A. Azovtsev, A. Barkov
Vibro Acoustical Systems and Technologies (VAST, Inc.),
22, Rosenshteina, St. Petersburg, 198095, Russia
It is well known that much of modern technology is derived from projects ordered by the military industry of the developed countries. The deepest research of machines and equipment vibration is done initially in countries with strong naval forces which have important needs to minimize noise and vibration. A number of naval research centers that deal with using and developing analytical tools for vibration measurement and reduction exist in Russia. These centers are equipped with modern instrumentation for measurement and analysis of signals ,mostly using instruments produced by Bruel & Kjaer (Denmark). A number of highly qualified experts work in these centers and prepare specialists for navy and shipbuilding industry.
In the early 1990s, the number of orders from Russian military customers decreased due to the changes in political situation, but, in exchange, the most qualified experts got the possibility to apply their knowledge in the civil industries. The greatest advances were made in the condition diagnostics of machines using vibration. VibroAcoustical Systems and Technologies (VAST, Inc.) became the leader in this field above the other research centers. VAST is located in St. Petersburg and pools a number of leading experts in vibration diagnostics with many year field experience on the naval ships. During a short period of time, they developed systems for condition diagnostics of machines using vibration that currently are successfully used in a number of civil industries including energy, nuclear power plants, paper and pulp, metallurgy, transport including aviation and railways.
The specifics of Russian economy restructurization had a significant influence on the development on condition diagnostics using vibration. One of the main problems in Russia is a great shortage of specialists who can efficiently use condition monitoring and diagnostics systems, including those with expert systems that are supplied by the leading Western companies. To prepare these specialists would take too much time and require significant changes in the work of the research centers. This fact has defined the main peculiarities of the vibration diagnostics in Russia. The strategy of VAST, Inc. is to develop systems for condition diagnostics that can be operated by a user with no special training in diagnostics or vibration analysis. It was decided not to initially develop common expert systems, but to start with higher level systems for automatic condition diagnostics by the application software. The first automatic diagnostics systems were developed in 1992 for the paper and pulp and energy industries. The main stages and achievements in the development of automatic condition diagnostics systems are analyzed in this paper.
The changes associated with the introduction of market relations in the number of industries in Russia were started at the end of the 1980s. Plans for restructuring the economy included plans to decrease production costs. The experts in condition diagnostics presented their own calculations for improving the efficiency of production processes based on three main assumptions:
The following reasons that limit the use of modern condition monitoring and diagnostics systems of American and West-European production in Russia.
This analysis of the current situation led to a recommendation to speed up the development of the methods for mass condition diagnostics that can be carried out by a user with no special training in condition diagnostics. These methods were developed over a period of several years based on the analysis of machine vibrations in the shipbuilding industry and the resultant extraction of diagnostic information signals. The methods developed were intended to be used for condition diagnostics of supplementary machines and equipment the maintenance of which was not governed by the existing standards and regulations.
This analysis was taken into account by VAST, Inc. At the same time, a number of problems became evident during the detailed market research stage:
The prognosis on the condition diagnostics development in Russia that was made in 1990 for the nearest five to ten years has come true in general, though some interesting tendencies may be observed now. One of them is the prospective increase of sales for the top level diagnostic systems of Western production caused by the increase in price for Russian made diagnostic systems. Currently, the Russian systems are only two to three times cheaper than the Western ones and the prices continue to grow. Another reason is the appearance of the instruments capable of making all the types of signal analysis required for automatic condition diagnostics in the product lines of the main Western companies. Among these companies, Bruel & Kjaer and Diagnostic Instruments (Great Britain)are very popular in Russia
Another notable factor is the increasing exposure of Russian customers to expensive on-line condition monitoring systems produced by the leading Western companies. Such systems are supplied to the environmentally dangerous enterprises, e.g. nuclear power plants, within a international programs of technical assistance to Russia. The most advantageous solution has turned out to be the supplement of such systems with the portable diagnostic systems of Russian companies. As a result, these companies get involved in the maintenance of the on-line systems that guaranties their efficient exploitation and training of personal.
