Thrust bearing adjustment

[RE-ENGINEER]

Dears,
We have some issues with a multi-stage centrifugal pump. First of,

The axial thrust is almost 0.2 mm. when the pump is off, the axial position does not read zero but it reads 0.2 mm, once the pump starts it reads 0.35 to 0.4 mm which is at the alarm. now we know the actual distance the shaft travels is around 0.2 mm. what should I do to adjust this to zero?

I have read the procedure is to push the shaft in both direction to get the total actual static float, make the shaft at the middle and set this as a zero, the other philosophy is to push the shaft to the end of the active side and again move it back with half of the specified float value and make this zero.

is it accurate to measure the axial bearing clearance by a feeler gauge?

so in this case is the solution to move the active and the non active bearings -0.2mm so that the rotor will be on center?

Please note that this issue is present on two pumps since a long time. so I am not concerned about the balance drum and the wear rings clearances yet.

 

[JJPellin]

You need to provide additional details about the pump and the bearings. Are the impellers aligned or opposed? Does the pump use a balance disk, drum or other thrust balancing device? Is the bearing a tilt-pad design with forced oil lubrication? Is there any spring mechanism built into the support for the active bearing? Is the normal thrust toward the drive end or non-drive end? What style of coupling is used.

We have pumps where this could be a result of a poor assembly. A thrust bearing assembled with a large number of thin shims can be springy. We have pumps with thrust bearings that use a Belville washer arrangement which is designed to give the bearing a springy response. These pumps use a balance disk arrangement rather than a piston or drum arrangement. We do not have any, but I have read about pumps of this type that have no true thrust bearing and rely entirely on the balance disk.

It is not normally possible to measure thrust clearance with a feeler gauge. A dial indicator is the correct tool in addition to reading the output of the thrust monitor. If this has been the issue for a long time and no failures have occurred, it is likely normal for this pump and all you need to do is change your alarm settings.


Johnny Pellin

 

[RE-ENGINEER]

JJPellin,
Thanks for your reply. regarding your first part, I will answer what I am sure about at the moment. The impellers are aligned and there is a balancing drum. The bearing is forced lubricated. not sure about a spring mechanism I need to double check. normal thrust is toward the NDE (why is that important?)

I have read one of your post where you mentioned a similar issue in a jet pump which I was interested to hearing more about. I am not sure whether there are shims installed or not. but I am a bit comfortable because the shaft travel distance is steady over the last years. secondly, because this is happening on both pumps. so I am more into the installation issue.

I agree the dial is the right tool, in fact I was thinking to avoid moving the whole drive train just to centre the pump shaft.

when I increase the alarm I am risking the equipment because the collar can physically touch the pad while the monitor does not say it alarming. I would rather set my zero at the center and run the pump.

 

[JJPellin]

I am not sure I understand. Correct any of these points where I have misunderstood. The total float is 0.2 mm from hard active to hard inactive. The thrust monitor was set to zero at the center of this float. If everything was perfect, the monitor would read +0.1 mm when running. But, you noted that it reads +0.2 mm when shut down and not running. This is already beyond the theoretical bearing clearance.

I am familiar with two methods to set the thrust monitor. One, as you describe, sets the monitor to zero at center of float. The other method sets the zero point at hard active. We normally set zero at center of float.

With aligned impellers and a balance drum, I would expect normal thrust to be toward the drive end. But, some pumps do thrust the other way. I assume the thrust probes are looking at a thrust target very close to the thrust disk.

If the pumps have run successfully for a long time without failing the thrust bearing, you know that a thrust indication of 0.4 mm will not indicate movement beyond the bearing clearance. Two possibilities come to mind. The techs may have set the monitor to zero when hard inactive by mistake. Or, as I already noted, the bearing is extremely springy as a result of the design or assembly with many thin shims.

I would set the thrust monitor to zero when thrust hard active or increase the alarm setting. You should consider removing the thrust bearing for inspection while you are down. Document the amount and number of shims and inspect the pivot points on the back of pads on both the active and inactive bearings. Recheck the thrust float and be sure they are making a firm bump.


Johnny Pellin