It is important to consider the consequences of a gas leakage from a pressurised gas cylinder as part of the risk assessment exercise. Leakage could reduce the concentration of oxygen to levels capable of causing asphyxiation (e.g. release of carbon dioxide or nitrogen). Equally, a release of oxygen could create an oxygen enriched atmosphere allowing fires to start more easily.
It is difficult to determine an accurate rate of gas discharge from a cylinder in the event of a tube blow-off. Figures supplied by Gas Safety UK Ltd estimate a discharge from between 1.2 to 84.0 m3 per hour could be anticipated. A rate of 1.2 m3 / hr could represent a slow release scenario whereby the contents of a cylinder would be discharged over the course of several hours (i.e. a working day or overnight), whilst 84.0 m3 / hr could represent a sudden release, whereby the cylinder contents are completely discharged within the space of a few minutes (the worst case scenario). Gas suppliers (e.g. BOC) will be able to supply information on the quantities of gas contained in different sized cylinders - it can therefore be estimated how long a cylinder would take to empty at a given release rate.
Since the process of risk assessment examines the likelihood of an event occurring, it is perhaps more appropriate to look at slow release scenarios as examples. These are more likely than a sudden or fast release, and there is also a greater chance that a slow release would go unnoticed (a sudden failure or rapid release would give rise to e.g. hissing, popping or other audible / visual warnings that would alert anyone in the vicinity to the fact that something was wrong).
Oxygen concentration resulting from a leakage of gas from a pressurised gas cylinder may be calculated as follows:
Resulting oxygen concentration (%) =
C = L
VN
Where:
C = gas concentration in room, L = gas release from cylinder (m3), V = room volume (m3), N = air changes per hour
Effect of slow CO2 gas release from cylinder in a laboratory
Effect of slow O2 gas release from cylinder in a laboratory