Page 253 - Design of Simple and Robust Process Plants

6.4 Application of Reliability Studies for a Process and Utility Plant 239
Operation by automation easily respects the design conditions of the equipment,
resulting in the avoidance of cavitation and deadheading of pumps, and surge or
choking of compressors. Frequency analysis of signals from instruments installed
close to rotating equipment can be applied to identify (alarm) any miss operation.
The lifetimes of these devices can exceed current MTBF values by using these more
robust operational practices. Another example is that of an agitator which must be
always submerged in service and provided with a proper vortex protection and signal
analysis warning to obtain longer stand time. The failures of rotating devices are
mostly related to its miss usage and less to wear out.
Examples of highly reliable rotating equipment include steam turbines, electrical
generators, and centrifugal compressors, for which the MTBFs exceed stand times
of five years.
The above points of better design and improved operation by automation will still
improve the failure rates.

. The evaluation and selection of the most economical alternative is the final
step for the design. This is the trade-off between investment cost and produc-
tion losses. The losses due to unplanned outages need to include any damage
inflicted as a result of process interruptions.
Validation is an important aspect of reliability study. The basis for the study is
formed by the failure rates and its distribution. An additional confirmation of the
results is still a valuable exercise. The best way is to compare the overall results of
the study by the evaluation of similar process plants during their history. The most
striking results were obtained by comparing the results of a polypropylene reliability
study with the data of an engineering company. This company licensed globally doz-
ens of similar plants, and collected for years all failure data from these processes.
The comparison that was made showed that the reliability and availability data had a
very good fit with the study results.

6.4
Application of Reliability Studies for a Process and Utility Plant

6.4.1
Application of a Reliability Study for a Continuous Process Plant

The production of 1-octene from crude C4, is subject to a reliability study. Thus con-
sists of three process sections, as presented in Figure 4.2 Chapter 4 (Koolen et al.,
1999). In the reliability study, we examine the mechanical reliability of the telomer-
ization section. The connection between process sections through intermediate stor-
age will be discussed with respect to availability in Chapter 7.
In this study, the utility failure rates were set at zero, while planned process stops
for catalyst regeneration and fouling were not considered as unavailability. This
gives a clear picture of the process itself.