OPT FOR SECURITY AND PERFORMANCE
Improve the security and performance of your plant with alarm management, performance supervision, asset supervision, web-based data visualisation, optimisation, advanced control, and tuning with the new TaiJi tool.
Rethink your working methods and obtain immediate results thanks to our services, trainings, and consulting expertise. Manage changes intelligently with our proven solutions.
certified service providerJanuary 2006
In this issue
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Classes and seminars |
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About us |
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Trick of the month |
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To contact us |
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Classes & seminars
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We offer a new 1-day class LT-110-1 PID Loop Tuning and
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PID Loop Tuning
February 2, Moncton, NB
February 22, Ottawa, ON
Seminar on modern control methods
February 14-15-16, Vancouver, BC
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About us
See our new calendar for 2006
See our previous issues
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Trick of the month : Mid-Range control revisited ("coarse/fine")
When a control loop requires high rangeability (maximum controllable flow/minimum controllable flow), a simple valve is not enough. The rangeability of a simple valve rarely exceeds 100 and in most cases, it is not easy to obtain more than 30.
To increase the valve rangeability, one can use 2 valves: a small valve to provide small flow and a large valve in parallel to ensure that high flow can be reached.
The controller (pressure, flow, level, etc.) sends its signal to the small valve. To manipulate the large valve, many strategies can be used but usually, the position of the small valve is selected:
- Ramp up/down when the small valve is below/above 50%
- Integral-only controller with SP=50%
- Switches to start a ramp when the valve is near its limits
- PID-gap controller
With solutions 1 and 2, the defects associated with the large valve will deteriorate the quality of the control. For example, if the large valve is 10 times bigger than the small one and has a stiction of 0.2%, this stiction will appear as 2% for the control loop.
With solution 3, disturbances in the line will probably be too high or too low.
Finally, with solution 4, the action on the large valve will depend on the process dynamics. Most of the time, the large valve is in a fixed position and moves only when the small one reaches its programmed limits. When this happens, the large valve opens gradually until the small one can fulfill the demand.
Extract from "Fundamentals of Process Control" by Michel Ruel.
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To contact us
Comments, suggestions and requests are welcomed. Please e-mail us at: info@topcontrol.com
You are welcomed to visit our web site :http://www.topcontrol.com
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49 Bel-Air Street, Lévis, QC
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1020 James Drive, Suite A
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