EXPERIMENTAL INVESTIGATIONS OF VIBRATIONS OF CENTRIFUGAL FIRE PUMP WITH THE DEFECT ( UNFASTENED SHAFT IN A BEARING SEAT )

Objective: The paper deals with the experimental investigations of the vibrations of the centrifugal fire pump with a hidden defect. The aim of the work is to identify the vibrational symptoms which accompany the operation of a fire pump with a defect (unfastened shaft in a bearing seat). This information is applicable for the technical diagnostics of the fire pumps in operation. Project and methods: The research consisted in a set of experimental tests of a centrifugal pump installed on the active fire engine. The experiments were carried out with the application of the measurement system “Ultra-V-I”. The system has a valid certificate of state metrological certification, issued in Ukraine, and allows to make measurements of vibration acceleration of the investigated object. All signals were spectrally analysed. The Fast Fourier Transform (FFT) was used for these procedures. A spectral analysis for transient regimes of vibrations was carried out with the application of short-time Fourier transform. Results: The changes in the spectrum and orientation of vibrations fire pump with the mentioned type of defect were identified. The analysis of the spectra of vibrations at the frequency of excitation of 2000 rpm in the vertical direction shows strong domination of 7 harmonic (233 Hz). Besides, the spectrum involves first (33.3 Hz), third (99 Hz) and fourth (133 Hz) harmonics of rotor rotation. However, the most important phenomenon consists in the presence of fraction harmonics in the spectrum, which in turn indicates system nonlinearities caused by loosening and contact impacts. These harmonics are 0.5 and 3.5 harmonics of rotor rotation (16.5 Hz and 116.6 Hz). A quantitative comparison shows that the largest vibration level is observed in the axial direction and the smallest one is observed in the horizontal direction. The comparison of vibrations of the defected and non-defected fire pumps shows that the defect makes an axial direction weaker and it leads to increasing axial vibration acceleration up to 80%. Vibrations in other directions are approximately the same in case of defected and non-defected pumps. Changes in the orientation angle of the vibrations ellipse trajectory were identified. Conclusions: The information about vibrational symptoms of the analysed type of hidden defect is useful in fire pumps technical diagnostics. This method will improve the quality of systems technical check-ups as well as their preventive maintenance. This in turn will contribute to the decrease in the amount of firefighting equipment left standing idle due to untimely repair works as well as prevent sudden failures of the utilised pumps.


Introduction
One of the most important elements of the emergency response divisions equipment is the fire engine with a centrifugal pump.Extreme operation conditions together with the long-term utilisation substantially increase the probability of hidden defects in the centrifugal fire pumps.These defects may significantly decrease the effi-ciency of the operational activities of the emergency response divisions, since they can lead to a sudden failure or negative influence on the fire-pump performance characteristics, such as discharge head, for instance.A current system of fire pump reliability control is based on the system of technical check-ups and preventive maintenance (repair).This system is effective if qualitative dia- The experience in the area of fire pump operation indicates that common defects usually lead to the higher vibration levels during the pump operation.Thus, vibration diagnostics can be used as a good tool for identification of technical state of centrifugal fire pumps, as it is commonly used [1] for the technical diagnostics of different rotor machines (turbines, compressors and etc).

Testing technique
The methodology of experimental studies is based on a series of measurements which allow defining a regularity of changes in vibration parameters caused by the presence of the hidden defects.Thus, a comparative analysis of changes in the dynamics of the systems with and without the defect provides a possibility to define hidden defects symptoms.In consequence, it provides us with tools for diagnosis of the device's technical state.

Experiment conditions and facilities
In the work, the experimental tests have been done with the centrifugal pump, which has been installed on an active fire engine.
Experiments were carried out on the base of a measurement system "Ultra-V-I" [2,3], which has been developed at the Department of the Dynamics and Strength of Machines of the National Technical University "Kharkiv Polytechnic Institute" (Kharkiv, Ukraine).The measuring system consists of (fig.1): a detector of vibration accelerations, an analogue-to-digital converter (ADC) and a portable computer."Ultra-V-I" has a valid certificate of state metrological certification and allows to make a measurement of vibration acceleration at the point of investigated object.Some details of experiments study facilities are presented in the paper [4].The detector of vibration accelerations was equipped with the MEMS sensor ADXL250 (Analog Device, USA).This is a capacitive sensor that allows to carry out measurements of the accelerations with amplitudes in the range from 0.5 to 40 m/s 2 and in the frequency range from 1 to 400 Hz.The measurements have been carried out on the body of a suction pipe of the fire pump (fig. 1) in 3 directions: axial (Z), vertical (Y) and horizontal (X).The registration of the vibro-acceleration signals were carried out simultaneously in 3 dimensions with sampling frequency of 25 kHz.A circumferential velocity of rotor rotation was also measured concurrently to the main signals.

