AXIAL THRUST A038
The pressures generated inside a centrifugal pump work on both the stationary and rotating components.
On single inlet impellers an axial force exists because of differences in pressure on the front and back of the impeller and the difference in the areas subject to the pressure. Axial thrust is the sum of these unbalanced forces acting in an axial direction.
At startup a centrifugal pump, with an overhung impeller, will thrust towards the thrust bearing. At about 65% of its efficiency the thrust will reverse towards the pump volute and remain their while running. That is the reason we often see a bent snap ring that is holding the radial bearing in its proper location. You need a better retention method
Theoretically a double suction impeller design would eliminate these forces, but there are some variables that interfere with the balance. The variables include:
- Unequal leakage through the stuffing boxes when packing is used instead of a mechanical seal.
- The two suction passages may not provide equal or uniform flow to both sides.
- An elbow located too close to the suction flange may provide non-equal flow to both sides of the impeller.
- The closed impeller may be located off center.
- The two sides of the discharge casing may not be symmetrical.
- Discharge or suction, stuffing box recirculation lines can affect the stuffing box pressure.
There are several approaches to compensating for this axial thrust:
- The installation of a double row thrust bearing in the end of the casing next to the coupling.
- Installing a wear ring on the back of the impeller with holes drilled through the impeller to equalize some of the forces.
- “Pump out vanes” or radial ribs on the back of the impeller to reduce the pressure behind the impeller.
- Some multi-stage pumps use a hydraulic balancing device mounted in a chamber that is connected to the suction side of the pump or a low point in the system