Page 376 - Reciprocating Compressors Operation Maintenance

Preventive Maintenance for Reciprocating Compressors 36 f

monthly sampling of the compressor lubricating oil and performing
spectrographic analysis that provides an accurate quantitative break-
out of individual chemical elements contained in the oil elements as
oil additives and contaminants. A comparison of the amount of trace
elements in successive oil samples can indicate wear patterns of all
wetted parts in the equipment and warn of impending failure.
Full benefits of oil analysis can only be achieved by taking fre-
quent samples and trending the data for each compressor. The basic
data on each compressor allow the laboratory to build a unique data-
base. Reports then include values from the current tests, the average
for the particular compressor, and values from previous tests.
A spike in the content of one element indicates a sudden change
in the conditions inside the compressor. A comparison with the plant
and laboratory averages provides a means of judging the signifi-
cance of the change.
Oil analysis can provide a wealth of information on which to base
decisions. However, major payback is rarely possible without a con-
sistent program of sampling in order that data can be trended. While
oil sampling and analysis can provide an additional capability to
existing preventive maintenance programs, it should not be depend-
ed upon to the exclusion of all other techniques. In other words,
there are documented instances of bearing failures taking place in
operating compressors that, for some reason, were not picked up by
sampling the lubricating oil.
5. Vibration monitoring. This monitoring is particularly useful on
those compressors that use anti-friction bearings that are the smaller
sizes of reciprocating compressors. Because reciprocating compres-
sors have relatively low rotative speeds, they produce low frequency
vibrations and unfortunately require more than the traditional vibra-
tion velocity monitoring or frequency analysis. However, monitor-
ing packages are available from experienced specialty firms (see pp.
244 to 245, also 262 to 268).
6. Acoustic emissions or ultrasonic detection of leaking gaskets, etc.
(see pp. 329 to 334).
7. Oscilloscope analyzers. These devices can be used to observe what
is happening internally in the compressor cylinder, and, by compar-
ing the actual pressure, volume, time indicator card, to the theoreti-
cal indicator card, the analysis determines if components are mal-
functioning.