Page 65 - Design of Simple and Robust Process Plants

3.3 Design Philosophies 49
3.3.7
Minimize Human Intervention

Human beings are both highly creative and intelligent ± characteristics which make
it possible to improve our way of life. Another characteristic is that humans are
always prepared to learn on a trial-and-error basis, in addition to receiving educa-
tion. Consequently, whatever is learned there is always the desire to confirm such
learnings. Another human characteristic is its difficulty in repeating things in con-
sistent manner; thus an airline pilot will conscientiously follow a checklist before
start-up, and this is cross-checked by the co-pilot.
Human beings are not consistent in performing tasks, and so it is necessary to
protect process systems for mishaps during operation of a process plant. The philo-
sophy to minimize human intervention is translated as:

design for total automation and robust hands-off control

of a chemical plant, and in this respect the operator will change to a supervisor with
limited operational tasks.
The advantages of this approach may be considerable:
. Man-power savings: it might be possible to use only one operator for two
mid-sized, continuous plants.
. Improved control, resulting in less off-specification products.
. Higher-capacity operation through the application of constraint controllers
on feed streams.
. Less maintenance due to mechanical failures as a result of operational errors
or uncontrolled stops/transients.
. Operation optimization, with its own savings, can be introduced on top of
the robust control layer.

3.3.7.1 Total automation
At present, the tools for total automation are available, and current instrumentation
systems have this capability. The automation strategies must be transparent and
may differ between plants, but this isn't preferable if they are planned to be operated
by one person.
An automation strategy must emphasize the following elements:
Level of automation: in an automated plant not all operations are automated. In
most cases it is preferable to provide a minimum of operation attention in a super-
visory role. To enable this, certain process actions must be fulfilled by an operator to
keep him actively involved in the operation.

. Level of process checking: in batch processes, reactor loads of different com-
ponents are often checked with a double independent measurement based
on a different measuring principle. This is in agreement with Total Quality
Control.