Page 248 - Machinery Component Maintenance

230 Machinery Component Maintenance ana‘ Repair

Thermal Growth-Tbelve Ways to Correct for It

Thermal growth of machines may or may not be significant for align-
ment purposes. In addition, movement due to pipe effects, hydraulic
forces and torque reactions may enter the picture. Relative growth of the
two or more elements is what concerns us, not absolute growth ref-
erenced to a fixed benchmark (although the latter could have an indirect
effect if piping forces are thereby caused). Vibration, as measured by
seismic or proximity probe instrumentation, can give an indication of
whether thermal growth is causing misalignment problems due to differ-
ences between ambient and operating temperatures. If no problem exists,
then a “zero-zero” ambient alignment should be sufficient. Our experi-
ence has been that such zero-zero alignment is indeed adequate for the
majority of electric motor driven pumps. Zero-zero has the further ad-
vantage of simplicity, and of being the best starting point when direction
of growth is unknown. Piping is often the “tail that wags the dog,” caus-
ing growth in directions that defy prediction. For these reasons, we favor
zero-zero unless we have other data that appear more trustworthy, or un-
less we are truly dealing with a predictable hot pump thermal expansion
situation.
If due to vibration or other reasons it is decided that thermal growth
correction should be applied, several approaches are available, as fol-
lows:

1. Pure guesswork, or guesswork based on experience.
2. Trial-and-error.
3. Manufacturers’ recommendations.
4. Calculations based on measured or assumed metal temperatures,
machine dimensions, and handbook coefficient of thermal expan-
sion.
5. Calculations based on “rules-of-thumb,” which incorporate the
basic data of 4.
6. Shut down, break coupling, and measure before machines cool
down.
7. Same as 6, except use clamp-on jigs to get faster measurements
without having to break the coupling.
8. Make mechanical measurements of machine housing growth dur-
ing operation, referenced to baseplate or foundation, or between
machine elements. (Essinger)
9. Same as 8, except use eddy current shaft proximity probes as the
measuring elements, with electronic indication and/or recording.
(Jackson; Dodd/Dynalign; Indikon)
10. Measure the growth using precise optical instrumentation.