S Rahmanovic Et Al_Mechatronics 2008-ID048_Ireland 2008

8/8/2019 S Rahmanovic Et Al_Mechatronics 2008-ID048_Ireland 2008

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Mechatronics 2008, June 23 25, University of Limerick, Ireland 1

VIBRATION SIGNATURES ANALYSIS FOR DETECTION OF

FAULTS IN AN INDUSTRIAL FAN

S. Rahmanovic*1, V. Dolecek2, E. Omerdic3

1Engineering and Investment Department of Cement Plant Lukavac, Lukavackih brigada bb, 75300 Lukavac,Bosnia and Herzegovina

2University of Sarajevo, Vilsonovo setaliste, Sarajevo 71000, Bosnia and Herzegovina

3Production Department of CP Lukavac, Lukavackih brigada bb, 75300 Lukavac, Bosnia and Herzegovina

ABSTRACT

The paper deals with detection of faults in a process fan of a rotary kiln plant for cement clinker production

by using vibration signatures analysis. The most common causes of vibration of this process fan arespecified and described. Velocity and acceleration of vibration as well as the enveloped acceleration havebeen measured at bearing housings in axial, radial and vertical directions. Prior to the measurement all forcing frequencies specific for this fan have been identified. The obtained vibration signatures in time and frequency domain have been analysed for fault detection in the process fan. It has been shown that faultssuch as imbalance of a fan and electric motor rotor, misalignment of a coupling or electric motor stator androtor eccentricity can be detected by using velocity spectra while the defects in rolling bearing in anincipient stage can be easier detected by using enveloped acceleration spectra.

1.INTRODUCTION

The industrial fans often operate under extremely difficult conditions. A process fan used in a rotary kiln

line for cement clinker production is an example of such a fan since it sucks the hot and dust laden gases from

the rotary kiln shell and transports them trough the preheating cyclones towards the electrostatic precipitator.

L L L

L

Figure 1: The mechanical system of the fan and points of measurement (left), Pictures of measurement (middle and right).

The process fan considered in this paper consists of an asynchronous squirrel cage electric motor rated 630

kW with speed of 994 RPM, as a driving, and of the process fan rotor as a driven machine, as shown in Fig. 1a

[1]. A flexible coupling connects the two rotors. The rotor of the electric motor is supported on the anisotropic

flexible bearings; cylindrical roller bearing NU1030M at back side and on bearings NU1030M and 6030 at the

front side. The rotor of the fan is non-symmetrically mounted on the shaft which is supported also on the

anisotropic flexible spherical self aligning roller bearings of type 23224 CCK, at both ends. Both rotors are

placed onto the pedestal that is fixed to the steel construction of the foundation frame. Between this frame and a

concrete plate of the building the cylindrical rubber pads are installed to reduce forces transmitted to the floor of

the building during the normal operation of the process fan.

*S. Rahmanovic: Tel.: (00387) 61 895-293; Fax: (00387) 35 553-580; [email protected]
mailto:[email protected]:[email protected]:[email protected]


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2 11th Mechatronics Forum Biennial International Conference

2.CAUSESOFVIBRATIONOFAPROCESSFAN

Due to thermal, chemical and mechanical causes an extremely hard coating, the build-up is formed on the

blades and other surfaces of the process fan rotor. During the operation of the fan the parts of build-up break-off

and cause serious unbalance that results in high vibration of rotor and other parts of the fan [1].Other sources of

vibrations of a process fan are caused by faults that are result of shortages in design, manufacturing and

installation of its components but also the result of stresses during operation, poor maintenance, etc.

The most common faults that can appear in this type of process fans are the following:

Imbalance caused by break-off of the parts of build-up, Misalignment of shafts of electric motor and the fan rotor, Eccentricity of rotating parts, such as coupling and fan impeller, Increased clearances in rolling element bearings, key and the keyway in shafts, Wear, cracks and other defects and faults in rolling element bearings, Rubbing between inlet orifice and the fan wheel, Mechanical looseness such as soft foot, looseness of bolts of fan casing and bearing housings, Permanent deformation of the shaft of the fan rotor, Fault in electric motor, such as stator and rotor eccentricity, loose, cracked or broken rotor bars, etc.All listed faults generate vibrations at specific frequencies. Linking them to the particular component it is

possible to identify problem. Amplitude of vibration at each specific frequency determines severity of the fault.

