Page 236 - Design of Simple and Robust Process Plants

222 Chapter 6 Process Design Based on Reliability
user software industry is lacking behind in that respect, but it will certainly need
to comply with the market demands, or it will go out of business. However, user
software is judged more on its on functional performance and robustness rather
than its reliability.
The nuclear power industry was the first to use reliability engineering techniques
on a large scale for the design of plants. However, it should be noted that this indus-
try is still using many design conventions such as triple redundancy for instrumen-
tation and the safeguard of equipment. Initially, some process and control designers
were in favor of triple redundancy for process plants, but the battle has now turned
in the delivery of safe plants complying with safety reliability criteria, as specified by
IEC 61508. This opens the way for reliable instrumentation and safeguarding
designs based on reliable and robust components in order to meet the criteria
defined. A parallel can again be drawn with the two-engine airplane flying reliably
and cost-effectively over the ocean!
The application of reliability engineering is the driving force in the development
of an optimized design between suppliers and users of equipment and supplies.
These optimal supplies can only be achieved in good partnership between supplier
and user based on reliability data. This requires clever solutions to maximize relia-
bility and availability at low cost (which is an evolutionary pathway), and not by
extensive redundancy provisions. An over-demand from the user's perspective will
result in more investment at the supplier, and this will ultimately result in higher
product prices.

6.2
Basic Theory of Reliability

In this section, the intention is to introduce the most important terms as used in
reliability engineering (Henley and Kumamoto, 1981 and 1992; Leitch, 1995; Red
book, 1997), rather than to provide the reader with derivatives for these equations.
However, this text is presented for use as a reference source for the methodology
presented herein. In other words, how to apply reliability engineering to process
design.

Reliability R(t) is defined a the probability that a component or system performs its
function over a time interval 0±t (in a particular environment), given that the compo-
nent or system was to be considered new at time zero.

Some definitions include the restriction ªin a particular environmentº. The charac-
teristics terms of the definition are:

. function
. time
. probability
. environment