Page 234 - Design of Simple and Robust Process Plants

220 Chapter 6 Process Design Based on Reliability
Reliability calculations can now support design decisions based on economics,
while in the past these were based on arbitrary design guidelines. All decisions
regarding the redundancy of equipment and other additional provisions can now be
evaluated based on the difference in up-time of a process (operational benefits) ver-
sus additional capital investment. In this respect:

. Is there any value in the installation of a spare power supply cable to a pro-
cess?
. Does the value of more up-time outperform the higher investment of the
cable and its adjacent provisions?
Safeguarding of the process can now be quantified and the design protection level
can be verified whether the required safety integrity level criteria are met, or not. In
the past, design philosophies were applied as the levels of defense installed.
Maintenance departments found a valuable tool in reliability engineering to pre-
dict the failure of components that were subject to aging. A careful analysis of fail-
ure rates based on life data are used from the so-called Weibull analysis, in order to
apply predictive replacement or repairs. The optimal amount of spare parts can also
be determined, based on the failure rates of the components installed.
The basis for any quantitative risk analysis is the likelihood of an event (a failure)
occurring, and this will be quantified, based on reliability data (CCPS, 1989).
From the above-mentioned application areas, only the first four have a direct
impact on process design, and are described herein. In this chapter, the basic princi-
ples of reliability engineering are discussed. The evaluation of process design alter-
natives based on cost±benefit analysis will also be discussed in this chapter. The spe-
cification of design items which have an impact on process reliability and availability
must be completed with a Reliability, Availability and Maintainability (RAM) specifi-
cation. The reliability and availability requirements need also to be part of the con-
tracts with suppliers, receivers and transportation companies, in order to enable an
optimal reliable design.
The evaluation of optimal storage and the vulnerability of a chemical complex will
be discussed in Chapter 7, safeguarding of the system being part of the instrumenta-
tion.

6.1.1
Reliability Engineering is an Evolution to More Optimal Designs

It must be said that reliability engineering techniques provide a support for eco-
nomic plant designs. However, high reliability can only be achieved by good engi-
neering practices. These practices are reflected in a reliability cycle (Figure 6.1),
which shows a chain of activities that reflect the evolution of a reliable design. When
we start at the top of the cycle, the process design takes advantage of databases to
achieve an optimal design. This will include a RAM specification, if applicable. The
design is translated by design engineering in selected equipment. After construc-
tion, the process will be taken in operation, after which maintenance becomes the
first line involved in any repair (ªDo maintenanceº). The failure data are also col-