Case Studies of Boiler Operation Accidents--2
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The water wall tube rupture accident occurred repeatedly in Boiler No. 4 of Shaoguan Power Plant. Accident details: On March 21, 1991, after the minor repair of Boiler No. 4 at Shaoguan Power Plant was completed and the turbine’s overspeed test was finished with preparations underway to connect it to the grid, a loud noise came from the furnace; the water level in the steam drum dropped rapidly and out of control, prompting an emergency shutdown of the boiler. Inspection revealed that one tube in the front wall water wall had burst; the rupture site was located about 100 cm near the flame shield zone. In the same circulation circuit surrounding the rupture site, a total of 25 tubes showed varying degrees of deformation. 14 water wall tubes that had been damaged by blasting or were severely deformed were replaced through emergency repairs. Ignition was attempted again at 18:00 on the 24th, and at 03:24 on the 25th the unit was operating at a load of 40 MW, with a main steam pressure of 9.3 MPa and a main steam temperature of 490°C. The electric contact level gauge indicated +30 mm; another loud noise was heard inside the furnace, and the water level in the drum dropped rapidly and could not be maintained, leading to another emergency shutdown of the unit. Inspection revealed that one water wall tube on the rear wall had burst, with the rupture site located about 80 cm above the flame retaining zone; more than 10 water wall tubes in the vicinity were deformed to varying degrees.
Cause analysis of the accident: The conditions of these two tube ruptures were essentially the same. After examining the characteristics of the rupture surfaces and conducting metallographic analysis, it was determined that the ruptures were caused by short-term overheating. The accident occurred because the operators failed to clean the local water level gauge of the boiler twice during its startup process, as required, and did not verify it against the electric contact water level gauge. As a result, the water level gauge in the control room could not operate properly, and there was a discrepancy between the electric contact water level gauge and the local water level gauge; this led to false water level readings that were not detected in time, causing severe water shortage in the boiler and subsequent tube rupture.
Accident summary: 1. Strictly prevent the boiler from operating with insufficient water level, as well as overheating or overpressure conditions; it is strictly prohibited to operate the boiler when there are not enough water level gauges (i.e., those capable of accurately indicating the water level). 2. Regularly calibrate the water level gauge to ensure that the on-site value is essentially consistent with the remote transmitted value. 3. There must be at least two reliable remote water level gauges in the boiler control room. 4. Flush the water level gauge in a timely manner as required. 5. When all the water level gauges in the boiler’s drum fail and it becomes impossible to determine the water level in the boiler drum during operation, the boiler must be shut down immediately as an emergency measure. 6. For tube ruptures caused by short-term overheating, measures such as preventing low water levels in the boiler drum, avoiding excessive use of desuperheating water, and stopping foreign objects such as water plugs, tools, and welding slag from entering the boiler pipes and causing blockages are generally required.
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