Hydrogen Flames

Hydrogen is flammable at concentrations between 4% and 75% in air, which is a very wide range compared to other common fuels (see Hydrogen Compared to Other Fuels).

• The hydrogen concentration could easily reach the lower flammability limit (4%) if there were a leak in a confined space with no ventilation. An outdoor leak would simply rise quickly and diffuse.
• Hydrogen burns with a pale blue flame that is almost invisible during daylight hours, so fires are almost impossible to see with the naked eye.
• Hydrogen fires have low radiant heat, so you can't sense the presence of a flame until you are very close to it (or even in it).
• Combustion can't occur in a tank or pipe that contains only hydrogen. Oxygen (or air) and an ignition source are required for combustion to occur.

Potential ignition sources include:

• Electrical
• Static electricity
• Electric charge from equipment operation
• Mechanical
• Impact
• Friction (rubbing surfaces)
• Metal fracture
• Thermal
• Open flame
• High-velocity jet heating
• Hot surfaces (e.g., an exhaust manifold)
• Vehicle exhaust
• Chemical
• Catalysts
• Reactants

Hydrogen Flame Detection

Thermal Imaging Camera in Use
Photo courtesy of HAMMER

• Because of these properties, use of a portable flame detector such as a thermal imaging camera may be the best way to detect flames. A simple flammable “indicator” tool such as a broom may also be used to check for hydrogen flames – when the broom comes in contact with the hydrogen flame, it will ignite with a visible flame.
• If flame detection equipment is not available, listen for venting hydrogen and watch for thermal waves indicating the presence of a heat source.
• Note that ignition of venting gaseous hydrogen from vent stacks is common. Vent stacks are designed to handle the ignition safely.