Page 212 - Sustainability in the Process Industry Integration and Optimization

Co m b i n e d P r o c e s s I n t e g r a t i o n a n d O p t i m i z a t i o n 189

where A is the availability of the components of subsystem i and y is
i i
the number of redundant components in subsystem i. Comparing
downtimes is another, intuitive way to express availability.
Maintenance covers those activities undertaken after a system is
in the field in order to keep it operational or restore it to operational
condition after a failure has occurred (Ireson, Coombs, and Moss,
1996). There are several classifications of maintenance, the most
important of which are listed as follows:

• Breakdown maintenance: An item of the system would be
repaired each time it breaks down (Mechefske and Wang,
2003).
• Condition-based maintenance (CBM): The critical components
are monitored for deterioration, and maintenance is carried
out just before the failure occurs (Mechefske and Wang,
2003).
• Preventive (scheduled) maintenance: The plant is stopped at
intervals, often annually, and is partly stripped and inspected
for faults (Mechefske and Wang, 2003).
• Reliability-centered maintenance (RCM): A procedure to identify
preventive maintenance requirements of complex systems
(Cheng et al., 2008).
Maintainability is the measure of an item’s ability to be retained in
(or restored to) a specified condition when maintenance is performed
by personnel having specified skill levels, using prescribed
procedures and resources, at each prescribed level of maintenance
and repair (Ireson, Coombs, and Moss, 1996). De Castro and Cavalca
(2006) defined it as the ability to renew a system or component in a
determined period of time, enabling it to continue performing its
designed functions.
For further information on reliability, availability, and mainten-
ance of waste management systems, see Sikos and Klemeš (2010a).
Another difficulty with large systems is that troubleshooting
usually requires several problems to be solved, often simultaneously.
Data collection is also difficult because of the variety of input data;
the characteristics (e.g., type) of waste; production that changes
seasonally, weekly, and also randomly or unpredictably (due, e.g., to
weather changes, price changes that lead to different consumption
priorities, unpredictable natural disasters such as volcanic eruptions);
and changes related to a special venue, where a gathering or migration
of a mass of people can cause substantial changes as at a football
match or rock concert. The associated danger and hazards mean that
waste materials have to be appropriately cared for. A wide variety of
possible failure causes have to be identified. There are many other
issues to consider, too. Waste management systems are quite complex,