Axial thrust

Axial thrust refers to the unbalanced force acting on the rotor of a pump that  tends
to displace it in an axial direction, or along the axis of its rotation. It is usually
expressed in pound unit.

In horizontal pump, the axial thrust is almost always hydraulic in nature, caused by
the pressure of the pumped liquid. (In very rare instances, the driver can also
transmit dynamic axial thrust to the pump.)

In vertical pump, the axial thrust is a combination of both hydraulic and static
forces. The static component is due to the rotor weight, and acts downward. The
hydraulic axial load 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.

The 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 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.

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?

My colleague advised that I should design for the lowest possible axial thrust, and
its direction is immaterial as long as the thrust bearing can handle the thrust load

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.