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How a mistuned temperature loop caused a total plant shut down

Problem: Recently, a waste management company in Texas experienced issues with the temperature control loop of their thermal oxidizer—resulting in an oxidizer shutdown. Thermal oxidizers are heated by natural gas to an operating temperature of 1400 ºF for oxidizing the waste gases. The temperature must be carefully controlled during startup, because rapid temperature changes can damage the unit’s firebrick; normal procedure calls for the temperature to be ramped up by 100 ºF per hour in steps.

 

However, as the loop was not tuned correctly, the oxidizer temperature was fluctuating by 125 to 150 ºF. Not only did this affect the oxidation process, but the uncontrollable 150 ºF swings prohibited proper startup of the oxidizer itself which caused thermal oxidizer shut down.

 

Solution: Installing an EnTech toolkit in on the customer’s DeltaV system, which uses lambda tuning techniques to recommend effective non-oscillatory tuning strategies, provided us with information to analyze the issue. Using this data, I was able to stabilize the oxidizer’s temperature control so that it held at 1400 ±8 ºF, thus enabling the thermal oxidizer to resume full operation.

 

Learning: It is common for a plant manager to become accustomed to a loop or valve not working “properly”. This type of myopia can prohibit the manager from investigating a solution or a better way to run things. Even, in this case, when the malfunction is cutting directly into the plant’s noncompliance to emission limits.

 

It took a major shutdown for the operation to reach out for assistance. However, our experts happily provide proactive guidance on how to properly tune loops, valves and more. Utilizing Emerson’s consulting expertise and technology, this plant is now equipped to properly tune other loops in advance of such incidents and resume profitable operations.