High Stack Temperature/High Convection Section Temperature
These two problems can be the result of common causes or they may be independent of each other. Some of the more common causes are:
- High excess air (high stack temperature)
- Positive pressure at the arch
- Coked convection tubes
- Fouled convection tubes
- Missing fins on the convection tubes
- Failed tube sheets or supports
- Process leak in the convection section
- Fouled or partially plugged shock / radiant tubes
Effect on Operations
High stack temperatures result in wasted fuel and poor heater efficiency. High convection section temperatures can cause serious problems with tubes, tube supports, failed refractory and overall heater operation.
Corrective / Preventive Actions
The most common cause of high stack temperatures is running with too much excess air If the process and temperature in the convection section are within specified limits the first item to check is the excess oxygen. If the excess oxygen is above the specified target then steps should be taken to reduce the air coming through the burners.
The culprit, in the air, that causes the problem is nitrogen. Since nitrogen is a “diatomic” molecule it does not transfer heat by radiation. The only way that nitrogen gives up the heat generated in the radiant section is by contact with tubes, refractory, supports and other gases. If the process has absorbed all of the heat it can then some of the remaining heat contained in the nitrogen is transferred by convection in the stack. The rest of the energy (heat) is carried out of the stack and lost in the atmosphere.
Not only will lowering the excess air reduce the stack temperature there will also be an increase in heat transfer in the radiant section of the furnace. This positive benefit is the result of increasing the percentage of carbon dioxide (CO2) and water vapor (H2O) in the flue gas as the amount of air is lowered. Both CO2 and H2O are “binary” gases that can absorb and transfer heat by radiation.
Positive pressure at the arch of the furnace can cause both high stack temperature and high convection section temperature. Closing the stack damper too far will impede the flow of the flue gas. This results in a “stagnant” zone in the convection section allowing the hot flue gas to be in contact with the convection tubes longer than necessary. This can cause high tube metal temperatures, high process temperatures, failed tube supports and failed refractory. This condition can also cause a reduction in the amount of air being supplied to the burners.
The condition of the tubes, in the convection section, is also very important. If the tubes are coking internally, fouled or developing scale build-up or missing fins then the flue gas temperature will be higher than designed.
A leak in one of the tubes will cause extremely high temperatures due to the oxidation of the fluid. These “pinhole” leaks take time to develop but once the fissure is open the hydrocarbon will spray into the firebox. Obviously this situation should be corrected immediately by shutting down the heater and making the necessary repairs.
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