Locating Blockages in Boiler Tubes

This article is provided courtesy of Snell Infrared, a thermographic training company.

Technical Service thermographers at Mohave Generating Station recently sent us images of problems they found in their boilers. The two 750 megawatt coal fired water tube boilers had been susceptible to startup problems after a scheduled shutdown for tube maintenance.

Any obstruction or lack of water flow to the 5/8" inside diameter tubes causes them to overheat and rupture-a costly proposition. The root cause of Mohave's troubles was unknown because the rupture would also blow away all evidence of what caused the obstruction. X-rays clearly showed the blockage, but they were expensive and time consuming to gather.

In 1990, the Technical Services Team (TST) began experimenting with and using thermal imaging to locate the blockage. After several false starts, the technique has been perfected.

During the process of tube repairs, welders often use dissolvable rice paper to temporarily seal off a tube. The paper generally disappears without a problem as soon as it gets wet, however, this is not always the case. They also found that scale may break loose and pack against the paper. In either case, the result is blockage in the area of the repair.

TST hypothesized that by circulating condensate (140-160 degrees F) through the tubes after repairs, the blockage should show up thermally. They found that it did not­everything was hot­until they introduced a draft of ambient air over the tubes! That was just enough to keep the blocked tubes a few degrees cooler than the normal tubes. Using these techniques, they show clearly on the thermograms as colder tubes.

Jim Hansen, Manager of the TST, and the thermographers are clearly thrilled to have found a solution to a problem that was very costly. Prior to using thermography, Mohave was averaging 2-4 tube failures a year. They now are up and running the first time. Savings have been estimated to be $48 million over the life of the plant.

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