machine design, Vol.6(2014) No.2, ISSN pp

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1 machine design, Vol.6(2014) No.2, ISSN pp Research paper CASE STUDY: VIBRATION ANALYSIS OF A VERTICAL PUMP OF COOLING SYSTEM IN FERO-NICKEL INDUSTRY Tale GERAMITCHIOSKI 1, * - Ljupco TRAJCEVSKI 2 1 University St. Klement Ohridski Bitola, Faculty of Technical Sciences, Bitola, Macedonia Received ( ); Revised ( ); Accepted ( ) Abstract: The basic goal of this investigation is analyzing the vibration condition of a vertical pump type 165 used for transportation of a hot water to the tower for natural cooling in fero- nickel industry Feni Kavadarci Republic of Macedonia. For that purpose was used VIBROTEST 60 (Bruel&Kjaer) instrument with appropriate modules. The paper shows the procedure for early detection of failure the rolling bearings using vibrating signature of the measurement and analyses the other problems of the elements as results of existing pump cavitation. Some proposal for constructive intervention on the pump-motor to decrease existing vibration amplitude values are given in the paper. Key words: pump, vibration analysis, bearing life 1. INTRODUCTION Production of nickel is an intensive process that uses robust and rotating equipment where it is exposed to intense workload and high temperatures, whereas the susceptibility of equipment failures poses a serious threat. In critical positions in flow of the production process include crushers, rotary dryers (x4), mills (x2), rotary kilns (x2) and the main hydraulic oil lubrication systems that use large electric motors and reduction enclosures, in addition to electric furnaces ( x2), converters (x2), arc furnace (x1), granulation system ( x1 ) and vats that are susceptible to decline bricks and large transformers [1]. By exploiting the plant comes to unwanted accidents due to the occurrence of illicit vibration. In order to avoid these phenomena are often performed unnecessarily replacing the elements of the plant, although they can still be used. The latest trends tend to maintain fixed time intervals for replacement of mechanical elements, to be replaced by interval control state of the plant. The basic principle of this system maintenance vibration diagnostics is to defining discrete air forces imply irregular operation of the plant. Modern equipment for measurement, analysis and diagnostics, along with knowledge and experience of professional staff who manage it, guarantee to resolve the complex side effects that occur during the operation of the plant. All rotating machines during operation generate vibrations that are inevitable occurrence in their exploitation. Their full elimination is impossible, but it is the same loft in boundaries defined by the equipment manufacturer or adopted global standards and norms, such as: VDI 2056 recommendations; DIN norms ISO standards; E French norm. The fact that irregular work of some mechanical element with particular intensity and frequency determined exactly unique to that part of the plant, is used for determining and defining exactly inertial forces that cause the dynamic state of the plant. Nature of inertial forces can be various different [2],[3],[7]: imbalance in the rotating masses ; misalignment - axis of the shaft ; mechanical damage in rolling bearings ; irregularity in the operation of the sliding bearings ; work in terms of resonance phenomena ; Excitation aerodynamic and hydraulic forces ; Electromagnetic vibration and more. The choice of balancing rotors in their own bearings has that advantage: Corrects imbalances of the total rotating mass of the entire composition. There are no transport costs; Avoided mistakes are repeated editing. The following example will show the measurement and diagnosis of vertical centrifugal pumps used permanently in Feni - INDUSTRIES- Kavadarci, R.Macedonia. Fig.1.Vertical centrifugal pump type 16 *Correspondence Author s Address: University St. Klement Ohridski Bitola, Faculty of Technical Sciences, Ivo Lola Ribar, 7000 Bitola, Macedonia, tale.geramitcioski@uklo.edu.mk