The third tendency is the joint supply, together with the Russian companies, of the diagnostic systems for the new Western machinery and equipment for the Russian plants. In this case, both the price and the structure of the diagnostic system can be optimized to fulfill the requirements of the customers. This tendency grows with the increase of Western investments in Russia as the investors acquire more understanding of the peculiarities of the situations in Russian industry. Just for condition diagnostic systems supplied for the Russian customers, our experts together with the American specialists of the Inteltech Enterprises, Inc., have developed application software and modules for vibration signal analysis, condition monitoring and diagnostics oriented to measurement instruments that incorporate personal computers with a standard operating system. This software and its modules are available and can be applied elsewhere as well.
Taking into account the modern level of vibroacoustical knowledge and the situation in the industry of the ex-USSR in the beginning of the 1990s, the experts of VAST, Inc. stated a number of problems having solutions which would allow the introduction of condition diagnostics in Russia. Among them were:
The solution of the stated tasks revealed a number of difficulties. The first one is the absence of the instruments for the analysis of envelope spectrum of the high frequency vibration selected by a band pass filter. This kind of analysis contains the most explicit information about the condition of the rotating machines' units with friction elements, e.g. bearings or impellers. Besides the existing diagnostic algorithms based on the analysis of the envelope spectra of the high frequency vibration could be easily automated.
The only instruments available in the beginning of 1990s capable of envelope spectra analysis were instruments made by Bruel & Kjaer with an accessory hardware envelope detector. That's why the first developments of VAST, Inc. were based on the instrumentation of this company. Later, instruments made by Russian companies and one more European company, Diagnostic Instruments, appeared. In addition, we have developed analog to digital converter boards with a digital signal processor for personal computers and accompanying software that can do the required types of signal processing. Starting in 1997 VAST, Inc. has introduced a Russian data collector that answers all the requirements for automatic condition diagnostics and monitoring. Some of the above instruments are presented on fig. 1.
Fig. 1. Data collectors that make all measurements necessary for automatic diagnostics and condition prediction:
DC-11 produced in Russia (top left), Type 2526 by Bruel & Kjaer (top right) and PL36 by Diagnostic Instruments.
The next problem was connected with the peculiarities in the automation of the algorithms for condition diagnostics and prediction. First of all, the diagnostic symptoms for the same defects differ with change of the machine operating modes. To take this fact into account, significant changes were introduced in the whole process of diagnostics so that the machine operating mode could be determined by the analysis of vibration during the first stage of diagnostics. The second difficulty was due to the fact that typically autospectra contain not only the components emitted by the vibration of the unit under control, but also by different units of this machine or even other machines operating in the same technological process. This was the main reason for making the first automatic systems that make diagnostics using the analysis of envelope spectra of high frequency vibration. These spectra typically contain only components emitted by the unit under control. These types of automatic diagnostic systems, in particular the systems for diagnostics and condition prediction of rolling element bearings by a single vibration measurement, were widely introduced in industry starting from 1992. Only in 1996 were automatic diagnostic systems for rotating machines based on the automatic analysis of autospectra developed. By this time, the problems of automatic detection of the components emitted by the diagnosed unit in the autospectra, detection of the machine operating parameters (operating mode) and identification of the defect types were successfully solved.
The developments carried out by VAST, Inc. resulted in the appearance of automatic portable systems for the rotating machinery diagnostics. Such systems were pioneered in Russia and are being successfully used in a number of enterprises in different industries. They have the same structure as the analogous condition monitoring systems supplied to Russia by CSI, Entek-IRD, SKF and include three main modules:
The systems of VAST, Inc. possess one more component which is:
This type of application software has also been developed for the portable diagnostic system of Bruel & Kjaer. This application software replaces the manuals for diagnostics typically supplied with the condition monitoring software or expert systems developed by some companies.
Table 1 contains the range of prices for the portable condition monitoring and diagnostic systems supplied by both Western and Russian companies. Separately, the prices for the systems supplied by VAST Inc. in Russia are presented. The price deviation is explained by the prices on the measurement instrumentation that can be either A/D boards for personal computers, the DC-11 data collector, or data collectors of other Russian manufacturers that are similar in their functionality and prices.
