Page 352 - Reciprocating Compressors Operation Maintenance

Troubleshooting Compressor Problems 337

able and, upon investigation, found that the compressor was designed
and built for a maximum air pressure of 40 psi. The operator was,
accordingly, cautioned that he was subjecting the machine to over twice
the load for which it was designed and that continued use under these
conditions might cause problems. That he had not already had trouble is
undoubtedly due to the conservatism with which compressors are rated.
Like many other users, this person believed that the air cylinder on the
machine appeared to be sufficiently heavy for 100 psi and that, therefore,
there was no danger in operating the compressor at this pressure. That
assumption brings us to the point: The maximum load allowable on a
compressor is determined more frequently by the strength of the frame
and the running gear rather than by the strength of the air or gas cylinder.
In the case cited above, it so happened that the 12x9 air cylinder is
sufficiently heavy for 100 psi pressure. As a general rule, air cylinders
are designed with quite a large margin of safety and are actually used
over a wide range of pressures.
Compressor ratings are typically limited by the load on frame and run-
ning parts. The three points in the frame construction that are under heavy
strain while transmitting the power from the pulley driving the machine to
the air cylinder are the main bearings, the crank pin, and the crosshead
pin. When these parts are under less than maximum load, they do their
work without appreciable wear or damage risk, but when they are subject-
ed to working strains greater than their ratings, the overload not only may
cause considerable knocking, but excessive heating as well. Serious dam-
age may be the logical outcome of operating with excessive load.

In this actual case, a plant had a system comprising four different recip-
rocating compressors feeding into a 100 psig receiver.
Reciprocating compressors at 100 psig

1-100 HP
1-60 HP
1-50 HP
1-40 HP
250 HP total, at approximately 5 CFM/HPor 1250 CFM