THE CAPABILITIES OF THE NEW GENERATION OF THE MONITORING AND DIAGNOSTIC SYSTEMS

Alexej V. Barkov,

General Director, VAST, Inc.

Published in magazine "Metallurg" ("Metallurgist") No. 11, 1998.

Introduction

The expenses on maintenance and repair are among the most important features of any technical system operation. Its minimization, when the system is maintainable, is practically impossible without the condition monitoring system. In modern means of control and diagnostics, at least for rotating machinery, the main type of analyzed process becomes the vibration that actively supersedes many other processes including the thermal one. The reasons of this is not only in that the vibration diagnostics is more efficient and there is a tendency that the cost of its realization is rapidly decreasing but also in that it is possible to begin the diagnostics by vibration any time even after several years of system operation when the expenses for maintenance and repair exceed the economically expedient value.

The capabilities of vibration diagnostics should not be identified with the capabilities of equipment vibration protection systems or vibration control systems. The means of protection systems consist of simple vibration power measurement instruments that detect the pre-accidental condition of a machine and stops its operation before or on the beginning stage of an accident. The means of vibration control (monitoring) are the instruments that measure the vibration power components and detect its variations during equipment operation. These means are not intended to be used for detection of incipient defects and because of this they some times register the vibration condition changes only after appearance of a chain of developed defects the last of which influences the vibration most severely. Such chains of defects appear shortly before an accident, so a good efficiency have only those monitoring systems that measure the vibration practically constantly.

Last decade several effective methods of machine and equipment main defects detection in their incipient stage of development were designed. These methods are based naturally on the analysis of high frequency vibration. To excite this vibration there is no need in large oscillation forces but it manifests only in the vicinity of their origin damping rapidly on the path of its propagation. Such methods become to be used by the diagnosticians in many countries when they transfer from monitoring to detailed diagnostics. Simultaneously the methods of diagnostic algorithms automation were developed. It enabled some of the diagnostic system manufacturers for typical diagnostic problems to substitute high qualified experts by their software. The part of such problems is very large and exceeds 90% of all the problems that can be solved by the vibration signal analysis. The first automatic systems for vibration diagnostics were designed in 1991-1992 and are constantly improving.

In 1998 a new generation of automatic machine diagnostic systems by vibration that were designed in cooperation by Russian and the United States specialists has appeared. These systems have included the best features of both the monitoring and diagnostic systems. A short analysis of the capabilities of such systems and the peculiarities of their design is discussed below.

The Aims and Objects of Monitoring and Diagnostics.

The machine and equipment condition monitoring is only one of the series of technical problems where vibration of the measurements and its analysis are used. Such types of problems can be divided into seven main groups including:;

Each group of problems has its limitation both on the diagnosed objects and the main vibration features used to gain the optimal results. For its description the vibration should be separated firstly by the frequency symptom and secondly by the method of its formation. By the frequency it is as a rule divided into four bands - low frequency, middle frequency, high frequency and ultrasonic frequency bands. By the method of vibration formation it is divided as vibration of natural origin (in machines and so on) and artificial, excited by a special source - oscillator. The vibration of the first origin is used to solve the problems of the first five groups, whereas the artificial vibration is used to solve the last two groups of problems.

The vibration monitoring. The object of this monitoring is mostly a machine and equipment - the sources of vibration. The peculiarity of such objects is the presence in them the oscillation forces, that appear for example during the motion of separate units or flows of fluid (gas), or as a result of action of alternative electromagnetic fields. Only very seldom the object of monitoring can be the equipment that is not a source of oscillation forces and vibration but is the path of vibration propagation from another source.

The aim of vibration monitoring is the detection of changes in the vibration condition of object under investigation during its operation. The cause of such changes is mainly the appearance of a defect.

The machine condition monitoring is conducted first of all by low frequency and middle frequency vibration that propagates very good from the point of its origin up to the point of its control. The number of such points can be minimized up to one-two for each object to be monitored if it has a common casing. The vibration measurements can be conducted without any change in the operation mode of the object. The monitoring system, if it is not included in the system of immediate protection, can have the hardware with one measurement circuit to which consequently are connected all the transducers used in the system. These measures enable to decrease significantly the price of the monitoring system without decreasing the reliability of the results received.

