponds to the good machine state and speaks for the high quality of the maintenance and installation works; 2 ‑ ACCEPTABLE (until the vibration parameter reaches the Alert boundary). This evaluation permits a long-term machine operation. It corresponds to an operable machine with a low probability of its failure; ‑ ACTION REQUIRED (until the vibration parameter reaches the Shutdown boundary). The evaluation permits a short-term machine operation. It warns of the condition coming close to the limit, hazardous status, progressing faults, gradual loss of availability and increased failure probability; ‑ UNACCEPTABLE (when the vibration parameter exceeds the Shutdown boundary). This evaluation prohibits the machine operation. It is indicative of developed defects or high rate of their growth, limit or hazardous machine status with a high failure probability. To rate the quality of the new equipment installation, it is advisable to introduce an EXCELLENT condition evaluation, to which refer the vibration parameters 30 % below the boundary between A and B evaluation zones [18]. Thus, a diagnostic system shall determine: 1. The condition evaluations as per GOST 27.002-2015 or GOST 32106; 2. The point of failure origin and failure type; 3. The running time before possible transition to the limit or other status. Systems that do not meet these requirements cannot be called diagnostic systems, and systems that do not indicate the condition evaluation cannot be called machinery health monitoring systems. Such systems shall be called condition monitoring systems. The existing standards and regulations rather ambiguously interpret the “monitoring” concept. Thus, in GOST R ISO 13372 the “machinery health monitoring” is defined as: “A process that provides the ability to determine the current machinery availability without requiring its disassembly or inspection”. In a general sense the definition is rightful, but it does not indicate the ways of solving the problem of current evaluation of machine state of health that would ensure its safe functioning [19]. A more precise definition for the “machinery health monitoring” is given in [12]: “Health monitoring means observation over the state of health of a machine train (structure, machine, component, mechanism) for determining and predicting the moment of its transition to the limit condition status. The result of machine train monitoring is a set of diagnoses for its constituents (structures, machines, components, mechanisms) obtained at adjacent time intervals, during which the machine train status does not significantly change”. However, hardware and software developers reluctant to use this definition, since specific instruments need specific engineering solutions requiring serious scientific and technological study and justification, which in fact lead to research 3 work involving significant material and intellectual resources. And this can only be done by large financially sound organizations and enterprises. Currently, the systems that meet the above terminology, both in Russia and abroad, are commonly referred to as real time health monitoring systems [5, 20]. Therefore, all systems measuring certain parameters of equipment, including machines and mechanisms, and monitoring their values should be called condition monitoring systems. The systems monitoring parameters at intervals in excess of the time these parameters reach their limit values should refer to condition monitoring tools. In order to avoid disagreements and doubts in the classification of monitoring systems, the criteria given in GOST R 56564 [21] should be used. This GOST is the only regulatory document that presents specific building principles and classification criteria of systems that allows us to objectively compare the technical capabilities of the systems monitoring health of different-purpose production and transport equipment. Historically, when measuring vibration according to GOST 2954 [22] the frequency range, in which the measurement error was normalized, is 10 to 1000 Hz. At the same time, most regulatory documents, including domestic and foreign standards [23, 24, 25, 26, 27], are not only limited to the range of measured frequencies, but specify only one parameter characterizing vibration – velocity (see Table 2). In some cases, the displacement is also normalized in a range up to 200 Hz. Such an approach to the analysis of equipment vibration behavior can be used only to detect low-frequency defects that appear in a frequency range up to 1000 Hz. But many defects and failures excite vibroacoustic oscillations at frequencies that significantly exceed the above range. For example, in induction motors, the electromagnetic field symmetry breaking occurs at frequencies multiple of the product of the number of slots (e.g., 48 pcs.) by rotational speed (e.g., 49 Hz). Thus, this motor problem will appear at a frequency of 2352 Hz and its harmonics (4704, 7056 Hz) and even at rotational speed modulated with harmonics and doubled network frequency (2352±49, 2352±100, 2352±98, 2352±200 Hz, etc.). The compressor multiplier with the output shaft speed of 12,000 min-1 will generate