- Email: [email protected]
Monitoring and testing of high power industrial fans vibration
Available Available online online at at www.sciencedirect.com www.sciencedirect.com
ScienceDirect
Available online at www.sciencedirect.com Procedia Procedia Engineering Engineering 00 00 (2017) (2017) 000–000 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 199 (2017) 2190–2195
X International Conference on Structural Dynamics, EURODYN 2017
Monitoring and testing of high power industrial fans vibration E. Rusinskiaa J. Czmochowskiaa, P. Moczkoaa*, D. Pietrusiakaa aa
Wroclaw Wroclaw University University of of Science Science and and Technology, Technology, Faculty Faculty of of Mechanical Mechanical Engineering, Engineering, Department Department of of Machine Machine Design Design and and Research Research Wybrzeże Wybrzeże Wyspiańskiego Wyspiańskiego 27, 27, Wroclaw Wroclaw 50-370, 50-370, Poland Poland
Abstract Abstract In In many many industries, industries, there there are are ventilation ventilation systems systems equipped equipped with with large large scale scale industrial industrial fans fans of of high high power power up up to to or or even even higher higher than than 33 MW. MW. These These are are both both axial axial and and radial, radial, which which in in most most cases cases play play aa key key role role in in the the production production process process like like in in underground underground mines mines ventilation ventilation systems systems or or power power blocks blocks of of power power plants. plants. Operation Operation of of this this type type suffers suffers of of many many vibration vibration problems, problems, resulting resulting from from the the large-scale large-scale (large (large rotating rotating masses) masses) as as well well as as the the complex complex flow flow phenomena. phenomena. Emerging Emerging negative negative phenomena, phenomena, mainly mainly in in the the form form of of excessive excessive vibration, vibration, they they are are often often difficult difficult to to identify identify and and eliminate. eliminate. The The paper paper presents presents examples examples of of such such events events present present in in the the operation operation of of large large scale scale fans fans along along with with numerical numerical and and experimental experimental methods methods for for their their identification identification and and subsequent subsequent elimination. elimination. © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © 2017 The Authors. Published by Elsevier Ltd. committee of EURODYN 2017. Peer-review under responsibility of the organizing Peer-review under responsibility of the organizing committee of EURODYN 2017. Peer-review under responsibility of the organizing committee of EURODYN 2017. Keywords: Keywords: condition condition monitoring; monitoring; industrial industrial fans; fans;
1. 1. Introduction Introduction Industrial Industrial fans fans operating operating in in various various flow flow systems systems are are in in most most cases cases core core plant plant equipment. equipment. Therefore Therefore they they should should be properly maintained and monitored. The most common way to assess condition is the use of be properly maintained and monitored. The most common way to assess condition is the use of vibrations vibrations measurements. measurements. There There are are many many different different approaches approaches to to do do such such tests tests varying varying mostly mostly on on the the maintenance maintenance approach approach applied. Vibration phenomena is relatively well recognized and its application provides many applied. Vibration phenomena is relatively well recognized and its application provides many advantages advantages like like faults faults recognitions, recognitions, problems problems identifications identifications etc. etc. However However we we can can observe observe many many unexpected unexpected and and often often unpredicted unpredicted problems problems in in industrial industrial fans fans operation, operation, which which can can lead lead to to long long and and expensive expensive repair repair stoppages stoppages or or even even catastrophic catastrophic failures, which are mostly caused by improper or lack of condition monitoring methods, operational failures, which are mostly caused by improper or lack of condition monitoring methods, operational faults, faults,
** Corresponding Corresponding author. author. Tel.: Tel.: +48 +48 71 71 320 320 4097; 4097; fax: fax: +48 +48 71 71 320 320 4097. 4097. E-mail E-mail address: address: [email protected] [email protected] 1877-7058 1877-7058 © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Peer-review Peer-review under under responsibility responsibility of of the the organizing organizing committee committee of of EURODYN EURODYN 2017. 2017.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017. 10.1016/j.proeng.2017.09.181
2
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Rusinski E. Czmochowski J., Moczko P. Pietrusiak D./ Procedia Engineering 00 (2017) 000–000
2191
especially present in case of large scale high power industrial fans. This is due to large-inertia effects (large rotating masses) as well as the complex flow phenomena present in steady but also unsteady conditions. Emerging negative phenomena, mainly in the form of excessive vibration, they are often difficult to identify and eliminate. In the paper examples of such events present in the operation of large scale fans along with numerical and experimental methods for their identification and subsequent elimination are presented. 2. Vibrations phenomena in the fans operation Vibrations occur in all cases of rotary machines operation. They are present in steady and unsteady states conditions [1] and might be caused by mechanical or inertia parameter such as unbalanced mass rotation, gear meshing or motor influence in the drive unit [2][3] . In the case of steady state conditions, vibrations occur during the nominal operation of the machine and, if the technical condition of the machine is satisfactory (proper design, good quality of manufacturing and installation, balancing parameters within acceptable limits), do not constitute a significant operational problem. The other type of vibrations occurs during the start-up and shut-down of a machine. In such a case, passing through resonant frequency may occur. The larger the machine the greater problems may be present due to lower self-frequencies and potential risk of high vibrations during slow start up or shutting down. Example of the start-up of the 2.5 MW radial fan is presented in figure 1 below, where axial vibrations on the main shaft bearing housing were measured. Al phases of the start-up are marked on the graph.