2 2. CASE STYUDY VERTIKAL PUMP Results of the measuring The task of this type vertical pump 165 (Fig.1,2) is heated water from the cooling system of the electric furnace to transport the cooling tower to perform again cooled naturally. The technical characteristics of the pump are: speed of the impeller pump n = 1450 rot / min pump bearings in points 1h/2h are SKF 7320 BGM bearings in electric motor in points 3h/4h are SKF 6319 CZ and 7319 B is 5h/6h (see Fig.2). First 10 harmonics of the characteristic frequency of SKF 6319 CZ on point s 3,4 in the motor (front side) c) First 10 harmonics of the characteristic frequency of SKF 7319 CZ on point s 5,6 in the motor (back side) Fig.4. Calculate general frequency with the GEARBOX Fig.2. Vertical pump type 165 for distribution hot water to cooling tower measurement point s The bearing frequencies can easily be calculated from the bearing geometry using the formulae given in Fig. 3[1]. To calculate the generate frequency we used the software GEARBOX for each of the bearings in pump and motor. (see Fig.4). The rolling bearings monitoring in this case is bearing number 1,2 SKF 7320BGM- (see Fig.5,6), measured vibrations with FFT analysis in two different periods. Measured vibrations and FFT analysis for bearings in electric motor in points 3h/4h (SKF 6319 C3) on the front side in two different way s (vertical and horizontal) shows on Fig.7,8. Fig.3. Formulas for calculating bearing frequencies First 10 harmonics of the characteristic frequency of SKF 7320 BGM on point s 1,2 in the vertical pump 54 Fig.5. Bearing SKF 7320BGM FFT on point 1-pump measuring on on

3 The same situation exist on another main point s where vibrations are measured in both directions, so, there are two figures for each characteristics point- two on pump 1,2 and fore to motor 3,4,5,6. Measured vibrations and FFT analysis for bearings in electric motor in points 5h/6h (SKF 7319 B) on the back side, where the motor cower exist, shows on Fig. 9, 10. Fig.8. Bearing SKF 6319 FFT analysis on point 4-motor measuring on on Fig.6. Bearing SKF 7320BGM FFT on point 2-pump measuring on on Fig.9. Bearing SKF 7319 FFT analysis on point 5-motor measuring on on Fig.7. Bearing SKF 6319 FFT analysis on point 3-motor measuring on on It s evident that FFT diagram established on had a big number of different frequencies as a sideband of the main frequencies as a result of a rapid progress the failure of outer ring (first step) and after that, on the inner 55

4 ring. The major amplitude (pick) show as a shaft imbalance. On FFT established on in the area of high frequencies, there is a lot of picks as a result of cavitation of the pump. severity of the machine condition with vibration. The standard uses the parameter of velocity-rms to indicate severity. The letters A, B, C and D as seen in Figure 2.13, classify the severity. Class I Individual parts of engines and machines integrally connected with a complete machine in its normal operating condition (production electrical motors of up to 15 Kw are typical examples of machines in this category). Class II Medium-sized machines (typically electrical motors with kw output) without special foundations, rigidly mounted engines or machines (up to 300 kw) on special foundations. Class III Large prime movers and other large machines with rotating masses mounted on rigid and heavy foundations, which are relatively stiff in the direction of vibration. Class IV Large prime movers and other large machines with rotating masses mounted on foundations, which are relatively soft in the direction of vibration measurement (for example turbogenerator sets, especially those with lightweight substructures). 3. RESULTS AND DISCUSSION Fig.10. Bearing SKF 7319 FFT analysis on point 6-motor measuring on on Norms and standards according ISO 2372 The most widely used standard as an indicator of vibration severity is ISO 2372 (BS 4675). The standard can be used to determine acceptable vibration levels for various classes of machinery. Thus, to use this ISO standard, it is necessary to first classify the machine of interest. Reading across the chart we can correlate the Fig.11. ISO 2372 standard for evaluate vibrations impact Fig. 5,6 represent the FFT spectrums on point 1,2 exists on the pump and shows the spectrum measured for the first bearing SKF 7320 in two different measuring times and ways. The measuring vibrations on point 1 realized on (Fig2 and on (Fig.2 shows FFT spectrum with several conclusions: rotational speed frequency of the pump shaft is 1450/60= Hz and this frequency is with the maximum peak (amplitude) of 1.48mm/s (fig.5.that means unbalance of the shaft and rotation elements exist because of equal the vibration frequency and rotational speed frequency. After balancing, the condition was better (Fig5 but some