Why can a CNG tank explode?
CNG is primarily methane stored in vehicles as a high-pressure compressed gas. Unlike LPG, which is stored as a liquid under pressure, CNG remains gaseous but is compressed to a very high pressure so that a large quantity can be stored in a relatively small cylinder.
The cylinder is designed to withstand this pressure, but a catastrophic failure can occur if its structural integrity is compromised. Potential causes can include physical damage, manufacturing defects, corrosion, improper maintenance or problems with the refuelling system. However, none of these has been established as the cause in the Tikri Kalan incident.
Refuelling can also involve changes in pressure and temperature. If a cylinder or its associated equipment is already compromised, the stresses involved can contribute to failure. This is why CNG cylinders and their valves are subject to safety standards and periodic inspection.
The danger is not simply that the compressed gas can escape. If a large quantity of CNG is suddenly released and encounters an ignition source, methane can burn extremely rapidly. In a confined or poorly ventilated area, a gas-air mixture can also ignite and produce a powerful blast.
A sudden rupture of a pressurised cylinder can itself be deadly because the stored pressure energy is released almost instantaneously, potentially producing a violent shock wave and sending fragments of the cylinder or surrounding equipment outward.
In the Tikri Kalan incident, however, it is too early to say whether the deaths were caused primarily by a pressure-vessel rupture, ignition of released gas, flying debris, fire or a combination of these factors. The investigation will have to establish the precise sequence of events.
The incident also highlights why CNG refuelling areas require strict control of equipment condition, pressure systems and ignition sources. The presence of people close to a vehicle during a catastrophic cylinder failure can leave very little time to escape.
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