Balancing Axial Thrust Devices

[PalomaP]

I would like if someone could clarify my doubts...

when a multistage pump has a balance axial thrust device (drum, sleeve or combination of both), is the axial thrust eliminated to the extend that it is not required axial thrust bearings?
If it should be required thrust bearings, what type of bearings? Hydrodinamic (kingsbury) or ball bearing? and the radial bearings?

I have a cross sectional drawing of a multistage pump with a hydraulic balance system and the only bearings that i can see are journal bearings, so It seems there is not any thrust bearings?¿?

thanks again in advance!

 

[byrdj]

I think you need a bearing and cannot depend of balanced flow. The type and size can vary depending on size and cost. It could be just a thrust washer. If the pump doesnot have one, then it might depend on the driver's. If so, the the driver to pump must be a rigid coupling

 

[sms]

You still need a thrust bearing... there must be a thrust bearing.

Can you tell if there might be a thrust face built into the radial bearing? And the bearings are journal bearings not rolling element?



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[cbrn]

Hi,
axial thrust is strongly dependent upon flow asset and so not only varies with rotation speed but also has some pulsating component. By consequence, a pump- or turbine-shaftline without any thrust bearing would be unstable in transients (run-up, coast-down), i.e. very dangerous.
In vertical machines, a resultant axial force directed downwards is required by design (obviously): the balancing devices are here to moderate the thrust and set it to the desired value(s), but the check is made that at any possible speed and working condition the axial thrust force never overcomes the gravity force (otherwise, the shaftline would "fly up").

Regards

 

[Cheddarcaveman]

Is there any chance you could post the pump cross section here so that we know what we are dealing with?

Most multistage pumps are designed so that the impeller and volute configuration also helps to cancel out axial thrust, so with this, and a balance disc/drum, there should be very little thrust left for the bearings to handle so it is probable that there is/are thrust faces built into the journal bearings.

 

[JJPellin]

Several manufacturers make pumps that have no thrust bearing. We only happen to have one pump in our refinery that would qualify. It may be a matter of terminology. These pumps use a thrust balancing device that is similar to ones used in most multistage horizontal pumps. This thrust balancing device is sometime called a balance piston or balance disk. Any pump with very many stages needs to compensate for the combined thrust of all of the impellers. It is not a very complicated step to design the balance disk to completely counter all of the thrust loads and eliminate the need for the thrust bearing completely. Most of these pumps would tend to be large, high energy pumps in services like boiler feed water.

Johnny Pellin

 

[daveboy]

When using a balance disk you tend to also include a small bump stop in the NDE bearing housing which allows contact during transients without damage.

A balance disk is effectivley a self actuating valve, as pressure builds in the gap to a level that the pressure over the disk area exceeds the axial thrust the faces open and leakage occurs and pressure drops. There is over shooting and then partial return until a balance is acheived between pressure build up and leakage /pressure loss.

As axial thrust is also dependant on the clearances between the impeller and side wall the shuttling of the rotor has varying effect on the actual axial thrust. Therefore it takes a few cycles of this shuttling until balance occurs.

The shuttling can cause the rotor to over shoot to a level that your shaft may contact the bearing housing hence a small plate is added usually of bronze or white metal coated steel to prevent any damage - this is effectivley a temporary thrust bearing.

The distance from the shaft end to the bump stop is less than any other axial clearance in the pump to prevent damage but large enough to allow the faces to open sufficently.


JJPelin is correct in the fact that it is a fairly easy design to balance out all the axial thrust. The difficult part is also minimising leakage losses to limit efficency loss and maintaining hydrodynamic stability.

 

[cbrn]

Hi,
yes, it seems we all agree. All centrifugal devices will nedd a "thrust-something". If we relegate the term "thrust bearing" to a "real", dedicated, device that acts permanently as a reaction against a thrust, then OK, most of the balanced-design pumps won't need this. Even single-stage pumps can be 100% balanced.
Anyway, a thrust-ring, thrust-collar, or any device able to act as an axial stop during transients, will always be needed (or, equivalently, the axial gaps could be so large as to completely "cover" the transient axial displacements "around" the equilibrium position; but this method would be tremendously unefficient from an hydraulic point of view...).

Regards