Effect Of Bearing Faults On Dynamic Behavior And Electric Power Consumption Of Pumps

Transcription

1 Effect Of Bearing Faults On Dynamic Behavior And Electric Power Consumption Of Pumps Abstract Samir M. Abdel-Rahman Dalia M. Al-Gazar M. A. Helal Associate Professor Engineer Professor Mechanical & Electrical Research Institute, National Water Research Center, Delta Barrage, P.O , Egypt Various problems are encountered in water pumping station that affect the performance and reduce the expected life of these stations. The sources of these problems vary from mechanical, hydraulic, thermal, structural, operation to environmental nature. Some of the resulting problems of such sources are energy losses, reduction in performance, and decrease of operation life, fatigue, wear and failure. Bearings are of the most important parts used in pumping. This research discusses the bearing defects and its causes and how to overcome these defects and minimize the influence of these defects on the pump performance through out the dynamic analysis of pumping stations to maintain their efficient, economic, and safe operation. The results reveal that the bearing defects have a major effect on the system dynamics behavior. Bearing faults increase vibration level 70, where power consumption increases 12 and pump efficiency decreases Introduction Bearings are of the most important parts used in pumping plants. The bearings used in these applications must be able to support the hydraulic loads imposed on the impeller, the mass of the impeller and shaft, and the loads from the drive system. The bearings must also keep the axial movement and lateral deflection of the shaft within acceptable limits to maximize the service life of the shaft seal. So bearings gained the researchers attentions for its great influence on the performance and operating efficiency. Where bearing faults are considered, a distinct indicator of decreasing pump performance and increasing the vibration and noise levels leading to failure and breakdown. Most pumping plants use rolling element bearings which are designed to operate for high speed and high performance conditions. Rolling element bearings are manufactured under very stringent quality control standards. Under ideal operating conditions, bearing can last through many years of continuous use. Operating conditions are rarely ideal, so most bearings never achieve their potential as far as useful life is concerned. The life of a rolling element bearing depends on the conditions under which it is manufactured, the care exercised in storing and handling it, installation practice, load conditions, and the operating environment [1]. Due to the fact that a rolling element bearing restricts rotor motion, forces generated by the rotor are transferred through the rolling elements to the bearing s outer ring which contains the outer race and ultimately to the bearing housing. Because of this transmission, a direct measurement at the bearing outer ring or casing (housing) is the primary accepted method for monitoring machines with rolling element bearings. Another characteristic that is a unique and normal to rolling element bearings is the generation of vibrations at specific bearing-related frequencies. These frequencies are generated by the bearing based on the bearing s geometry, number of rolling elements, and the 1

