Auxiliary boiler uptake or flue gases normally pass through economisers or air pre-heaters or feed water heaters. All these types of heat exchangers are fin type. Hence large amount of carbon or soot deposits on the fins. Along with the flue gases some un-burnt carbon particles also get deposited on the economisers. Normally dry soot deposits have a very high ignition temperature. But when the soot gets wet with hydrocarbon vapours, their ignition temperature comes down to around 150 degree Celsius. This may result in boiler soot fire or boiler uptake fire. When an uptake fire is suspected, never soot blow the economiser. Soot blow with steam causes steam to convert as hydrogen gas and result in metal fire.
Preventive Actions
§ Regularly inspect boiler flame to ensure correct air fuel ratio is maintained. This makes complete combustion with clean flue gas.
§ Carry out boiler burner and swirler routines as per PMS.
§ Regularly soot blow economisers and water wash periodically to remove soot deposits.
§ Use proper grade of fuel oil for boiler.
§ Fuel oil used for boiler to be treated well.
§ Avoid low load operation of boiler for long period.
§ Running boiler near full load periodically, helps in expelling out carbon deposits adhered in uptake passages and reduces chances of boiler uptake fire considerably.
What is exhaust gas economiser (EGE) fire and how it occur ?
§ A flame appear at the economizer coil during running of main engine at sea while the EGE is put into service with circulation water passing through the coils, it is called economizer fire.
§ It is actually cause by soot fire at the economizer coils.
§ For the economizer coils fire, heat is already presented due to passing the gases of main engine.
§ Air is available due to excess supply of scavenging air into the unit combustion chamber.
§ Thick deposit of unburnt fuel, carbon residue (soots) are sticking at the economizer coils .
§ Soots are formed from incomplete combustion and use of low grade fuel and high carbon content fuel.
§ At the manoeuvring time, the more incomplete combustion may occur and at that time leaving flue gas velocity is very low, thus unburnt fuel can be adhering on the economizer coils.
§ A flame is produced when air and fuel are proportionally mixed in the sufficient heating temperature.
What are the indications of EGE fire ?
1. Smoke smell will get from the economizer.
2. Over heat at economizer body ( external casing of uptake.)
3. Heavy smoke and sparks will emit from the funnel.
4. Sudden unexpected increase in uptake gas temperature ( Abnormally high stack pyrometer reading)
5. Flame visible in the smoke indicator.
How to prevent EGE fire on ships ?
1. To get complete combustion of fuel, maintain the main engine (ME) in optimum combustion condition.( governor, fuel injection pump, fuel injection valve, fuel injection timing, fuel condition, air cooler, turbocharger, do not run too long ME with slow speed)
2. Regular open up & cleaning smoke side of economiser depending upon soot accumulation. (water washing)
3. Regular soot blowing operation. Check & maintain soot blowing equipment.
4. Regular overhaul boiler burner, correct air fuel ratio & damper.
What are the actions to be taken during economizer fire?
1. Inform to the bridge and chief engineer. Take the permission to stop main engine.
2. Gradually slowdown the ME and then stop.
3. If fire fighting equipment is fitted, it should be brought into operation.
4. Cool down the economizer by means of boundary cooling.
5. Circulation pump must be run but, if large amount of water lost it must be stopped.
6. There is no heat source and fire is out.
7. When economizer fire occurs never do soot blow to economizer.
8. It can cause hydrogen fire and melt down the coils stack. It can cause deformation and tube bulking because intensive heat and high thermal stress formed at economizer coils.
What are the actions to be taken after economizer fire?
1. Cool down to ambient temperature.
2. Open the economizer cover.
3. Clean inside parts of the economizer.
4. Check the any defective coil and repair for temporary used by welding or plugging the tubes.
How to effectively fight the economiser fire?
1. Slow down the engine.
2. Shut off oil burners, draught fans, dampers & air register.
3. Raise water level full and blow down continuously so as to maintain good flow of water.
4. Reduce boiler pressure by easing gear.
5. Spray water on the external casing of the uptake to cool the affected area.
6. A few times starting and stopping of main engine should be done to blow out collected soots at the uptake.
What is self perpetuating fire or hydrogen fire on ships ?
§ When the fire causes the metal itself burning at about 700 °C, if steam smothering soot blowing system or water jetting system have been attempted, the big hydrogen fire may result.
§ The applied steam dissociates into hydrogen and oxygen and accelerating the fire.
§ Once such a fire has started, resulting two kind of fires may take place simultaneously, one kind, iron burning in steam, and the other, the hydrogen burning in an air exothermic way.
§ This combined fire being self supporting and lasting until the supply of steam is exhausted.
§ The primary objective of dealing the fire is to cool the surface and burning material as quickly as possible.