vibration at frequencies significantly higher than 1000 Hz: multiplying 30 teeth by 200 Hz we obtain a tooth meshing frequency of 6,000 Hz, which is also modulated by the rotational speeds of the toothed wheels (6000 ±200, 6000 ±400 Hz, etc.) and have harmonics (12000 ±200, 12000 ±400, 18000 ±200, 18000 ±400 Hz, etc.). From the given examples it follows that to monitor the health and diagnose most types of pump and compressor equipment, the vibration shall be measured 4 with accuracy normalized over the range up to 3 kHz, and for certain vibration components up to 10 kHz [18, 28]. These simplest examples reveal another important problem that today is tacitly bypassed by many vibration analysis specialists. It is a reminder that nowadays in most cases, vibration is measured with piezoelectric accelerometers. So, we have the sensor output signal proportional to the vibration acceleration of the machine on which the sensor is mounted. The vibration velocity and displacement are obtained by integrating the original signal. Meanwhile, the velocity is often used for the analysis, believing that since this parameter is derived from the vibration acceleration, which is also “noised” with “unexplained” high frequency components, the velocity adequately characterizes the machine’s vibration behavior. However, the above examples show that, in addition to low frequency harmonics and subharmonics of the shaft speed, the signal also contains high frequency components that are integral multiples of the shaft speed, and for rolling bearings – non-integral multiples. And, as is known from the probability theory [29] and the statistical radio engineering [30, 31], the signal being the sum of at least 5-6 segments of the same sinusoidal oscillation taken with different initial phases is a stationary random process close to normal [9]. Here it should be kept in mind that when summing equifrequent harmonic oscillations, although the resulting process is stationary, but not ergodic. In this case, each realization of the resulting process is a harmonic oscillatory process that differs from other realizations (i.e., resulting processes obtained at another time) only in amplitude and phase (depending on how the phases of the initial oscillatory processes are in the given realization). When summing the harmonic processes not only with random initial phases, but also with different frequencies, we obtain a process not only stationary, but ergodic [30]. Thus, the vibroacoustic signal should be a priori perceived as a random process and, to analyze and process it, we shall apply methods of the theory of random functions and processes with all the consequences that come with it. Integration of such a process (stationary random close to normal), again, as it is known from the statistical radio engineering [30, 31] leads to a nonstationary process, and, to be more precise, to the Wiener process, which after removal of the low frequency and constant components, possesses all the properties of a stationary random process, which in turn is uncorrelated with the input and usually statistically independent of the input process [4, 5, 31, 32]. In practice, it comes to the fact that knowing the root mean deviation of the vibration velocity signal (rootmean square value in terms of vibration) we cannot unambiguously and reliably determine the value of the r.m.s vibration acceleration or displacement. This is true for all vibration parameters: acceleration, velocity, and displacement that, as other 5 vibration parameters obtained from the indicated through differentiation or integration, are statistically independent or orthogonal parameters [4, 5, 6, 27, 29, 30, 31, 32]. As an example we show the signals and spectra of vibration acceleration (see Figure 1 and Figure 2) and velocity (see Figure 3 and Figure 4) produced by a piezoelectric accelerometer mounted on the motor bearing having unacceptable defects. The major portion of the oscillatory energy is concentrated in the high frequency area within 4 to 5 kHz, which is shown by the vibration acceleration spectrum (see Figure 2). The r.m.s. vibration acceleration is 37 m/s2, which exceeds more than twice the value of the evaluation zone matching the UNACCEPTABLE status [18]. At the same time, the r.m.s. vibration velocity is 6.3 mm/s, which is lower than the ACTION REQUIRED boundary for motors having a center height of less than 225 mm (see Table 1). After the bearing was replaced, the r.m.s. vibration acceleration decreased 10 times and amounted to 3.7 m/s2 (see Figure 5). During the repair the motor was dismantled and aligned. This could entail the shaft misalignment and loose fastening of the motor to its foundation. It means the repair could cause changes in the low frequency vibration of the machine. However, the r.m.s. vibration velocity changed only by 5% and amounted to 6.12 mm/s (see Figure 6). It is evident that the motor repair was performed well. Such change in vibration is consistent with the measurement error of the monitoring and diagnostic system [4, 5, 18, 28] and the statistical variation of the signal. So, condition deterioration and a critical defect of the bearing