2 speed at which the shaft is rotating [2]. Monitoring, analyzing and correcting bearing problems are critical operations in modern industry. Without the help of a good predictive maintenance program, vibration problems associated with bearings of critical machines can be difficult to understand and analyze [3]. 2. Problem Identification A main axial flow pump inclined 45 degree, with head 2m and discharge 16 m 3 /s working in one of the most irrigation projects in Egypt, is considered in this study. The motor power is 650 kw, speed 1485 rpm and the gearbox reduction speed ratio is 1: The bearing of the motor-driveend was damaged for one pump during operation. Primary investigation showed a high level of vibration and noise at the housing of the motor-drive-end bearing. A defect was expected for the bearing of the motor-drive end. Exciting frequencies of the pump unit were calculated to define the source of high vibration prior to dynamic analysis. Frequencies of the components of the pump system are Motor frequency : Hz Pump frequency :1.492 Hz Gear mesh frequency : Hz Vane passing frequency : 5.96 Hz Vibration measurements were taken at the housing of the motor-drive-end bearing (MDEB) before and after replacement of the damaged bearing. This damaged bearing results mainly due to poor lubrication of the bearing where the pump operated for long time higher than the designed operating conditions. In this study, vibration analysis was focused on the motor-drive-end area in the radial and axial directions to evaluate the dynamic behavior of the motor bearing. Because the axial flow pump fixation is at the bottom of the pump system, so the pump system acts as a cantilever fixed at the pump end and free at the motor. As a result and due to the high torque of the motor operation, the motor becomes more sensitive to vibration than other components. Electrical energy to drive a pump unit represents the major factor in the whole life operating cost of the pump system. The total operating cost includes 87 to power consumption, 8 to maintenance and only 5 to the equipment. With this high cost of power consumption, it is important to avoid any operating conditions that impact pump performance and to improve the operation profitability of the pump. Vibration is considered to be a major factor in the loss of performance of pumps [4]. 3. Vibration Level Measurements Overall vibration levels measured on the damaged motor-drive-end bearing (MDEB) in the radial direction show that the overall vibration level is about 4.5 mm/sec as shown in Figure 1. This level is dangerous according to the standard specification [5] which specifies 1.4 mm/sec for long life bearings in pumps as a maximum value for working with efficient performance rates. These results indicate that there was a defect in this motor drive end bearing which working in the primary operational stage. After replacement this defective bearing with a new one, the results of the measurements showed a decrease of vibration level to values less than 0.7 mm/sec. As a result of replacing the bearing, the vibration level decreases about 85 in the radial direction. On the other hand, the vibration level measured in the axial direction of the damaged bearing was about 1.9 mm/sec and it decreases to 1.3 mm/sec after replacing the bearing. This means the vibration level 2

3 decrease about 32 in the axial direction and 85 in the radial direction. Hence, the radial direction can be considered a good indicator for measuring the vibration level compared with the axial direction. The overall vibration level on the MDEB measured in terms of displacement ( m) show the same trend. It is found that the vibration level measured in the radial direction, as shown in Figure 2, decreases about 70 after replacing the defective bearing, however, in the axial direction the vibration level decreases about 11. This confirms that the radial direction is an effective indicator and sensitive to bearing state than the axial direction. Figure 1 Vibration level (mm/s) in the radial direction before and after replacing the bearing Figure 2 Vibration level ( m) in the radial direction before and after replacing the bearing 4. Diagnosis of MDEB Rotational Frequencies Knowing the ball passing frequencies for the outer and inner races and the rolling elements rotational frequency is often helpful for analyzing the vibration generated from the rolling bearings and providing additional clues to the cause of defect. The rotational frequencies generated by a defective bearing include four major frequencies: ball passing frequency of outer race, ball passing frequency of inner race, ball defect frequency and cage defect frequency. These rotational frequencies can be calculated according to the dimensions of the bearing components, including the ball diameter (Db), number of balls (n), the pitch diameter (P), the contact angle ( ) and shaft r.p.m (N) as follows [6]: n N Db Rotational frequency of outer race F (outer) = (1 cos β ) (1) 2*60 P 3

