Some Consideration for the Damage of Exhaust Gas Economizers Caused by Soot-fires

The damage of exhaust gas economizers due to the burning of accumulated sooty deposits have often occurred, and this type of damage is generally called a "soot-fire." Soot-fires have tended to increase in recent years. Nippon Kaiji Kyokai (NK) conducted an analytical assessment on soot-fires in NK classed ships. As a result, it was revealed that this trend in failures of exhaust gas economizers due to soot-fires has been encouraged by the increasing use of degraded fuel oil for main engines and the over-sophistication in the efficiency of machinery and equipment in ships, which is particularly evident today. Although the outline of the mechanism of soot-fires has been grasped, many details are left to be clarified. There is very little research work on the specific mechanism in the sequence from soot-burning to failure of heating tubes, in particular, even though it is suggested that the effect of the flow conditions at the water side of heating tubes on the mechanism is significant. An investigation was conducted on this point from the viewpoint of a phenomenon called "dryout" of a two-phase liquid-vapour flow, and our understanding of soot-fires has been deepened by collating the results of investigation with actual design conditions. Furthermore, a study was also made on the mechanism generating meltdown failures of heating tubes often seen in large exhaust gas economizers, and descriptions are given on the results of the study referring to the results of analytical assessments on the failed items.