PRACTICAL PROCESS CONTROL TUNING AND TROUBLESHOOTING

After working almost exclusively in process control for over 40 years, I have
developed a strong opinion that is refl ected throughout this book: The process
aspects of process control are the key to success; the systems aspects (the bits
and bytes) are largely irrelevant. If you can do an application with one com-
mercial system, then you can do it with any of them.
The practice of process control involves two intertwined endeavors:
1. Developing a process and instrumentation (P & I) diagram for the
process.
2. Proportional - integral - derivative (PID) controller tuning and
troubleshooting.
Defi ciencies in the P & I diagram invariably lead to controllers that cannot be
successfully tuned. The root problem is with the P & I diagram; tuning diffi cul-
ties are merely symptoms. This makes troubleshooting an essential component
of controller tuning, and vice versa.
Both endeavors are process (as opposed to systems) issues. The PID control
equation is implemented in a variety of commercial control products, including
a distributed control system (DCS), programmable logic controller (PLC),
single - loop (or multiloop) controller, PC - based control system, and so on.
A few implementations have a “ wart ” or two, but in the end, all are largely
equivalent.
Automation of any industrial process should be undertaken with a simple
objective — achieve a high degree of automatic control. That means controls
that deliver the required performance a very high percentage of the time. This
capability is a prerequisite to obtaining the greatest economic returns from a
continuous process and the minimum product variability from a batch process.
The P & I diagram must be correct, and the controllers must be properly
tuned.
What does every plant manager know about the controls? The cost! Control
system maintenance and support costs usually appear as line items in the
budget. But ask the same plant manager how (or even if) these controls are
making money. Most managers do not have a clue. The plant manager under-
stands that controls are necessary, but with no understanding of the benefi ts,
he/she does what all good managers do with regard to costs — manage the costs

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PRACTICAL PROCESS CONTROL TUNING AND TROUBLESHOOTING