Vibration diagnostics. Its objects are the same machines and equipment that are investigated by the vibration monitoring systems. The vibration diagnostics most often is used both for quality control of manufacturing (repair) and machine assemblage or for flaw detection before maintenance or to detect the defects and watch their development during operation. For each of listed cases different diagnostic methods can be used. In the last case the diagnostic measurements can be made without any changes in the diagnostic object operation mode.

In contradistinction to monitoring the vibration diagnostics during equipment operation is intended to be used to detect the changes and predict not the vibration state but machine condition state and more over of each of its element that has a real probability of failure between maintenances. For this purpose not only the low and middle frequency vibration is measured but the high frequency vibration too. Also more complicated methods of vibration analysis than for monitoring are used. They enable to receive all the necessary diagnostic information. The vibration is measured on each unit to be diagnosed or at least in the points of high frequency vibration transaction from the diagnosed unit to other units that have access for vibration measurements. The hardware can have also one channel of vibration measurements and analysis.

Rotor Balancing. During machine operation vibration on the rotating frequency can be increased and to decrease it it is necessary to balance the rotor of each machine in field. The objects for balancing in field are usually machines where there is access to the balancing planes, e.g. to the places where it is possible to fix the balancing weights to the rotating parts.

The main purpose of balancing is the rotor equilibration and thus the decrease of low frequency machine vibration. But the vibration on the rotating frequency is not always defined by centrifugal forces, so to achieve good and stable results in most cases is possible with the help of means that define the reasons of vibration increase in each case, e.g. means that solve also the diagnostic problems. Balancing in field is made by vibration with the help of the same instruments as are used for vibration diagnostics.

The measurement instruments for rotor balancing must have at least two channels one of which defines the vibration amplitude on the rotating frequency and the other (the channel with the tacho probe) - its phase against a marker on the rotor. For the search of defects that restrict the ability of balancing in addition to the low frequency the high frequency vibration of the rotor supports and machine casing can be measured. The vibration is measured in typical modes of machine operation where the machine has to be balanced.

During machine operation vibration on the rotating frequency can be increased and to decrease it it is necessary to balance the rotor of each machine in field. The objects for balancing in field are usually machines where there is access to the balancing planes, e.g. to the places where it is possible to fix the balancing weights to the rotating parts.

The main purpose of balancing is the rotor equilibration and thus the decrease of low frequency machine vibration. But the vibration on the rotating frequency is not always defined by centrifugal forces, so to achieve good and stable results in most cases is possible with the help of means that define the reasons of vibration increase in each case, e.g. means that solve also the diagnostic problems. Balancing in field is made by vibration with the help of the same instruments as are used for vibration diagnostics.

The measurement instruments for rotor balancing must have at least two channels one of which defines the vibration amplitude on the rotating frequency and the other (the channel with the tacho probe) - its phase against a marker on the rotor. For the search of defects that restrict the ability of balancing in addition to the low frequency the high frequency vibration of the rotor supports and machine casing can be measured. The vibration is measured in typical modes of machine operation where the machine has to be balanced.

The Detection of Vibration (Noise) Sources. Such a problem in the industries appears first of all when there is a leakage, especially on long pipelines inaccessible for visual inspection. It is solved as a rule during different tests with increased pressure in the pipeline or after detection in the controlled equipment some changes in its state by means of monitoring systems.

In this case when it is necessary to find the source of increased vibration among a large number of objects, inaccessible for mounting a vibration transducer, the detection is conducted by noise emitted by this source. To do such job, aimed to detect the coordinates of the source, the instruments are used with at least two channels that measure either the noise in gas or fluid, or high frequency vibration, for example on the walls of the pipeline.

