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

Abstract

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.