Fig. 1. Start-up of the 2.5 MW radial fan – axial vibrations measured on the main shaft housing [4]
Another reason of vibrations presence is medium flow through the fan. Flow can be source of vibrations or can amplified vibrations caused by other reasons (mechanical as for example). Here there are complex and multiply phenomena. The first is operation of the fan on the stable side of the fan curve. As per theory and standard rules, fans shall operate on the right side of the fan characteristic curve as shown in figure 2. There is additional 10% safety margin required, which is measured from the top of the curve. If operation of the fan is not assured in the safety range, there will be excessive vibrations present due to lack of stabilizing flow. Another factor in fans operation is to assure matching flow parameters of ventilation system with fan characteristic. Significant changes in the ventilation system must be considered in the fan operational parameters by rpm change or regulation flaps adjustment. Usually rpm change is more efficient but expensive frequency inverters or multi speed engine motors are required in such a case. If no action is taken, there is a risk to operate the fan on the left hand side of its characteristic curve, which may lead to excessive vibrations.
2192
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Author name / Procedia Engineering 00 (2017) 000–000
3
Fig. 2. Example of the fan characteristic curve – operating range marked
The next trigger of vibrations can be ventilation system itself, causing flow fluctuations in a certain frequency. Usually low frequency of flow fluctuation is not dangerous, but as soon as it matches with own natural frequency of the fan or with rpm frequency or blade frequency, it might cause high fan vibrations and its failure. Natural source of vibrations, which is related to flow and fan design is pulsation effect coming from finite number of blades. The more blades the less flow pulsation and vibrations as an effect are present. The characteristic blade frequency is always present in the vibrations spectrum graphs. Its influence on the overall vibrations (RMS) is usually low but must be considered in the design to prevent possible matching with natural frequency of the fan. Very important action on the designing stage of rotating machine is specifying the modal properties such as normal modes frequencies and shapes and assuring them to be out of characteristic frequencies of the fan. As already mentioned the example of such a load, in terms of fans, is the rotational frequency of the fan rotor and blade frequency as well. The standards concerning individual types of devices define ranges, in which there should be no natural frequency. If they are not met, the object can be prone to resonance vibrations, possibly leading to failures or even catastrophes [5][6][7][8][9]. The usual effects of vibrations are cracks of fan’s rotor. Examples of such cases are shown in figures below.
