Rolled LPG Tanker Beside a Market: Cordon, Cloud and the Ignition Hunt
At 06:40 a 32,000 L road tanker carrying 18 t of commercial propane/butane has rolled onto its side at a busy road junction. The shell looks intact, but the gland of a top-mounted liquid withdrawal valve is leaking and a white cloud is forming and drifting north-north-east towards a weekly market 150 m away where traders are setting up; a housing colony lies 400 m beyond the market. Wind is estimated at 3 m/s, it is 22 °C, the ground is flat, and there is no fire. The driver is out with a scalp wound. A 9,000 L water tender is on scene, a hydrant 400 m back gives 900 L/min, and the crew has DCP extinguishers and a foam branch. A diesel generator still runs at a shop 30 m away, an excavator is working 200 m off, and traffic is still passing on the far side of the junction. Technical support is 40 minutes away.
1. Set the initial cordon and evacuation, and state the first two actions in order, with your reasoning.
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Product reasoning. LPG is a flammable gas, heavier than air (vapour density roughly 1.5–2 times air), flammable across roughly 2–10% by volume, colourless, and visible only where moisture condenses. The visible white cloud is therefore smaller than the flammable cloud, and the gas will travel downhill and collect in low spots, drains and basements. Eighteen tonnes is a large release.
Cordon. Treat this as a large spill. Published emergency response guidance for a large LPG release is in the order of 800 m isolation with evacuation downwind of a similar distance; that figure must be checked against the current guide in use, but any such figure already covers both the market at 150 m and the colony at 400 m. Approach and park upwind or on the upwind flank. Do not enter the cloud for any reason.
First action - people, not the tanker. Evacuate the market now, moving people crosswind and upwind, not straight downwind; then the colony. At 3 m/s the cloud covers 150 m in well under a minute, so this is a race that must be started before any leak control is attempted.
Second action - remove ignition sources. Shut down the generator 30 m away and the excavator 200 m away, stop and divert traffic on the far side, and prohibit vehicles starting, smoking and mobile phone use inside the cordon. A drifting gas cloud that finds an ignition source becomes a flash fire across the whole downwind area.
2. What leak-control and vapour-management actions are appropriate in the first 40 minutes, and which attractive-looking options should be rejected?
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Appropriate:
- Keep everyone out of the vapour cloud, monitor with a calibrated flammable-gas meter, and set the no-entry boundary at a low percentage of LEL (commonly 10% LEL for unprotected entry, with withdrawal well before any reading approaches the lower flammable limit).
- Use water spray/fog from behind cover and from upwind to knock down and disperse the cloud, to protect exposures, and to keep the tank shell cool if a fire starts. Spray is used at a distance and directed at the cloud, not into the liquid.
- Let the product vent and disperse in a controlled way while the area is cleared of ignition sources - this is the safest of the available options here.
Rejected:
- Righting the tanker or reaching the top-mounted valve to plug it: no fire cover, a rolling load and a live leak; that is a job for specialist technical support.
- A solid water jet on the liquid or the tank: it spreads and entrains vapour, and adds heat to liquid at about −40 °C, increasing vaporisation.
- Foam on the vapour cloud: foam is for liquid pools, not for a gas cloud drifting downwind.
- Flaring, transfer or hot-tapping in the first 40 minutes with one appliance and no technical support.
- DCP on the cloud: it does nothing to a dispersing gas; DCP is only relevant if a small fire is burning at the valve and the supply can be shut off.
If the leak ignites, do not extinguish it unless the leak can be positively stopped - an extinguished leak rebuilds an explosive cloud. Cool the shell and withdraw.
3. Assess the BLEVE risk and decide where the appliance and crews should be positioned and why.
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Risk assessment. A BLEVE requires a pressurised liquid container heated by fire until the shell loses strength and the contents flash. At present there is no fire, and the tank is being cooled by its own leaking product, so a BLEVE is not imminent. That changes quickly if the vapour finds an ignition source and a pool fire or jet fire impinges on the shell; from that point the time to failure can be short, and there is no reliable instrument reading that predicts it. The risk is therefore potential and condition-dependent, not zero - which is exactly why ignition exclusion is the priority control.
Positioning:
- Appliance upwind and, where possible, at or beyond the outer cordon - 800 m if the space allows, otherwise behind substantial solid cover such as a building or embankment.
- Do not site in line with either end of the tank. The end caps are the weakest part of a tank shell, and a BLEVE throws the fireball and fragments along the tank's axis.
- Crews in full PPE, working in the open only where they have a protected route out, and never between the tank and an exposure they are trying to protect.
- Water is set up for cooling and exposure protection only, from a position that can be abandoned quickly.
A distant, upwind, protected position is not caution for its own sake: it is the only posture from which the crew can still act if the situation changes.