Boiler Failure: A Case Study

-Joe Gisoti

The boiler data plate was intact and showed a manufacture date of 1969. When arriving on site the boiler fireside was open and the vessel had no water. The low water float controls were removed on both sides but the waterside hand hole plates were still installed.

Boiler Failure

At first look the boiler tubes looked to be in acceptable condition. The customer stated that he had been informed that the boiler had run out of water. I do not believe that to be the case. The tubes in the boiler in the 2nd, 3rd, and 4th passes showed no signs of over-heating. From front to back there appeared to be virtually no “sag” which is not indicative of tubes that have been run dry.

Boiler Tubes are still straight from front to back.

Both low water float controls were tested and both the primary and secondary float switches break on a low water condition.

The view from the rear of the furnace showed a collapse of the fire tube. This was due to overheating inside the furnace in the front portion directly in front of the burner.

There were also signs of flame impingement on the walls of the fire tube. The front view also showed the flame hitting the walls.

The boiler leak was also located at the front of the fire tube. The original shell thickness was not shown on the vessel any were. However, the thickness where the crack occurred was less than 1/8”.

Material was thin at the time of failure. The pressure inside the shell (120 psi) was able to collapse the weakened shell and cause a fracture due to over-heating. This was caused by lack of heat transfer to the water inside the shell of the boiler along with the excessive flame temperature inside the furnace. This portion of the furnace was not repairable.

The tube ends at the rear turnaround area showed signs of blistering. There was also the appearance that the refractory inside the rear door on the first pass may also have been seeing excessive heat.

Summary

This boiler failure was due to excessive heat inside the furnace. This was caused by poor combustion and flame impingement. The boiler was clearly over-fired. The size of the flame and the over-heating of the rear door support this. Without a fuel flow meter on the burner, it was difficult to say how far over-fired it was.

The customer stated that when the Honeywell fuel/air ratio controller was installed the contractor had a difficult time selecting a fuel metering valve with correct cv. The customer also stated that this boiler would keep up easily with the heavier plant steam loads while the other boiler, the same size and input, would not. This also supports the theory that the input on this unit was beyond an acceptable level. It was unclear how long this condition existed. I was not able to obtain a view of the waterside of the vessel. However, the low water float chambers looked to be clear and there was no scale build up.

The boiler vessel in my opinion had been stressed beyond design criteria and just replacing the furnace tube and the 2.5” fire tubes will not address the problem. The rear tube sheet and the shell itself was exposed to excessive heat, causing the boiler life to be shortened. Due to its current age of 43 years, this boiler should be replaced not repaired.

Joe Gisoti is the manager of the Preferred Utilities upstate New York sales/service office in Clifton Park, NY.