Fig. 3. Examples of fans failures caused by vibrations – cracks of rotors
Apart of vibrations influence on fans integrity it is also important to assure high efficiency of fans operation. High efficiency means lower power consumption, less pollution and thus lower overall costs of operation. Fans should be designed to operate in optimum point of characteristic and in such conditions shall not suffer any excessive vibrations related problems. However in complex ventilations systems or in case of change of flow parameters this requirement may not be fulfilled. In such a case fans operators want to assure operation with acceptable vibrations and as second priority consider efficiency. This approach is correct of course due to high costs
4
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Rusinski E. Czmochowski J., Moczko P. Pietrusiak D./ Procedia Engineering 00 (2017) 000–000
2193
of fans stoppages and repairs. As already mentioned, in such a case the optimum solution is to have efficient regulations systems to adjust ventilator characteristic curve to flow condition and obtain high efficiency with low vibrations. The resonance vibration or in general excessive vibration phenomenon is exceptionally dangerous to large scale rotating machines such as high power industrial fans. It is for that reason why testing whether the machine operates in such unfavorable conditions is crucial. 3. Condition assessment Measuring methods in fans operations are strongly related with the maintenance approach taken into account. There are the following approaches commonly used: • corrective maintenance - operation until failure occurs, • preventive maintenance - condition-based operation , • predictive maintenance - operation which prevents and correct failures. The first two operation strategies are the most popular. Considering that these machines are crucial to the production process, the first strategy is obviously not efficient enough. It often leads to unplanned downtime which significantly reduces production efficiency. Preventive maintenance may be divided in to two sub-categories such as operation with regularly scheduled overhauls and operation based on the condition. The second relatively popular approach, which is present in industrial fans operation, requires preferably continuous monitoring of the condition of the structure under test. If symptoms of damage are detected, an immediate analysis of the situation follows and decisions are made to perform corrective actions. This method helps to detect failures early, but its efficiency is strictly determined by the quality of condition assessment. This pertains to the assessment methodology, the implemented monitoring techniques, computational methods and indicators of the process of degradation of the fan. Early detection of faults helps to reduce the costs of failure repair and decrease machine downtime. The predictive maintenance provides a significant upgrade to machine operation but is still not as popular as preventive maintenance. The fundamental difference between preventive and predictive approach, is that the latter approach can predict the occurrence of failure. Based on such information, users can take preventive steps in advance to postpone, or eliminate entirely, the expected failure. However, the predictive maintenance strategy requires the application of the most advanced methods of condition monitoring. Therefore this method is still not commonly used. As already mentioned preventive maintenance approach requires continues monitoring of fans vibrations and it is the most commonly present approach in high power industrial fans operation. Periodic measurements might not be efficient enough to discover change in vibrations or other measured parameters of fan at the early stage to prevent serious failure. The most popular measuring technic used in condition monitoring of fans is to measure vibrations in location of main shaft bearing housing. The most useful signals related to fan rotor are obtained from the bearing on the rotor side in horizontal or vertical direction. Axial vibrations measurements shall be taken from the axial bearing location. The most common parameter in fans condition monitoring is velocity of vibrations in mm/s. Standard accelerometers in above mentioned locations are used for such measurements. Their range as well as sensitivity must be properly adopted to technical parameters of the fan such as rotational and blade velocity. Such signals enable to monitor condition of the fan rotor, shaft and roller bearing as well. Monitoring of plain bearings of main shafts is more complex and expensive because we have to perform orbit analysis [10]. This type of measurements is still not commonly used in industrial fans operation. Therefore plain bearings condition is usually monitored with the use of their temperature measurements and sometimes with the oil tests (presence of metal particles etc.) Another issue is to use proper sampling frequency to be able to detect failures symptoms at the early stage. The required sampling frequency is strictly related to the frequency of the monitored signal. This rate should be set in such a way so as to enable reconstruction of analog signal from discrete data without losing information on the measured parameter (Shannon-Kotielnikov condition must be fulfilled). However, for condition monitoring the minimum sampling rate must be much higher. In the stationary signals obtained from relatively low speed machines such as high power industrial fans (radial type especially) this frequency is 5–10 times higher than Nyquist frequency.
2194
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Author name / Procedia Engineering 00 (2017) 000–000
5
Another method, investigated by authors of this paper, which is still not used for fans condition monitoring is to measure flow on the inlet and/or outlet of the fan and correlate it with vibrations and fault recognition [4]. There are relations between flow and certain directions of vibrations. We can also use vibration of the fan rotor housing for recognition of flow problems (operation on the left hand side characteristic curve for example). Based on performed researches it is clear that data from both vibration and flow signals can be used in parallel to improve condition monitoring of industrial fans. 4. Case studies In case of faults detection in the standard monitoring system, sometimes it is necessary to perform more detailed analyses to identify root causes of problems and made optimum action decisions. Such analyses can be made based on another, more advanced tests results with the use of more advanced methods. In this chapter examples of various experimental technics are presented as a case study of faults recognitions of industrial fans. The first case refers to high power 3MW axial fan vibrations. The fan operates in exhaust system of power block. It is core equipment, which failure may cause entire block emergency stop. During operation of the fan there are periodical vibrations observed in amount exciding alarm threshold limits. There is standard continuous monitoring system present and it bases on vibration measurements of main shaft bearing housings. The amount of vibration exceeds 10 mm/s. Such vibrations may lead to failure of the fan. In order to identify root causes of excessive vibrations few measuring technics and numerical methods were used. They are described shortly below together with obtained results. Scanning measurements were made with the use of 3D laser on the fan inlet/outlet ducts. The main purpose was to identify modes, frequencies and amounts of vibrations for further analyses. Example of results from tests are presented in figure 4. Teste were made during start-up of the fan as well as during its normal operation. The most dominating frequency in all recognized vibration modes is rotational frequency of the fan, which equals fr = 12.5 Hz.
