Page 267 - Reciprocating Compressors Operation Maintenance

252 Reciprocating Compressors: Operation and Maintenance

components have been affected as well. Careful inspection of the area
around a replaced bearing is an essential step in preventive maintenance.
Crankshaft straightness, bedplate saddle alignment, and the bore diam-
eter of the connecting rod bearing bores are potential trouble spots. The
localized overheating that often accompanies bearing failure creates
dimensional changes that can lead to distortion or cracking in related
parts. Be sure to consult the manufacturers' manuals and check all related
components for damage whenever bearing problems occur.
Regardless of other factors involved, most severe compressor align-
ment deficiencies will ultimately lead to destructive overheating of major
components.

BEAMING MAINTENANCE

Many serious problems can remain hidden long enough to cause dam-
age. Good bearings may actually conceal a bearing saddle problem. A
bent crankshaft can, in some cases, give "zero" deflection readings.
Sometimes the lack of normal wear can indicate a problem. Thus inspec-
tions should be made a regular part of the maintenance program. Be sus-
picious when anything out of the ordinary is seen—even if the overall
condition of the machine seems acceptable.
Expect wearing parts to wear out. In one catastrophic failure event, a
compressor broke a crankshaft. There was no abnormal crankshaft
deflection at the last inspection, and it had run for over 15 years on the
same bearings.
During disassembly, it was discovered that, while the bearing surfaces
of the mains appeared to be in good condition, their backsides had
become worn into the bedplate. Large compressor components such as
crankshafts are subject to considerable flexing despite their mass. They
should be supported along their entire length, or else severe damage can
result. In this case, the worn bearings failed to provide that support. The
distortion and fretting on the backs of the bearing shells also impaired
heat transfer.
These bearings should have been removed, inspected, and replaced
thousands of hours earlier. Spending about $12,000 on new bearings to
protect a $400,000 crank would have been a wise trade-off. This unfortu-
nate story is instructive. Compressor bearings should be replaced every 5
to 8 years, or at least thoroughly inspected.