The Detection of the Acoustical Emission Sources. The detection of leaks in the high pressure vessels and pipelines usually occurred when the defect is already so dangerous that requires immediate interference. On the early stage of defects development the defects that lead to leakage are detected by acoustical emission of faulted parts of loaded case and construction. During the development of such micro-cracks the ultrosonic vibration appears. It propagates very good in heterogeneous material, that has no welded or other type of couplings and can be registered by high frequency vibration sensors even on large distances from the source of emission.

By acoustical emission the incipient defects not only of the vessels under pressure can be detected but also of the metallic building constructions, bridges, cranes and so on. To increase the sensitivity of the detection means for to define the place of defect appearance the multi-channel systems for simultaneous high frequency vibration measurements are used. They are of increased complexity. The methods of acoustical emission detection are efficient when in the object under control there is no high frequency vibration excited by other sources, for example, by the gas or liquid flows as it is in the pipelines under pressure. In the latter cases it is necessary to use special means of vibration sources separation that not always occurred to be sufficient.

Vibration model analysis. The objects of this type of analysis are not only machines, equipment and their units, but also composite parts of different constructions, buildings and so on. This analysis is used to define such features of objects as resonance frequencies, oscillation wave forms and so on.

The designation of model analysis can be both the design of new machine and equipment constructions and the quality control of the serial manufactured products first of all what concerns the magnitude of resonance frequencies of either the object as a whole or its separate parts. For machine and equipment diagnostics during operation the model analysis systems are not used often first of all because of the complexity of the systems itself and their maintenance. Such systems include vibration sources and several (not less than two) channels of vibration measurements and analysis. As vibration sources of permanent operation are used special machines - exciters, as sources of pulse vibration are used hammers with systems to measure shock pulses inserted in them.

Ultrasonic flaw detection. The objects of inspection are separate units of machines, equipment, constructions or buildings that are as a rule in the stage of manufacturing or restoration. The tools of inspection, that use external sources of ultrasonic vibration, by their structure and designation are similar to the means of model analysis "in miniature", but they have also other distinctive features besides the frequency band of measured vibration.

The ultrasonic flaw detection uses the wave features of vibration, particularly its reflection from different heterogeneous volumes and losses during propagation. It enables to detect and localize faulty elements inside the details or their blanks and this is the main designation of the ultrasonic flaw detection. Such means, just the same as the model analysis, are rarely used for machine diagnostics during operation. One of the reason is that other methods and means that use other types of radiation for example electromagnetic, X-ray and other types are much more efficient.

The Main Methods of Monitoring and Diagnostics.

The diagnostic problems included in the mentioned groups can be solved by different methods often duplicating each others. Below are considered the most efficient methods requiring minimum expenses for hardware and teaching the operators to conduct measurement and make the diagnostics.

Naturally, this approach requires certain limitations on the types of equipment to be diagnosed. From all the machine and equipment types nowadays most fully are solved the diagnostic problems for rotating machines the methods of monitoring and diagnostics of which are discussed below.

The main method of vibration monitoring is the observation of machine vibration energetic parameters and first of all the power (level) of separate vibration components. The peculiarities of any approach to solve the monitoring problems are defined by three main factors. The first one - the selection of the points where to measure the vibration, the second - the number of separated components, and the third one - the intervals between measurements.

Nowadays the machine monitoring is developing in two main directions. In the frame of one of it the problem to minimize the number of vibration control points and intervals between the measurements is solved first of all by using on-line continuous monitoring systems. In the frame of other direction the intervals between the measurements are maximized by controlling the vibration in many points including the points on each unit of equipment that is a source of vibration. The increase of the intervals enables to use portable hardware for vibration measurements and analysis.

In both cases the monitoring efficiency depends on the number of vibration components available for measurement. To increase their number for machines with rotating parts a narrow band vibration analysis is conducted. It is most efficient for analysis of periodical components of the signal. As an example on Fig.1 are shown the spectra of machine (gear box) vibration when there is no defect and after appearing of a defects of gear wheels, that is accomplished by the increase of the level (power) of many vibration spectrum components.