Fig. 4. Results of the 3D vibrations scanning – vibrations of fan outlet duct, VRMS,1 = 5.2 mm/s
Dynamic, modal simulations of the fan inlet/outlet ducts were made on the validated (by experimental tests) models of ducts to enable their future modifications due to discovered resonance problems. Similarly rotating unit (shaft and rotor) was validated with the use of experimental modal analysis (EMA) of the main shaft and rotor blades. Based on the results also here possible resonance problem was found due to overlapping of bending modal mode frequency f1 of the shaft with rotational frequency: f1 = 14,43Hz = 1,16 fr. Additional under pressure tests in the inlet and outlet of the fan, during start-up and normal operation were made in order to investigate if the root cause of the vibrations lie on the flow or mechanical vibrations phenomena. As a conclusion it was confirmed that the root casus of the fan vibrations are related to improper design of the shaft and ducts of the fan. There are normal mode frequencies overlapping the shaft rotational frequency, which results in resonance and beating effects.
6
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Rusinski E. Czmochowski J., Moczko P. Pietrusiak D./ Procedia Engineering 00 (2017) 000–000
2195
Another case study refers to cracks recognition of the fan housing with the use of 3D laser scanning method. As shown in figure below, if there is through crack, the modal parameters differs significantly from the surrounding area, what can be easily detected by scanning method.
Fig. 5. Results of the 3D vibrations scanning with the use of laser scanning method – detection of crack in the fan housing
5. Conclusions Continuous or periodic condition monitoring of industrial fans is carried out using many experimental and numerical methods and techniques. These methods and techniques are used to analyze the obtained information and, consequently, to determine the condition of the machine in the operation. Reliability of the monitoring and subsequently condition assessment process is in this case closely related to the adopted methodology and scope of testing. Considering importance of high power industrial fans as plant’s core equipment, it has a great importance to develop existing and create new methods of their condition monitoring. References [1] Parszewski Z. Drgania i dynamika maszyn. Warszawa: WNT, 1982 [2] Głowacz, A., Diagnostics of DC and Induction Motors Based on the Analysis of Acoustic Signals, MEASUREMENT SCIENCE REVIEW Volume: 14 (2014) Issue: 5 Pages: 257-262 [3] Głowacz, A., Diagnostics of Synchronous Motor Based on Analysis of Acoustic Signals with the use of Line Spectral Frequencies and Knearest Neighbor Classifier, ARCHIVES OF ACOUSTICS Volume: 39 (2014) Issue: 2 Pages: 189-194 [4] Czmochowski J, Moczko P, Odyja s P, Pietrusiak D. Tests of rotary machines vibrations in steady and unsteady states on the basis of large diameter centrifugal fans. Eksploatacja i Niezawodnosc – Maintenance and Reliability 2014; 16 (2): 211–216 [5] Czmochowski, J. 2008. Identyfikacja modeli modalnych maszyn urabiających w górnictwie węgla brunatnego. Oficyna Wydawnicza Politechniki Wrocławskiej [6] Fry P.R. Loads and Stresses—The Real Cause of Failures in Surface Mining Machinery, Practical Failure Analysis 3(2) 2003 [7] Rusinski, E.; Moczko, P.; Pietrusiak, D., Low frequency vibrations of the surface mining machines caused by operational loads and its impact on durability, PROCEEDINGS OF INTERNATIONAL CONFERENCE ON NOISE AND VIBRATION ENGINEERING (ISMA2014) AND INTERNATIONAL CONFERENCE ON UNCERTAINTY IN STRUCTURAL DYNAMICS (USD2014) Pages: 683694 [8] Rusiński E., Moczko P., Pietrusiak D., Przybyłek G., Experimental and numerical investigations of the jaw crusher supporting structure fatigue failure, Strojniški vestnik - Journal of Mechanical Engineering 59 (2013) 9, 556-563, DOI:10.5545/sv-jme.2012.940 [9] Kokociński J., Wibroakustyczna diagnostyka maszyn. Remonty i utrzymanie ruchu 11/2009 [10] Stamboliska Z., Rusiński E., Moczko P., Proactive Condition Monitoring of Low-Speed Machines, Springer International Publishing Switzerland 2015,
ScienceDirect
Available online at www.sciencedirect.com Procedia Procedia Engineering Engineering 00 00 (2017) (2017) 000–000 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 199 (2017) 2190–2195
X International Conference on Structural Dynamics, EURODYN 2017
Monitoring and testing of high power industrial fans vibration E. Rusinskiaa J. Czmochowskiaa, P. Moczkoaa*, D. Pietrusiakaa aa
Wroclaw Wroclaw University University of of Science Science and and Technology, Technology, Faculty Faculty of of Mechanical Mechanical Engineering, Engineering, Department Department of of Machine Machine Design Design and and Research Research Wybrzeże Wybrzeże Wyspiańskiego Wyspiańskiego 27, 27, Wroclaw Wroclaw 50-370, 50-370, Poland Poland
