Hydrocarbon Processing - May 2021 - 79

Process Controls, Instrumentation and Automation

Plant test. A ramp horizon control was deployed on a feed drum

to a reactor in an industrial plant. The feed to the drum varies and
the existing PI gap controller was tuned to keep the OP as steady
as possible while maintaining the level within limits of 30%-70%.
If the low level is exceeded, the reactor feed pumps will cavitate;
if the high level is exceeded, a high-level protection controller
will send reactor feed material to another process where the feed
material would be converted to a lower value product.
The ramp horizon controller results were compared to the
original PI gap controller and analyzed using the VOD and
AAM, as well as visual inspection of the PV and OP trajectories.
Data of the feed to the drum were collected and used as disturbance data to a simulated level controlled by a ramp horizon
controller. Tuning was then refined to minimize the standard
deviation of the OP.
The controller was commissioned, and data was collected
for a period of 1 mos. This was compared to data collected during the same month of the previous year to eliminate seasonal
influences, as far as possible. The data sets show that the ramp
horizon controller managed to maintain the level between 30%
and 70%, while the PI gap controller allowed the level to vary
between 36.7% and 84.7%.
Data where the controllers were switched to MAN were excluded. This accounted for 11.9% of the data for the ramp horizon controller and 19.7% for the PI gap controller.

FIG. 8 shows one day's data while the ramp horizon controller
was used. When comparing this with one day's data from the PI
gap controller in FIG. 9, the ramp horizon controller moved the
OP less than the PI gap controller, but it had to move it faster
when it was required to remain between limits.
All metrics shown in TABLE 6 indicate that the ramp horizon
controller outperformed the PI gap controller. The simulation results show how the non-linear PI controller will perform similarly
to the ramp horizon and SOALC when a disturbance of the exact
size that was used to tune the non-linear PI controller occurs.
When a disturbance occurs that is larger than was assumed to
tune the controllers, the non-linear PI controller will allow the
level to move outside the limits. As the ramp horizon and SOALC calculate a response that is not based on an assumed maximum disturbance, they will maintain the level within limits.
When a smaller disturbance occurs, the ramp horizon may
make no changes initially and the SOALC will make small
changes throughout. They both allow the level to reach the limit. In this case, the non-linear PI controller will prevent the level
from going to the limit, using only part of the available buffer
capacity offered by the drum. Consequently, the non-linear PI
controller will move the OP more aggressively than the ramp
horizon and SOALC controllers.
The plant test shows how the ramp horizon controller outperformed the non-linear PI controller by:
* Keeping the level within limits
* Implementing smaller and fewer moves.
50
49
48
47
OP, %

Positive step disturbance of 3.75 m3/hr. If the magnitude
of the disturbance is decreased by 25%, FIG. 6 shows how the
non-linear controller did not use the full range allowed for level
buffering. FIG. 7 shows how the ramp horizon controller initially
did not move its OP because a limit violation was not predicted
within the defined time horizon. Later, the controller then had
to move the OP faster to prevent a limit violation, which increased its VOD (TABLE 5).

TABLE 3. Performance metrics for a step disturbance of 5 m /hr
3

Non-linear

Ramp horizon

SOALC

VOD

0.073

0.074

0.067

AAM

0.041

46
45
44
43
42
Non-linear OP

TABLE 4. Performance metrics for a step disturbance of 6.25 m3/hr
Non-linear

Ramp horizon

SOALC

VOD

0.098

0.115

0.097

AAM

0.051

Ramp horizon OP

SOALC OP

FIG. 5. OP response to a step disturbance of 6.25 m3/hr.
75

TABLE 5. Performance metrics for a step disturbance of 3.75 m3/hr
Ramp horizon

SOALC

0.05

0.056

0.041

AAM

0.031

TABLE 6. Performance metrics comparing ramp horizon
controller and PI gap controller
Ramp horizon

PI gap

VOD

0.09

0.11

AAM

0.013

0.018

65
Level, %

Non-linear
VOD

50
Non-linear PV

Ramp horizon PV

SOALC PV

FIG. 6. Level response to a step disturbance of 3.75 m3/hr.
Hydrocarbon Processing | MAY 2021 79

Hydrocarbon Processing - May 2021

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