Gamma scanning distillation and absorption towers is one of the most powerful diagnostic techniques available. Most gamma scans are shot through the active areas. In many cases, invaluable insight to the nature of the problem or bottleneck can be gained by also scanning the downcomers.
Due to the narrow width of downcomers (compared to active areas), scanning downcomers can be trickier than scanning tray active areas. If attention is not paid to the pitfalls, one can end up with a misleading diagnosis. Misinterpretation of downcomer gamma scans is not uncommon.
This paper presents cases where downcomer scanning was central to obtaining a good diagnostic of a tower problem, and other cases where pitfalls veered an investigation in a wrong direction. The paper presents guides of how to avoid such pitfalls, properly interpret downcomer scans and promote correct diagnostics.