Diesel spray onto hot exhaust lagging in the engine room
A 4,800 TEU container ship is on passage with six engine-room day workers on board. At 14:20 the second engineer notices a light haze of smoke and the smell of hot oil near No.2 diesel generator. The fuel-oil booster pump discharge pipe has a 0.3 mm pinhole leak, and for several weeks a fine diesel mist has been playing on the exhaust manifold lagging, which is dark and oil-soaked. The manifold surface reads 335 C on the infrared thermometer. There is no visible flame, only a dull red glow deep in the lagging and a faint crackling sound. The generator is still running and the fuel booster pump is still pressurised. The engine-room smoke detector is in alarm on the panel in the ECR. The chief engineer wants to shut down the generator and strip off the lagging to find the source, and asks whether the fixed CO2 should be released now. Crew is 21 and nobody is missing.
1. Using the fire tetrahedron, explain how ignition occurred with no flame, spark or hot work anywhere near the leak, and why oil-soaked lagging is a recognised ignition point.
Read the worked answer
All four sides must be present:
- Fuel. Diesel escaping through a 0.3 mm pinhole leaves the pipe at pressure and atomises. Mist droplets have a huge surface-to-volume ratio, so they vaporise and mix with air far better than a pool of liquid; the fuel is already a vapour-air mixture before it reaches the hot surface, which is why its flashpoint is no longer the controlling number.
- Heat. 335 C is above the auto-ignition temperature of diesel, roughly 210-260 C. No external ignition source is needed: contact with the hot metal is enough. This is why a spray fire on a hot surface is one of the classic machinery-space scenarios.
- Oxygen. Ordinary engine-room atmosphere.
- Uninhibited chain reaction. The branched radical reactions that make combustion self-sustaining.
Oil-soaked lagging is dangerous because it insulates (heat cannot escape, so the metal beneath runs hotter), it is porous (it holds oil and admits air), and the absorbed oil can carbonise and self-heat. The glow and crackling are smouldering inside the lagging, not a flame in the open.
2. Classify the fire and set out the correct sequence of action. Explain specifically why opening the lagging and releasing the fixed CO2 now are both wrong.
Read the worked answer
It is a Class B fire around live machinery, with a Class C (electrical) element while the generator is energised, so the first agent must not conduct.
Why the two proposals are wrong:
- Opening the lagging while it is hot admits oxygen to a smouldering, fuel-rich mass and can produce a sudden flare straight into the person's face. The oil in the insulation is still above its auto-ignition temperature.
- Releasing fixed CO2 now wastes the system: the space is open, the generator is running and drawing air, and the concentration will not be held long enough to extinguish.
Correct sequence:
- Raise the alarm, muster, inform the bridge, and stop the fuel-oil booster pump. The fuel is the side of the tetrahedron you can actually remove.
- Trip the generator and de-energise the affected circuits; the emergency source carries the safety loads.
- Cool the surroundings with water spray or fog from a boundary, not a solid jet, staying clear of live electrical equipment.
- For the small fire itself use CO2 or dry powder at the base of the glow; dry powder also breaks the chain reaction.
- Cool adjacent lagging and hot surfaces, because hot metal will re-ignite any oil left on it.
Fixed CO2 remains the last resort, used only with the space sealed and everybody mustered.
3. The fire is reported out. What must happen before the ship resumes normal operation, and what would you record for the investigation?
Read the worked answer
Before normal operation:
- Keep a re-ignition watch on the area with a thermal imager for several hours. Metal below the oil's auto-ignition temperature can restart the fire after the space is reopened and air returns.
- Confirm the fire is out by two independent means, thermal and visual, before anyone re-enters without BA.
- Do not disturb the origin: the pinhole, the lagging, the spray direction and the temperature readings are the evidence.
- Recharge extinguishers and the fixed CO2 system, and replace the damaged pipe and lagging before the generator is returned to service.
Record and report: time of detection, the alarm and muster times, the last person out, the isolation steps taken, the agents used, thermal readings, the position of the leak and the condition of the lagging. Report through the SMS; a fire affecting safety is also notified to flag State, class and the insurer as required.
Root causes are the real deliverable: why a pinhole went undetected (no line inspection in the PMS, no spray shield, lagging never checked for oil soaking), why the lagging was allowed to become saturated, and why the immediate reaction was to open it up.