Centrifugal pump - How to balance axial thrust - Part 1/2
In centrifugal equipment, axial forces are generated by the rotating parts due to the difference of pressures that apply on them. if uncompensated, these forces can largely exceed the capabilities of a reasonably sized bearing.
Example: We consider a pump of a few kilowatts with a impeller diameter of 250mm, an inlet of 100mm and a shaft diameter of 30mm, with a suction pressure of 1barG and a discharge pressure of 6barG. In absence of balancing thrust mechanism, suction pressure would apply on the inlet side and discharge pressure would apply on both shrouds at the back of the discharge side of the impeller.
F1= (12,5^2-5^2) x pi() x 6,0 = 2474 daN
F2= (12,5^2-1,5^2) x pi() x 6,0= 924 daN
Fm= 5^2 x pi() x 1,0 = 78 daN
Resulting axial thrust = 2474-78-924= 1472 daN or about 1.4 tons.
This value well exceeds the capability of a standard roller bearing of this size. We will be looking at reducing this axial force by a factor 5 to 10 to find a roller bearing that would last the 25 000 hours required by API 610.
This simplistic example gives an overall idea of the magnitude of forces that can develop and the necessity to compensate them to avoid overdesign or unreliability of the thrust bearing.
Here are the most common ways to compensate the axial thrust:
Wear rings and balancing holes
Wear rings allows the designer to reduce the flow circulation between an high and a low pressure area. They therefore introduce a step change in the pressure that the rotating element are exposed to.
API 610 specifies the wear rings to be renewable, meaning that they can be replaced during maintenance to restaure the pump performance.
On single stage pumps, wearing rings are used in combination with balancing holes that bring the requird area of the impeller shroud around suction pressure.
This allows to reduce the axial thrust by 75 to 90%.
Radial back vanes
The pressure at the impeller shroud is reduced by vanes that pump out the liquid and create a zone of low pressure at the impeller shroud.
The vane design require relatively small clearances between the impeller and the pump casing.
This design is applied to small industry standard pumps and is not allowed by API 610 (ref to 7th ed. sect 6.7.1)
Go to next page to read about back to back impellers, balance drum and sizing tips
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