Fig.1. Vibration spectra of the bearing unit in the gear box.
a) - a gear box without any defect; b) - a gear box with a defect of a gearing wheel.
frot - rotating frequency or the gear box shaft; fz - teeth frequency.

Vibration machine condition monitoring in this case consists of comparing the levels of separate components with the levels of the components shown on fig.2 and analysis of changes in the levels of separate components during the time of operation.

Fig.2. Machine vibration condition monitoring:
a) - low vibration signal level; b) - the level of medium vibration signal; c) the level of severe vibration signal.

In practical machine diagnostics by vibration there exist two main approaches in solving the diagnostic problems. In the first approach the diagnostics is conducted only after detection of the changes in the machine vibration condition by means of monitoring and its objective is to interpret these changes. But as a rule the monitoring is conducted by low and middle frequency vibration that responds only on appearance of mainly developed defects.

Just deeply developed defects lead to perceptible changes in the energetic parameters of vibration that exceed their natural fluctuations as a result of mode operation change.

The second approach consists of using the methods and means of diagnostics that detect the main defects types on their incipient stage before the essential energetic changes in machine vibration signal are produced. The detection of defects on their incipient stage enables to watch their development and plan the machine maintenance and repairmen. Such approach often is named as defect monitoring and it can be conducted during several years up to the time when the combination of defects by its development and number will not lead to an almost dangerous situation. Naturally that such approach is possible if only all the potentially dangerous defects can be detected and identified on an early stage of their development. For many years there was no such a possibility. But as a result of intensive development of methods and technical means of diagnostics and first of all by the vibration signal nowadays the defect monitoring systems become real and begin to spread widely.

To detect incipient defects in the rotating units the natural diagnostic symptoms are used. They are defined by spectral analysis of the vibration signal itself or the oscillation of power of its components (spectral analysis of its envelope).

For example the main symptom of a bearing defect is the change of the friction forces features and the high frequency vibration that they excite. These changes, consisting either of micro-shocks appearance or of periodical changes of friction coefficient in the defective friction contact surfaces. They are detected very simply by the spectral analysis of the envelope of high frequency vibration components of the bearing shield excited by friction forces. (See Fig. 3.)

Fig.3. Envelope spectrum of bearing vibration without defect (a) and with a cavity on inner race (b).
frot - rotating frequency of the inner race, fi - BPFI

When some defects in mechanical transmissions are detected, in particular, gearing, belt transmission, flexible couplings and others, the main attention is paid to such defect symptoms as the appearance of dynamic loads, applied on the transmission elements, for example on bearings. These loads lead firstly to the increase of certain vibration components, and secondly, to the fluctuations of friction forces in the bearings and hence to the oscillations of power of the high frequency vibration. So by the combination of measurement results of vibration spectra and envelope spectra it is possible to detect and identify the incipient defects of the mechanical transmissions.

The main diagnostic symptoms of defects in flow creating units of rotation types such as impellers of centrifugal pumps and compressors and blade wheels of turbines are similar to the defect symptoms of mechanical transmissions and bearings. They are the result of changes in the turbulent flow features in the zone of the distributor and wheels. The pulsation of pressure there, firstly, excite high frequency vibration of the case and, secondly, create dynamic loads applied to the bearings. So the defects of the systems that create flows are detected by collateral analysis of the machine vibration spectra and the envelope of the high frequency vibration of the case and bearing shields.

The friction forces in the bearings and the pulsation of pressure in the liquid and gas flows significantly change their properties as a result of many defects appearance but not all types of defects in the rotating machines. For example, some defects in electric machines change the parameters of magnetic field in the air gap and additional forces and pulsating moments begin to apply to machine rotor. As in machines without defects there are no pulsating moments, particularly they and excited by them vibration are used very efficient to detect incipient defects.

The spectral analysis of the electric machine vibration on low and middle frequencies measured in different directions enables to identify practically all defects of the electric machine magnetic system except isolation aging as up to the moment of the insulation breakdown the magnetic field and the machine vibration do not change.

