Hydrogen Properties & Leak Detection Considerations
An understanding of the properties of hydrogen is critical for the proper design of a facility or workspace. A workspace can be configured to mitigate hazards by understanding and taking advantage of some of the characteristics of hydrogen.
Hydrogen is Flammable and Ignites easily
Designers and operators of hydrogen systems should be aware that hydrogen's flammability range is very wide compared to other fuels. Additionally, under optimal combustion conditions (at a 29% hydrogen-to-air volume ratio), the energy required to initiate hydrogen combustion is much lower than that required for other fuels (i.e., it only takes a small spark).
For most of the incidents involving hydrogen fires and explosions, the ignition source is never identified. Because of the hydrogen’s low ignition energy and the many possible sources of ignition, it is customary to expect any hydrogen leak to be ignited. Mixtures near optimal combustion conditions should be considered prone to spontaneous ignition. Ignition sources include:
- Static electricity
- Electrical charge from equipment
- Metal fracture
- Open flame
- High-velocity jet heating
- Hot surfaces
- Vehicle exhaust
- Chemical reactions
A large, high-pressure hydrogen release could also auto-ignite due to shock wave formation.
NFPA standards stipulate how far any hydrogen storage vessel must be separated from combustible materials including construction materials, vegetation, vehicles, etc.
An example practice that reflects these requirements is that there should be no grass or shrubs planted near areas where hydrogen might be released to prevent the need for using powered garden tools in the area.
Leaks are Hard to Detect
Hydrogen is colorless and odorless, so human senses can't detect it. Personnel should use caution when approaching an area where there is the potential for a hydrogen leak. Best practices include the following:
- Listen for the sound of high-pressure gas escaping.
- Gas detectors should be permanently installed indoors where leaks may occur or where hydrogen may accumulate.
- Listen and watch for alarms.
- Use a portable hydrogen detector.
Flames are Hard to See
Hydrogen burns with a pale blue flame that is nearly invisible in daylight. The flame may appear yellow if there are impurities in the air (e.g., dust or sodium from ocean spray). A pure hydrogen flame will not produce any smoke. Hydrogen flames also emit low radiant heat, so a person may not feel heat until they are very close to the flame. Best practices include the following:
- Flame detectors should be permanently installed to monitor areas where hydrogen is dispensed or handled.
- A portable flame detector (e.g., thermal imaging camera) may also be used.
- Listen for venting hydrogen and watch for thermal waves that signal the presence of a flame.
- Use a combustible probe made of materials that will easily ignite if they contact a flame (e.g., a broom).
Hydrogen is Buoyant
Hydrogen gas is much more buoyant that other gases, so when it leaks into air, it will rise rapidly. This is an advantage for hydrogen systems installed outdoors because the leaking hydrogen is rapidly dispersed. In enclosed spaces, leaking hydrogen will accumulate in the ceiling pockets, potentially creating an explosive mixture. Ventilation systems should be designed to prevent such accumulations.