The Strange Case of the Failed Boiler Tube

Several years ago, I was contacted by a small college who had an unusual boiler tube failure problem. The unit was an old fire tube boiler that was rated at about 350 HP and operated at 60 psig.

The boiler had operated for at least 20 years and had never had a tube failure. When I arrived on the scene to survey the situation, the boiler operator told me the boiler had three tube failures caused by oxygen pitting in the last 3 years. The unusual thing was it was the same tube that was failing. During my visual inspection, I did not observe pitting on any of the other tubes.

The feedwater tank temperature was 130 F (typical for summer) and was about 170 F during winter operation. Checking a dissolved oxygen versus temperature chart, the dissolved oxygen was in the 3.5 to 6 ppm range for the feed water temperature operating range. There was no steam sparge line as the temperature was determined by the amount of condensate returning from the campus.

I asked about the feedwater pumps which appeared to be oversized compared to the feed water tank volume. I was told a ‘new’ maintenance director, had been hired and he immediately changed out the original pumps and installed the larger pumps that had been sitting in the boneyard. His line of reasoning for this change being he did not want to buy new pumps and he felt the old pumps were running too long in order to keep up with steam demand in the winter months. After the change was made, the larger pumps developed a cavitation problem as they were starving for water. To correct this problem, a larger makeup water line was installed on the tank which increased the rate at which cold water was added. This caused the dissolved oxygen in the feed water to increase rapidly.

The following recommendations were made and implemented:

1. The larger feed water pumps were replaced with new pumps which were the same size as the original pumps. The new maintenance manager was irate about this as it went against his decision to go with larger pumps.
2. A steam sparge line with a temperature regulator was installed on the feedwater tank.
3. A flow regulator valve was installed on the larger makeup water line to prevent sudden dumping of cold water into the feedwater tank.

These changes resulted in no further tube failures and a reduction in sulfite usage as the average feedwater temperature was consistently in the 185 F to 190 F range.