Ventilation

Any structure containing hydrogen components should be adequately ventilated when hydrogen is present. Suspended ceilings, inverted pockets, confining cowlings, or covers that might accumulate hydrogen should be avoided or adequately ventilated. The local release of hydrogen into the laboratory should be controlled by enclosures (for example, a fume hood) and vented to the outside to prevent it from reaching an ignition source. Ventilation systems should not be used for the disposal of hydrogen, e.g., pressure relief lines, purging and cryogenic boil-off; this should be managed through a separate vent system. See Venting.

• Ventilation rates should be sufficient to dilute hydrogen leaks to less than 25% of the LFL; which is about 1% by volume in air.
• Where passive ventilation is used, 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 in the 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 according to 29CFR 1910.106.
• Normal air exchange should be 0.3 m3 of air per minute per 1 m2 of solid floor space, or 1 ft3 of air per minute per 1 ft2 of solid floor space.
• Laboratories and laboratory hoods in which hydrogen is present should be continuously ventilated under normal operating conditions.
• The minimum ventilation rate should safely dilute hydrogen buildup due to leakage from system components. Ventilation should not shut down in an emergency.

• Laboratory ventilation systems should be designed to ensure that supply air to the laboratory does not contain flammable gas or other hazardous material re-circulated from another system’s exhaust.
• The air pressure in the laboratory should be negative with respect to corridors and non-laboratory areas.
• The location of air supply diffusion devices should be chosen to avoid air currents that would adversely affect the performance of fume hoods and exhaust systems.

• Air exhausted from fume hoods should be released above the roof in such a way that it will not enter other ventilation intakes. See ANSI/AIHA Z9.5 Laboratory Ventilation.
• Air from laboratories in which hydrogen is present should be continuously discharged through duct systems maintained at a negative pressure relative to that of normally occupied areas of the building.
• Positive-pressure portions of the laboratory hood exhaust systems (e.g., fans, coils, flexible connections, and ductwork) located within the laboratory building should be sealed airtight or located in a continuously mechanically ventilated room.
• Fume hood face velocities and exhaust volumes should be sufficient to contain hydrogen released within the hood and exhaust it outside of the laboratory building.
• Special local exhaust systems (e.g., snorkels) should have sufficient capture velocities to entrain the hydrogen being released.
• Duct airflows must be sufficient to keep hydrogen concentrations below the LFL during probable release scenarios.
• Canopy hoods should not be used unless an evaluation shows that they can adequately capture released hydrogen gas.
• Automatic fire dampers should not be used in fume hood exhaust systems.

• Fans should be selected to meet requirements for fire, explosion, and corrosion.
• Where hydrogen is passed through the fans, the rotating element should be of non-sparking construction.
• Nonferrous or spark-resistant materials should have a flame spread index of 25 or less when tested in accordance with ASTM E84, UL 723, or NFPA 255.

• Designers of hydrogen laboratories should know that hydrogen, the lightest and most buoyant of gases, is prone to leak through the smallest piping or fitting defect and should not be allowed to collect in confined spaces or poorly ventilated areas, such as

  • high areas under ceilings
  • pockets between ceiling beams
  • suspended ceilings
  • confined spaces with hydrogen piping within, such as wall cavities, enclosed floor drains, covered pipe trenches, and sealed enclosures

   

  The best practice to avoid this is to determine where hydrogen leaks are likely to occur and how they may disperse. Computational fluid dynamics (CFD) may be the best method, but other methods include using IEC 60079-10 and adherence to building code ventilation requirements. Eliminating leak sources is the best way to avoid leaks – for example, continuous piping or welded fittings instead of mechanical fittings – but may not be practical in many settings.

  In addition to ventilation, dilution, and exhaust, many labs and fume hoods use H₂ gas detectors and/or H₂ flame detectors as part of their safety system. Thoughtful analysis can help determine where hydrogen sensors are best placed for the earliest leak detection. The number and distribution of detection points and time required to shut off the hydrogen source should be based on factors such as leak rates, ventilation rates, and the volume of space in the lab.

  New facilities can avoid confining spaces and can optimize natural and forced ventilation to properly prevent accumulation. Where an existing space must be retrofitted to accept hydrogen use, some features such as suspended ceilings and enclosed drains and covered piping may require modification or installation of ventilation and gas monitors.