One more feature of vibration is used for rotor defects detection especially for high speed rotating machines that is sensitive to the changes in value and structure of centrifugal forces. Depending on the defect type the low frequency vibration increases and its spectral composition changes. This feature is used to identify the defect type.

So the vibration signal enables to detect practically all types of incipient defects in the rotating machinery without attracting other physical processes for diagnostics. But this means that for to maintain and repair the equipment according to its real condition the portable vibration diagnostic systems can be used. They do not require to maintain fixed transducers in the equipment and its units.

The Structure of Monitoring and Diagnostics Systems

The new generation of rotating machinery monitoring and diagnostic systems differ from the current ones by three main features.

Firstly - they use for detailed diagnostics only the vibration signal without attracting such machine parameters as temperature, pressure, spectral composition of lubrication oil and other features. It is the result of the fact that, though the expenses for the measurements and analysis of these parameters are high, their results mainly only duplicate the information received by the vibration analysis.

Secondly - for detailed diagnostics they use the measurements and analysis of the high frequency vibration of the main machine units which changes in the most degree its features with the appearance of defects in their incipient development stage.

The third - the systems make automatic diagnostics and identification of defects after each vibration measurement. Without this it is impossible to transfer from the vibration monitoring to the monitoring of defects.

So the main changes in the new systems concern only the detailed machine diagnostics. And this ensures the natural way of development of monitoring systems into the diagnostic ones. The current systems of rotating machinery condition monitoring that use the transducers of vibration and other physical values can simply be added with new means of detailed diagnostics of machines and their units.

Figure 4 shows a simplest structure of a portable system for detailed diagnostics of rotating machinery. It consists of vibration sensor, an instrument for vibration signal analysis, a computer and software for diagnostics of machine units. For the machines whose rotating speed during measurements is unknown with acceptable accuracy besides the vibration sensor a tacho measurement instrument is also used.

Fig.4. A simplest portable system for detailed diagnostics of rotating machinery
a) data collector - analyzer DC-11; b) a PC - notebook type; c) an interface cable for connecting the DC-11 with the computer; d) vibration transducer.

The main requirement concerning the vibration sensor is its possibility to measure both the high frequency and low frequency components of vibration acceleration. The instrument for vibration analysis has to ensure the narrow band spectral analysis both the vibration signal and the envelope of its high frequency components preliminary extracted from the signal by a band pass filter. This instrument can be made as a separate instrument, as it is shown on the picture or as a circuit board for installation into the personal computer.

The systems for detailed machine diagnostics by vibration can be produced for machine repair determination, for its final test after production or maintenance, for machinery condition monitoring during operation. They differ from each other only by the peculiarities of the software. For example, the machines after maintenance and assemblage in the field have not enough run in period, so for their diagnostics is used mainly low frequency and middle frequency vibration and the pattern of a non-failure machine (machine unit) is automatically structured by an array of similar machines. For machine repair determination are fully used the diagnostic symptoms of defects, that are contained in the high frequency vibration, and the pattern is structured also using an array of machines. In operation conditions, in contradiction to the repair determination case, the patterns are constructed by the first three measurements and for the following measurements they are automatically adapted.

A simplest system for rotating machinery detailed diagnostics can easily fulfill the functions of a monitoring system. It is necessary only to utilize additionally the software for monitoring which will plan the measurements, save the measurement results in the data base, compare them with the threshold values, set up automatically or manually by the user, and watch the changes in the vibration signal or in its spectral components.

This simplest system can be also used for machine balancing in field. It has to be only added by a balancing software that calculates the balancing weights and detects the operator's mistakes and the defects that restrict the efficiency of balancing.

The described system of detailed diagnostics can be widened up to on-line monitoring and diagnostic system. To do so, besides the mentioned earlier monitoring program, the system has to be complemented with a group of vibration sensors and electronic multiplexer (between the sensors and the vibration signal analyzer), with an automatic control system, that ensures planning and automatic control of measurements and an alarm system as a termination block (fig.5).

Fig.5. On-line system for rotating machine monitoring and detailed diagnostics.
a) Computer with software, b) Data collector analyzer DC-11 controlled by computer with 8 channel multiplexer and tacho sensor, c) interface cable to control and transfer the data, c) tacho sensor, d) vibration transducer.

