We do not know how the fire started, but its unlikely to have been a gas cylinder problem that caused it. It could be a pigtail that has time expired and started to leak, or the old low pressure flexible pipes.
Its more likely the fire started for other reasons, and becasue gas cylinders were still in some caravans, as the bottles heat up the pressure rises and bottles conforming the UK standards do have an over pressure relief valve built into the vapour take off valve. When the pressure is exceeded , the valve releases and vents gas. The internal pressure will drop quite quickly so the valve will close again, but in the heat of fire it will repeatedly vent gas as indicated by the whooshes and the pulsating ball of flames.
The problem the fire service has, is first of all knowing if any cylinders are still in any caravans, secondly they don't know if they are UK spec, and Ill come back to this point in a moment.
Thirdly they don't know if the cylinders are secured or loose or upright (as the should be)
They don't know if the cylinders are full or empty.
If a cylinder is on its side, and there is sufficient liquified gas to cover the valve, of the over pressure valve operates, then it wont vent gas it will vent liquid. It can physically project liquified gas over quite distance. and what ever it lands on can suffer significant frost burns as the liquid tries to vapourise, and of course once its in its vapour stated and mixed with enough air it is highly flammable.
Non UK spec bottles may not have the pressure relief valve, which means the pressure inside the bottle can rise potentially enough to reach the hydraulic rupture pressure. This would then spray a mixture of vapour and liquid LPG all around. You do not want to be anywhere near when that happens.
The other failure mode of some bottles is the brass valve may soften enough in the high temperatures to allow the pressure to eject the valve, Which would be like a missile, and then the vapour will be released violently and with considerable thrust. If the bottle is secured upright, the vent will be directly upwards and the cylinder will be pressed to the ground, the fire service prefer to allow that to burn off rather than having unburnt gas sloshing around under foot. If the cylinder is loose and can fall over or its already on its side the sudden loss of the valve
assembly releasing the pressurised gas will tun the cylinder into a ballistic missile that can do a lot of damage.
The fire service are most likely to spray water onto gas bottles to try and keep them cool. If they are already venting and a flame is established, provided its not likely endanger people or set anything else alight they would prefer to leave the vent burning so there is no residue of flammable gas left lying at low points or collecting in drains in the area.
Whilst Rays point about cylinder being robust enough to survive the time it takes for one caravan to burn down may true, the continued exposure due to neighboring units succumbing could well exceed the safety margins. And that is high probability in a storage yard especially where there is little gap between units to provide a fire break. Also whilst it may be true for UK spec. bottles Foreign imports may not be built to the same standards.
What to do with gas bottles. Places that store gas bottles have to follow a code of practice, where by the storage facility is away from other flammable or vulnerable materials and open drains. Bottles have to be stored in a covered secured well ventilated space often a an open mesh cage, and and bottles must stored upright and prevented from falling over.
Such cages are a common sight at LPG dealers, and at many recycling centers. If caravan storage sites were required to have such cages, and system that would allow them to identify each customer's bottle(s) the might reduce the scale of damage when such sorry events occur. It would have to be a condition of storage that all gas bottles are removed from caravans and stored in the cage.
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