Abstract Abstract In In many many industries, industries, there there are are ventilation ventilation systems systems equipped equipped with with large large scale scale industrial industrial fans fans of of high high power power up up to to or or even even higher higher than than 33 MW. MW. These These are are both both axial axial and and radial, radial, which which in in most most cases cases play play aa key key role role in in the the production production process process like like in in underground underground mines mines ventilation ventilation systems systems or or power power blocks blocks of of power power plants. plants. Operation Operation of of this this type type suffers suffers of of many many vibration vibration problems, problems, resulting resulting from from the the large-scale large-scale (large (large rotating rotating masses) masses) as as well well as as the the complex complex flow flow phenomena. phenomena. Emerging Emerging negative negative phenomena, phenomena, mainly mainly in in the the form form of of excessive excessive vibration, vibration, they they are are often often difficult difficult to to identify identify and and eliminate. eliminate. The The paper paper presents presents examples examples of of such such events events present present in in the the operation operation of of large large scale scale fans fans along along with with numerical numerical and and experimental experimental methods methods for for their their identification identification and and subsequent subsequent elimination. elimination. © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © 2017 The Authors. Published by Elsevier Ltd. committee of EURODYN 2017. Peer-review under responsibility of the organizing Peer-review under responsibility of the organizing committee of EURODYN 2017. Peer-review under responsibility of the organizing committee of EURODYN 2017. Keywords: Keywords: condition condition monitoring; monitoring; industrial industrial fans; fans;
1. 1. Introduction Introduction Industrial Industrial fans fans operating operating in in various various flow flow systems systems are are in in most most cases cases core core plant plant equipment. equipment. Therefore Therefore they they should should be properly maintained and monitored. The most common way to assess condition is the use of be properly maintained and monitored. The most common way to assess condition is the use of vibrations vibrations measurements. measurements. There There are are many many different different approaches approaches to to do do such such tests tests varying varying mostly mostly on on the the maintenance maintenance approach approach applied. Vibration phenomena is relatively well recognized and its application provides many applied. Vibration phenomena is relatively well recognized and its application provides many advantages advantages like like faults faults recognitions, recognitions, problems problems identifications identifications etc. etc. However However we we can can observe observe many many unexpected unexpected and and often often unpredicted unpredicted problems problems in in industrial industrial fans fans operation, operation, which which can can lead lead to to long long and and expensive expensive repair repair stoppages stoppages or or even even catastrophic catastrophic failures, which are mostly caused by improper or lack of condition monitoring methods, operational failures, which are mostly caused by improper or lack of condition monitoring methods, operational faults, faults,
** Corresponding Corresponding author. author. Tel.: Tel.: +48 +48 71 71 320 320 4097; 4097; fax: fax: +48 +48 71 71 320 320 4097. 4097. E-mail E-mail address: address: [email protected] [email protected] 1877-7058 1877-7058 © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Peer-review Peer-review under under responsibility responsibility of of the the organizing organizing committee committee of of EURODYN EURODYN 2017. 2017.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017. 10.1016/j.proeng.2017.09.181
2
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Rusinski E. Czmochowski J., Moczko P. Pietrusiak D./ Procedia Engineering 00 (2017) 000–000
2191
especially present in case of large scale high power industrial fans. This is due to large-inertia effects (large rotating masses) as well as the complex flow phenomena present in steady but also unsteady conditions. Emerging negative phenomena, mainly in the form of excessive vibration, they are often difficult to identify and eliminate. In the paper examples of such events present in the operation of large scale fans along with numerical and experimental methods for their identification and subsequent elimination are presented. 2. Vibrations phenomena in the fans operation Vibrations occur in all cases of rotary machines operation. They are present in steady and unsteady states conditions [1] and might be caused by mechanical or inertia parameter such as unbalanced mass rotation, gear meshing or motor influence in the drive unit [2][3] . In the case of steady state conditions, vibrations occur during the nominal operation of the machine and, if the technical condition of the machine is satisfactory (proper design, good quality of manufacturing and installation, balancing parameters within acceptable limits), do not constitute a significant operational problem. The other type of vibrations occurs during the start-up and shut-down of a machine. In such a case, passing through resonant frequency may occur. The larger the machine the greater problems may be present due to lower self-frequencies and potential risk of high vibrations during slow start up or shutting down. Example of the start-up of the 2.5 MW radial fan is presented in figure 1 below, where axial vibrations on the main shaft bearing housing were measured. Al phases of the start-up are marked on the graph.
