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 small leak in a confined space with no ventilation. A small leak outdoors would likely rise quickly and diffuse.
- 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.
- Hydrogen-air mixtures are easily ignited. For recorded incidents, the ignition source is frequently unknown. Potential ignition sources include:
- Spark producing electrical equipment
- Spark from electrostatic discharge
- Mechanical impact
- Open flame
- Hot surfaces (e.g., an exhaust manifold
- 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 like dust or sodium.
- A pure hydrogen flame will not produce smoke.
- Hydrogen flames have low radiant heat. Unlike a hydrocarbon fire, you may not feel any heat until you are very close to the flame.
- 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.