Abstract: By collecting and analyzing vibration signals, identifying abnormal vibration faults in water pumps, and solving them, we provide guarantees for the safe, reliable, and stable operation of oxygen generating units.
Keywords: water pump; Bearing vibration; Signal analysis; Spectrum; fault
The auxiliary equipment water pump plays an irreplaceable and important role in the operation of the oxygen generation unit. The quality of the water pump device directly affects the safe operation of the oxygen generation unit.
The commonly used water pumps in oxygen production systems include nitrogen water pumps, cooling water pumps, room temperature water pumps, and lifting water pumps. The different working states of each water pump are determined based on their power, efficiency, head, flow rate, and other factors. Ma Steel Gas Sales Branch has nearly 50 large and small water pumps. In addition to monitoring large oxygen concentrator units, we also spend a lot of time and energy testing the water pumps. The faults of the water pumps can be basically determined through five senses and spectrum reading methods.
Rolling bearings are important components of water pumps, and damage to rolling bearings accounts for over 70% of water pump failures. The fault manifestations of rolling bearings include peeling, dents, fractures, poor lubrication, corrosion, overload, wear, and foreign object intrusion on the surface of the ball and raceway. The causes include improper installation, poor alignment, tilting of the bearing, incorrect selection of the bearing, lubrication or sealing failure, inappropriate load, and manufacturing defects. Z will eventually cause peeling pits on the surface of the rolling element or raceway, and develop into large peeling areas, leading to damage to the inner ring, outer ring, retainer, and rolling element, ultimately resulting in bearing failure. The analysis of multiple cases of water pump bearing failures encountered in work is as follows.
Case 1
The working parameters of the faulty small water pump are as follows.
Motor power: N=75kW
Speed: n=2975rpm
Flow rate: Q=13.3m3/min
Head: H=125m
On the morning of May 24, 2007, during the inspection of standardized operations, it was found that the vibration of the water pump (WP1103) was abnormal (see Table 1).
Figure 1 WP1103 Water Pump Layout
Table 1 Vibration values at various measurement points of WP1103 water pump
The vibration value of 4A on the load side of the water pump reached 13.1mm/s (see Figure 2 and Table 2), and the amplitude of FTF17.5Hz was 4.09mm/s. The frequency of the cage passage was more obvious, and the low-frequency signal was concentrated. We believe that the looseness or wear of the rolling element increased, resulting in a significant increase in the axial vibration value compared to the previous stable operation. The frequency characteristics of the faults are relatively easy to determine. We arranged for maintenance and replacement of the bearings. During the inspection, it was found that the bearings were worn for a long time, causing the gap between the retainers to increase and resulting in looseness. The fault diagnosis is basically consistent with the maintenance results.
Figure 2 Frequency spectrum of WP1103 water pump bearing cage passing through
Table 2 Frequency spectrum and amplitude of WP1103 water pump bearing cage passing through
The vibration amplitude at the frequency of 7.5Hz on the power spectrum diagram 4 after partial amplification and maintenance is only 0.02 (mm/s).
After maintenance, data collection was carried out on the water pump, and it was found that the frequency amplitude of the retainer decreased significantly, and other vibration frequencies of the water pump also decreased (see Figure 3, Figure 4, and Table 3 after maintenance). The operation of the water pump returned to normal. By replacing the rolling bearings, hidden dangers and defects were eliminated, and daily maintenance achieved the goal.
Figure 3 Frequency spectrum of WP1103 water pump bearing retainer after maintenance
Figure 4: Partial amplification spectrum of the passing frequency of the WP1103 water pump bearing retainer after maintenance
Table 3 Vibration frequency and amplitude of WP1103 water pump after maintenance
Case 2
On the early morning of January 12, 2007, at 6:00 am, operators discovered during an inspection that the oil sight glass of the WP1104 water pump in an oxygen production unit had fallen off, and there was a large pool of oil stains underground. The sight glass had a burnt mark, and there was a crack about 1.5cm long at the bottom of the tooth buckle. Observing the pump body, there are signs of peeling and peeling of paint at the bearing end covers at both ends, and overheating on the coupling side. The water pump had already been shut down immediately, and upon receiving dispatch instructions, the inspector arrived at the site and was unable to collect data. After arranging the disassembly of the water pump, it was found that almost all of the bearing balls and retainers on the impeller side of the water pump were burnt out, and the retainers had been scattered. The outer raceway (ring) of the rolling bearing was severely burnt and turned blue locally. The back measurement of the impeller showed severe carbon deposition on the bearing, and cracks appeared on the bearing end cover. This malfunction is caused by a lack of oil in the bearing, causing the bearing to overheat instantly. Heat is transferred to the pump body. When the temperature reaches a certain level, the sight glass melts at high temperature, and oil flows out of the sight glass, burning the rolling bearing due to the lack of oil. Figure 5 shows poor lubrication causing bearing damage.
