Page 39 - Design of Simple and Robust Process Plants

Design of Simple and Robust Process Plants. J. L. Koolen 23
Copyright 2002 Wiley-VCH Verlag GmbH & Co. KGaA
ISBNs: 3-527-29784-7 (Hardback); 3-527-60047-7 (Electronic)

Chapter 2
Simple and Robust Plant Design

2.1
What is ªSimpleº?

Definition: a system or device is simple if the user understands its purpose and is
able to operate it with few manipulations, while any wrong or unstructured manip-
ulation will not result in any damage. Restart of the system should be easy and real-
ized with one-button operation, while the safe guarding should result in a fail-safe
situation
Some examples will be used to explain simplicity. Electrical lighting is a simple
device: the user understands its function of ªlightº, and its operation with a single
switch. The safeguarding is done with a fuse. If the bulb does not light, the user
action may be to install a new bulb, but all other actions are for an ªexpertº. Another
example is the water reservoir of a toilet cistern, which has a level control for the
water supply. The water level in the reservoir is automatically kept constant through
a level-actuated valve in the water supply. The reservoir has an overflow protection to
a safe location. If the system does not flush, the user understands that he/she must
call for assistance, because they know that the basic function of flushing is not avail-
able. A third example is a television set: even elderly people understand the function
of the device and its three basic operational functions (on/off switch, program selec-
tor, and volume button). The fact that many TVs have a complicated handset is a
problem that some suppliers are solving by hiding function under a special cover, or
by using a special color for the essential buttons (Freudenthal, 1999).
The above examples show devices that are simple for the user to operate, but on
the other hand they can be very complex for the designer, the manufacturer, or
maintenance person. Just consider the difficulties and complications involved in the
design and manufacture of a TV set!
Let us compare this situation with some process plant details. For example, a line
connects two vessels with the purpose of transporting liquid from vessel A to B
(Figure 2.1). When this line is introduced, we need to have back flow protection.
The standard solution for this is a check valve, although we know it will always leak
and so it is not fail-safe. A solution in such a case is always to keep the up-flow vessel
at a higher pressure then the down stream vessel.