Page 308 - Design of Simple and Robust Process Plants

294 Chapter 8 Instrumentation, Automation of Operation and Control
. Control valves need to be adequately sized (do not oversize), and manual
bypasses are to be avoided. Control valves are very reliable, but we should not
operate without them as this will lead to uncontrolled situations.
. Switches on block valves are advised for fouling or higher temperature sys-
tems, where the likelihood of sticking valves is relative high. For emergency
block valves these are advised only on the failsafe position.
. Diagnose instrument signals by spectrum analysis to detect/alarm opera-
tional or mechanical problems. Instrument fieldbus or other digital commu-
nication systems makes this a possibility.
Simplification finds it way in the above guidelines by single reliable and robust
instruments, which are properly selected, installed, operated and provided with
auto-check.

8.3
Automation of Operation

Automation takes care of implementing the operation strategy which plays an
important role in simple and robust design concept (CCPS, 1993; Rijnsdorp 1991).
As ªWhy automation?º is a legitimate question to ask, we have to look at the charac-
teristics of human beings as mentioned earlier (Chapter 3) under the design philo-
sophy: minimize human intervention (CCPS, 1996).
One of the main human characteristics which justifies it is:

Human beings cannot consistently perform tasks.

Each time that people need to repeat certain activities, they will perform the task ±
but always with some interpretation of the way this task can be completed. They will
not react timely and follow different sequences, but are also handicapped by human
errors. In the process industry it is necessary to make products of consistent quality
with a minimum of variation, and such an objective can only be met by automated
processing and hands-off operation. Safety systems are designed hands-off based on
the same reasoning.
Another quality of character is that:

If it is to complicated, no one will be able to figure it out.

When something is difficult to understand or is complex, people will either not
touch it, or will switch it off. In the discussion on simplicity and complexity
(Chapter 2) it was said that complex operations require total automation. A model-
based controlled or an operation optimization-controlled process is an example of
where the design of these controllers require a high level of robustness and availabil-
ity. If these requirements are not met, they will be ªswitched offº by operation. Com-
plex operations demand for total automation.