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SOOT FIRES CANNOT OCCUR IN A HEAT EXCHANGER WHICH IS FREE OF SOOT DEPOSITS

Soot deposits occur in diesel engines running on heavy fuels generally when running at light load, or when manoeuvring where no exhaust gas bypass exists. These deposits may also contain excess cylinder lubricating oil and are highly susceptible to ignition once engine power and exhaust temperatures increase. It is therefore imperative that tubes are maintained in a clean condition by the effective use of soot blowers. In particular, following periods of slow running or manoeuvring for extended periods, it is essential that soot blowing operations are carried out prior to an increase in power, or in the case of shutting down an engine. Dry soot deposits normally have a very high ignition temperature. However, when the soot gets wet with hydrocarbon vapours, their ignition temperature reduces to around 1500C. This may result in boiler soot fire or boiler uptake fire.

Reasons for Exhaust Gas Economizer to Occur:
• Soot gets deposited on economizer coils. Soot deposits become high when using high carbon content fuel
• Soot is formed due to incomplete combustion
• Flue gas velocity is low and incomplete combustion may occur at the manuvering time, thus, unburnt fuel can adhere on economizer coils
• Air is available due to excess supply of scavenging air into the unit combustion chamber
• A flame is produced when air and fuel get mixed proportionally in the sufficient heating temperature

Indications of Economizer Fire:
• Signs of overheating and peeling of paint from the outer economizer body
• Sudden increase in uptake gas temperature (Abnormally high stack pyrometer reading)
• Heavy smoke and sparks are released from the funnel
• Flame is visible in the smoke indicator

Actions to be taken during Economizer Fire:
Fire level 1: an initial soot fire has just been discovered:
• Gradually slow down the main engine by informing the bridge and chief engineer and stop the main engine at earliest opportunity. This will cut off the oxygen supply to the fire.
• Stop ME aux blowers. Ensure ME exhaust valves are in shut condition. Stop Blower supplying air to ME turbocharger.
• Continue operating the water circulating pump.
• Never use soot blowers for firefighting, as air will feed the fire with oxygen, and steam will involve a risk of high temperature fire.
• Use water washing, if fitted, to extinguish the fire. This is normally connected to the ship’s firefighting water system.
In a well-run plant any fire that starts will be small, and if the above emergency action is taken immediately, the fire will be damped down quickly, and water circulated by the pump will help keep the tubes cool and reduce any heat damage caused by the fire.

Fire level 2: TUBE MELT DOWN.
Turning of soot fire into an iron fire can be indicated by sudden rise of temperature, a temperature sensor (normally max. 400°C) will not normally be able to measure the high temperatures. a loss of water, if the feed water consumption increases very much and/or if a low level alarm in the steam drum is activated. A temperature sensor (normally max. 400°C) will not normally be able to measure the high temperatures.
A. Stop the main engine, if it is not stopped already.
B. Stop the circulating water pump.
C. Close valves on the water circulation line.
D. Discharge the (remaining) water from the exhaust gas boiler sections.
E. Cool down with plenty of splash water directly on the heart of the fire
• Operate firefighting equipment if it is fitted on the economizer
• NEVER soot blow while economizer is on fire, it can cause hydrogen fire and result in metal fire leading to melt down the coils stack.
• If soot fire is considerably big, shut down the main engine and boiler
• Spray water on the external casing to cool the affected area
• Start and stop the engine a number of times to blow out collected soot at the uptake

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