1 Overview
Our factory has five centrifugal induced draft fans for boilers, with fans Y6-51№25D. These fans are the key equipment in the dry purification process of electrolytic flue gas. Only when the fans operate normally, the flue gas purification system can operate normally and the gas collection efficiency of flue gas can be guaranteed, can the electrolytic flue gas emission reach the national flue gas emission index. The imbalance of induced draft fan is an important problem in the normal operation of the equipment. In order to reduce the failure rate of induced draft fan, the author makes some discussions on the imbalance of induced draft fan.
The vibration spectrum analyzer is used to monitor the vibration of the bearing seat, and the fault of unbalanced fan is found and solved, which reduces the vibration of the induced draft fan and makes the vibration value within the normal operation range. It ensures the normal operation of the fan and the normal production.
2 Relative phase balance method analysis
The relative phase balance method is usually used in the field dynamic balance test of fan rotor. That is, by applying a trial mass to the fan rotor, the vibration vector of the rotor changes, and the size and position of the original unbalanced mass of the rotor are calculated by using the relative change of this vector. Then, a balance mass is welded at an appropriate position in the opposite direction of the original imbalance to balance the rotor.
From the above, it can be seen that the relative phase balance method must have the process of trying to add mass. Even to balance the simplest rotor, it is necessary to stop the machine at least twice on the basis of the original unbalanced vibration data, so that the rotor can be balanced.
First shutdown: measure the vibration data after installing the trial mass.
Second startup shutdown: according to the measured vibration data, the size and position of the balance mass should be calculated by using the influence coefficient method, and then added to the rotor to measure the vibration data.
If that measure vibration data meets the vibration standard, the production is run. If the measured vibration data does not meet the vibration standard, the third start-up and shutdown shall be carried out. Take the measured residual vibration as the base, calculate, add weight and carry out dynamic balance again.
In this way, the field dynamic balance test of the fan rotor is completed, and the number of fan starts and stops is more, which not only affects the production, but also affects the service life of large motors. If the dynamic balance test of fan rotor can be completed with only one weighting, it will not only reduce the number of motor starts, improve the service life of the motor, reduce the impact on the power grid, but also greatly reduce the downtime and improve the output. Therefore, after careful study and exploration, a weighted balance method is summarized.
3 One-time weighted balance method
Since 1998, the field dynamic balance test of fans has been carried out, and the relative phase balance method has been adopted. After several years of field dynamic balance test of fan rotor, abundant information and data have been accumulated. Through careful arrangement and analysis of these materials and data, some laws of field dynamic balance of fan rotor are found out, and on the basis of summarizing the laws, a weighted balance method is explored. That is, the magnitude and phase of the unbalanced mass of the fan rotor can be found out only by measuring the original unbalanced vibration data of the fan rotor. In this way, the field dynamic balance of the fan rotor can be completed only by stopping the machine once.
During the dynamic balance of the fan rotor, the measured vibration quantities: amplitude and phase, are the vibration displacement and phase of each bearing bush of the fan, but the magnitude and phase of the unbalanced mass cannot be measured, but there is a specific law between the measured vibration vector and the original unbalanced mass. namely
α = ψ-ψ
Where α is the unbalanced mass phase; ψ measurement is the measured vibration phase; ψ is a constant value for a fixed vibration measuring system.
The position where the balance mass should be added should be in the opposite direction of α, that is, β = α+180.
Therefore, the phase β = ψ-ψ+180 of the balance mass should be added, and the determination of ψ value is an important link of the first weighted balance method. In recent years, the primary weighted balance method has been applied to the field dynamic balance of Y4-73-11№29.5D and Y4-73-11№23.5D induced draft fan rotors, and good results have been achieved.
4 the practical application of the first weighted balance method (analysis example)
4.1 Main parameters of induced draft fan and motor
Flow: q = 300,000m3/h
Total pressure: 4.488kPa
Medium density: 0.745 kg/m3.
Motor power: 630kW
Motor speed: 990r/min
Lubrication method: 30 # mechanical oil is thrown.
Bearing inner diameter: 210mm.
4.2 Find the problem
During the condition monitoring of the exhaust fan, it was found that the radial vibration of the bearing seat at the impeller end of the 4# exhaust fan suddenly increased. After collecting the vibration data with the data collector, we can see that the vibration has the following characteristics:
The waveform of vibration is close to sine wave;
On the vibration spectrum, the 1× power frequency is abnormally high, as high as 6.32 mm/s. Although 2×, 3× and 4× power frequencies appear, they are all low.
4.3 Solve the problem of dynamic balance
According to the above two points, it is judged that the fan is an unbalanced fault. The fan was measured and processed by dynamic balance instrument, and the field dynamic balance test was carried out by once weighted balance method. Specific steps:
(1) measure the original value, and the vibration data obtained is 6.32 mm/s rms57;
(2) Take any point A on the impeller as 0, add a 290g trial weight block here, and start the fan. The vibration speed and phase (degree) of the front bearing of the fan are measured by the field dynamic balance instrument: 11.4 mm/s rms 49;
(3) According to the above two measurement data, the dynamic balance instrument obtains 352.2g Location 197 after calculation by the dynamic balance instrument;
(4) Remove the 290g steel plate;
(5) According to the data obtained from the dynamic balance instrument, weld 320g (352.2 minus the covered electrode mass) of the counterweight at the corresponding position of the fan blade at the position of reverse rotation of 197 from point A.. After welding, restart the fan. The corresponding data is 1.26 mm/s RMS 140 (Table 1), the fan vibration value reaches the normal range, and the dynamic balance fault is handled.
Table 1 Data on Dynamic Balancing Instrument
Before treatment
6.32
57
Add test weight
11.4
forty-nine
After treatment
1.26
140
5 Economic benefit analysis
One-time weighted balance method is used for dynamic balance, which reduces the starting times of large fans, improves the service life of fan bearing shells and motors, and reduces the impact on power grid. Compared with the phase balance method, it takes at least 8 hours to stop the machine twice, while it takes only 2 hours to complete the dynamic balance with the one-time weighted balance method, thus reducing the downtime by 6 hours each time. Due to the wear of the fan rotor, the dynamic balance should be carried out at least once a year on average, and the downtime should be reduced by 6 hours every year. Due to the continuity of electrolytic production, if the fan abnormally affects the feeding, it will cause the reduction of electrolytic cell production. Based on the loss of 2.5t primary aluminum per hour and the profit of 8000 yuan per ton of aluminum, the loss of 6 hours is 2.5× 8000× 6 = 120000 yuan = 120000 yuan. Using the dynamic balance instrument, the unbalanced fault of the fan is dealt with by the one-time weighted balance method, which is basically one-time balancing to realize the balance of the fan, which is very simple and practical.