3.CHARACTERISTICFORCINGFREQUENCIES

The running speed frequencies of the electric motor rotor and the fan rotor are in this case equal, since theserotors are coupled. This frequency is the fundamental one and it is present in all vibration spectra of the processfan. Forcing frequencies of all rotating parts should be determined prior to vibration measurement in order toprovide the proper choice of the measuring parameters regarding frequency range [2].

3.1.FORCING FREQUENCIES OF THE ROLLING ELEMENT BEARINGS

As it is well known, the rolling element bearings generate four specific forcing frequencies or bearing tones.

These are FTF, BSF,BPFI, BPFO and they are specified in Table 1.They are not synchronous with rotor runningspeed frequency, but they are function of that running speed and of the geometry of each rolling bearing. Thesefrequencies can be calculated according to the formulae given in literatures, as for example in [3]. The specificforcing frequencies for all bearings installed in the process fan assembly are given in the Table 1.

Table 1: Rolling element bearings specific forcing frequencies

Bearing forcing frequency, Hz Rolling element bearing

NU1030 ECMA SKF6030 23224 CC/W33

Rotational Speed Frequency, RPS 16.6 Hz 16.6 Hz 16.6 Hz

Fundamental Train Frequency, FTF 7.49 Hz 7.3 Hz 7.16 Hz

Ball Spin Frequency, BSF 85.50 Hz 68.90 Hz 58.5 Hz

Ball Pass Frequency Inner Race, BPFI 218 Hz 148 Hz 179 Hz

Ball Pass Frequency Outer Race, BPFO 180 Hz 117 Hz 136 Hz

Amplitudes of vibration of the new bearings at the specific frequencies are very small. But, if the defectsuch as a crack appears, for example, at the outer race of the bearing, the amplitude of a peak at the frequencyBPFO will increase each time the rolling element passes over a crack. Impacts of rolling elements will excitesome of the rolling bearing natural frequencies that are much higher then the BPFI or BPFO frequency [4].Thanks to this fact the defect can be detected using high frequency methods, such as shock pulse method,demodulation of a spectrum, acceleration enveloping method and other advanced methods for rolling elementbearing faults detection. In this paper the spectra of envelope of the vibration acceleration are used to detectdefects in bearings and possibly in other components, for example problems with electric motor rotor. If one ofthe rolling elements would be deformed an increased peak of vibration at the ball spin frequency, BSF can be

expected in a spectrum. This peak may also be surrounded by sidebands spaced at frequency FTF, since thecage rotation is a carrying motion for the balls and therefore frequency FTF modulates their vibratory motion.


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3.2.FORCING FREQUENCIES OF THE ELECTRIC MOTOR

The specific forcing frequencies at which vibrations occur in an electric motor are given in the table 2.

Table 2: Electric motor specific forcing frequencies

Frequency Symbol ValueElectric line frequency FL 50 Hz

Motor slip frequency FS = NS-RPM/60 0.13 Hz

Pole pass frequency, FPP = NP x FS 0.8 Hz

Rotor bar pass frequency FRB = NRB x RPM 927.92 Hz

where:NS synchronous speed of motor,NP number of poles,NRB number of rotor bars.

Frequencies that are linear combination of frequencies FL, FS, FPP and FRB can also be found in a spectrumof an electric motor.

3.3.FORCING FREQUENCIES OF THE FAN ROTOR BLADES

This frequency is called blade pass frequency (FBP), and it is calculated as the product of number ofrevolution of the fan rotor multiplied with number of blades on the rotor. In our case FBP = 11994/60=182.23

Hz. Fan rotor system can generate large amplitudes at blade pass frequency FBP if the gap between rotating vansand stationary diffusers is not equal. This is similar with the problem of electric motor in a case of unequal airgap between rotor and stator described above.

4.APROCCESFAN VIBRATIONMEASUREMENTANDANALYSISOFRESULTS

The vibration signatures are obtained by measuring vibration on the housings of bearings of the fan.Measurement has been done with the portable frequency analyzer Microlog GX-CMXA 70/S051138 and an

accelerometer CMSS2200/S2712, both products of SKF. At all measuring locations L1, L2, L3 and L4 marked inFig. 1, the measurements have been performed in three directions: axial horizontal (A), radial horizontal (H)and radial vertical (V). Measured variables were velocity (VEL), acceleration (ACC) and the envelopedacceleration (ENV) of vibration. Recorded vibration signatures and interpretations are shown in the following.