other problems show later. On Fig.5a all bearing frequency exists with different peaks. The limited frequency of each of the element bearing is calculated by the instrument for measuring vibrations and shows on the right side of the picture of the spectrum. When the frequency of the measured vibration of the elements coincided with border (red lines), it is a signal that element of bearing failure. In this case, that happened in line 1 with the outer race of the bearing with the frequency Hz and the amplitude of 0.95 mm/s (Fig.13). As bearing wear progresses, defect shows up first on the outer race as spalling. The amplitude of outer race frequency increase as the spalling gets deeper. In the final stages of failure, the rollers transfer the spalling to the inner race and inner-race frequencies increase (Fig.5. Definitely, the situation in fig.5 main problem is imbalance, but also misalignment of the rotating elements with the bearing axis. This conclusion is supported by the large number of peaks of different frequencies in the width of the frequency range. 56

5 If you compare Fig.5a and Fig.5b, it is evident that in the range of high frequencies Fig.5b peaks occur with significant values of the amplitudes. It is proof that appeared in pump cavitation. Cavitation is the vaporization of fluid within the pump. It occurs when the fluid pressure is less than the vapor pressure at the temperature. Technically, cavitation is the boiling of fluid at the ambient temperature due to the reduced pressure. Prolonged cavitation caused erosion damage to a pump impeller (Fig.10). Cavitation causes vibration in the frequency range of HZ. Dominant peaks occur at 845 HZ with amplitude of 0.62mm / s at 883Hz with amplitude of 0.48mm / s and the highest amplitude of 0.74mm / s at 910Hz.Problem of existing cavitation will be solved later. On FFT analysis on fig.5a its shows a bearing wear progresses defect show up first on the outer-race as spalling. The amplitude of the outer-race frequency increase as the spalling gets deeper Fig. 7,8 represent the FFT spectrums on point 2 exist on the motor of the pump from the front side and shows the spectrum measured for the first bearing SKF 6319 in two different measuring times in two directions horizontal and vertical. The measuring vibrations on point 2 realized on (Fig7ahorizontal direction, and Fig.7a. vertical direction) and on (Fig7b-horizontal direction, and Fig.8b. vertical direction) shows FFT spectrum with several conclusions: On were measured 4.9mm/s amplitude of the vibrations on pump motor point 4 with rotational speed frequency of the motor (pump) shaft 1450/60= Hz. Other amplitude on other characteristics bearing frequencies is minor. So this amplitude shows the imbalance existing on that point. In point 3 at the same date, FFT diagram (Fig.7a shows a lot of pick s with the significant amplitudes and wide frequency range. That implicate of existing the misalignments in rotation parts of the motorpump system. On that diagram there are a various number of the picks of vibrations on the frequency range of Hz with the dominant amplitude 1.45mm/s on frequency 169.2Hz which is the 7-th harmonic of the rotational speed frequency of Hz. It implicated the existing of a significant misalignment in the phase of under mounting rolling bearing in the housing. Amplitude with the peak of 1.38 mm/s on frequency 118.5Hz implicates the failure of the inner ring of the bearing because it touched the red line no.2 (limited frequency of the inner ring). It has a huge impact on the rolling elements because the amplitude of 1.58mm/s with frequency 99.8Hz is near by the limited rolling elements ball-pass frequency (red line no.4) and exist the failure of the rolling elements. Fig. 9,10 represent the FFT spectrums on point 5,6 exist on the motor of the pump from the back side and shows the spectrum measured for the first bearing SKF 7319 in two different measuring times in two directions horizontal and vertical. The measuring vibrations on point 2 realized on (Fig.9ahorizontal direction, and Fig.9a. vertical direction) and on (Fig10a-horizontal direction, and Fig.10b. vertical direction) shows FFT spectrum with several conclusions: Evident is the dominance of the frequency of the working speed of HZ which indicates imbalance, but also the resonance of the cower shit of the motor. This conclusion is supported by the fact that some basic frequency came into range of harmonic