Passive Ventilation

Proper ventilation can reduce the likelihood of a flammable mixture of hydrogen forming in an enclosure, following a release or leak.

Hydrogen is unlike other fuels such as gasoline vapors or propane, which are heavier than air and tend to accumulate at ground level. Hydrogen is lighter (less dense) than air and will accumulate near the ceiling, roof area, or in pockets at these locations.

When the buoyancy of hydrogen is not properly taken into account in the design of facilities, hydrogen leaks can become more dangerous than leaks or spills of conventional fuels such as gasoline, LPG and propane. The building codes of many countries require garages to have ventilation openings near the ground to remove gasoline vapor, but ventilation high in the workspace is not always addressed. As a result, even slow releases of hydrogen in such buildings could lead to the formation of a flammable concentration at the ceiling.

Passive ventilation features such as roof or eave vents can prevent the buildup of hydrogen in the event of a leak or discharge. Note that outdoor installations offer the best passive ventilation (see Proper Storage, Use & Venting and Hydrogen Storage and Piping Systems).

In designing passive ventilation, ceiling and roof configurations should be thoroughly evaluated to ensure that a hydrogen leak will be able to dissipate safely. Inlet openings should be located at floor level in exterior walls. Outlet openings should be located at the high point of the room in exterior walls or roof. Inlet and outlet openings should have a minimum total area of 0.003 m2 per 1 m3 of room volume, or 1 ft2 per 1,000 ft3 of room volume, per 29CFR 1910.106