Page 75 - Machinery Component Maintenance

60 Machinery Component Maintenance ana' Repair

line or off-line washing, in order to delay a subsequent machine over-
haul. While compressor manufacturers shy away from on-line full-speed
washing, knowledgeable users have had very good luck with both this
technique and with off-line washing when the machine is slow rolled
while half full of the wash liquid. When choosing a wash fluid be sure it
is compatible with all components in the machine, such as O-rings, as
well as the process. On-line abrasive cleaning with walnut hulls, etc., has
found wide acceptance with gas turbine users, but is not without its prob-
lems. Plugged orifices, airbleed passages, and the like are common. The
total subject of on-line or off-line cleaning is beyond the scope of this
text, but it is well worth considering in specific situations as it is a real
time and money saver.
The bearings, journals, and seals should be visually checked for signs
of distress. One frequent problem has been that of babbitt fatigue. While
the aftermarket has been offering bearings with babbitting less than
0.010 in. thick for a number of years, some machinery manufacturers
have resisted change in this area. Nevertheless, industry experience with
thin babbitt bearings has been excellent to date.
Labyrinths can also tell a story which needs to be read and analyzed.
Deep grooves in the impellers or shaft spacers are indications of a shaft
excursion at some time in the operating cycle. Worn or corroded laby-
rinths indicate loss of efficiency, and, if found over the balance piston,
could lead to a thrust bearing failure. As with bearings, new materials
able to combat the corrosion problem are now coming into the after-
market. Rubs could indicate misoperation, such as running at or near a
rotor critical speed or in surge; a rotor dynamics problem; a thermal
bow, or similar difficulty. Observations regarding location, depth, and
distribution of the rubs are the keys to a proper analysis.

Cleaning

When cleaning fouled components-rotors, diaphragms, etc. -make
sure the work is done in a remote location. Sand or nut hulls used for this
purpose will usually find a way of invading the wrong parts of the ma-
chine, such as bearings and seals. The rotor should be carefully checked
at this time for debris lodged in the gas passages. We know of instances
where a rag or piece of metal was jammed in an inaccessible place in an
impeller. The use of a small dental mirror and a thorough inspection by
hand can reveal much of this debris.
It is fairly common practice to inspect a rotor using magnetic particle
or dye penetrant techniques. This is a strongly recommended step; it can
turn up defects which could otherwise prove to be highly damaging dur-
ing a subsequent running period. In one such instance we uncovered an