Figure 5 Bearing damage caused by poor lubrication
Case 3
The 350S-75 single-stage double suction, horizontal split centrifugal water pump is responsible for the cooling water supply of the second nitrogen compressor and second liquefaction station of the 20000 oxygen concentrator and 35000 oxygen concentrator units of Ma Steel Gas Sales Branch.
The working parameters of the 350S-75 water pump are as follows.
Motor power: N=360kW
Shaft power: N1=304kW
Speed: n=1450rpm
Flow rate: Q=21m3/min
Head: H=70m
The impeller of the water pump undergoes static balance inspection and is fixed with shaft sleeves and shaft sleeve nuts on both sides. Its axial position can be adjusted through shaft sleeve nuts. The axial force of the impeller is balanced by the symmetrical arrangement of its blades, and some remaining axial force is borne by the bearings at the end of the shaft. The water pump shaft is supported by two single row radial ball bearings, which are installed in the bearing shells at both ends of the pump body and lubricated with grease. The water pump is directly driven by an elastic coupling and an electric motor, with a tolerance of 0.1mm for different centricity on the outer circle of the two couplings, and a tolerance of 0.3mm for unevenness of the end face clearance along the circumference.
Since its production in July 2003, the pump has been operating unevenly and has experienced shaft breakage accidents. On March 8, 2007, during the inspection, it was found that the 2 # water pump had high noise. A vibration signal was collected using a vibration meter (see Table 4, Figure 6, and Figure 7), and the values fluctuated greatly. The vibration value at the 3H position of Z was as high as 22mm/s. Note: FTFI cage passage frequency; BSF rolling element passing frequency; BPFO outer ring passing frequency.
Table 4 Vibration frequency and amplitude of 2 # water pump
Figure 6 Vibration Spectrum of 2 # Water Pump
Figure 7 Partial Enlarged View of Vibration Spectrum of 2 # Water Pump
We believe that the fault characteristic frequencies of the cage passing through frequency, rolling element passing through frequency, and outer ring passing through frequency are particularly obvious, that is, the FTFI amplitude is large, and the BPFO also shows abnormalities. Therefore, we arrange to stop the machine for maintenance and replacement of the bearings.
During the disassembly of the water pump, it was found that the bearings were damaged, the retainer had broken into several sections, the ball bearings were missing, and there were cracks on the outer ring. Due to the thorough inspection and specialized inspection of the operation position, accurate judgment, and timely maintenance, no major equipment accidents were caused.
Since the water pump was put into production in July 2003, due to various problems such as improper bearing assembly and installation quality of the entire water pump system, the motor foundation vibration has been large. The vibration of the water pump bearings has intensified in the short term, the wear of the impeller has increased, and the torque deformation of the shaft has occurred. Even in May 2004, the fault of the water pump shaft fracture occurred, seriously affecting the normal and safe operation of the 20000 oxygen production unit. For this reason, professional equipment inspection personnel will focus on monitoring the water pump. On March 6, 2007, during the daily inspection of the pump, the inspector used a 701 spectrum data collector to collect vibration data from points 3 and 4 of the water pump bearings. It was found that the vibration value of the bearing on the load side of the water pump was too high, the sound was abnormal, and the motor current increased accordingly. Then closely monitor the pump for three consecutive days. On the 9th, when collecting vibration data at inspection points 3 and 4, the vibration value significantly increased compared to the previous days, accompanied by fluctuations. The bearing made a "choking" sound. Through data comparison analysis and abnormal sound reference, it was preliminarily determined that there may be a defect in the outer ring or retainer on the load side of the pump, and it was immediately shut down for maintenance. The maintenance personnel disassembled and inspected the pump, and found three irregular cracks on the outer ring of the load side bearing and several segments of broken cage, which basically confirmed the diagnosis made by the diagnostic personnel. This method of using detection instrument data analysis and comparison can detect and predict faults in advance and in a timely manner, avoiding the expansion of the entire water pump fault. It not only saves costs, improves work efficiency, but also extends the operating cycle of the equipment.
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2024-07-07