Page 338 - Improving Machinery Reliability

304 Improving Machinery Reliability

One issue that is hidden in Figures 5-17 and 5-19 raises its head in Figure 5-18.
It’s the cost of electrical power to drive the pump. Power consumed is a direct result
of work performed, energy lost in inefficient motordbearing, and energy lost in
pump dynamics. Energy savings by use of high efficiency motors can save 2%-5%
of the total power cost, and choosing high-efficiency internals for the pumps can
save 5%-10% of the total power cost. In short, purchase high-efficiency motors and
high-efficiency internals carefully matched to the task to achieve a short payback
period. If pump internals were selected for 80% pump efficiency rather than the 70%
efficiency used for the calculations, the lower power consumed would be US$16,500
* (70%/80%) = $14,438, which results in a savings of US$2,062 each year, or about
equal to all maintenance labor efforts spent to correct failures! The point is this:
Examining cost-reduction possibilities by use of LCC details can be productive for
discovering real savings opportunities rather than following the old recipes. In short,
creating wealth for shareholders often means stop doing some things the old way
and start doing new things in smarter ways.
Using the overall ANSI pump failure rates and a mission time of one year, the
reliability at the end of one year is calculated as 37% (which is about the same as
saying one pump in three will operate for a one-year interval without some type of
failure). The chance for failure-free intervals is low. Much of this poor reliability is
driven by how the pump is operated. Optimum conditions are rarely achieved in pro-
duction plants because of variations in operating conditions and operating styles,
Figure 5-20 illustrates the sensitivity of pump reliability to pump curves and other
well-known problems. The shape of the reliability curve is dependent upon many
pump features and operating conditions. Figure 5-21 shows other possible sensitivity
studies that combine multiple features.
Of course, the effectiveness equation offers good information because the largest
single variable is reliability. The other components of the effectiveness equation in
Table 5-18 have minor variations.
The life cycle cost shown in Figure 5-22 is the NPV result of the alternatives to
put LCC into business terms. The shape of the curve is decided by selection of alter-
natives and cost drivers.
Step 10 Study Risks of High Cost Items and Occurrences. Failure data is
available from many sources to test whether the assumptions made in the analysis
are valid or if unusual risks have been taken with numbers used in the study. Consid-
er the failure rate values given in Table 5-19.
An example of the conversion from failure rates to mean time between failures is:

MTBF = U((4E-06 failures/hour) * (8760 hourdyear)) = 28.5 years

Compare Table 5-19 to Table 5-12 and Table 5-14 for ANSI pumps and the data
look comparable except that the failure rate for impellers may have been selected too
high and thus the MTBF is lower than shown in Table 5-19. Let local operating con-
ditions and experience decide the correct value. When comparing Table 5-19 to
Table 5-16 for the API pump, the results look okay.