Page 299 - Improving Machinery Reliability

270 Improving Machinery Reliability

80
70

u:
m 40
E
4 30
20
10

Figure 5-4. Improved mean-time-between-failure (MTBF) at three British oil refineries
were attributed to seal upgrade and selection ~trategy.~

MTBF increases of (80-57)/57 = 40% in 4 years (refinery B) and (50-33) 33 = 51%
in 2 years (refinery C).
It makes good sense to see more substantial improvement possibilities for the
refinery that has the lower MTBF to start with. We note that refinery C started with
33 months seal MFBF and that our refinery is presently at 28 months MFBF.
Returning to refinery A and their overall pump MTBF, which had increased from 30
months at the end of year 2 to about 71 months at the end of year 7, we calculate an
MTBF increase of (71-30)/30 = 36% in 5 years. If we take into account the observa-
tion that refineries starting with MTBF figures of 30 months have experienced
MTBF increases around 25% per year, we should feel little reluctance assuming that
our own plant could go from an MTBF of 28 months to one of 56 months in the span
of five years.
Let us, therefore, assume that a refinery wanted to embark on such a mechanical
seal MTBF Improvement Program and management requested an appropriately doc-
umented and referenced cost and benefit projection.
We have 1,474 centrifugal pumps at our plant site. Our seal MTBF was originally
calculated from (1,474 pumps installed) (12 months/yr)/632 seal failures/year = 28
months. Furthermore, it is known that upgrading to superior seal configurations and
improved seal materials would add $1,700 to each pump repair and that typical
pump repairs, using traditional grade seals, would cost approximately $5,000.
Assuming a linear MTBF increase from 28 months today to 56 months five years
from now, we might now opt to calculate our yearly repair cost outlay in the most
straight-forward manner and list our results in Table 5-6.
Table 5-7 highlights a similar approach that can be used to estimate the economic
justification for retrofitting small pumps with new casing covers.1o These “back-pull-