Balance Disc Assembly

In boiler-feed pumps, whether it's stages are in-barrel or held by the tie-bar arrangement (for holding all the stages in place). In each stage a certain amount of axial thrust is generated due to the hydraulics in a given pump, regardless of the nature of pump construction as given above.

If the axial thrust per stage is X lbs. and N is the number of stages in a boiler-feed pump, then, the total hydraulic thrust would be XN lbs. keeping in mind that all the the pump Impellers are mounted in one direction only, on the shaft and not back to back.

To counter this total thrust, the balance thrust-disc is mounted on the discharge end of shaft prior to it's bearing, which are normally of journal type. the end-float of the shaft assembly is controlled by having a stationary-disc next to the balance thrust-disc which is rotating. The two mating faces of the balance thrust-disc and the stationary-disc is separated by pressurised water film, when in running condition.

There is a small gap or cavity, left between the stationary-disc and the rotating balance thrust-disc, but still keeping a reasonable width of contact face which is on the outer periphery of the rotating balance thrust-disc.

The high-pressure water generated by the multi-stage pump is then directed, in small quantity to this gap or cavity just detailed above. Once the pressure is built up in the gap or cavity of the balance thrust-disc and when this exceeds the axial-thrust, it releases this high pressure water between the balance thrust-disc mating faces and lubricates the same and keeping the faces apart. This whole process repeats all the time and constantly lubricates the mating faces in question, while the pump in operation.

Hence, one can conclude that the Kingbury thrust bearing float and the balance thrust-disc end float are not related to one another but are independent of each other. The float between the stationary disc and the rotating balance disc is a self-adjusting feature, provided that there is a large enough end-float within the Kingsbury thrust bearing. Hope the above will answer your queation.

Thanks for your help. This is useful. Regarding the "pressurized water film" that lubricates and keeps the disc away from the stationary bushing, what would you guess the thickness of this water film to be?

As for your answer of the film thickness, I have no idea, but you can be sure of that the mating faces won't run dry, provided enough water is directed to the cavity, I had indicated. The separation between the mating face of the balance thrust-disc is self adjusting or regulating, hence the water-film can be a smaller or a larger one depending upon the gap opening between the mating faces.

Thank you for your response. This is of great help in understanding root cause of failure.

One to Two Thous= 0.001">0.002"is the optimum. Enough leak past-but not a lot!

Best

This is interesting. Since my message to you all I have had two different employees of a Pump OEM give me conflicting information about this running clearance. This matches one of them. Now I feel better about who I can trust for assistance. Our biggest problem has been root cause of failure. We are getting better at. Thanks again.

Note that this clearance is not fixed (otherwise it would not work). With increased thrust, the axial clearance between the faces will get smaller, reducing the leakage across the faces thus reducing the pressure on the back of the disc. This will lead to the faces opening again to restore the balance. Once steady head is established the system reaches equilibrium and a pretty consistent gap is maintained. Also note that the thrust bearing must be set up such that at no stage can the balance disc faces come into contact. You do not mention the make of pump, but it may use a spring loaded thrust bearing, in which case the bearing would be set up such that the gap between the faces of the balance discs is never less than about 0.8 mm. (1/32 for the uninitiated).

The gap of 1 to 2 thou mentioned is not in my opinion realistic. It would not be possible to set up a thrust bearing that accurately to stop the faces touching. The thrust bearing must stop the faces touching under all circumstances but allow relatively free axial movement once pressure is established to allow the balance device to function.

Is your thrust bearing failing or are you destroying your balance disc?

Thanks for your message. The issue is that with such a small running gap between disc and balance sleeve suggests that the only way the Kingsbury Thrust bearing does ANYTHING is when the balance disc fails. There does not appear to be any way the Kingsbury and the balance disc can share the thrust load.

Thrust bearing and balance disc can and will "share" the thrust load under certain circumstances. The thrust bearing takes ALL of the axial load under certain circumstance such as at startup. I repeat, the running gap is not that small and the thrust bearing is set up such that the thrust disc and balance sleeve can never come into contact. If for some reason the thrust is not balanced by the thrust disc then the bearing will take all the load. The maintenance manual for your pump will give you the correct clearance for the balance disc with the shaft hard up against the thrust bearing. If you do not have this then contact the OEM.