Proper ventilation can reduce the likelihood of a flammable hydrogen-air mixture from forming in an enclosed area.
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 can accumulate near the ceiling, under the roof, or in pockets at these locations.
When the buoyancy of hydrogen is not properly considered in the design of facilities, hydrogen leaks can result in dangerous conditions resulting from trapped hydrogen. 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 buildings without proper high space ventilation 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 .
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 0.003m2 per 1m3 of room volume, or 1ft2 per 1,000ft3 of room volume, according to 29CFR 1910.106.
If sufficient passive ventilation is not sufficient, active (mechanical, forced) ventilation can be used to prevent the accumulation of flammable mixtures. Active ventilation can be used to ensure sufficient air flow is maintained to keep the concentration of hydrogen below that which will burn in air. Normally, air flow is selected to ensure hydrogen concentration is less than 25% of the lower flammability limit = 1% hydrogen by volume. For example, NFPA 52 (para. 22.214.171.124.5) requires a minimum of 1m3 per minute per 1m2 of floor area (1ft3 per minute per 1ft2 of floor area) and at least 1m3 per minute per 12m3 of room volume (1ft3 per minute per 12ft3 of room volume) for proper hydrogen ventilation. Note that no practical ventilation rate can effectively disperse hydrogen from a massive release from a pressurized vessel, pipe rupture, or blowdown.
Equipment used in forced ventilation systems (fan motors, actuators for vents and valves, etc.) should have the applicable electrical classification (class, division, group, and operating temperature) and should be approved for hydrogen use. Systems that recirculate air should be avoided (see Electrical.)
If active ventilation systems are relied upon to mitigate gas accumulation hazards, procedures and operational practices should ensure that the system is always operational when hydrogen is present.
Hydrogen equipment and systems should be shut down if there is a loss of the ventilation system. If the hazard is substantial, an automatic shutdown feature may be appropriate.