Fig. 1. Start-up of the 2.5 MW radial fan – axial vibrations measured on the main shaft housing [4]
Another reason of vibrations presence is medium flow through the fan. Flow can be source of vibrations or can amplified vibrations caused by other reasons (mechanical as for example). Here there are complex and multiply phenomena. The first is operation of the fan on the stable side of the fan curve. As per theory and standard rules, fans shall operate on the right side of the fan characteristic curve as shown in figure 2. There is additional 10% safety margin required, which is measured from the top of the curve. If operation of the fan is not assured in the safety range, there will be excessive vibrations present due to lack of stabilizing flow. Another factor in fans operation is to assure matching flow parameters of ventilation system with fan characteristic. Significant changes in the ventilation system must be considered in the fan operational parameters by rpm change or regulation flaps adjustment. Usually rpm change is more efficient but expensive frequency inverters or multi speed engine motors are required in such a case. If no action is taken, there is a risk to operate the fan on the left hand side of its characteristic curve, which may lead to excessive vibrations.
2192
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Author name / Procedia Engineering 00 (2017) 000–000
3
Fig. 2. Example of the fan characteristic curve – operating range marked
The next trigger of vibrations can be ventilation system itself, causing flow fluctuations in a certain frequency. Usually low frequency of flow fluctuation is not dangerous, but as soon as it matches with own natural frequency of the fan or with rpm frequency or blade frequency, it might cause high fan vibrations and its failure. Natural source of vibrations, which is related to flow and fan design is pulsation effect coming from finite number of blades. The more blades the less flow pulsation and vibrations as an effect are present. The characteristic blade frequency is always present in the vibrations spectrum graphs. Its influence on the overall vibrations (RMS) is usually low but must be considered in the design to prevent possible matching with natural frequency of the fan. Very important action on the designing stage of rotating machine is specifying the modal properties such as normal modes frequencies and shapes and assuring them to be out of characteristic frequencies of the fan. As already mentioned the example of such a load, in terms of fans, is the rotational frequency of the fan rotor and blade frequency as well. The standards concerning individual types of devices define ranges, in which there should be no natural frequency. If they are not met, the object can be prone to resonance vibrations, possibly leading to failures or even catastrophes [5][6][7][8][9]. The usual effects of vibrations are cracks of fan’s rotor. Examples of such cases are shown in figures below.
