Hydrocarbon Processing - November 2021 - 28

Process Control, Instrumentation and Automation
FIG. 2. The APC tool gap. Traditional advanced regulatory control (ARC) has limited capabilities,
while the high ownership costs of conventional MPC make it suitable mainly for large
applications. This has left a large tool gap for industry's many midsized multivariable control
applications, which can be filled by more cost-effective and agile APC tools based on insights
from traditional manual multivariable control practices.
ile-primarily due to the unreliability of
the models upon which they depend-to
evolve into low-cost,
low-maintenance,
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Customised solutions thanks
to the modular scheme
high-agility, long-lifecycle technologies
(i.e., into core competencies). Several articles
report that more than half of MPC
applications perform at pre-installation
levels or have been removed within
18 mos-24 mos.2
now show that multivariable control, just
like single-loop control, can be readily accomplished
using feedback control algorithms
with or without the selective use of
key feedforward models.
" Real-time " optimization, defined as
This combination of high
cost, high maintenance and short lifecycle
is increasingly considered unsustainable.
Model-based control and real-time
optimization are often considered synonymous
with multivariable control, but
they are actually just part of the way conventional
MPC solves the multivariable
control problem. Other ways exist to do
multivariable control that do not entail
these elaborate and fragile methods. For
example, FIG. 1 outlines a method of multivariable
control that basically mimics (automates)
traditional manual multivariable
control methods that do not depend on
detailed models or embedded optimizers.
In retrospect, industry should not be
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surprised by the pitfalls of large-scale
model-based control, because the same
lesson was learned in single-loop control
long ago. Feedforward is the single-loop
equivalent of model-based control and
is equally powerful-in concept. However,
industry uses feedforward very sparingly-less
than 5% of loops-due to the
added cost, risk and maintenance associated
with any model. MPC was expected to
overcome this limitation by virtue of more
careful plant step tests and better model
identification tools, but instead industry
discovered that models change frequently
for a wide variety of reasons, so that model
identification is a shifting target. Rather
than solving process control tuning and
performance issues once and for all, MPC
took application support to vast new levels.
Fortunately, experience and insights
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optimization deployed at the control layer
in conjunction with multivariable control,
also may be on a declining trajectory. Aside
from its part in MPC complexity and fragility,
optimization at the control layer
cannot begin to compare with modern optimization
practices at the business PP&O
layer in terms of sophistication of tools,
site-wide breadth of scope, appropriate
optimization time scales, etc. At the same
time, modern connectivity makes it easy to
share information between layers, so that
multivariable control can directly utilize
PP&O results, rather than be burdened
with its own inferior optimizer. As a further
emerging concern, the high support
and maintenance of model-based control
and real-time optimization put them at
odds with modern control network reliability
and cybersecurity principles.
Benefits and metrics. Industry adopted
the intrinsic value of closed-loop control
over open-loop control decades ago, so
that " loops in manual " is one of the most
common process control metrics among
top-tier operating sites today. Industry
also adopted the idea that console operators
should not be distracted by excessive
numbers of low-value alarms, so that " bad
actor alarms " is another nearly universal
alarm management metric.
This says a lot about the number of
loop interventions (i.e., the number of
controller setpoint, output and mode
changes) that occur daily at an operations
console. High numbers indicate excessive
multivariable loops in manual and/
or bad actor loops. Such a " loop interven

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Hydrocarbon Processing - November 2021

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