Vibration analysis of the non-defected centrifugal fire pump
The analysis deals with an experimental study of vibration occurring during the operation of the fire pump PN-40UV that has no defects.The study of pump vibrations has been carried out on the most loaded regime, that provides a pump with an opportunity to realise different dynamic peculiarities.
The Measurements have been carried out on the body of the suction pipe of the fire pump.During these tests, an angular velocity of rotor was set near the maximum operational value that was slightly more than 2000 rpm.It should be noted that the angular velocity of the pump rotor could not be precisely fixed due to the presence of irregularity of the engine motion.Moreover, the angular velocity of the pump is controlled manually, so a human element is always present.
Therefore, it is necessary to control the angular velocity of the pump rotor rotation during measurements.It was decided to use frequency counter during the current investigation.The counter was installed at the connection of the pump rotor with a fire engine shaft.The data from frequency counter have been transmitted to the ADC and recorded on a computer in real time.Fig. 2b presents vibrational acceleration obtained in the vertical direction at the measurement point.A presence of the random fluctuations and polyharmonic nature in the signal can be observed.At the same time, the signal has no outliers, periodic impacts or other evident signal distortions.This confirms the absence of the defects in the pump and its well performed assembling.Analogous tests with the over values of pump rotor angular velocities have been carried out to obtain more detailed information about the vibrational processes.Generalization of these results can be obtained by special tests, when vibrational signal is analyzed during continuous gradual increasing of the frequency of excitation.So, the test was made with the pump that operates with the angular velocity, which is gradually increased from 800 rpm to 2100 rpm.
The obtained signals show dependence on time of angular velocity of the pump rotor (Fig. 3,a), and vibrational acceleration (Fig. 3,b).The test was held for 30 seconds.
All the obtained signals were spectrally analyzed.A Fast Fourier Transform (FFT) is used for these procedures [5,6].A vibration acceleration signal, which was obtained at the test with continuous excitation frequency increasing, was analyzed using spectrograms [7].The signal was divided into separate fragments (windows).FFT analysis has been carried out with the signal fragment on each window.The map of spectrums has been built in the next step.The axis X of spectrum map presents the rotor angular velocity (or the equivalent time of test process), the axis Y corresponds to the frequency of obtained vibrations and the different colours shows the level of the amplitude at these frequencies (the largest amplitude in black and the smallest one in white).This procedure is called short-time Fourier transform and resulting map is called spectrogram.
The width of the window was set to 0.5 seconds.This value on the one hand is sufficient to obtain correct results with FFT transform within each window (lowest operating frequency of the signal corresponds to 800 rpm, so equal to 13.3 Hz, that means that at least 10 periods analy-sed on each window).On the other hand, this width of the window allows to get results in more than 60 windows at the signal and obtain a continuous field of changing of the vibration spectrum over the time with increasing frequency of load.
Fig. 4 presents vibration spectra at the fixed value of the pump rotor angular velocity (2000 rpm) and spectrograms for the vibration accelerations in three different directions.It should be noted that high harmonics have essential amplitudes in the spectrums of pump vibrations that even exceeded first based harmonic.It has become apparent especially for the 7 harmonic.This harmonic dominates all others in spectrums across the range of frequency of perturbation, especially for vertical and axial directions.
Existence and domination of 7 harmonics are associated with turbulent flows in the chamber of the pump that forms near the edges of driven wheel blades.The pump driven wheel has 7 blades.
A quantitative comparison shows that the largest vibration level is observed in the vertical direction and the smallest one is observed in the horizontal direction.
A detailed analysis of the orientation of vibration at the point of the pump can be made from the spatial trajectory of the acceleration vector.Corresponding results are shown in the Fig. 5.
All projections of trajectory form elliptic shapes.A major axis of the ellipse of the YZ projection of the trajectory of the vibration acceleration vector shows an inclination from the axial direction.The angle of the inclination is around 55 o .

Investigations of the fire pump vibrations that has unfasten shaft in a bearing seat
A defect consisting in the unfastened shaft installation in a bearing seat is a widespread imperfection that accompanies the centrifugal fire pumps in operation.The shaft which has a smaller radius on 0.03mm at the place of his landing on the first bearing was installed into a new non-defected fire pump in order to study vibrations of the pump with only unfastened shaft defect inside it.