The velocity spectrum of a vibration signal measured on the housing of bearing at L-1A (location L1, axialdirection) is shown in Fig. 2. The prominent peaks are present at the rotor running speed or fundamentalfrequency 1x and at its 2nd harmonic 2x. As it is well known, a peak at the frequency 1x means an imbalance,which in this case is small. One can notice that the second harmonic peak is higher than the first one. It can be asign of misalignment of the bearing or of the shaft. A peak at the rotor bar passing frequency FRB=927.11 Hz ispresent but its amplitude is very small. The amplitudes of all peaks which appear in the spectrum in Fig. 2 andFig. 3, are inside the limits determined by standards. The velocity spectrum of a vibration signal acquired at L-1H (location L1, radial horizontal direction) is given in Fig. 3. A small peak at a low frequency can be observed.It is fundamental train frequency of the bearing SKF NU1030, FTF=7.49 Hz, given in the Table 1. Although the

amplitude of this peak is small, it should be observed and checked again in the next measurement, since the


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Figure 2: Velocity spectrum of a signal from L-1A. Figure 3: Velocity spectrum of a signal from L-1H.

Figure 4: Velocity spectrum of a signal from L-1V Figure 5: Acceleration spectrum of a signal from L-1H

bearing can fail even with such a small amplitude peak in the velocity spectrum. The peaks at specific bearingfrequencies are very difficult to detect in vibration velocity spectra because they are small for bearings with

defect in incipient stage and therefore they are hidden in the noise floor. This is the reason that special methodsfor extraction of bearing forcing frequencies out of the raw vibration signal have been developed. One of suchmethods is the acceleration enveloping method [5]. It is based on the measuring of high frequency naturalvibration of the bearing components that are excited by consecutive impacts of rolling elements when they passover the defected spot. Vibration signal is filtered to remove all lower frequencies, such as rotor running speed,electric line frequency, blade and electric motor bar pass frequencies and similar ones as well as the very highnatural frequencies of the structure. The filtered signal is then rectified and enveloped by an unbroken line. Thespectrum of this enveloping line for the damaged bearing shows peaks at the specific bearing frequenciesdepending on location of damage [5]. Further in Fig. 3 we see the prominent peak at frequency 1x. It has almostreached limit for the class of machines to which this fan belong. The velocity spectrum of a signal from L-1V isgiven in Fig. 4. The peak at 1x is now smaller than 1x peak in Fig. 3. Reason for this is that the most of therotating machines with horizontal rotor are less rigid in horizontal than in vertical direction. Due to this anunbalanced force produces higher peak in horizontal then in vertical direction, as clearly is seen from Fig. 3 andFig. 4. The 2nd harmonic of running speed frequency, 2x, shown in Fig. 4, is higher than the first one and itindicates the problem of misalignment of the coupling between motor and fan rotor. In both Fig. 3 and Fig. 4the peaks at the FRB frequency are present, but very small in amplitudes.

Spectrum of acceleration acquired at location L-1H, given in Fig. 5, shows prominent peak at frequency FN= 1128.9 Hz which is not one of the previously determined forcing frequencies. Most probably it is the naturalfrequency of inner- or outer ring of bearing or of the bearing housing, at location L 1. To find out this, theadditional measurement like a bump test or a detailed analysis of natural frequencies of the entire system shouldbe performed. Spectra of acceleration measured at locations L-2H, L-3H and L-4H are quite similar to thespectrum in Fig. 5; they all show the peak at FN= 1128.9 Hz but their amplitudes and overall trends are lowerand therefore those spectra are omitted. In Fig. 6 is given the spectrum of the envelope of acceleration measuredat L-1H. Besides the first three harmonics of the rotor running speed which are small in amplitude, thedominating peak is present at frequency 2FL. Also are visible peaks at 4FL, 6FL, 8FL and at FRB frequency. Thepeak at 2FL is related to the electric motor. An open rotor cage, rotor out-of-round, elliptical stator core, statorand rotor misalignment caused by stator or rotor eccentricity, are the electrical problems that produce a peak atfrequency 2FL. In our case the high peak at frequency 2FL is caused by the stator eccentricity which producesunequal but stationary air gap between rotor and stator causing vibration in a specific direction.


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Figure 6: Acc. envelope spectrum of a signal from L-1H Figure 7: Velocity spectrum of a signal from L-2A

Stator eccentricity usually is caused by soft foot due to looseness of connecting bolts, as it was the case withour fan. Rotor eccentricity however produces variable, rotating air gap inducing pulsating vibration between2FL and nearest harmonic of rotor running speed, [6]. In Fig. 6 we see that the frequency 2FL is followed bypole pass frequency FPP which, according to [6] indicates an eccentricity of the electric motor rotor. The FPP

sidebands that surround also the frequency 1x and its harmonics support this assumption.

