Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

Alignment 510
parallelism/perpendicularity. The T 4- B vs. S 4- S test, which is described
in Murray [6], will detect inclined-plane error if this is present.
Numerical Examples
Numerical examples will not be given here due to lack of space to give
proper coverage. Those wishing to examine a variety of such examples
are urged to see Murray [6].
Thermal Growth
When machines are operating, their temperatures usually change from
that at which they were aligned. In most cases they get hotter, although
the opposite is occasionaly true. In the case of pumps, turbines, and compressors,
the temperature of the connected piping also changes. These
temperature changes cause expansion or contraction in the metal. In most
cases, this expansion or contraction is not uniform and differs significantly
for driver and driven machines. Their relative positions therefore
change during operation, causing changes in the alignment relationship.
In addition to temperature effects, torque, hydraulic effects, and oil film
thickness may contribute to alignment changes. Because of these factors,
a coupled machine train with good alignment in its "cold" or non-operating
state may have less precise alignment under operating conditions.
For a pump that operates at conditions subject to little temperature variation
and aligned at an ambient temperature that also varies little, the
relationship between cold and hot alignment will be constant and repeatable.
If the amounts and directions of growth can be determined, they can
be used in the form of thermal offsets to include a deliberate misalignment
when the machines are aligned. Then, under operating conditions,
the growth will act equal and opposite to this misalignment, causing the
machines to become aligned during operation.
So much for the theory. We live in an imperfect world, and things do
not usually behave exactly as we might wish. The best laid plans of mice
and men oft go astray, and thermal growth offsets are a good example of
this.
There are two common approaches. One, favored by certain authorities,
is to use formulas or rules of thumb involving fluid or metal temperatures,
coefficients of thermal expansion, and machine geometric measurements
to calculate predicted growths. These are then converted to
offsets and incorporated into the initial machine alignment. If the machine
runs well, with low vibration and without premature failure, the
offsets are assumed to be correct, and are used thereafter. If the machine
does not run satisfactorily, and the problem is attributed to misalignment
caused by thermal growth other than that used to derive the offsets, the