Table I. Instrumentation and Application Software for Condition Monitoring and Diagnostics.
|System (Part) Description||Price for the Systems of the Established Western Leaders||Price for the Systems of the Russian Vendors|
|Other Companies||VAST, Inc.|
|Data Collectors (Spectrum Analyzers) and other means of measurement and signal analysis||$5,000 - 15,000||$3,000 - 6,000||$3,000 - 5,000|
|Condition monitoring application software||$5,000 - 15,000||$3,000 - 5,000||$2,500 - 3,500|
|Complete system for condition monitoring||$15,000 - 30,000||$6,000 - 10,000||$5,500 - 8,500|
|Application software for condition diagnostics||$5,000 - 15,000
|-||$5,000 - 12,500
(automatic diagnostic systems)
|Complete condition monitoring and diagnostic systems||$15,000 - 40,000
||-||$10,500 - 21,000
The table does not contain instruments and condition monitoring systems based on the shock pulse method. This method has many limitations of both nomenclature of defect types and prediction of the defect development. Consequently, the prices for such systems are significantly (2-3 times) less that the ones presented in the table. So on the first stage of the development of means for condition monitoring and diagnostics in Russia there were solved the main problems that limited the wide use of such systems in the industry. These are:
As a result, two main directions of introduction of portable condition monitoring and diagnostic systems were formed in Russia. The first direction is the use of technical methods of the leading Western manufacturers and their training of qualified personnel to work with such systems. The second way is the use of Russian automatic systems for condition monitoring and diagnostics for the enterprises that do not have qualified experts in diagnostics. Both ways exist in tight competition on the Russian market, thus accelerating the process of condition monitoring introduction in the main enterprises in Russian industry. Condition monitoring and diagnostic systems monitor and predict the development of the defects and allow the minimization of maintenance and repair work in the enterprises.
Experience shows that the frequency of unpredicted failures of the equipment being diagnosed by the automatic diagnostic systems has decreased at least in 10 times. At the same time, maintenance costs, including expenses on the condition monitoring systems and its operation, have decreased about 30%.
Most of the users of the condition monitoring and diagnostics systems have previous experience with portable condition diagnostic systems and many of them are users of the automatic condition diagnostic systems produced by VAST for which they already had a high degree of confidence. During the first months of portable system operations, the high efficiencies and economic benefits of such systems were demonstrated, proving the desirability of extending the use of such systems to other parts of the enterprises. In the earliest stage of portable system use, some of the customers installed permanent transducers on some units of the machines that are not accessible for temporary mounting without shutdown of the equipment and, in many cases, made some provision for electronic selection of the transducers. Then for important machinery, especially those with variable operation modes, e.g. rotation speed, they connected these transducers to a personal computer having a means of signal analysis and software for automatic condition prediction and diagnostics. This was a common way for a customer to obtain an on-line condition monitoring and diagnostic system rather than making an initial investment of $100,000 or more. In Russia, costs for the various steps of this approach might be:
In this manner, the customer can acquire a small, complete system for on-line monitoring and diagnostics for more important or inaccessible equipment and a portable data collector for for ancillary equipment for $50,000 to $60,000 and acquire the system in more economically affordable stages. The customer can then add new measurement points as needed. The turn-key costs for such a system expanded to roughly 300 points is about $180,000 to $300,000.
Analysis of the reasons for a customer to switch from portable to on-line systems led to the following conclusions:
In enterprises where all the machinery works in stable loads and speeds and measurement points are available for temporary mounting of accelerometers without interfering with the operation of the machines, the portable systems are usually preferred by the customers. Portable systems continue to be used along with the on-line systems in enterprises where the on-line systems monitor the condition of the main equipment and machinery and the portable systems are used for condition diagnostics of the supplementary equipment.
As soon as the groups of the permanently mounted transducers could be united in one condition monitoring and diagnostic system, a number of questions were raised.
