Page 168 - Improving Machinery Reliability

140 Improving Machinery Reliability

leads to certain extrapolations and occasional risks for the purchaser who may end
up serving as the vendor’s test bed or research auxiliary.
Evidently, careful review and selection procedures may be in order. Presented are
sample component analysis procedures which serve to identify components that are
potentially limiting in uprate or extrapolation situations. In turn, these analyses will
lead to a more objective evaluation of competitive bids for major reciprocating com-
pressors from several manufacturers.

Manufacturers’ Scale-up Philosophies Differ. Generally speaking, compressor-
frame ratings are fixed by component sizing criteria used in the initial design of the
machine. With few exceptions, the first several compressor frames of a given model
or frame designation are sold for, or operating with, throughput rates which keep
component loading conservatively low. In an uprate situation, the manufacturer will
have to investigate the suitability of a given frame for higher throughput rates than
are indicated by prior experience. If higher throughput is to be achieved merely by
pressure increases while maintaining the actual inlet volume relatively unchanged,
the effect on component loading can be readily assessed and the adequacy of the
components in question analyzed.
However, increased throughput must often be achieved without changes in pres-
sure levels. In this case, the manufacturer has one or more design options to choose
from. Among these options are:

Increasing the compressor rpm.
Increasing the piston or plunger diameter.
Increasing the piston or plunger stroke.
Adding more cylinders.

The purchaser is now faced with the task of analyzing the potential risks associat-
ed with the scale-up approach used by a given manufacturer. The machinery engi-
neer will immediately recognize that:
Increasing the compressor rotative speed will result in higher linear piston veloci-
ties, higher inertia loading of compressor components, and more valve cycles over
a given time period.
Increasing the piston or plunger diameter will result in higher component loading
and may raise some questions as to proven manufacturing experience with unusu-
ally large piston or plunger diameters. In some cases, the heat dissipation achiev-
able with large-diameter pistons or plungers may become a point of contention.
Increasing the piston or plunger stroke will result in higher linear piston velocities,
crankshaft redesign, and different component loading.
Extending the compressor frame length to add more crank throws (cylinders)
requires primarily a statistical assessment of the added downtime risk resulting
from more parts, and also an analysis of increased costs of foundation and piping.

Detailed Component Analysis Recommended. The potential purchaser of
extended-size reciprocating compressors should familiarize himself with the experi-