Hydrocarbon Processing - May 2021 - 80

Process Controls, Instrumentation and Automation
80

49.5

50
Level SP and PV, %

48.5
OP, %

48
47.5

40
30

46.5

46
Non-linear OP

Ramp horizon OP

SOALC OP

FIG. 7. OP response to a step disturbance of 3.75 m /hr.
80

Level OP, %

20
55

10
0

50
SP

FIG. 8. Drum level controlled with ramp horizon controller.

Takeaway. The proposed SOALC and ramp horizon controllers will maintain levels within limits when disturbances occur that are larger than what was assumed during tuning and
commissioning. Both controllers will outperform a traditional
PI controller in minimizing OP movement. Additional control
hardware and software will not be required to implement them
if the plant is controlled by a modern DCS.
Because the level and rate-of-change of the level are calculated at every cycle, and they run at a high execution rate, noise
and tuning errors can be accommodated.
As these controllers do not attempt to return the level to SP,
consecutive disturbances in the same direction will allow the
level to move outside the set limits. When this happens, the
ramp horizon controller will return the level to within limits
while the SOALC will have to revert to PID control until the
level is within limits again.
When attempting averaging level control on a plant with
disturbances of varying sizes, ramp horizon control or SOALC
can be used. This will ensure that levels remain within desired
limits while moving the OP less than traditional PID-based
methods, while minimizing OP movement.
However, some challenges exist in the implementation of RH
and SOALC. Since both the ramp horizon and SOALC will allow the level to reach limits while making the smallest OP moves
to ensure the limits are not exceeded, the risk remains that disturbances that push the level in the same direction as before
may occur once the level has reached the limit. If this occurs, the
80 MAY 2021 | HydrocarbonProcessing.com

Time, hr
PV

FIG. 9. Drum level controlled with PI gap controller.
80

50
SP

Level SP and PV, %

Level OP, %

SOALC will not return the level to the limit that was exceeded
using the same algorithm. Eqs. 9 or 10 will move the OP in the
wrong direction, aiding the disturbance that pushed the level
outside its limits. When this happens, the error will grow and
the SOALC will accelerate the OP movement in the wrong direction. While this might occur infrequently, reverting to PID
control when the PV is outside limits is recommended.
LITERATURE CITED
1 Lindholm, A., " Buffer management strategies for improving plant availability, " MSc
Thesis, Department of Automatic Control, Lund University, 2009.
2 King, M., Process control: A practical approach, Wiley, December 2010.
3 McDonald, K. A., T. J. McAvoy and A. Tits, " Optimal averaging level control, "
AIChe Journal, Vol. 32, No. 1, 1986.
4 Kotze, G. and F. Pieterse, " ROC level control, " Honeywell internal document, 2011.
5
AspenTech DMCplus Advanced Concepts Course, MA305.06.10, 2007.
6 Reyes-L´ua, A., C. F. Backi and S. Skogestad, " Improved PI control for a surge tank
satisfying level constraints, " 3rd IFAC Conference on Advances in Proportionalintegral-derivative control, Ghent, Belgium, May 9-11, 2018.
7
Taylor, A. J. and T. G. la Grange, " Optimize surge vessel control, " Hydrocarbon
Processing, May 2002.
8 " Experion control builder components theory, " Vol. 1, Honeywell 2005.
9 Shinskey, F.G., " Special rules for tuning level controllers, " 2005, online: www.controlglobal.com/articles
10 Friedman, Y. Z., " Tuning of averaging level controllers, " Petrocontrol, 1994, online:
http://www.petrocontrol.com/papers/1994
11 Ogawa, S., B. Allison, G. Dumont and M. Davies, " A new approach to optimal averaging level control with state constraints, " 41st IEEE Conference on Decision and
Control, Las Vegas, Nevada, U.S., December 2002.
12 Rosander, P., A. J. Isaksson, J. Löfberg and K. Forsman, " Robust averaging level
control, " AIChe Annual Meeting, Minneapolis, Minnesota, U.S., 2011.
13 Sidhu, M., " Multivariable averaging level control, " BSc Thesis, University of British
Columbia, Canada, 2001.
14 Ye, N., T. J. McAvoy, K. A. Kosanovich and M. J. Piovosv, " Optimal averaging level
control for the Tennessee Eastman problem, " The Canadian Journal of Chemical
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Complete Litature Cited available online at www.HydrocarbonProcessing.com.
GUSTAF GOUS works as Senior Manager Electrical and Instrumentation at Sasol's
Secunda Chemical Operations. He graduated from the University of Pretoria in
South Africa with a BS degree in chemical engineering in 1987, followed by a
BA degree in English (honors) and an MS degree.
PHILIP DE VAAL is an Emeritus professor of chemical engineering at the
University of Pretoria in South Africa. He was Head of the Department of Chemical
Engineering from 2004-2020. He earned a BS degree in chemical engineering,
followed by an MS degree and PhD.
GERT VAN COLLER graduated from the Department of Chemical Engineering at
the University of Pretoria with a BS degree in chemical engineering. He completed
his engineer in training course at Sasol in 2018 and now works as a process control
engineer at the Solvent Units at Sasol's Secunda Chemical Operations.

http://www.controlglobal.com/articles http://www.controlglobal.com/articles http://www.petrocontrol.com/papers/1994 http://www.HydrocarbonProcessing.com http://www.HydrocarbonProcessing.com

Hydrocarbon Processing - May 2021

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