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

520 Centrifugal Pumps: Design and Application
growth is then measured by some means, and the offsets are revised accordingly.
The other approach, favored by this author for most pumps (but not
necessarily for large turbomachinery), is simpler. This is to say that formulas
and rules of thumb are unreliable because they fail to account for
piping growth effects that may influence machine movement more than
the growth of the machines themselves. The formulas also fail to account
for uneven temperature rises in different parts of the machine frames,
which is quite common. In effect, formulas are unlikely to give good figures
for amounts of relative growth and are equally unlikely to get the
directions of such relative growths correct. This being the case, our most
conservative course of action, as well as the easiest, is to align everything
to zero offset, cold. Then, at least, the machines are starting from a
good alignment condition, and relative growth in one direction is no
more harmful than in another. If alignment-related vibration and reliability
prove satisfactory, as they do about 90% of the time, we have simplified
our alignment task by eliminating offsets. In the other 10% of the
cases, the growths should be measured, not calculated, and offsets applied.
Several methods of growth measurement are described in Table
19-5.
Table 19-5
Methods for Determining Thermal Growth Offsets for Machinery
Alignment
Method Advantages Disadvantages
Manufacturers' When available, these
Recommendations require little effort to obtain.
Sometimes they give good
results.
Calculations
based on
temperatures,
thermal
expansion
coefficients,
and machine
geometry,
using formulas
and rules of
thumb.
Shut down and
"quickly" take
alignment
measurements.
Easy to obtain, and very
scientific-looking.
Makes the measurer feel he
is doing something
worthwhile.
Often not available, and
often unreliable when they
are available.
Unreliable. Likely to do
more harm than good by
getting the relative growth
direction wrong and thus
increasing the misalignment.
Formulas fail to account for
nonuniform temperature rise
in machines and effects of
temperature rise in
connected piping.
Utterly useless and
self-deceiving. The majority
of the thermal movement
occurs during the first