Balance disk (disc)



A pump rotor is subjected to axial thrust due to the pressure of the liquid acting
axially on the radial surface area of the rotor elements - its impeller, wear rings,
shaft sleeves, bushings, etc.


The axial thrust is directly proportional to the elements' radial surface areas and
the pressure acting on those areas. The force acting on one side of the rotor
elements are usually different in magnitude from the force acting on the opposite
side. The summation of the opposing forces  results in a net thrust imbalance on
the rotor.


In a single stage unit the thrust imbalance is usually small enough that the thrust
bearing is more than sufficient to handle the thrust load. In single stage overhang
unit operating under very high suction pressure the thrust imbalance can be very
high and should be reduced in magnitude by changing an area subjected to
pressure such as by changing an impeller wear ring diameter (usually on the
back side of a single suction impeller), or by reducing the shaft diameter under
the seal to reduce the mechanical seal size.


In a multi-stage unit where the thrust imbalance can be very high due to the high
differential pressure, reducing the thrust imbalance may require altering either (1)
a pressure area, or (2) the amount of pressure acting on that area, or (3) both.

  • Item (1) can be done by changing the diameter of a balance disk
    specifically designed for this purpose. The disk can be a shaft sleeve
    located adjacent to the high pressure stuffing box, or at the center of the
    rotor, or both.

  • Item (2) can be done by providing a flow bleed-off line to reduce the
    pressure acting on a rotor element located near the high pressure stuffing
    box.



R: 0909-BADI
C: design, operation
F:  thrust disk
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Balance disk (disc)