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c) 20 s after release start

These results indicate that, whenever possible, hydrogen processing systems or storage at high pressures should be placed outdoors in well ventilated areas. If, for some reason, hydrogen systems have to be located indoors, it is very important that the risk of leaks and gas accumulation is assessed. If the risk is not acceptable risk reducing measures such as gas detection coupled to automatic activation of emergency ventilation, relief of hydrogen to safe area, purging etc. should be implemented.

Relief of hydrogen to atmosphere from safety valves

Hydrogen may be relived to atmosphere from time to time, caused by pressure build-up and opening of safety valves or by controlled ventilation in case of maintenance. Relief through safety valves may cause rather high release rates when the back pressure is high. The flammable gas cloud may reach several meters away from the outlet. It is therefore important that the relief point is located so that releases will not lead to hazards in the vicinity. This should especially be taken seriously in case of location in large cities with a high density of high buildings.

Hazards during refuelling

As pointed at, leaks from process equipment containing hydrogen at high pressures may lead to significant flammable gas clouds even in unconfined areas. Releases during refuelling where persons may be exposed to the gas jet were identified to represent a significant hazard. Hydrogen gas and hydrogen releases are virtually invisible in daylight. It is therefore very important that all precautions are taken to avoid hazardous situations during refuelling of vehicles where public customers may be exposed to the consequences of a leak in case of ignition. Risk reducing measures were suggested as follows:

Only use high quality equipment that are documented for the expected pressure, temperatures, cycling etc.

Regular inspection Fast leak detection and shutdown

Design and layout so that the probability of hazardous situations close to the dispenser is reduced to a minimum

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    Automatic retraction of refuelling hose after refuelling

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      Design of drive in to minimise the probability of collisions

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