Active (mechanical, forced) ventilation can be used to prevent the accumulation of flammable mixtures, if passive ventilation is insufficient. Active ventilation can be used to ensure sufficient vent flow is maintained to keep the concentration of hydrogen below that which will burn in air (suggest setting vent flow to ensure concentration is less than 25% LFL = 1% H2 by volume). For example, NFPA 52 (para. 22.214.171.124.5) requires a minimum of 1 ft3 per minute per ft2 of room area and at least 1 ft3 per minute per 12 ft3 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 (see Proper Storage, Use & Venting).
Active ventilation may be required if the configuration of the room may cause hydrogen to accumulate in the ceiling or roof area. Hydrogen accumulation may be a problem, for example, in rooms with a peaked roof or in rooms with dropped or false ceilings.
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 Electrial Classification.)
If active ventilation systems are relied upon to mitigate gas accumulation hazards, procedures and operational practices should ensure that the system is operational at all times when hydrogen is present or could be accidentally released.
Hydrogen equipment and systems should be shut down if there is an outage or loss of the ventilation system. If the hazard is substantial, an automatic shutdown feature may be appropriate.