Boiler Uptake Fire Risk During Soot Blowing
An auxiliary oil-fired boiler has been running on HFO with a history of smoky combustion at low load. The economiser uptake temperature has been creeping up over two weeks from 320 °C to 370 °C at constant steam demand. The engineer plans to soot-blow the economiser during the next watch. The boiler is fitted with steam soot blowers, and the uptake has no fixed temperature alarm, only a local thermometer. The ship is on a long ocean passage with no port for five days.
1. Assess the significance of the rising uptake temperature and the risk associated with the planned soot blowing.
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A rise of 50 °C in economiser uptake temperature at constant steam demand is a classic indicator of increasing fouling on the gas side - soot and unburnt fuel accumulating on the economiser and boiler tubes. The history of smoky combustion at low load supports this: incomplete combustion produces soot, and soot deposits insulate the surfaces, raising the gas temperature downstream.
The risk is an economiser or uptake fire. Soot deposits can contain unburnt oil and carbon, and when the deposit is disturbed or when a soot blower is used, the loosened material can ignite, particularly if the blower steam is wet or the blower is operated with the boiler at low load. A local thermometer with no alarm means no automatic warning if the temperature runs away during or after blowing.
Judgement: soot blowing is the right action to remove the deposit, but it must be done under controlled conditions, not casually during a watch. The reasoning to show is: interpret the temperature rise as fouling, link the smoky combustion to soot formation, identify ignition of deposits as the hazard, and recognise that the absence of an alarm raises the required supervision rather than removing the need to act.
2. Set out the procedure and precautions you would require for the soot blowing, and the monitoring afterwards.
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Precautions before starting: raise the boiler load to a steady, reasonably high firing rate, because low-load, smoky firing is exactly the condition that leaves unburnt material; confirm the blower steam is dry and at the correct pressure, since wet steam can quench and disturb deposits without clearing them; confirm the blower rotation and that the blowers are not seized; and position a person at the local thermometer or arrange frequent readings, because there is no alarm. Inform the bridge and engine room and record the start time.
During blowing: blow in the correct sequence, one blower at a time, watching the uptake temperature continuously. If the temperature rises sharply or the funnel emits dense black smoke, stop and investigate - a rising temperature during blowing can indicate a fire rather than cleaning.
After blowing: continue monitoring the uptake temperature for at least 30 minutes. A fall confirms successful cleaning; a continued rise or a new high reading suggests a fire, in which case stop firing, shut off fuel, use the fixed firefighting arrangement or steam smothering as fitted, and do not open the uptake until it has cooled.
Over the passage: improve combustion at low load (atomisation, excess air, fuel temperature) to reduce further soot, and record the uptake temperature each watch so the trend is visible. The answer must show the load condition, steam quality, sequence, continuous temperature watch, post-blow monitoring and the fire response.