The main question was how to balance the efficiency of the system vs. the costs of its purchase, installation and maintenance. Three main approaches were considered during the definition of technical specification for the systems to be supplied in different industries in Russia.
Experience in using on-line condition monitoring systems with minimum numbers of permanently mounted transducers and portable diagnostic system revealed a number of problems. One of them occurs in the monitoring of multimode machines when the change of operating mode is taken for the change in the vibration state of the machine and the on-line monitoring system requests measurements by the portable diagnostic system. A possible solution of this problem is rather simple, combining a number of small monitoring systems working on the main equipment in one technological process into one computer net. This allows comparison of the vibration state of several machines and distinguishes between changes in the vibration state of the machine due to defect appearance and changes in the operating mode of the machine.
Another problem is connected with the fact that, to reduce the number of transducers in the condition monitoring systems and to get the information about the less noisy units of the machine, it is necessary to install transducers close to the less noisy units and far away from the most vibration active units. This configuration of the installed transducers results in the loss of high frequency components of the vibration signal with the result that a number of the defects can not be detected before they reach a dangerous stage of their development. In this case, after the defect was detected by the small condition monitoring system, there is left inadequate time for decision making about the maintenance of the machine before possible break down.
The small condition monitoring systems have a number of benefits that makes them most widely used. The most important advantage is relatively low cost. This allows monitoring the vibration state of all of the most important machines and equipment. The second important feature is that such a system is a damage proof, because all the transducers and communication lines are installed far away from the machine parts that require maintenance with partial disassembly. Most of the mechanical damage of transducers and communication lines is done just during this sort of maintenance. The next benefit is the possibility of integrating several monitoring systems into one computer network.
The on-line condition diagnostic and prediction systems are designed in quite a different way and have their own advantages and disadvantages. They differ from all other systems in the fact that transducers are mounted on each unit of the machine that produces noise or vibration or may have a defect that can be dangerous for the machine operation. The second feature is very wide frequency band for noise and vibration measurements which includes the high-frequency domain. This type of the condition monitoring systems provides detection of the defects in the early stage of their development, thus the intervals between vibration measurements may be longer. The system may include only one circuit (device) for signal measurement and analysis and all transducers may be connected to it via a commutation device. The measurements are done in series, but not parallel in this case. One more interesting use of this system is a "Stand System" for the diagnostics of different machines. In this case, the transducers of the on-line condition diagnostic system are temporarily mounted on the machine for the time required for measurements and diagnostics. When another machine comes to the diagnostic stand, these transducers are mounted on another machine. One of applications for such a system is the output or periodic production check-out. This system became widely used in transport for the vehicle's diagnostics on special stands.
Three basic limitations on use of such a system for the monitoring and diagnostics of main equipment on the enterprise should be emphasized. The first one is the high cost of the system with the great number of transducers and communication lines. The second is possible frequent failures of the system due to mechanical damages of transducers and communication lines during maintenance with the machine partial disassembly. To minimize the influence of mechanical damage on the process of condition monitoring, special attention is paid in Russia to the automatic diagnostics of transducer condition and communication lines performance. The third limitation is defined by need for individual adaptation of diagnostic software to the peculiarities of the objects under monitoring and diagnostics. The adaptation is usually carried out by the vendor representative in two steps. First, the algorithm of data collection is adapted. Then the diagnostic algorithms and programs are adapted using the data collected at all operating modes of the object under control.
The configuration of a typical on-line monitoring system for the condition monitoring and diagnostic of machines and equipment is shown in Fig. 2. The number of vibration transducers in similar systems may exceed 500. For example, systems with such large numbers of transducers are used in diagnostics of paper machines. One of the problems we faced in negotiations with the prospective customers was the need to explain to them that vibration vibration transducers are sufficient for reliable condition diagnostics. Other types of transducers just increase the system's cost and often make them unprofitable. Analysis for this is usually done by the diagnostics of the customer equipment by the portable diagnostic system.
Fig. 2. Configuration of a typical on-line condition monitoring and diagnostic system
In this manner, the following problems have been solved at the initial stage of the introduction of the on-line condition monitoring systems in Russia.
