Page 47 - Design of Simple and Robust Process Plants

2.4 What is Robustness? 31
is a rotating device that is installed without spares, and has a very long stand time,
without maintenance.
Developments in the seals of centrifugal pumps is another good example of an
evolving device. Next to the requirement for higher availability and reliability, the
development was also driven by a need to reduce environmental load. This all began
with the stuffing boxes, followed by liquid seals as labyrinth seals, single mechanical
seals, and double mechanical seals. The introduction of seal-less pumps (and lately
of gas seals) has provided a clear demonstration of how to achieve higher reliability
and availability, and lower environmental loads. Similar developments relate to bear-
ings, the evolution of which has included greased journal bearings, hydraulic jump
(oil lubrication) bearings, rolling bearings, life-time greased bearings, and no-con-
tact bearings (electromagnetic bearings).
The progress made in the mechanical reliability of components and units makes
the introduction of the design philosophy ªdesign for single reliable components
unless ¼º, as well as the concept of ª¼ maintenance-free operation ¼º become rea-
lity. Techniques of reliability engineering which help cope with the uncertainties of
reliability and its probability in design have been published (Henley and Kumamoto,
1992; Leitch, 1995; The ªRed Bookº, 1997). In addition, reliability modeling can be
performed using standard tools such as the software ªSparº(Malchi Science,
Herzliya, Israel; Clockwork Design, Inc., Austin, Texas).
At present, the market has been opened up for RAM (reliability, availability, main-
tenability) specifications of critical equipment and supplies and services.

2.4
What is Robustness?

Reliability, which was discussed in the previous section, has been defined as ª¼ the
probability that a component or system performs its function over a projected time period if
used in a proper wayº.
Robustness, however, is defined as ª¼ the property of a component to remain healthy
and operable if it is not utilized in a proper way for a certain timeº. (Note that ªwrong
usageºis purposely excluded here).
Robustness might be achieved in two different ways: (i) the component is resis-
tant to improper usage; or (ii) it is made less likely to occur. In general, we prefer to
have reliable and robust components, but robustness is difficult to quantify, and
may be better envisaged by the help of some examples.
Many men use an electric razor that is very reliable, but when their hands are wet
they might drop the razor onto the floor, where it will break. This problem has been
solved by most razor suppliers by covering the hand-grip with small rubber nipples
to make it less slippery. The casing of the razor also has a certain shock-resistance to
reduce the likelihood of damage if dropped. In the same way, the design of a
chemical complex must be prepared for mis-operation, as well as for failures of
equipment and instruments. Apart from the safety provisions which must always
form part of the design, we can anticipate any mishaps that might occur under the