The term "plant-wide control" is used here to describe the use
of advanced software that sits above (or on top of) the individual PID controllers
running a number of process units in a plant.
Depending on the technology employed, this advanced process control
software can perform a variety of predictive, scheduling, supervisory and/or
optimizing computations. The most common architecture in industrial practice
has the plant-wide
software compute and transmit set point updates
to a traditional platform of individual PID controllers.
Notes:
·
Plant-wide
control, where one package computes updates for an entire facility, is
widely discussed in
scholarly articles but is rarely found in industrial practice.
·
While it is
possible for an advanced control package to completely eliminate the
need for individual PID controllers, this is also a rare practice.
One reason is that advanced software is often
an add-on to a plant already in operation. The existing PID controllers provide a distributed backup
infrastructure that enable continued operation, including an
orderly shutdown, in the event of a computer problem. Even in
new construction, plants are normally built with a traditional platform
of PID
controllers underneath the advanced control software.
|
Plant-wide process control holds great allure. The
promise is an orchestrated operation for
addressing upsets, maximizing throughput, minimizing energy and
environmental impact, making scheduling more flexible, and keeping
production to a tighter specification.
The figure below illustrates the
hierarchy of a more complex multi-level implementation.
Higher Level Software Uses Longer Time Scales
The individual PID loops of a traditional control system provide the
firm foundation for advanced process control. They are designed, implemented and validated as discussed
in the
dozens of
articles on this site. Typical loop sample times
for PID
controllers on processes with streams comprised of gases, liquids, powders,
slurries and melts are often on the order of once per second.
Advanced software sitting above the individual controllers computes and
transmits values less frequently, perhaps on the
order of once every ten seconds to several minutes. High level optimizers
output commands even less often, ranging from once per hour to even once per
day. The frequency depends on the
time constants of the process units in the hierarchy, the complexity of the plant-level control
objectives, the numerical solution methods employed, and the capabilities of
the installed hardware.
Project Stages For a Retrofit
Though each implementation is different, projects on existing plants
tend to follow a standard progression:
1) Validate all sensors and associated instrumentation; replace where
necessary.
2) Service all valves, pumps, compressors and other final control elements
to ensure proper function. Replace or upgrade where necessary.
3) Upgrade the DCS (distributed control system) with latest software
releases; update hardware if it is aging.
4) Tune all low level PID loops, including cascade, ratio and similar
architectures.
5) Upgrade the computers in the control room to modern standards; ensure
enough computing power to handle the plant-level software.
6) Design and deploy the plant-level control software.
Note that step 6 is presented as a simplistic single step. In reality, the
design and implementation of plant-level software is a complex procedure
requiring tremendous experience and sophistication on the part of the project
team. Step 6 is presented as a summary bullet item because the purpose of
this post is to separate and highlight the vital importance of a properly
operating platform of individual PID controllers in any control project.
Steps 1-4 Provide Profitability
Software vendors who suggest that a prospective company consider all steps 1-6 as part of an
"advanced process control" project are not being completely transparent.
The return on investment (ROI) may appear attractive for the complete
project, but it is appropriate to determine what portion of the return is
provided by steps 1-4 alone. The profit potential of these first steps, reasonably characterized as
traditional control tasks, can be responsible for well over half of the
entire revenue benefit on some projects!
Arguably, it is a better business practice to work through steps 1-4 and then
reevaluate the situation before making a decision about the need for and
profit potential from plant-level control software.
When the base level instrumentation is in proper working order, the
PID loops can be tuned to provide improved plant performance. Rather than using historical
tunings, the project team should use
commercial software to quickly
analyze process data and compute appropriate tuning parameters.
PID Control is The Foundation
As the figure at the top of this post illustrates, the PID loops provide the
strong foundation upon which the plant-level software sits.