Different approaches should be used to introduce and adapt the condition monitoring systems at enterprises in different industries to the various operating conditions for both machines and the condition monitoring systems. But, if they have a common application software which can be adapted to real conditions by a user himself, the cost of the monitoring and diagnostic system may be considerably decreased. The group of specialists of VAST, Inc. has solved the challenges of designing mathematical and application software of this kind.
The application software, intended for use in an integrated system of monitoring, diagnostics and condition prediction of machines and equipment, will consist of five components, each of which is easy to add to increase capability of the system. The component parts are:
The diagram of a combined system for condition monitoring, diagnostics and condition prediction of machines and equipment which the specialists of VAST, Inc. and Inteltech Enterprises, Inc. have recently been designing, is shown in Fig. 3. It can be considered as a group of small monitoring systems combined into one network and a diagnostic center including the portable devices for signal measurement and analysis as well. Our experience in introduction of the functional diagnostics of the machines and equipment on the enterprises of different industries in Russia shows that it is worth starting with small condition monitoring and diagnostic systems for the main equipment, which then can be united in one network.
Fig. 3. Configuration of an on-line monitoring and diagnostic system that consists of a network of independent small condition monitoring systems and one diagnostic station.
The small system may contain up to 12 to 16 vibration transducers, providing the condition monitoring and diagnostics of a standard machine with 6 to 7 bearing units. One third of the transducers is used for monitoring the condition of the machine as a whole and others for monitoring and diagnostics of the parts with the highest risk of producing breakdowns, most importantly, bearings. The large on-line condition monitoring system may consist of 20 to 40 small systems. Each small system may have the software of its own for condition monitoring, but it is better to have only one application software package for diagnostics for the whole large system.
Recently the beta versions of the main programs, made for MS Windows-95 and Windows-NT environment, are being tested in a number of enterprises in Russia. The complete software package may be offered to the users in the middle of 1997. Today, the finished modules for the condition monitoring, diagnostics and condition prediction of the machines and equipment can be used in different applications.
Components of rotating machines that are sources of steady vibration can be completely diagnosed by the vibration signal. These are:
Vibration diagnostics cannot provide complete results for the machines with reciprocating parts, for example, internal combustion engines, piston pumps or compressors. This is caused by the presence of strong impact loads while inertial elements such as pistons and valves change the direction of their motion. The vibration levels generated by these parts are so high that it is impossible to measure the vibration of the other parts, for example, background vibrations of the bearings. As a result, the analysis of vibration in the reciprocating machines may reveal only the information about reciprocating mechanisms and their defects, but it is practically impossible to detect incipient defects of the other parts, in particular the bearings.
On the other hand, nearly all types of defects of any machines, at least shortly before a breakdown situation, have a considerable influence on the vibration of either the defective units or the machine as a whole. Therefore, condition monitoring systems are efficient and widely used in many countries for breakdowns prediction of various objects. If a monitoring system doesn't involve the means of deep diagnostics of breakdown prone units and machines, i.e. those where not all possible defects are detected on the early stage of their development, such a system must provide periodic vibration measurements in short intervals, from seconds to hours. In this situation, it is obvious that the monitoring system must be the on-line one.
A portable diagnostic system must solve another problems, namely, to detect dangerous defects at any development stage and make it possible to observe their development and passage from one stage to another. The use of such a system may significantly extend the intervals between measurements up to several days or even several months. But, the number of measurement points at each of the machine under diagnostics may increase in comparison with on-line system, since for deep diagnostics of any machine unit, it is necessary to control vibration on the housing of this very unit.
For machines with no impact loads, practically all types of defects can be detected in the early stage of their development using the analysis of vibration signals. In the following paragraphs, the main units of rotating machines are described together with possible defects can influence the service life of the machines.