Fig. 3. Examples of fans failures caused by vibrations – cracks of rotors
Apart of vibrations influence on fans integrity it is also important to assure high efficiency of fans operation. High efficiency means lower power consumption, less pollution and thus lower overall costs of operation. Fans should be designed to operate in optimum point of characteristic and in such conditions shall not suffer any excessive vibrations related problems. However in complex ventilations systems or in case of change of flow parameters this requirement may not be fulfilled. In such a case fans operators want to assure operation with acceptable vibrations and as second priority consider efficiency. This approach is correct of course due to high costs
4
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Rusinski E. Czmochowski J., Moczko P. Pietrusiak D./ Procedia Engineering 00 (2017) 000–000
2193
of fans stoppages and repairs. As already mentioned, in such a case the optimum solution is to have efficient regulations systems to adjust ventilator characteristic curve to flow condition and obtain high efficiency with low vibrations. The resonance vibration or in general excessive vibration phenomenon is exceptionally dangerous to large scale rotating machines such as high power industrial fans. It is for that reason why testing whether the machine operates in such unfavorable conditions is crucial. 3. Condition assessment Measuring methods in fans operations are strongly related with the maintenance approach taken into account. There are the following approaches commonly used: • corrective maintenance - operation until failure occurs, • preventive maintenance - condition-based operation , • predictive maintenance - operation which prevents and correct failures. The first two operation strategies are the most popular. Considering that these machines are crucial to the production process, the first strategy is obviously not efficient enough. It often leads to unplanned downtime which significantly reduces production efficiency. Preventive maintenance may be divided in to two sub-categories such as operation with regularly scheduled overhauls and operation based on the condition. The second relatively popular approach, which is present in industrial fans operation, requires preferably continuous monitoring of the condition of the structure under test. If symptoms of damage are detected, an immediate analysis of the situation follows and decisions are made to perform corrective actions. This method helps to detect failures early, but its efficiency is strictly determined by the quality of condition assessment. This pertains to the assessment methodology, the implemented monitoring techniques, computational methods and indicators of the process of degradation of the fan. Early detection of faults helps to reduce the costs of failure repair and decrease machine downtime. The predictive maintenance provides a significant upgrade to machine operation but is still not as popular as preventive maintenance. The fundamental difference between preventive and predictive approach, is that the latter approach can predict the occurrence of failure. Based on such information, users can take preventive steps in advance to postpone, or eliminate entirely, the expected failure. However, the predictive maintenance strategy requires the application of the most advanced methods of condition monitoring. Therefore this method is still not commonly used. As already mentioned preventive maintenance approach requires continues monitoring of fans vibrations and it is the most commonly present approach in high power industrial fans operation. Periodic measurements might not be efficient enough to discover change in vibrations or other measured parameters of fan at the early stage to prevent serious failure. The most popular measuring technic used in condition monitoring of fans is to measure vibrations in location of main shaft bearing housing. The most useful signals related to fan rotor are obtained from the bearing on the rotor side in horizontal or vertical direction. Axial vibrations measurements shall be taken from the axial bearing location. The most common parameter in fans condition monitoring is velocity of vibrations in mm/s. Standard accelerometers in above mentioned locations are used for such measurements. Their range as well as sensitivity must be properly adopted to technical parameters of the fan such as rotational and blade velocity. Such signals enable to monitor condition of the fan rotor, shaft and roller bearing as well. Monitoring of plain bearings of main shafts is more complex and expensive because we have to perform orbit analysis [10]. This type of measurements is still not commonly used in industrial fans operation. Therefore plain bearings condition is usually monitored with the use of their temperature measurements and sometimes with the oil tests (presence of metal particles etc.) Another issue is to use proper sampling frequency to be able to detect failures symptoms at the early stage. The required sampling frequency is strictly related to the frequency of the monitored signal. This rate should be set in such a way so as to enable reconstruction of analog signal from discrete data without losing information on the measured parameter (Shannon-Kotielnikov condition must be fulfilled). However, for condition monitoring the minimum sampling rate must be much higher. In the stationary signals obtained from relatively low speed machines such as high power industrial fans (radial type especially) this frequency is 5–10 times higher than Nyquist frequency.
2194
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Author name / Procedia Engineering 00 (2017) 000–000
5
Another method, investigated by authors of this paper, which is still not used for fans condition monitoring is to measure flow on the inlet and/or outlet of the fan and correlate it with vibrations and fault recognition [4]. There are relations between flow and certain directions of vibrations. We can also use vibration of the fan rotor housing for recognition of flow problems (operation on the left hand side characteristic curve for example). Based on performed researches it is clear that data from both vibration and flow signals can be used in parallel to improve condition monitoring of industrial fans. 4. Case studies In case of faults detection in the standard monitoring system, sometimes it is necessary to perform more detailed analyses to identify root causes of problems and made optimum action decisions. Such analyses can be made based on another, more advanced tests results with the use of more advanced methods. In this chapter examples of various experimental technics are presented as a case study of faults recognitions of industrial fans. The first case refers to high power 3MW axial fan vibrations. The fan operates in exhaust system of power block. It is core equipment, which failure may cause entire block emergency stop. During operation of the fan there are periodical vibrations observed in amount exciding alarm threshold limits. There is standard continuous monitoring system present and it bases on vibration measurements of main shaft bearing housings. The amount of vibration exceeds 10 mm/s. Such vibrations may lead to failure of the fan. In order to identify root causes of excessive vibrations few measuring technics and numerical methods were used. They are described shortly below together with obtained results. Scanning measurements were made with the use of 3D laser on the fan inlet/outlet ducts. The main purpose was to identify modes, frequencies and amounts of vibrations for further analyses. Example of results from tests are presented in figure 4. Teste were made during start-up of the fan as well as during its normal operation. The most dominating frequency in all recognized vibration modes is rotational frequency of the fan, which equals fr = 12.5 Hz.
