(67d) Chemical Tracers Find Elusive Leak in Hydrotreater Feed/effluent Exchangers

High sulfur content in the effluent of a hydrotreater stream is a signal that the unit has a problem with either the reactor catalyst bed or the feed/effluent exchangers. Plant resources can often isolate the offending vessel. These approaches take time and can be expensive when they involve a shutdown to search for the problem. At times, conventional efforts fail to locate the offending vessel. On-line techniques, such as radioisotope tracers, have been used for many years to leak test these exchangers, avoiding the maintenance and lost production costs of a search during a shutdown. But, even radioisotope tracers have limits and some leaks are too small for that technique. A new service using stable chemical tracers has been employed to find very small leaks.

A refinery in the Great Lakes area operates a Naphtha Hydrotreater with a bank of four feed/effluent exchangers. Results of effluent sample analysis showed that sulfur levels which had been staying around 1 to 2 ppm, were now fluctuating between 1 and 10 ppm. Operations personnel had tried to determine which exchanger was leaking by sample analysis, but had been unsuccessful. These exchangers were tested with four chemical tracers. One of the exchangers was found to have a leak of 44 ppm. When this exchanger was pressure tested, five tubes were found to be leaking. The tubes were plugged and the unit was brought back on line. The sulfur levels returned to the 1 to 2 ppm level. Leak rates as low as 5 ppm can be detected by this technique.