Damage to the boiler feed pump bearings
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I would appreciate it if experts could analyze the reasons for the damage to the bearings of the boiler feedwater pumps. Two boiler feedwater pumps failed within about 4 days; in most cases, the bearings were damaged after just two days of operation after installation. The balance drum is used to counteract most of the axial forces. The bearing types used are NU series bar bearings on the drive end, while on the non-drive end, one pump uses 7309BM/DB bearings installed back-to-back, and the other pump uses 31309 bearings installed face-to-face. The condition of the bearings that bore the main stresses is shown in the diagram below. Please note that the bearings were left on the ground and are quite dirty
The boiler feed pump has hardly ever been used properly; if it functions fine for about two days after installation, it can generally operate normally for over 3 months
For back-to-back mounted angular contact ball bearings, is the outer bearing more prone to damage compared to the bearing on the drive drum? What is the clearance of the balance drum?
Unit 7309 has severe damage on the outside, while unit 31309 has damage on the inside. The other bearings in both pumps are generally in good condition; there should be no issue with the balance drum clearance, as long as the bearings do not get damaged after about two days of operation, the pumps can continue to function for over three months. This situation has occurred several times already, and the main problem is the rapid wear of the bearings under stress during the initial phase of operation
The load on the rear thrust bearing is too high; the balance drum fails to perform its proper balancing function!
Tapered roller bearings are installed face to face, and the damaged ones are those bearing forces directed toward the side of the inlet. If poor lubrication is ruled out, then it’s an issue related to axial forces. The balance drum is designed to counteract axial forces; if the clearance in the balance drum is large, the axial forces will be significant
The clearance of the balance drum is relatively small, and there are no obvious signs of wear; the new pump performs well. However, after maintenance, severe wear of the bearings often occurs
This post was last edited by chuanhengmpq on 2018-3-17 07:55; 7309BM – let’s have an interview
It’s just that the axial force isn’t properly balanced; take it apart to check the wear on the balance disc. For the nine-stage pump I have here, the radial clearance on one side of the balance drum is 0.2–0.3. It shouldn’t be too large or too small – this can serve as a reference. After all, in most multi-stage pumps, the axial force is eliminated mainly by the balance drum and balance disc, with some of it being handled by the bearings. The drive and non-drive ends in our pumps are both equipped with NU cylindrical roller bearings. Are the pumps you have now manufactured with angular contact bearings paired with NU bearings, or tapered roller bearings paired with NU bearings? Did you change it yourselves?
Regarding additional details: the quality of lubricating oils (greases) and maintaining cleanliness during maintenance. To assess the quality of a bearing, one should consider the surface condition of the bearing after wear, based on these two factors.
Sometimes, after replacing the bearings, the pump can operate properly for several months