Page 288 - Design of Simple and Robust Process Plants

274 Chapter 7 Optimization of an Integrated Complex of Process Plants
It can be concluded that, in this case, a rather small storage gives an increase in
capacity of 2.5%. The total process failures show a spread, although the failures
should not be affected by storage capacity. The spread is caused by the same reason
as discussed previously. For the storage capacity of 24 h, the number of trains that
were partially loaded were counted and expressed as % partially loaded trains. These
are only summarized data. Many details might be obtained, as was illustrated in the
previous example. To mention some of these, a ranking can be made of causes of
failures and down-times, including tank-empty and tank-full situations. The level of
storage can be plotted over time. The methodology applied in this case did not differ
from the previous example in this section ± only the size of he problem was
increased.
The variables for such a chain of production facilities are: process reliabilities and
availability; storage volume of each intermediate tank; set point of the level in stor-
age; and transport reliability and availability. The set point of storage level is impor-
tant when the availability of the supply and receiving streams shows a large differ-
ence.
The last example can easily be extended with transportation data of raw material,
but also with supply and delivery requirements. Intermediate product streams
might be loaded or unloaded and can be included in the scope. Particular options
for supply and delivery requirements might be the subject of further study. This
makes the cost of these (mostly contractual) terms viable, and enables the settlement
of an agreement on optimal terms for both partners.
In conclusion, one can say that reliability studies are an excellent tool for the
determination of the impact of process reliability and storage capacity on production
availability and capacity. The data form a sound basis for a cost±benefit analysis of
investment in this field. In the next section, the extension to a supply chain at a site
is discussed.

7.7
Site Vulnerability

The objective of a site vulnerability study is to identify weak points in the infrastruc-
ture of a complex which could lead to an unacceptable level of unplanned partly or
total outages and related losses (Figure 7.8).
Economic criteria can be applied when the losses that may occur during an out-
age can be quantified in relation to the investment cost to reduce them. The order of
magnitude for a chemical complex experiencing a total unplanned outage might be
once every 20 years. This depends a great deal on the availability and failure rates of
utilities. It must be clear that specifically for power, one might have a large availabil-
ity but still a large number of relatively short power losses. It is the number of these
power losses which sets the stage in such a case. For a plant, an unplanned outage
may occur somewhere between a few times per year or even less, and this largely
depends on the design of the plant. A quantitative method is shown here of how to
determine the vulnerability, and ways in which it can be improved. This avoids the