Figure 8: Velocity spectrum of a signal from L-2A Figure 9: Velocity spectrum of a signal from L-2A

Faults such as loose, cracked or broken rotor bars cause vibrations at rotor bar pass frequency, FRB. Peakscaused by these faults are always surrounded with sidebands spaced at 2FL. Such sidebands we see in thespectra of enveloped acceleration which are shown in Fig. 6 and Fig. 10 and in velocity spectra given in Fig. 7and Fig. 8. According to [6], cracked rotor bar generate FPP sidebands around 3

rd, 4th and 5th harmonics of rotorrunning speed. In Fig. 6 such sidebands are present only around rotor running speed frequency and its 2 nd and3rd harmonics, but they are difficult to notice due to insufficient resolution of the spectrum. The appearance ofsidebands in the spectrum spaced at 2FL around FRB, is an indication of the slight looseness of a rotor bar. Thus,it should be regularly checked to follow development of this fault. It can be noticed that the frequency FRB ispresent only in spectra of signal acquired at housings of bearings of electric motor, locations L1 and L2.

Figure 10: Acc. envelope spectrum of a signal from L-2H Figure 11: Velocity spectrum of a signal from L-3A


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Figure 12: Velocity spectrum of a signal from L-3H Figure 13: Velocity spectrum of a signal from L-3V

Figure 14: Acc. envelope spectrum of a signal from L-3H Figure15: Velocity spectrum of a signal from L-4A

Figure 16: Velocity spectrum of a signal from L-4H Figure 17: Velocity spectrum of a signal from L-4V

The spectra in Fig. 7, 8, 9, 11, 12, 13, 14, 15, 16 and 17 show imbalances that are indicated by peaks at 1x,looseness and clearances indicated by 2nd, 3rd, 4th and other harmonics and misalignments that exist if the 2ndharmonic is higher than the fundamental frequency, 1x. Spectra in Fig. 10, 11 and 14, contain yet the specificbearing frequencies FTF, BSF, BPFI and BPFO, whose values are listed in Table 1. Peaks at blade passingfrequency FBP = 182.27 Hz appear in spectra given in Fig. 13 and Fig. 15. These peaks could be caused byunequal gap between fan wheel and its casing or by rubbing between inlet orifice and the hub of the fan wheel.

5.CONCLUSIONS

From the presented experimental results it can be concluded that the faults such as imbalance of the fan andelectric motor rotor, coupling misalignment between rotors of the electric motor and the fan, stator and rotor

eccentricity, loose or broken rotor bars and similar faults, can be detected by using vibration velocity andacceleration spectra together with overall readings that have been given in each vibration signatures. The


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defects in rolling bearings that can be recognized at bearing specific forcing frequencies in an incipient stage aredifficult to detect by using velocity and acceleration spectra. They can be easier detected by using envelopedacceleration spectra since this method enables extraction of specific forcing frequencies of rolling bearings.

REFERENCES

[1] S. Rahmanovic, V. Dolecek, "The Influence of The Build-up of Firm Particles on Impeller on the Vibration of aProcess Fan", Proceedings of 6

thInternational Scientific Conference on Production Engineering Development and

Modernization of Production, pp. 103-104, Plitvice, Croatia, 2007.[2] V. Wowk, Machinery Vibration-Measurement and Analysis, McGraw Hill inc.,USA, 1991.[3] S. Sassi, B. Badri and M.Thomas ,"A Numerical Model to Predict Damaged Bearing Vibrations", Journal of

Vibration and Control, 2007; 13; 1603

[4] Barron R: Engineering Condition Monitoring-Practise, Methods and Applications, Addison Wesley Longman, NewYork, 1996.

[5] http://www.ludeca.com/casestudy/vibration_uptime0206.pdf [6] http://www.sea.siemens.com/motorsbu/product/white papers/vibration problems.pdf
http://www.ludeca.com/casestudy/vibration_uptime0206.pdfhttp://www.ludeca.com/casestudy/vibration_uptime0206.pdfhttp://www.sea.siemens.com/motorsbu/product/white%20papers/vibration%20problems.pdfhttp://www.sea.siemens.com/motorsbu/product/white%20papers/vibration%20problems.pdfhttp://www.sea.siemens.com/motorsbu/product/white%20papers/vibration%20problems.pdfhttp://www.ludeca.com/casestudy/vibration_uptime0206.pdf

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S Rahmanovic Et Al_Mechatronics 2008-ID048_Ireland 2008