Waterwall tube failures are not uncommon in coal fired boilers. The failures reasons can be many starting from short term overheating to long term overheating, corrosion, erosion, etc. Waterwall tube failure in the burner panel corner in a tangential fired boiler and waterwall tube failure in the burner opening of a front wall fired boiler can lead to minor to sever explosion in the boiler. If the location of failure of tube, load in the boiler, and the temperature conditions are such that the steam from the leaky tube can reach the pulverized coal at a location where it is just starting to burn, then a large amount of CO, etc.-like producer gas can be formed. This gets ignited leading to heavy disturbance in the furnace leading to further production of unburned gases and volatiles, which then explodes. The magnitude of explosion can vary from a low to very high.
The sequence of events at full load operation of a boiler can be visualized as given.
- Boiler running at full load and with no specific disturbance
- One of the corner water tube fails with a burst opening
- A loud sound is heard
- The boiler tripped and the operator boxed up the boiler
- The walk through inspection reveal that there was an explosion in the boiler
- When boiler is examined after it was safe to enter into the boiler, a tube failure very close to the fuel nozzle or burner can be seen
- The water wall tubes on the other panel can have sever bows depending on the intensity of the explosion
- The corner could be ripped open and the buckstay shear pin sheared, typical of an explosion / high pressurization of furnace
- It can then be believed that the cause of this could be an atmosphere conducive to produce large volume CO and or producer gas at the point when the water wall tube started opening has occurred.
- This large quantity of these gases would, along with the pulverized coal entering the furnace, could have exploded very locally or in the furnace and resulted in the damage.
If the boiler is a tangential fired type with tilting arrangement, then the chances are more for such phenomenon to occur. However cannot be fully ruled out in front wall fired designs.
About the Author
Dr V T Sathyanathan is a boiler consultant with 35 years of experience in various areas of high pressure boiler trouble shooting. He holds a PhD in coal combustion in boilers.
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