frequencies with smaller amplitudes in the range of Hz. Fig.9a and 10a show the emergence of a number of peaks equally spaced frequencies in the frequency band width of and Hz suggesting massive failure of all rotating bearing elements [6],[7]. Fig.9b. and 10b section at high frequencies showed great influence on the cavitation chamber, and the Fig.12 reflected damage of the propeller pump as a result of the action of cavitation. Fig.12. Damage on the pump propeller as an influence of the cavitation Fig.13. Corosion on outer race of the bearing SKF 7320 on point 1,2 in pump as a result of cavitation impact 4. CONCLUSION On FFT spectra diagrams of all points of the system motor vertical pump showed the existence of imbalance of the rotating parts of the system, i.e. the shaft of the pump. In the future, it must be made completely balancing the entire rotating parts of the system. 57

6 On the FFT diagrams is evident existence of eccentricity, i.e. misalignment on the rotating elements in terms of housing existence through a series of significant vibration amplitude across the width of the entire spectrum. This is due to unprofessional and incorrect mounting of the bearings in the housing which occurs misalignment of the outer ring relative to the axis of house. It happened at the mounting bearing on the shaft with alignment not be achieved. It needs to be fully dismantled and reinstalled with full attention to avoid misalignment and control that with the comparators. SKF bearing - mounted in 6319 there point 3.4 quart of inner ring and rolling-ball elements. Bearing must be replaced with new. Recent measurements showed the occurrence of cavitation due to the presence of very hard particles in the form of dirt that passed the existing filter and contaminated water are transported to the cooling towers, and causing an imbalance in the flow of water provided by the pump. Cavitation damage the fins of the impeller of the pump. To avoid this phenomenon, it is necessary to clean or replace the filter to prevent penetration of dirt in the water. Taking into account all these considerations, in October 2012, there was a complete overhaul of the vertical pump 165th Run a balancing shaft, changed three pairs of bearings and re-mounted refrigeration system. After that, measurements were made of the peak amplitudes of the three characteristic points in two directions at four different time periods-table 1. Table 1. Measured vibration values in bearings after repair Amplitude (mms) in measured point s on date s: 1H 2H 3H 4H 5H 6H The results made the following conclusions: Values of the peak amplitudes of vibration in bearings are quite high ; compared with the recommended values of the standard ISO 2372, worrying values in section 6 ( slot on the back side of the motor). According to these values, the motor work in unacceptable area of dynamic stability. Requires further constructive intervention of the pump by inserting rubber washers under the flange contact surfaces of the motor and the pump housing (3.4 points), better anchor the back cover of the engine, and shall re- balancing the rotor of the motor due to the occurrence of capture the fins of the fan cover for imbalance. The pump should be set on the ground through the rubber that will cushion the impact of vibrations from adjacent units. REFERENCES [1] Wowk,V., Machinery Vibration Measurement and Analysis, McGraw-Hill Inc. [2] Lindh, T., On predictive bearing condition monitoring of induction motors, Ph.D. dissertation, Lappeenranta University of Technology, Isbn , Isssn ,2003. [3] Henderson, D.S., K. Lothian and J. Priest, Pc based monitoring IEE/IMechE International Conference on Power Station Maintenance Profitability through Reliability, no. 452, March/ April 1998, pp: [4] Li, Y. and C. Zhang, 2004, Dynamic Prognostic Prediction of Defect Propagation on Rolling Element Bearing. Journal of Vibration and Acoustics, Trans of ASME, vol 85, no. 1, pp: July [5] Igarishi, T. and Hiroyoshi., Studies on Vibration and Sound of Defective Rolling Bearing. Bulletin JSME, vol 25, no. 204, pp: [6] Chaudhary, A. and N. Tandon, A Theoretical Model to Predict Vibration Response of Rolling Bearings to Distributed Defects under Radial Load. Journal of Vibration and Acoustics, Transactions of ASME, vil 120, no. 1, pp: [7] Taylor, I.J., Identification of Bearing Defects by Spectral Analysis. Journal of Mechanical Design, Transaction of ASME, Vol. 120: