During morning attendance of engine room, a water leak from the exhaust gas boiler was discovered. In order to get to port, the water circulation was shut off, and the instruction manual on "emergency dry running" was followed. When arriving at anchorage, the exhaust gas boiler overheated, and the crew found that a soot fire had occurred.
Tube stacks at gas outlet of exhaust gas boiler were found heavily damaged. More than half of the tube stack sectional area was melted down and the tubes of the remaining area were found sagging.
In this case, it seems that the circulating pump was stopped almost immediately, while the main engine was left running. This action left the tube stack fully uncooled and the soot deposits on the tubes eventually ignited and a fire occurred.
Apart from the above the exhaust gas boiler damage may have also been caused from:
- Insufficient sootblowing and water washing in general, especially sootblowing prior to and after manoeuvring and after water washing of turbo chargers in service.
- Sootblowing with wet steam, which leaves the soot in the middle of the boiler.
- Sootblowing with too low pressure, which leaves the soot in the middle of the boiler.
- Poor routines in observing the exhaust gas pressure differential across the boiler.
In order to operate the exhaust gas boiler in dry running mode the following should be taken into consideration:
- When shutting off the water circulation, the main engine should also be shut down so the exhaust gas boiler can cool down and any smouldering soot deposits on the boiler tubes can die out.
- The heating surface should be inspected carefully for soot deposits, and water washing performed, both for cleaning and cooling.
- Make every effort to re-establish the water circulation to the boiler and thereby reduce the dry running period to a minimum.
- Boiler manufacturers allow dry running of exhaust gas boilers only in the case of emergency and with a clean boiler. In addition they emphasize that every possible precaution must be observed to prevent soot fire.
Left – Sagged economizer tube nests after soot fire, finned tubes.
Right – View inside economizer with melted tubes.
Actions to be taken during dry running:
- Increase the frequency of sootblowing considerably, and perform sootblowing several times prior to manoeuvring.
- Inspect the boiler frequently, and if any soot is present, then water wash the boiler and increase the sootblowing frequency.
- The boilers instruction manual must be read carefully and its instructions are always to be followed.
Even though usually the tubes seem to be clean from soot and oil residues at visible inspection, soot deposits will always be present to some extent. Boilers with finned tube design are even more exposed to accumulation of soot than boilers with plain tube design, and the consequences of a fire are often more severe.
Source: DNV
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