a b
Spectrums of pump vibration at rotor angular velocity of 2000 rpm and spectrograms at different frequencies of excitation are shown in Fig. 6.
An analysis of vibrations at the frequency of excitation of 2000 rpm in the vertical direction which is presented on the Fig. 6,a shows a strong domination of 7 harmonic (233 Hz).
Besides it, in the spectrum exists first (33.3Hz), third (99 Hz) and fourth (133 Hz) harmonics of rotor rotation.However, the most important phenomenon is in the pre-BiTP Vol.34 Issue 2, 2014, pp.133-141 DOI:10.12845/bitp.34.2.2014.13sence of fraction harmonics in the spectrum, which indicates the presence of system nonlinearities caused by the presence of looseness and contact impacts.These harmonics are 0.5 and 3.5 harmonics of rotor rotation (16.5 Hz and 116.6 Hz).
The same result can be seen on the spectrogram, which is shown in Fig. 6,b.The analysis of presented measurements indicates a significant domination of the 1st (main) and 7th harmonics of rotor rotation.
Besides, the presence in the spectrum of fractional harmonics is also confirmed.The harmonic of the 0.5 of rotor rotation is observed in the all range of frequency of excitation.The harmonic with frequency 3.5 of rotor rotation appears only at high levels of vibration that is at the highest frequencies of excitation.
Analysis of spectrum and spectrogram for vibrations in the axial direction are close to the corresponding spectrum and spectrogram of non-defected fire pump vibrations at the appropriate regime.The harmonic with the frequency of 0.5 of rotor rotation is also presented in the spectrogram of vibrations in the axial direction.
A quantitative comparison shows that the largest vibration level is observed in the axial direction and the smallest one is in the horizontal direction.A comparison of vibrations of the defected and non-defected fire pump showed that defect makes the axial direction weaker and this leads to increasing axial vibration acceleration up to 80%.Vibrations in other directions are approximately at the same level when it comes to defected and non-defected pumps.
Another feature that accompanies the unfastened shaft defect are changes in the spatial form of vibration.Indeed, the presence of such defect leads to the rearrangement of system stiffness that becomes a cause of the changing of vibration modes.An analysis of this phenomenon can be realised relying on the comparison of the spatial trajectory of the vector of vibration acceleration for defected and non-defected pumps.

Conclusions
Thus, in the current experimental studies of vibrations of centrifugal fire pump PN-40UV without defects and with unfastened shaft in a bearing seat.A spectral analysis of vibration and investigation of the forming of spatial trajectory of vibration acceleration vector have been carried out.It should be noted that obtained results on spatial trajectories as well as conclusions about levels of vibrations magnification cannot be generalized for other types of a centrifugal pump and should be used for diagnostics of the PN-40 UV fire pump which is installed in an emergency vehicle.
It was determined that the spectrum of vibrations of the pump with mentioned type of defect have fractional harmonics (0.5 and 3.5 of the rotor rotation).The changing in the spatial orientation of the vibration of the defected pump has been defined.So, a non-defected pump has vibrations with domination in the vertical direction, but a defected fire pump vibrations are axially oriented.
These vibrational symptoms of the analyzed type of hidden defects can be used in the procedures of fire pumps technical diagnostics.A usage of this technique will im-prove system technical check-up and preventive maintenance, that, on the one hand, leads to a reduction of the idle standing of fire equipment that are associated with untimely repair work, and on the other hand, will prevent sudden failures of pumps in operation.

Fig. 2 ,
a shows a typical realisation of time depending changing of the rotor angular velocity during the test.Average frequency was <ω> = 37.73 Hz that corresponds to 2264 rpm.Variations in the frequency was observed in the range from 35.67 Hz to 39 Hz (2140 rpm -2340 rpm) with dispersion σ 2 ω = 0.71 Hz 2 .

Fig. 2 .
Fig. 2. A typical obtained signals during the tests of fire pump vibration a -time varying angular velocity; b -recorded vibration acceleration signal

Fig. 3 .Fig. 4 .
Fig. 3.The test of the pump vibration with the continuous gradual increasing of the frequency of excitation a -registered signal from the frequency counter (rotor angular velocity in time); b -registered vibration acceleration signal in the vertical direction

Aleksandr Larin -
Professor of the Department of General, Emergency and Rescue Engineering of the National University of Civil Defence of Ukraine, Doctor of Science, Professor.Sphere of scientific interests: operation and repair of vehicles, fire and emergency engineering, diagnostics of fire and rescue equipment.The author (co-author) of more than 200 scientific publications.Aleksii Larin -Associated Professor of the Department of Dynamics and Strength of Machines of the National Technical University "Kharkiv Polytechnical Institute", PhD.Sphere of scientific interests: prediction of the reliability of mechanical systems, nonlinear and random vibration of machines and machine elements, vibration measurements, technical diagnostics.The author (co-author) of more than 70 scientific publications.Іgor Ushapivsky -Head of the department of emergency response of the General Directorate of State Service for Emergency Situations of Ukraine in Lviv region.Sphere of scientific interests: operational activities in emergency elimination, fire engineering, diagnostics of fire and rescue equipment.