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Axial thrust
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thrust due to the pressure of the pumped liquid acting on the radial surface area
of the rotor elements - on the impeller shrouds, on the annulus area of wear
rings, shaft sleeves, bushings, etc., that tends to displace the rotor in an axial
direction or parallel to the shaft axis.

The magnitude of the hydraulic axial thrust is proportional to the elements'
diametrical surface areas and the pressures acting on those areas (force x
area). The forces acting on one side of the rotor elements are usually different
in magnitude from the forces acting on the opposite side in opposite direction.
The summation of the opposing forces results in a net axial thrust imbalance on
the rotor, and is expressed in the unit of pound force (lbs).

In horizontal pump, the net axial thrust is almost always hydraulic in nature
although, in some rare instances, the driver can also transmit dynamic axial
thrust to the rotor.

In vertical pump, the net axial thrust is a combination of both hydraulic and static
forces. The static component is due to the weight of the rotating assembly that
acts downward. The hydraulic axial thrust component acts in either upward, or
downward, direction depending on suction pressure and differential head of the
pump.

The direction of axial thrust can be toward one side only (along the axis of its
rotation), or it can reverse direction alternately if there is a change in the pump's
operating conditions. Although it is generally assumed that a single stage pump
with double suction impeller has balanced axial thrust this is not true most of the
time.

The axial thrust load is carried by the pump's thrust bearing which is usually of
the anti-friction (ball bearing) type. The thrust bearing should be capable of
carrying reversing thrust, if applicable.

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 an overhang pump with high suction pressure, and in multistage between-
bearing pump, the axial thrust imbalance can be very high. 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.

In situations where the thrust load is high, two or more bearings may be stacked
together to carry the thrust load. If it is not possible to do this because of space
limitation in the bearing housing, some other design changes can be done.
These may include the addition of balance holes on the impeller, changing the
balance diameter of the impeller wear rings, changing the diameter of balance
drum, changing the size of the mechanical seal, adding bleed-off connection,
etc., depending on pump type and size. These actions are collectively referred
to as
axial thrust balancing.


Dashboard:

Q - As design engineer in a pump company, one of my projects was to destage
a multistage pump with ball bearings. I know that I should recalculate the axial
thrust and rebalance the pump hydraulically because of the destaging. Being
new on the job, I sought guidance from a fellow engineer on two questions:

1. How much axial thrust imbalance is acceptable?
2. In which direction should the axial thrust be?

I was told the thrust direction is immaterial as long as the thrust bearing can
handle the thrust load and still meet the required bearing life. Your comments
please.


A - It is not advisable to either design for the lowest possible axial thrust, or to
preload the bearing with high thrust load. Both actions can result in premature
bearing failure. There are also good reasons for ensuring the axial thrust load is
in the right direction. These are discussed in the second part of this article.