Page 255 - Design of Simple and Robust Process Plants

6.4 Application of Reliability Studies for a Process and Utility Plant 241
Several alternatives were evaluated for different pump configurations, as these
had the major contribution to down-times and the number of unplanned stops per
year. The alternatives evaluated were:
. Case 1: the same as base case (0), but with four pumps identified as noncriti-
cal. Noncritical is defined as a situation where a process can stand a failure
without interruption within the MTTR of the item and its distribution.
. Case 2: as Case 1, with two additional spares for those cases which had rather
long process recovery times.
. Case 3: as Case 2, with pumps in which the MTBF was extended two-fold by
the application of gas seals.
. Case 4: all pumps spared.
The results of the reliability calculations are listed in Table 6.4 for the contribution
of pumps. It is clear from these results that single pumps provided with gas seals
(Case 3) give a large reduction in down-time as well as in the number of unplanned
stops per year, which clearly will have a positive economic outcome. The investment
cost of gas seals is marginal compared with the savings in reliability and availability
The spare pumps have the highest reliability and availability, but that is not necessar-
ily the best choice. In most cases, investments in reliable components have a shorter
payback than an increase in redundancy, this is already practiced for years in case of
compressors.

Table 6.4. The contribution of pump reliability for several design cases on: down-time/year, num-
ber of stops/year, overall process unavailability, including all equipment.

Alternative Down-time (h/year) No. of unplanned Unavailability
stops/year as % of time

0 61.9 5.1 1.12
1 48.6 3.20.92
234.2 2.45 0.80
3 16.1 1. 0.57
4 0. 0. 0.42

The above illustrates that reliability engineering techniques can easily be used to
design an optimal reliable process. In the next chapter, the reliability of a process
will be studied in the context of a complex of integrated process plants including
feed supply, product delivery, and utilities.

6.4.2 Application of a reliability study for a utility steam plant
Reliability engineering studies for utility plants are, in essence, not different from
those for process plants, but the economic optimal levels of reliability and availability
are much higher. Any outage on utilities ± whether power, steam, process water, cen-
tral cooling water system, fuel-gas, or instrument air ± may lead to total site outages.