Hot Spots – External Causes
The only cause of hot spots that can be fixed by changes in burner design, operation or maintenance is flame impingement on the tubes. Possible hardware-related causes are:
• Burner plugging,
• Burner misalignment,
• Individual tip misalignment,
• Ports enlarged by reaming,
• Burner refractory damage and
• Damaged furnace refractory or other debris falling into the burner block.
Flame impingement can also be caused by adverse flow patterns of combustion gases inside the firebox. This occurs particularly in reformer plants, for example, in reactor charge heaters of the double-arbor design. The flames in this type of heater are prone to “lean” (to being deflected) through the central double row of tubes. Burner adjustments to correct flame lean have been only marginally successful at best.
The most successful correction to existing double-arbor designs has been physical separation of the two arbors by means of a ceramic fiber panel wall inserted between the two central tube rows. While about 30% of double-arbor designs have been afflicted with flame lean problems, no single-arbor designs have. We recommend single-arbor designs in new construction.
Heat transfer in the radiant section of a fired heater is limited by the amount of absorbing tube surface that was provided in the original design. If we try to transfer more heat than the tube surface can absorb, we will cause hot spots and tube ruptures.
To prevent overfiring we pay attention to the design firing rate of the heater. If we try, by overfiring, to put more heat into the charge than the radiant section can transfer at given flow rate, we are bound to exceed the design temperature of the tubes. Hot spots appear where this first begins to happen. The solution is to back off on the firing rate or increase the charge rate. But if an increase in charge results in an increase in firing, this could worsen the problem.