When working with gas there are a number of situations of especially high risk.
Changing portable gas cylinders:
In general, a static situation is not dangerous.
The danger is created during connection or disconnection, an insufficiently tightened connection to the cylinder, a faulty tap left open – all these can cause a gas leak.
Also, the cylinder connection pipes (the "coil") can crack or split and cause a leak when it's hot and they expand too much. If the leak isn't located, it could cause a buildup over time.
Overfilling gas cylinders:
If the cylinder has been filled to over its allowed limit, the excess liquid could spread, under the influence of temperature changes, and cause the excess to be expelled through the safety valve (or another dangerous situation where liquid gas enters the supply system).
Filling a central gas tank from cylinders:
In this situation, large quantities of gas are present. One of the tanks is on the delivery vehicle, above ground and exposed. During filling, a high pressure pump and flexible piping are used.
When the flexible pipe is disconnected, gas is expelled into the air and is used as an indication to prevent over filling.
Once the filling process is completed, the tanks filling valve may not close completely, causing a leak.
Close to the tank there could also be sources of fire such as evaporators or the consumer's gas burners.
Replacement of an underground gas tank:
This method of filling a gas tank is extremely complicated and lack of care could easily cause damages to parts and a leak of substantial quantities of gas.