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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.
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