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7.6 Optimization of Storage Capacity 265
7.5.2.4 Utilities availability
The sensitivity of the site utility system has a major impact on the vulnerability of
the site. Therefore, an evaluation of availability of the site utility systems requires a
careful quantitative basis. A trade-off must be made between investment cost for
redundancy/ back-up provisions versus the quantified risk of production losses. The
availability calculations should be done in the same way as for the plants (see Sec-
tion 6.4 in Chapter 6).
The involvement of the availability of external supplies must be included. During
this study, the load-shedding options need to be explored and prioritized, and specif-
ic attention must be given to common cause failures. This goes beyond the study as
described in Section 6.4, which assumed no common cause failures of the individu-
al steam generators.
The output of the utility reliability and availability must be used as input for the
site vulnerability study (see Section 7.7).
7.5.2.5 Summary utility integration
The design principles for heat integration at the site are:
. All plants must have a pinch analysis and heat exchanger network design.
. The optimal energy system for a complex based on fuel is a co-generation
system for power and steam.
. Heat integration between plants is preferably done through a utility system,
but in case of direct exchange the philosophy of independent operation of the
plants must be respected.
. A quantitative reliability study must be available for the utilities, with a thor-
ough analysis for common cause failures of utility generators. This study
needs to include the reliability of the different load-shedding levels for applic-
able utilities such as power and steam.
7.6
Optimization of Storage Capacity
The optimization of the storage capacity will be illustrated through analyzing a tank,
which is located between two processes (Koolen et al., 1999; Mihalyko, 1999). In this
particular example, the tank has only one inlet stream from the upstream process
and one outlet to the down stream process (Figure 7.5). The size of the tank must
Utilities Utilities
Plant Storage Plant
Fig. 7.5. Optimization of storage capacity.