Hydrocarbon Processing - May 2021 - 75

Process Controls,
Instrumentation
and Automation
G. GOUS and P. DE VAAL, The University
of Pretoria, Pretoria, South Africa; and
G. VAN COLLER, Sasol, Secunda, South Africa

Implementation of averaging level control
using native DCS functions
When applying averaging level control in a process plant,
engineers can typically choose between proportional integral
derivative (PID)-based control, such as p-only, gap and non-linear, or a model-based control like optimal averaging level control. PID controllers allow the level to go outside of limits when
disturbances are larger than the assumed maximum disturbance
used during tuning. Model-based controllers will be more successful in keeping the level between limits, as no assumed maximum disturbance is used during tuning. However, model-based
control requires additional hardware and software and typically
runs at a slower execution interval of approximately 1 min.
Two simplifications of model-based averaging level controllers are proposed here. They ensure that levels stay within
limits in the same way as model-based controllers. They can
run at fast execution intervals, as they can be directly implemented on a standard distributed control system (DCS).
The first method is a simplification of the control of imbalanced ramps in standard advanced process control (APC)
software, called the ramp horizon controller. Here, the process
model is also simplified to use a gain only. The controller uses
a pre-defined time, called the ramp horizon, during which the
limits may not be exceeded. If no limits are exceeded within
the ramp horizon, then no controller moves are implemented.
If a limit violation is predicted, a move is implemented that is
large enough to prevent the calculated violation only.
The second method is a simplification of the optimal averaging level controller, called the simplified optimal averaging
level controller (SOALC). By simplifying the process model to
a gain only, the calculations required by the controller are decreased enough to enable implementation on a standard DCS.
The SOALC then calculates the minimum controller moves
that must be made continuously to reduce the rate-of-change
of the level to zero at the time it reaches the approached limit.
Averaging level control. Many plants have feed drums with
tightly tuned level controllers that essentially reduce a drum
to nothing more than a pipe, wasting the initial capital outlay
of installing the drum. Averaging level control will allow the
level to move away from setpoint to allow the controller to re-

duce movement of the output, which should stabilize downstream processes.
Averaging level control is typically done by using properly tuned PID-based controllers1,2 or by using model-based
controllers3,4,5,6 designed to minimize controller output (OP)
movement. The PID controllers include p-only,7 proportional-integral (PI), gap and non-linear techniques,2 typically
implemented as standard algorithms on modern DCSs8 and
programmable logic controllers (PLCs) and running at execution cycles of 1 sec or faster.
A shortcoming of PID-based controllers is that they are typically tuned to accommodate an assumed maximum size disturbance.2,9,10 If a larger disturbance is encountered, the desired
upper or lower limits for the level will be exceeded. This often
leads control engineers to make conservative estimates for the
maximum size disturbance that may be encountered. If smaller
disturbances than these occur, these controllers will not make
full use of the buffer capacity.
Model-based control is performed using APC software that
will normally operate on dedicated control servers that communicate via a network connection to the DCS or PLC. These
controllers typically run at 1-min execution cycles.
Model-based controllers will make full use of the buffer capacity by letting the level move between the high and low limits. Disadvantages of model-based control include:
* Additional hardware and software are required.
* A network connection is used to link the controller
to the DCS or PLC.
* Once the controllers have allowed the level to
move up to a limit, consecutive disturbances in the
same direction will cause the level to exceed the limit,
albeit not by a large amount.
Two new control methods are proposed that are simplifications of model-based averaging level control techniques. Benefits include:
* They run on a modern DCS using its standard control
functionality.
* They run at ≤ 1-sec execution cycles.
* They take full advantage of the available buffer capacity.
Hydrocarbon Processing | MAY 2021 75

Hydrocarbon Processing - May 2021

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