Page 170 - Improving Machinery Reliability
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142 Improving Machinery Reliability
Cyl. c
Figure 3-38. Forces acting on a second stage of typical hyper compressor.
design conservatism for selected components. The actual factor of safety built into a
given component may be the subject of debate; however, comparing the ratios of
maximum allowable loading versus actual loading in the designs of manufacturers X
and Y does indeed give a relevant indication of comparative strengths.
Other Factors to Be Considered. In addition to performing detailed analyses of
component strength, the potential purchaser should review the safety and mainte-
nance requirements of a given design.
Safety reviews must include an assessment of the likelihood of piston or plunger
failures occurring during machine operation. The consequences of such failures must
be studied and understood. Failure likelihood and failure consequences are influ-
enced by elements such as cylinder misalignment; provisions to effectively vent
leakage flow from packing and frame areas; provisions to permit cylinder head bolts
to lift off, or stretch, well within the material elastic limit during potential decompo-
sition events; and so forth.
The relative ease and projected frequency of reciprocating-compressor mainte-
nance may prove to be a significant cost factor. The review should focus on such
potential needs as cylinder support, cylinder alignment, bearing and slide replace-
ments, valve maintenance, and hydraulic tensionsing provisions. Feedback from user
installations may be helpful to a degree, but experience shows that variations in cata-
lysts or lubricants used by different installations often represent enough variables to
question the validity of direct comparisons of, say, maintenance frequency for valves.
Vendor Standings Can Be Compared. Vendor selection based purely on
installed cost of compressors may be quite misleading because overall operating
costs are obviously affected by many of the factors mentioned earlier. However, an
effort can be made to compare vendor standings by using a tabular rating similar to