4 n N Db Rotational frequency of inner race F (inner) = (1 + cosβ ) 2*60 P P N Db 2 Rotational frequency of rolling element F (ball) = (1 + cos β ) d * 60 (3) p N Db Rotational frequency of cage F (cage) = (1 cosβ ) (4) 2*60 P This method expected to be more accurate compared to the previous methods and the calculated frequencies are most closely to the actual rotational frequencies because it takes into consideration more specified dimensions like the contact angle and the pitch diameter. The calculated frequencies are: Rotational frequency of outer race F (outer) : 345 Hz Rotational frequency of inner race F (inner) : 460 Hz Rotational frequency of rolling element F (ball) : 145 Hz Rotational frequency of cage F (cage) :16.3 Hz The defective part in the bearing has the most severe influence on the vibration amplitude and causes bearing deterioration. As shown in Figure 3.a for the defective bearing, the rotational frequencies of the rolling elements are defined in the spectrum with high level of vibration. The vibration velocity at the rolling elements rotational frequency reaches a value of um/s at 145 Hz in the radial direction. This significant vibration velocity level implies the problem in the rolling elements. On the other hand in the axial direction, it could be seen that the vibration velocity at the frequency of the inner race reaches a value of um/s at 460 Hz. This is because the defect in the rolling elements affects the inner race during they pass it and causes initial deterioration appearing as an increase in the vibration amplitude at the frequency of the inner race. When this defect appears, the predicted residual service life of the bearing decreases by not less than two times [7]. The recommendation is be to replace the bearing or correct the defect soon. After replacing the defect bearing with a new one the vibration signal is measured in radial and axial directions as shown in Figure 3.b. The results reveal that the vibration velocities are significantly decreased at the rotational frequencies, and the motor is operating satisfactorily. But it is obvious that the most significant decrease of vibration velocities is at the rotational frequency of the rolling elements and reaches 90. This ascertains the problem is in the rolling elements. When the defected bearing replaced, the visual diagnosis indicated that the rolling elements showed a severe wear. This class of bearing problem is due to various passive treatments such as improper lubrication, which lead to the wear in the rolling elements. The contacts areas of the rolling elements are dull and roughened. Abraded matter turns the lubricant dark in color. The grease is also solidified. In many cases, moisture leads to the consistency growing watery lubricant. If foreign particles are the cause of wear, the rolling element surfaces will be particularly badly scored. Vibration level change with motor drive end bearing frequencies before and after replacement is shown in Table 1. In the view of the results, it is obvious that the defected bearing has a significant effect on the ball rotational frequencies while it has a negligible effect on the cage rotational frequency in both the radial and axial directions. Also the influence of the defected bearing is considerable on the rotational frequencies of the ball and the inner race compared to the 2 (2) 4

5 rotational frequency of the outer race. Thus the vibration level variations on the ball rotational frequencies are good indicators of the state of bearings. (a) Before replacement (b) After replacement Figure 3 Bearing rotational frequencies in the radial direction before and after replacement Table 1 Vibration level changes with MDEB frequencies before and after replacement Bearing rotational frequencies (Hz) Frequency for the outer race 345 Hz Frequency for the inner race 460 Hz Ball rotational frequency 145 Hz Cage rotational frequency 16.3 Hz Vibration levels measured in radial direction (um/s) Damaged bearing New bearing Vibration levels measured in axial direction (um/s) Damaged bearing New bearing Dynamic Analysis of the Pump System Frequency analysis is very important to define the exciting frequencies and determine the level of vibration at each specific frequency. Also it is required to define all the working frequencies to control vibration levels and solve vibration problems. So it is evident that the dynamic analysis is important to increase operation life and performance of the station and to decrease losses [8]. Vibrations spectra were taken at the location of the MDEB in the radial and axial directions after and before replacing the defective bearing. The measurements show that the vibration level measured is in the order of 1 mm/sec at different exciting frequencies and the machine is running in a smooth condition for such large machines according to the ISO standard [5]. 5

6 Vibration spectra measured on the pump system before changing the defective bearing shows exciting frequencies at the motor running speed and its harmonics with vibration amplitude about 1 mm/sec in the radial and axial directions as shown in Figure 4.a. This gives an indication that there was a mechanical problem. This may be due to unbalance and misalignment resulting from fixture and assembling operations. So it is important to confirm of the safety of assembling operations by vibration measurements and construct a vibration spectrum for the new pumping stations. After changing the defected MDEB, vibration measurements, shown in Figure 4.b, indicate that the vibration level at the exciting frequencies decreases obviously as the amplitude of vibration decreases from 1 mm/s to 0.7 mm/s. The vibration level at the running speed decreases about 73 after replacement. Also the vibration level decreases at the frequencies of the rotating parts and its harmonics by percentages within 10 to 93 at high frequencies (688 Hz). The data show that the decreasing percentages are very obvious at the high frequencies and lay between 78 to 92 as indicated in Table 2. As expected this indicates that the defected bearing was a significant main reason of increasing the vibration level in the pump. (a) Before replacement (b) After replacement Figure 4 Vibration spectrum before and after replacement bearing in the radial direction 6. Electric Power Consumption Measurements Increasing efficiency of the pumping stations is necessary to optimize the operational functions of these stations. Efficiency of the pumping stations is greatly affected by bearing problems. Bearings defect is a symptom of less performance efficiency and high vibration level. Electric power consumption and pump performance measurements were recorded for the defective bearing before replacement and after replacement. The results shown in Table 3 reveal that the electric power consumption before changing the defective bearing was high and the pump efficiency was low due to high vibration results from the defective bearing that lead to high losses of energy. After changing the bearing, it is obviously that the electric power consumption decreases and the pump efficiency increases. This ascertains that with the utilization of good bearing, the pump will run smoother with less vibration. The result shows that reduction of vibration level reduces consumption power and requires less maintenance. So study the bearing faults effect on power consumption is useful for efficient and reliable pumping stations. By comparing the measurements results that taken before and after replacement the defective bearing, it could be seen that there was a significant decrease in the electric power consumption in 6

