Petrol Wash-Down Next to a Live Heater
At 9:40 on a cold morning, a two-bay vocational workshop has a Maruti 800 on a two-post lift with its gearbox removed. An 18-year-old first-year apprentice is washing the bell housing in an open steel tray holding about 3 litres of petrol, using a nylon brush; the roller shutter is open, so a 2 kW electric heater stands 1.5 m away. A 180 mm angle grinder on the bench has a visible crack across its disc and its guard has been removed. The compressor quick-coupler near the tray leaks continuously at 8 bar. The only extinguisher is a 5 kg CO2 unit in the office, 25 m away behind a closed door. The instructor has stepped out for 20 minutes and no other staff are on the floor. The apprentice has gloves but no goggles, and the tray sits directly under the lift's hydraulic hose.
1. Rank the petrol wash, the cracked grinder disc and the leaking air coupler by risk, showing severity x likelihood reasoning, and state which activity must be stopped first and why.
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Risk = severity x likelihood, not 'how dangerous it looks'.
Petrol wash - rank 1. Petrol has a flash point near -43 C and vapour about 3-4 times denser than air, so vapour creeps along the floor to the heater element 1.5 m away; the lower explosive limit of about 1.4% by volume is reached from a small spill. Severity: major/fatal flash fire and burns. Likelihood: high, because a live ignition source is already within the vapour path. Stop this immediately - it is in progress, not just planned.
Cracked 180 mm disc - rank 2. At roughly 8,500 rpm the rim speed is about 80 m/s; a cracked disc bursts and throws fragments. Severity: fatal. Likelihood: only when switched on, so lower than the petrol. Still lock it out and tag it before any other work.
Air coupler - rank 3. An 8 bar jet can inject air under the skin (air embolism) and an unrestrained hose whips. Severity: serious. Likelihood: medium-low if the coupling is pinned. Isolate the compressor and fit the correct coupling.
Conclusion: petrol first (already burning-capable), grinder locked out second, air line isolated third.
2. Give controls for the petrol hazard in hierarchy-of-control order, most effective first, and explain why PPE is the weakest control here.
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1. Eliminate: ban petrol as a cleaning solvent. This removes the hazard completely - the only control that gives a guaranteed result.
2. Substitute: use a non-flammable aqueous degreaser (or a solvent with flash point above 60 C) in place of petrol. Kerosene or diesel is not acceptable - they still burn.
3. Engineering controls: a closed, lidded recirculating parts washer; local exhaust ventilation; bonding and earthing of the tray; removing all ignition sources within 6 m (heater, grinder, welding, smoking); RCD protection.
4. Administrative controls: written permit for solvent work, no lone working, direct supervision of a first-year apprentice, a short toolbox talk, and a documented procedure for spills.
5. PPE: nitrile gloves and chemical goggles are necessary but last.
Why PPE is weakest: gloves do not stop vapour reaching the heater, do not stop ignition, and petrol degrades many glove materials. PPE protects one person from splash only; it does not reduce the fire or explosion risk to the apprentice or anyone else in the bay.
3. If the petrol in the tray ignites, what extinguisher would you use, and why is the workshop's 5 kg CO2 unit a poor choice for this fire?
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Petrol is a Class B flammable-liquid fire: the correct media are foam, dry powder or (for a small, contained spill) CO2. For 3 litres of burning petrol the practical choice is foam or dry powder.
Why CO2 is poor here: a 5 kg CO2 extinguisher empties in roughly 10-15 seconds; it smothers by displacing oxygen but provides no cooling or sealing, so hot metal and the tray can re-ignite the remaining vapour. It is also stored 25 m away behind a closed door, so it will not be in hand during the first minute, when the fire is still small.
Correct actions: raise the alarm and start evacuation; switch off the heater at the breaker only if this can be done safely; do not attempt to carry the burning tray outside; attack from the doorway with foam/powder, keeping an exit behind you and a second person at the door; call the fire service; account for everyone at the muster point. A solid water jet must never be used - it spreads burning petrol.