Page 147 - Improving Machinery Reliability

Machinery Reliability Audits and Reviews 119

experience of the analyst. The manufacturer produces many machines that have no
problems and thus has the confidence that the machinery will run successfully. The
independent consultant usually is asked to look only at machinery with some kind of
prloblem. Therefore, consultants probably look at the analysis from a slightly different
viewpoint. The goal of the manufacturer, the user and the independent consultant who
make audits is the same: everyone wants a trouble-free machine.
The independent audit generally occurs after the manufacturer has finalized his
drawings. In many cases where problems were found during the audit, it turned out
that although the manufacturer had made an analysis in the initial stages, some
dimensions were changed during the manufacturing phase, causing significant
change in the calculated responses. If the independent audit is made, any differ-
ences between the manufacturer’s and the consultant’s calculations can be resolved
before installation.

Guidelines as to when an independent audit should be obtained are influenced by
many factors. Generally audits should be performed on:

1, New prototype machines that are extrapolations in horsepower, pressure, num-
ber of impellers, bearing span, or incorporate new concepts.
2. Machines which, if unreliable, would cause costly downtime.
3. Machines that are not spared (no backups).
4. Expensive machines and installations in which the cost of the audit is insignifi-
cant compared to total cost.

Typical costs for analyses are difficult to specify since the scope of the work
depends upon the adequacy of the supplied information, the complexity of the
machinery, and the number of parametric variation analyses required. If accurate
drawings and system information can be supplied to independent consultants who
perform these studies, then accurate cost estimates can be given. Exxon’s experience
may, however, give the most powerful incentive to studies of this type: It has been
estimated that for every dollar spent on machinery reliability verification before
machinery commissioning, ten dollars were returned at plant startup, and one hun-
dred dollars were saved during the life of the plant.

Failure Statistics for Centrifugal Compressors

For any type of machinery considered, knowledge of failure causes and downtime
statistics allows us to determine which components merit closer review. Also, prop-
erly kept records could alert the review engineer to equipment types or models
which should be avoided. In some cases, failure statistics might provide key input to
a definitive specification. In other words, “you learn from the mistakes of others.”
All of this presupposes that “others” saw fit to record their experiences. If an engi-
neer has these data available he will no doubt use them before selecting machinery.
Whenever specific data are lacking, the review engineer has to resort to general
statistics. These statistics are typically available in the form given for centrifugal
compressors, Table 3-6 or in the form of availability tables (see Chapter 4). Probable