Editor's Choice

Case history 189: Poor furnace temperature control.

September 2023 Editor's Choice PLCs, DCSs & Controllers

Many process engineers do not appreciate the importance of flow loops in their unit controls.

A senior process engineer once told me that flow loops need not be tuned well, as they generally have little effect on the more important and much slower loops like temperatures and pressures. In actual fact, flow loops can be vitally important for ensuring that other controls work properly. The example given here is an excellent illustration of this.

The process unit used for this example is the steam desuperheater on a boiler in a petrochemical refinery. The water is heated using gas as the fuel. The final desuperheater outlet temperature is a cascade secondary loop, with the primary control being the outlet temperature control of the upstream desuperheater. Just out of interest, there was a measurement of the outlet steam flow after the second desuperheater, but they had not used this as a further cascade to ensure the temperature control worked properly, which is something I recommended they do. A secondary flow cascade loop takes care of any valve problems as it can quickly ensure that the flow of steam keeps to the amount demanded by the temperature controller. In actual fact, there were several problems with the steam desuperheater valve.

The problem being experienced on the steam temperature was that it was cycling quite badly, and they had not been able to establish the cause. Investigations were carried out as to what was causing the cycle, by checking all the various controls that could have had an effect on the desuperheaters’ outlet temperatures. It was found quite quickly that the culprit causing the problem was the combustion air flow control loop.

Figure 1 is a Closed Loop as Found test on the airflow, with the controller in automatic, and using the original tuning parameters. A series of setpoint changes was made, as can be seen in the figure. The flow was apparently in a continuous slow cycle, with a 20% amplitude and a period of roughly 100 seconds. The other two traces shown in the figure are the outlet temperatures of the two desuperheaters in cascade. It can be clearly seen what a large influence the cycle of the air flow was having on them, particularly on the secondary cascade desuperheater.

Figure 2 shows an Open Loop test performed on the air flow where steps are made on the PD (controller’s output). This shows some remarkable things, which explain why the control was cycling in automatic:

• The moment the PD dropped below 41%, the flow immediately dropped by about 20%, and one could not control the flow once the PD had gone below that value.

• When the PD was moved up in small 3% steps from 41%, the flow responded pretty well. However when we stepped the PD down again, the flow did not move at all until the PD went below 41%. Then it immediately dropped by the 20% as mentioned above.

• When repeating the test it was found on a couple occasions that the flow did occasionally slowly move a little on downward steps of the PD in the upper region.

These observations show that there were very serious problems with the air damper, and proved that no real control was possible until it could be fixed or replaced. It also conclusively showed why the cycle occurred in automatic, with the controller trying to get the flow to a value in the region over which the valve just jumped up and down. This is a good example of how poor control on a flow can seriously influence other more important and slower controls.

Michael Brown is a specialist in control loop optimisation, with many years of experience in process control instrumentation. His main activities are consulting and teaching practical control loop analysis and optimisation. He now presents courses and performs optimisation over the internet. His work has taken him to plants all over South Africa and also to other countries. He can be contacted at: Michael Brown Control Engineering CC, +27 82 440 7790, michael.brown@mweb.co.za, www.controlloop.co.za


Share this article:
Share via emailShare via LinkedInPrint this page

Further reading:

WEG Africa applauds energy standards for motor efficiency
WEG Africa Editor's Choice Electrical Power & Protection
In a country where most electric motors are not energy efficient, the announcement by South Africa’s government of its plans to introduce Minimum Energy Performance Standards is not a day too soon.

World first planetary gearbox customisation
SEW-Eurodrive Editor's Choice Motion Control & Drives
When a mining operation in Mpumalanga found that one of its planetary gearboxes on a high-torque conveyor was failing, it called SEW-EURODRIVE for a solution. Leveraging the latest intelligent technology, the solution was a pioneering innovation providing a cost-effective replacement that will ensure reliable performance.

Thermoforming machine automation
Beckhoff Automation Editor's Choice Motion Control & Drives
Hamer has joined forces with Beckhoff to develop a thermoforming machine for water-based cellulose pulp to produce fully recyclable and environmentally-friendly cellulose packaging.

ABB and Gravitricity to collaborate on energy storage systems
ABB South Africa Editor's Choice News
ABB has signed an agreement with UK-based gravity energy storage firm Gravitricity to explore how hoist expertise and technologies can accelerate the development and implementation of gravity energy storage systems in former mines.

Process instrumentation that delivers
Editor's Choice
The incorporation of Honsberg, Greisinger, Martens, Val.Co and Delta OHM into the Senseca brand has enabled Senseca to expand its research, development and manufacturing operations globally. This has resulted in state-of-the art flow, level and environmental measurement instrumentation that is suitable for applications throughout industry.

Level sensing, conveyor control and grid resistors
DRH Components Editor's Choice Level Measurement & Control
DRH Components is a specialist electric component and systems supplier with a focus on the control of electric motors. As part of its wide variety of products, the company markets the entire Bindicator range. Bindicator manufactures sensors for dry bulk and liquid level measurement.

Greenhouse gas monitoring satellites
ABB South Africa Editor's Choice
ABB has secured a third contract with GHGSat, the global leader in high-resolution greenhouse gas monitoring from space, to manufacture optical sensors for its satellites set to launch into orbit in 2024.

Reminiscences of a life in control
Michael Brown Control Engineering SAIMC
Reminiscences from Michael Brown on a long and rich journey in the world of automation together with SAIMC.

Next-generation hybrid power unit for Formula 1
Siemens South Africa Editor's Choice IT in Manufacturing
Red Bull Ford Powertrains has leveraged the Siemens Xcelerator portfolio of industry software from Siemens Digital Industries Software to develop the next-generation hybrid ICE/electric driven power unit for the 2026 Formula 1 racing season.

Schneider Electric connects to the future
Schneider Electric South Africa Editor's Choice News Electrical Power & Protection
Schneider Electric had a major presence at the recent MESA Africa International Summit, which focused on ‘accelerating the journey to smart manufacturing’. SA Instrumentation & Control’s editor caught up with Dr. Suven M Ramsunder, Digital Transformation Expert, Anglophone Africa, to find out how the company is approaching the integration of artificial intelligence (AI) technologies into the manufacturing industry, both in its own factories and for its customers.