Question about centrifugal pump thrust bearing temperatures.

[smrodriguez4]

Hi all,

This might be a dumb question but I am looking for some feedback from a more seasoned engineer regarding centrifugal pump bearing temp alarm limits.
The pumps in question are between bearings, single stage centrifugal pumps and there are about 2 years old. These pumps are mostly used with a VFD so they never see full load. However, we started to run them on bypass at full operating speeds and I have been starting to notice that our pumps are running really close to Thrust bearing shutdown temperature (208 F), a couple of them have tripped due to high temp. Regardless, we changed the thrust bearings on the pump that was running the hottest and did a complete inspection. We didn't find anything out of the ordinary; it wasn't misaligned, the motor was running on magnetic center and all the dimension were within tolerance. Furthermore, none of the bearings showed signs of overheating. Of course we checked the RTD's and they are working properly. Regardless, we changed the bearings. The temperature seems to be running cooler but still, I have my doubts that in a hot summer day the pump will reach shutdown temperature.

What caught my attention that the RTDs are not measuring the housing nor oil temp they are making direct contact with the sleeve and the outer ring of the trust ball bearings. By trending the data it seems that the thrust bearings will always run about 15F hotter that the sleeve bearing. I am planning on increasing the shutdown by 15F to 224F on the ball bearing, without modifying the 208F shutdown on the sleeve since I read on SKF that their bearings are good to 300F. Do this shutdown temperature seems unreasonable to you guys? I am also planning on changing the oil from a Tellus 32 to a Tellus 46... Any thoughts will be greatly appreciated.

Thanks!

 

[romke]

the fact that the sleeve bearing runs cooler then the thrust bearing may be due to the fact that the whole sleeve is covered with through flowing oil that is better capable of diverting the heat to surrounding hardware. in the thrustbearing there is only a very small area of contact between the rolling elements and the races and a very thin oil film to carry the heat away. changing from a ISO VG 32 to a ISO VG 46 fluid might well be a good idea - you will increase the thickness of the lubricant film in the thrustbearing and that might both decrease the amount of heat generated and increase the amount of heat that is carried away by the lubricant.

be aware that at the temperatures you mention both ISO VG 32 and ISO VG 46 are very thin. If you do experience lower temperatures with a ISO VG 46 it might well be interesting to see whether operating temperature can be brought down somewhat more with a ISO VG 68 oil.

 

[Tmoose]

Does the pump manufacturer have anything to offer?

 

[1ton]

There is a possibility that during the bypass mode the circulating fluid is much hotter. Is that another possibility that you have not look into ?