Superheater Tube Failure After Burner Tip Change
A 60,000 kg/h shop-assembled boiler with a convection superheater has run for years at 415 °C final steam temperature at the superheater outlet, controlled by a desuperheater. Maintenance replaced worn burner tips with a set drilled to a slightly different spray angle and pattern, and the boiler was returned to service without a combustion tune. Within three weeks the superheater outlet temperature climbs to 445 °C at the same firing rate and steam flow, and the desuperheater spray valve is nearly wide open. The stack temperature has risen about 25 °C, and CO readings at the economizer outlet are 350 ppm where they used to be under 50 ppm. A superheater tube in the second pass shows a bulged, thin-walled rupture with a longitudinal split and heavy external scale on the fire side.
1. Explain how the burner tip change plausibly caused both the rising superheater temperature and the elevated CO, and why the tube failed where it did.
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The new tips changed the flame shape and mixing. A spray angle that pushes flame closer to or longer along the furnace, or that degrades fuel/air mixing, raises the gas temperature entering the convection superheater and lengthens the flame. More heat is absorbed in the superheater, so outlet steam temperature rises and the desuperheater must spray more. Poor mixing also produces CO (350 ppm vs <50 ppm) because unburned fuel and CO leave the furnace before completing combustion; that CO then burns in the convection pass, adding heat exactly where the superheater sits and raising stack temperature.
The tube failed by long-term overheating/creep: a bulged, thin-walled longitudinal split with heavy fire-side scale is the classic signature of metal temperature above design for an extended period. The second-pass location is consistent with afterburning and higher gas temperature in the convection bank. Scale on the fire side insulates the tube, pushing metal temperature even higher. This is not a short-term overpressure burst (which would show little thinning) and not caustic gouging (which would show internal deposits).
2. List the corrective sequence to restore reliable operation and prevent recurrence.
Read the worked answer
- Reduce firing rate/load to bring superheater outlet temperature back within design and stop further damage; do not rely on maximum desuperheater spray as a permanent fix. 2) Shut down and inspect the superheater fully, not just the failed tube; replace the failed tube and check neighbors for bulging and wall loss, with thickness surveys. 3) Restore the original tip specification or obtain manufacturer-approved tips, and perform a proper combustion tune: measure O2, CO, and stack temperature across the load range, adjust air/fuel and burner registers, and verify flame shape and furnace exit gas temperature. 4) Recheck desuperheater spray flow and superheater outlet temperature at design load; confirm CO is back below target. 5) Investigate whether the fire-side scale indicates chronic high metal temperature; review water/steam chemistry and prior operating logs. 6) Document the tip change and tune in the management-of-change record, and add a post-outage combustion verification step to the restart procedure.