Fig. 4. Results of the 3D vibrations scanning – vibrations of fan outlet duct, VRMS,1 = 5.2 mm/s
Dynamic, modal simulations of the fan inlet/outlet ducts were made on the validated (by experimental tests) models of ducts to enable their future modifications due to discovered resonance problems. Similarly rotating unit (shaft and rotor) was validated with the use of experimental modal analysis (EMA) of the main shaft and rotor blades. Based on the results also here possible resonance problem was found due to overlapping of bending modal mode frequency f1 of the shaft with rotational frequency: f1 = 14,43Hz = 1,16 fr. Additional under pressure tests in the inlet and outlet of the fan, during start-up and normal operation were made in order to investigate if the root cause of the vibrations lie on the flow or mechanical vibrations phenomena. As a conclusion it was confirmed that the root casus of the fan vibrations are related to improper design of the shaft and ducts of the fan. There are normal mode frequencies overlapping the shaft rotational frequency, which results in resonance and beating effects.
6
E. Rusinski et al. / Procedia Engineering 199 (2017) 2190–2195 Rusinski E. Czmochowski J., Moczko P. Pietrusiak D./ Procedia Engineering 00 (2017) 000–000
2195
Another case study refers to cracks recognition of the fan housing with the use of 3D laser scanning method. As shown in figure below, if there is through crack, the modal parameters differs significantly from the surrounding area, what can be easily detected by scanning method.
Fig. 5. Results of the 3D vibrations scanning with the use of laser scanning method – detection of crack in the fan housing
5. Conclusions Continuous or periodic condition monitoring of industrial fans is carried out using many experimental and numerical methods and techniques. These methods and techniques are used to analyze the obtained information and, consequently, to determine the condition of the machine in the operation. Reliability of the monitoring and subsequently condition assessment process is in this case closely related to the adopted methodology and scope of testing. Considering importance of high power industrial fans as plant’s core equipment, it has a great importance to develop existing and create new methods of their condition monitoring. References [1] Parszewski Z. Drgania i dynamika maszyn. Warszawa: WNT, 1982 [2] Głowacz, A., Diagnostics of DC and Induction Motors Based on the Analysis of Acoustic Signals, MEASUREMENT SCIENCE REVIEW Volume: 14 (2014) Issue: 5 Pages: 257-262 [3] Głowacz, A., Diagnostics of Synchronous Motor Based on Analysis of Acoustic Signals with the use of Line Spectral Frequencies and Knearest Neighbor Classifier, ARCHIVES OF ACOUSTICS Volume: 39 (2014) Issue: 2 Pages: 189-194 [4] Czmochowski J, Moczko P, Odyja s P, Pietrusiak D. Tests of rotary machines vibrations in steady and unsteady states on the basis of large diameter centrifugal fans. Eksploatacja i Niezawodnosc – Maintenance and Reliability 2014; 16 (2): 211–216 [5] Czmochowski, J. 2008. Identyfikacja modeli modalnych maszyn urabiających w górnictwie węgla brunatnego. Oficyna Wydawnicza Politechniki Wrocławskiej [6] Fry P.R. Loads and Stresses—The Real Cause of Failures in Surface Mining Machinery, Practical Failure Analysis 3(2) 2003 [7] Rusinski, E.; Moczko, P.; Pietrusiak, D., Low frequency vibrations of the surface mining machines caused by operational loads and its impact on durability, PROCEEDINGS OF INTERNATIONAL CONFERENCE ON NOISE AND VIBRATION ENGINEERING (ISMA2014) AND INTERNATIONAL CONFERENCE ON UNCERTAINTY IN STRUCTURAL DYNAMICS (USD2014) Pages: 683694 [8] Rusiński E., Moczko P., Pietrusiak D., Przybyłek G., Experimental and numerical investigations of the jaw crusher supporting structure fatigue failure, Strojniški vestnik - Journal of Mechanical Engineering 59 (2013) 9, 556-563, DOI:10.5545/sv-jme.2012.940 [9] Kokociński J., Wibroakustyczna diagnostyka maszyn. Remonty i utrzymanie ruchu 11/2009 [10] Stamboliska Z., Rusiński E., Moczko P., Proactive Condition Monitoring of Low-Speed Machines, Springer International Publishing Switzerland 2015,