Bearings are the main units that cause the breakdowns of the machines. For them, it is possible to detect all the defect types of both their assembly and operation during the first hours of machine operation. Installation and assembly defects that decrease the service life are:
The defects of each friction surface or lubrication can be considered as defects of the bearing itself or wear defects (defects developed with bearing operation). Two different defect groups can be defined for each of the friction surfaces. The first is rather even wear of friction surface, the second is flaking, spalling and cracks on it. Only cage defects in a rolling element bearing cannot be divided into groups by means of vibration diagnostics. The wear defects in rolling element bearing include:
The number of detected defects in the journal bearings is smaller with the lower number of friction surfaces in this type of bearing. Three installation defects that influence the service life can be named which are:
During operation, two defects can appear which are bearing wear and lubrication defects. They manifest themselves not only directly, but also as a self-sustained shaft vibration in bearings and as impacts between outer and inner races. The total number of detected defects in journal bearings is six, i. e. all the defect types considered above.
In high speed machines such as fans, turbines, pumps, compressors, the defects of impellers and deformation of gas or liquid flow caused by them may significantly reduce the service life of the machine. Among the installation (assembly) defects :
During machine operation, all the above defects can appear and also other defects as well which are:
All of the above defects can be detected on the early stage of their development by the analysis of vibration of the housing of the machine or the flow noise.
The service life of machines with the gearboxes is mainly determined by the conditions of the gears and other components of the gear assemblies. In these assemblies, all types of the bearings defects and additionally three type of the gearing defects that may significantly decrease the service life can be detected. These are:
The first two defect types can appear during both gearbox assembly and operation. The third defect type includes not only the uniform wear, but spalls, cracks on gear teeth and also absence of some teeth or their parts. This usually appears during the gearbox operation.
All the above defects are detected by vibration of either the entire gearbox or of bearing shields. The peculiarities of geared couplings diagnostics by the analysis of their vibration do not allow the division of defect types into several groups. Therefore, for the gear couplings all defect types, detected by vibration of the machines or bearing shields are gathered in one group.
Gearboxes and gear couplings are just two types of mechanic transmissions that can be diagnosed by the analysis of noise and vibration signals. Other transmissions that can be successfully diagnosed are worm gearing, belt and others, as well as the transmission devices without mechanic contact including magnetic or hydrodynamic couplings.
During recent years, methods for the diagnostics by noise and vibration signals of the electromagnetic systems, including electric machines, were developed. Unlike diagnostic methods based on current or electromagnetic fields measurements, vibration and acoustic methods make possible the diagnosis of machines using transducers mounted on the machine housing without the need to stop the machine. At the same time, all the diagnostic information and accuracy remains the same or even increases compared to current measurement techniques. Defects of both installation (assembly) and operation that can be referred to as defects of electromagnetic system of the AC-machine and can be detected and identified by vibration are
Similar defects can be detected for DC machines together with the defects of the slip ring and brush parts.
Note that disorders in the power supplies are included in the defect list. In some cases, the mains supply distortion can be so great, in particular if machines are supplied by static electric converters, that the diagnostics of electromagnetic system of the machine may be very difficult using both vibration and current (electromagnetic field) methods, and at very strong distortions, may be even impossible.
The list of the unit type and the defects detected by vibration analysis shows the high efficiency of detailed diagnostics of machines machinery by noise and vibration. Note that these diagnostics cannot be expressed quantitatively because the defect value is not expressed in particular measurement units, but, instead, must be described by in the type of condition. The typical number of defect conditions, categorized by the defect severity, is three:
Sometimes it is necessary to introduce one more defect condition, dangerous, the presence of which requires an efficient control of its development up to machine shut-down for maintenance.
When the first medium defect appears in the machine unit, it is possible to predict its development, and thus the residual service life of the machine. Before this, it is possible to define a non-failure operation time for a machine. It is a minimum interval from defect appearance up to its breakdown/dangerous stage of development. Such an interval can be typically estimated as a 10-20% of an average service life of the part under diagnostics, i.e. over several months.
Experience shows that practical efficiency of the diagnostics and condition prediction of the rotating machines by vibration is so high that it doesn't require any measurements of the other physical parameters of the machine. For some units, it may be supplemented with noise measurements.
This brief analysis of vibration diagnostics in Russia and perspectives of its development enables the following conclusions.