Generally, end-users shouldn't have to worry about the axial thrust generated in the pump. As pump failure due to axial-thrust problems are rare and relatively uncommon. However, that doesn't mean you can rule out the possibility of an axial-thrust issue completely.
It always pays to be aware of all aspects of your pump, so that you can operate it efficiently. Here, in this guide, we help you understand the role of axial thrust in pumps and how to deal with axial-thrust problems.
Why are axial-thrust problems challenging to manage?
In a pump, a rotating single-entry impeller moves in an axial direction, generally with lots of force. The principles of the axial thrust are quite easy to understand. However, there are several complicating factors and uncertainties that are difficult to pinpoint. Hence, if an axial thrust problem occurs, it's challenging to identify the cause and rectify it.
Understanding Hydraulic Thrusts in Centrifugal Pumps
During centrifugal pump operation, the kinetic energy of the pumped liquid is converted into pressure. The high-pressure fluid flows continuously along the circumference of the impeller. The liquid also gets entrapped between the clearances in the impeller and the casing covers.
The flow of the high-pressure liquid generates two forces – one in the longitudinal direction and the other in the lateral direction, with respect to the shaft axis.
- Radial Thrust – The force generated in the lateral direction is known as radial thrust. The radial thrust is due to different pressure generation in the volute.
- Axial Thrust – This is the pressure generated in the longitudinal direction. Different areas of the impeller are exposed to the pressurized liquid, leading to axial thrust.
Why do Axial Thrusts occur in centrifugal pumps?
Axial thrusts arise in centrifugal pumps due to asymmetry. The clearances between the impeller back shroud and casing cover, the impeller front shroud, and housing are filled with fluid at high pressure. The pressure generated acts on the impellers. Since the back shroud has a larger surface area than the front cover, a net thrust is created that acts in the direction opposite to the flow of the liquid. Thus, the net axial thrust is the difference in the forces acting between the front and back shrouds.
Possible Reasons for Axial-thrust Related Problems
Several factors can alter the thrust force generated in a centrifugal pump. Generally, axial thrust related problems occur due to the following reasons:
- Unbalance in the thrust generated in the front and back shrouds, which, in turn, leads to an excessive thrust bearing load.
- Failure of the thrust compensating disc
- Transient or cyclic rotor axial excursions
How to identify if your pump has an axial thrust related problem?
The symptoms of high axial thrust include:
- Premature failure of the thrust bearing loads
- Failure of the thrust bearing and thrust discs
- A high temperature of the bearing
- Excessive wear and tear in the internal pump clearances
- Rotor axial shuttling
- Reduced pump flow
- Elevated pump vibrations
Various Methods to Rebalance Axial Thrust
If you notice unbalance in axial thrust, you can rebalance it using any of the following methods:
- Adding a plain bearing, rolling-element bearing, or ball thrust bearing is one of the most efficient ways to eliminate axial thrust. It helps in the complete absorption of axial thrust.
- By arranging the impellers back to back, thereby the thrust bearing absorbs any residual axial thrust.
- Balancing axial thrust by using balancing holes on the impeller.
- Automatic balancing of the thrust with the help of a balancing device.
- Reduction of the thrust at the back vanes of each impeller.
As an end-user, you need not worry excessively about axial thrust problems, as they are uncommon and occur very rarely. However, it's always helpful to have a clear understanding of axial thrust – its symptoms and related problems so that you can take the right measures if a problem occurs.