Analysis and treatment methods for excessive vibration of condensate pump bearings

Abstract: The vibration of the condensate pump unit of the steam turbine exceeds the standard, and the wear rate of the bearings is also relatively fast, which causes serious interference to the stable operation of the pump unit. In order to eliminate the hidden dangers of the equipment, the author analyzed and dealt with the causes of vibration to ensure that the pump unit operates in a safe and stable state.

Keywords: condensate water pump; Vibration reasons; Bearing wear

1. System Introduction

The specification of this unit is 500MW beyond the Russian limit. Unit 1 was put into operation in May 1998, while Unit 2 was put into operation half a year later than Unit 1. This steam turbine is produced by the Leningrad Metal Factory in Russia. The specific specification is the K500-240-4 type supercritical single shaft four cylinder four exhaust condensing steam turbine with intermediate reheating. Its Z-high power can reach 525MW, while the rated power is 500MW. Three stage condensate pumps will be equipped in each unit, with two in each stage. If during normal operating hours, one is running while the other is a backup.

Each stage of the condensate water pump is a Russian made vertical pump with different lifting pressures, so the structure is also different. The first stage condensate pump has two sets of impellers, while the second stage condensate pump has one set of impellers, and the third stage condensate pump has two sets of impellers. We can install an inducer in front of the first stage impeller at the inlet of each pump, so that the anti cavitation ability of the impeller will be improved.

2. Operation of condensate pump

Our company uses a Russian made 500MW supercritical unit, which was put into operation in May 1998 and November 1998, respectively. Therefore, it has been in operation for over a decade. After a long period of operation, we found that there were problems with the operation of the KCB series Russian made vertical condensate water pump, and unstable abnormal conditions occurred frequently. Excessive vibration is particularly prominent. In order to complete on-site vibration monitoring, we used a handheld vibration method. The vibration ranges of the first, second, and third stage condensate pumps are 11-0.15mm, 0.08-0.12mm, and 0.08-0.13mm, respectively. It can be seen that the range of these vibrations exceeds the regulations (the standard should be less than 0.08mm), which will pose a great threat to the safe operation of the pump unit. During the monitoring of the first stage condensate water pump RM840 by the staff in January 2010, it was found that there was still abnormal noise in the internal rolling of the bearing when the bearing vibration did not increase. In the following month, we conducted monitoring again and disassembled the pump. We found that there was a local detachment of the bearing track alloy and the cage was also broken. In addition, due to wear and overheating of the bearings, there are signs of blackening on the surface of the local alloy.

There is also an abnormal phenomenon, which is that the oil and metal wear monitoring in the bearing chamber repeatedly exceeds the standard. Our company uses a new type of oil quality monitoring device, which can complete the oil quality monitoring of attached rotating machinery. In addition, the staff also conducted a sampling inspection on the oil quality of the bearing chamber of the condensate pump. The inspection result showed that the oil quality of the condensate pump contained significantly more metal particles than other pump groups, and also exceeded the index 50 standard for rare lubricating oil iron loss. Therefore, it is necessary to replace the pump sets that exceed the standard with new oil. After running for two months, the metal particle size in the oil gradually increased and Z eventually exceeded the standard.

3. Analysis and handling of abnormal situations in condensate water pumps

After analyzing the causes of excessive vibration in rotating machinery, it was found that there are two main reasons: imbalanced rotor quality and poor installation quality. Unbalanced rotor mass refers to the situation where the center of mass of all accessories is not on the centerline of the rotor during installation. Therefore, stable power frequency vibration is the main component in all signals of the vibration spectrum. The causes of rotor mass imbalance include bending of the rotor, loosening of components during rotation, and original imbalance. The different forms of unbalanced vibration result in different characteristics: (1) The original mass imbalance occurs before the rotor rotates, which means that the imbalance has already occurred before the rotation. This imbalance often occurs during the manufacturing process, some during maintenance, or during component replacement and rotation. The characteristic of this imbalance is that when monitoring the amplitude deterioration phase at a certain speed, it is generally relatively stable. (2) Loose rotating parts. The rotating components include all the components assembled on the rotor, while the condensate pump mainly includes the following components: water tiles, spiral sleeves, impellers, inducers, bearings, and various positioning sleeves. Power frequency vibration is largely generated by loosening, and this type of vibration is sudden. The rapid increase in vibration of a certain bearing to a fixed value can occur within a few seconds. In addition, there will also be a fixed change in phase. The vibration of adjacent bearings may also increase, but the magnitude of the change is relatively small. (3) Bending of the rotor can also cause mass imbalance. When this situation occurs, the power frequency vibration will change over time. The longer the operation time, the more the power frequency amplitude will gradually increase, and therefore the phase will also change.