7 a percentage within 10 to 14. Also the overall pump efficiency increases significantly after replacement the defective bearing to reach a good values within 71 to 84 after it was fallen within 63 to 66 before replacement the defective bearing. That is due to reducing the mechanical losses that was produced from the high vibrations resulting from the defective bearing. This ascertains that the power consumption increases considerably by machines operating in roughly bearing conditions. Table 2 Vibration level decrease with exciting frequency after bearing replacement frequencies Excitation frequency (Hz) Axial Vibration level decrease Radial Vibration level decrease Table 3 Electric power consumption before and after replacement Electric power consumption (kw) Discharge Head Water Power (m 3 /s) (m) (kw) Before After Decrease replacement replacement

8 7. Conclusions 1- Defective bearings cause considerable increase of vibration amplitudes and decrease the service life of bearings at high speed pumping stations. 2- The statistical study of the recorded vibration level of the pumping station measured before and after replacement of the defective bearing shows considerable decreasing in the vibration level reaching 70 in the radial direction and about 11 in the axial direction. 3- From the results it is noticed significant variations in the vibration level measured in radial direction compared with axial one, which indicates that the measurements in radial direction are good indictor for describing the degree of defectiveness of rolling bearings. While the measurements in the axial direction are good indicators for the occurrence of damage due to mechanical problems. 4- Utilization of safety rolling bearings decreases significantly the power consumption of pumping stations. From the experimental analysis, it is found that replacement of the defective bearings reduces the electrical power consumption by 10 to 14 and increases the overall pump efficiency from 71 to The present work provides an efficient detection tool for preserving the life operation of the pumping stations in Egypt as well as early diagnosis of the initial deterioration of the performance of the pumping stations due to the defects of rolling element bearings. 8. References [1] Ming, X.U.; Bleu, J.R. Condition Monitoring of Seals Pumps Using Spike Energy. P/PM Technology December [2] Harker, R.G; Sandy, J.L. Rolling Element Bearing Monitoring and Diagnostic Techniques. Journal of gas turbine April Vol.111/251. [3] Spike Energy Detecting Rolling Element Bearing Defects. Spike Energy Manual Catalogues [4] Abdel-Rahman, S. M. "Cavitation Detection in Centrifugal Pumps By Monitoring Vibration..1 st International Conference On Green & Advanced Technologies, National Research Center, Dokki, Cairo, Egypt, 3-6 Jan [5] Bruel & Kjaer. Machine Condition Monitoring. Application note BR , B&K, Denmark, [6]- Foiles, B., 1987, Rolling Element Bearing Frequencies, Technical Report Edited by Bently Nevada Corporation. [7] Azovtsev, A.Y., Barakov, A.V., Yudin, A. I. Automatic Diagnostics and Condition Prediction of Rolling Element Bearings Using Enveloping Methods 18 th Annual Meeting of the Vibration Institute, June, 199 [8] Birds, C.F., and Arnold, E. Engineering Vibration Analysis with Application to Control Systems. Adivision of Hodder Headline PLC, London,