In such system the number of sensors fixed on the machine or group of machines can be minimum but after the alarm system gives its signal the operator must conduct with a portable part of the system additional vibration measurements in the points where it is necessary to analyze the vibration signal for machine or its units detailed diagnostics.

The Peculiarity of Vibration Diagnostics in Metallurgical Industry

The means and methods of machine vibration monitoring are practically the same in all the industries and ensure similar results. The specific peculiarities for different industries have only the methods of detailed diagnostics.

In metallurgical industry the first problem is concerned with a large number of very heavy loaded low speed rolling element bearings. The traditional diagnostic methods by appearance of shock pulses for such bearings very often do not work because even in nondefective bearings on low speeds there appear many brakes of lubrication film and the presence of defects does not change its features significantly. The only method of detecting the incipient defects in such bearings is the spectral analysis of the envelope of high frequency vibration (see fig.3). Just this method is used in up-to-date systems for detailed diagnostics.

The second problem also concerns the rolling element bearing diagnostics when for to receive reliable results it is necessary to measure the vibration during several dozens of revolutions with constant load and rotating frequency. On the rolling mills the load and rotating frequency, for example of the rolls, is constant only during several revolutions which is insufficient for reliable bearing diagnostics. To solve this problem in diagnostic systems are used several accumulated vibration time runs when the measurements are made simultaneously with the measurements of rotating frequency and load.

One more problem can appear in diagnostics of low speed gears of multi gear gearboxes where the rotating frequencies of the driving and driven gears can differ from each other more than in ten times. In this case on the housings of practically all the bearing shields the vibration is defined mainly by the forces that are acting in the most high speed gearing. To diagnose low speed gearing if is necessary to measure only local high frequency vibration in the frequency band higher than 10 kHz which by its amplitude is thousand times weaker than the low frequency vibration. To do so it is necessary to use analyzing instruments with high linearity and dynamic range, exceeding 70-80 dB. Such instruments is produced only by few manufacturers.

And finally in metallurgical industry very often is used a powerful, regulated by frequency, electric driver with direct current machines where the regulation is made by static voltage converters. This voltage contains many alternating components that create pulsating moments which excite significant vibration of the direct current machine. To detect on the background of this vibration the vibration that is the result of defects in the machine magnetic system, it is necessary to have the information about the spectral composition of voltage during the diagnostics and automatically deduct from the vibration spectrum the components that are exited the power mains supply.

The means and methods of machine vibration diagnostics with consideration of the metallurgical industry specifics were worked out by the Russian and American specialists while designing the new generation systems for monitoring and diagnostics during five years. In particularly, in Russia such work was conducted for West-Siberian metallurgical plant where about twenty portable systems and several on-line systems work beginning from 1994. The first portable diagnostic systems of the new generation that take into account the specific conditions of the metallurgical industry were implemented in 1997-1998 on the Severstal complex in Cherepovets. According to the information of the diagnostic service department of this complex each system ensures the years economic effect more than 100 thousand USD that is several times more than the price of the system.

Summary

New generation of the rotating machinery monitoring and detailed diagnostics systems by vibration enables to transfer in the nearest future the maintenance and repair of the main and auxiliary equipment in the metallurgical industry according to its real condition.

It does not require to install the expensive on-line systems of continuous monitoring on each type of equipment. It is sufficient to use portable monitoring and detailed diagnostic systems that enable to detect the defects of practically all types on the incipient stage of their development many months before an incident and to plan in time the time and volume of the equipment maintenance.

It is possible to begin the implementation of the new generation systems at any time of equipment operation. To operate this systems, that make the diagnostics and condition prediction automatically, can a person without special education in diagnostics, just after several days of training in the system operation.

Bibliography

A.V. Barkov, N.A. Barkova, A.Yu. Azovtsev "Monitoring and Diagnostics of Rotating Machines by Vibration"
VAST, Inc., Saint Petersburg, 1997

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