Due to the occurrence of excessive vibration in each water pump, our company has carried out a large amount of maintenance work, which includes the following aspects:

(1) Perform low-speed dynamic balancing on the rotor. When disassembling and overhauling the condensate water pump, low-speed dynamic balancing work must be carried out for all components to eliminate the original imbalance.

We have installed inducers at the inlet of the first stage impeller of the Russian made condensate pump, and by adding them, the anti cavitation ability of the impeller will be improved. After a long period of operation, the staff conducted an inspection. It was found that cavitation occurred in all inducers, but to varying degrees. The processing method used is to repair, polish, and weld the parts with light cavitation. When the cavitation situation is severe, it is necessary to replace the inducer and use a new inducer. Therefore, when dismantling and repairing the condensate pump, low-speed dynamic balancing should be carried out. The condensate pump unit needs to undergo vibration testing during major repairs, and the vibration value is generally slightly better than before the overhaul, with no significant changes.

(2) Prevent the detachment of rotating parts. When dealing with the detachment of rotating parts, we found after inspecting the condensate pump that the phenomenon of loose rotating parts did not occur in the condensate pump. When assembling the pump shaft, it is necessary to strictly follow the maintenance process, whether it is the fastening or locking of the components, in order to avoid the occurrence of loosening.

(3) When there is a mass imbalance caused by the rotor, the pump unit needs to be disassembled. When measuring the curvature of the rotor, it is necessary to strictly follow the relevant regulations. The bending value generally cannot exceed 0.02 millimeters. If it exceeds the value required by the regulations, it needs to be replaced again. After using the above measures, we found that the problem of excessive vibration still exists. Therefore, we used a rotating mechanical vibration treatment professional to conduct multiple inspections and treatments. The following is the process of detecting and analyzing the online vibration of Class III condensate pump No. 21. The staff measured multiple measurement points of the motor, and we can see the measurement values of measurement point 1 in Table 1.

Table 1 Vibration Measurement Values

From Table 1, we can see that the vibration in the Z direction of measurement point 1 is in Zone C, which is not suitable for long-term operation. Using a spectrum analyzer to analyze the vibration results, it was found that the spectrum of measurement point 1 in each direction mainly has the following characteristics: (1) 1 times the rotational speed frequency has an advantage in the vibration spectrum. (2) The vibration in the Z direction is greater than the other two perpendicular directions. The suggestions and methods adopted are: (1) Focus on inspecting the lower bearing of the motor, especially the clearance of the bearing, to see if it is suitable. Wear or looseness must not be taken lightly. (2) The pump unit has been inspected for wear and looseness of bearings, and a key inspection should be carried out. Due to prolonged operation, the fit between the bearing chamber and the bearing is not very tight, and the gap between the two is 0.12mm, which seriously exceeds the specified standard by 0-0.02mm. During processing, use bearing chamber brush plating. Afterwards, the situation improved slightly, although it did not exceed the limit, it was still within the prescribed upper limit of 0.07-0.08mm.

4. Conclusion

The author analyzed and discussed the analysis and treatment methods of excessive vibration of condensate pump bearings, mainly from three aspects: system introduction, operation of condensate pump, and analysis and treatment of various abnormal situations of condensate pump. It is hoped that these analyses and suggestions can be used for reference by peers, which is conducive to the safe and stable operation of pump units and production.

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