Hydrogen is unlike conventional fuels such as gasoline or propane, which are heavier than air and tend to accumulate at ground level. Hydrogen is lighter than air and will accumulate near the ceiling or 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. As a result, even slow releases of hydrogen in such facilities could lead to the formation of a flammable concentration at the ceiling.
The three events listed below occurred in laboratories:
The four events listed below occurred in industrial or power plant facilities:
Some of the key lessons learned from these events include:
- Active venting, warning signs, and local alarms are essential for safe plant operations.
- Experienced engineers should be involved in the design of ventilation systems.
- Potential hydrogen leaks must be considered during the design of the facility's ventilation system.
- Maintenance on ventilation systems should occur at regular intervals.
- An air flow monitor with a low-level hydrogen alarm would provide a reliable indication of ventilation system performance.
- It is a good practice to check ventilation system integrity and functionality before starting any activities involving hydrogen.
Proper ventilation can reduce the likelihood of a flammable mixture of hydrogen forming in an enclosure following a release or leak. At a minimum, ventilation rates should be sufficient to dilute a potential hydrogen leak to 25% of the lower flammability limit (LFL) for all operations and credible accident scenarios.
Passive ventilation features such as roof or eave vents can prevent the buildup of hydrogen in the event of a leak or discharge, but passive ventilation works best for outdoor installations. 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, and outlet openings should be located at the highest point of the room in exterior walls or the roof. If passive ventilation is insufficient, active (mechanical, forced) ventilation can be used to prevent the accumulation of flammable mixtures. However, no practical indoor ventilation features can quickly disperse hydrogen from a massive release by a pressurized vessel, pipe rupture, or blowdown.
Equipment used in active ventilation systems (e.g., fan motors, actuators for vents and valves) should have the applicable electrical classification and be approved for hydrogen use. 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 LFL quantities of hydrogen could accumulate due to the loss of ventilation. If the hazard is substantial, an automatic shutdown feature may be appropriate.
More detailed information about ventilation system design is available on the Hydrogen Safety Best Practices web site.