manifested itself in the acceleration and absolutely did not appear in the velocity. Therefore, when measuring only the vibration velocity, this defect could be missed, which would result in a sudden incident or accident. At the same time, when using the vibration parameters given in [18, 28], this situation is excluded, since the stated standards require measuring acceleration, velocity and displacement, time rates of their change with the upper limit of a frequency range from 3 to 10 kHz. This example shows that vibration acceleration and velocity are noninterchangeable from the viewpoint of evaluating the stochastic properties of oscillatory processes. Similarly, velocity and displacement, acceleration and displacement are pairwise uncorrelated and independent at the same time. These vibration parameters independently characterize different disjoint classes of faults [4, 5, 6, 9, 10, 11, 12, 13, 14, 15, 32, 33]. Summarizing the above, we can state as follows: 1. Today, the regulatory documents introduce the “vibroacoustic nondestructive testing method” concept that is understood as a method based on the parameter analysis of stochastic characteristics of signals measured during the 6 recording of vibroacoustic oscillations that occur while the observed machine is running [34]. 2. The real-time health monitoring systems are systems that: - Provide the machine condition evaluation, - Determine the point of failure origin and failure type, - Calculate the machine residual life, - And perform all these functions in real time, i.e. at the rate of the measurements at adjacent time intervals, during which the state-of-health of the machine cannot change from the normal to limit [5, 12, 21]. 3. In order to minimize the probability of missing the hazardous equipment state of health (not more than 0.05), in particular, in terms of vibration parameters, it is necessary that the monitoring and diagnostic system has the probability of errors in static recognition of equipment condition no more than 0.01, dynamic – no more than 0.001 [35]. 4. In order to increase the reliability of the machinery condition monitoring, it is necessary to measure all three basic parameters of vibroacoustic oscillations – displacement, velocity, and acceleration, the latter parameter shall be measured in the frequency range up to 3000 Hz minimum. It is preferable to carry out measurements up to 10 kHz with an accuracy of few percent [4, 7, 18, 28]. 5. Taking into account the range of normalized values of the vibroacoustic signal, their frequency ranges, evaluation criteria, GOST 32106 [18] shall be used for health monitoring of centrifugal pumps and compressors, and GOST R 56233 [28] – for the health monitoring of stationary piston compressors. 7 Table 1 Evaluation Zone Boundaries [18] Pump Electric Motor Power, kW Shaft Axis Height, mm <50 <200 >200 <132 <225 <400 G G G G G G aRMS, ARQ* 8 12 16 8 12 16 m/s2 1.50 1.33 1.50 1.50 1.33 1.50 UAC** 12 16 24 12 16 24 ARQ 6.3 8.7 11.2 4.5 7.1 11.2 vRMS, 1.26 1.38 1.29 mm/s UAC 8.7 11.2 14.1 7.1 1.58 11.2 1.58 18 1.61 ARQ 18 28 36 14.1 28 36 dRMS, µm UAC 28 1.56 36 1.29 45 1.25 23 1.63 36 1.29 57 1.58 * ARQ – Action Required ** UAC – Unacceptable. Parameter Zone Boundaries Table 2 Vibration Parameters Used to Evaluate Machinery Vibration Behavior [27] Standard Year Criteria Frequency Range Machine Type VDI 2056 1964 dP-P vRMS 2.5 to 10 Hz 10 to 1000 Hz Reciprocating machines type K, M, G, T, D, S ISO 2372 1974 dPEAK vRMS 2.5 to 10 Hz 10 to 1000 Hz Reciprocating machines type I, II, III, IV, V, VI DLI Eng. Corp. 1988 dP-P, vPEAK, aRMS 10 to 1000 Hz Reciprocating machines ISO 10816-6 1995 dRMS, vRMS, aRMS 2 to 1000 Hz Reciprocating machines type 1, 2, 3, 4, 5, 6, 7 ISO 10816-1 1997 dRMS, vRMS, aRMS 10 to 1000 Hz Stator elements of machines GOST 25364 1997 vRMS, 10 to 1000 Hz Steam-turbine stationary units GOST 30576 1998 dRMS, vRMS, aRMS 10 to 1000 Hz Centrifugal feed pumps for thermal stations ISO 10816-3 1999 dRMS, vRMS GOST 31351 2007 GOST 31349 10 to 1000 Hz Centrifugal pumps, electric motors dRMS, vRMS, aRMS dPEAK, aPEAK 10 to 1000 Hz Industrial fans 2007 dRMS, vRMS, aRMS 10 to 1000 Hz Reciprocating internal combustion engine driven alternating current generating sets GOST R IEC 60034-14 2008 dRMS, vRMS, aRMS 2 to 1000 Hz Rotating electrical machines GOST 32106 2013 dRMS, vRMS, aRMS 2 to 200 Hz 2 to 1000 Hz 2 to 3000 Hz Centrifugal and screw pumps and compressors ISO 10816-8 2014 dRMS, vRMS, aRMS 2 to 1000 Hz 120 … 1800 min-1 reciprocating compressors dRMS, dα Reciprocating compressors 2 to 200 Hz, vRMS, 2 to 1000 Hz, aRMS , aα 2 to 10000 Hz The notations used in the table have the following meanings: dRMS, vRMS, aRMS – root-mean square vibration displacement, velocity and acceleration; dPEAK, , vPEAK, aPEAK – peak values: the largest absolute extremum of the oscillating quantity at time interval under consideration; GOST R 56233 2014 8 dP-P – peak-to-peak: the difference between the largest and smallest values of the oscillating quantity at time interval under consideration; dα, aα – α- quantiles of displacement and acceleration. 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