Hydrogen is flammable at concentrations between 4% and 75% in air, which is a very wide range compared to other common fuels. (See the Flammability Range chart for hydrogen and other common 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 that contains only hydrogen. Oxygen (or air) and an ignition source are required for combustion to occur.
- Potential ignition sources include:
- Static electricity
- Electric charge from equipment operation
- Friction (rubbing surfaces)
- Metal fracture
- Open flame
- High-velocity jet heating
- Hot surfaces (e.g., an exhaust manifold)
- Vehicle exhaust
Thermal Imaging Camera in Use
(Photo courtesy of HAMMER) Hydrogen Flame Detection
- 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 a portable flame detector, such as a thermal imaging camera, when possible. If flame detection equipment is not available, listen for venting hydrogen and watch for thermal waves.
- Note that vent stacks are standard in storage facilities, and the ignition of venting gaseous hydrogen is common. Systems are designed to do this safely.
- Flame detectors may be installed in storage facilities and fueling stations. Listen and watch for audible or visual alarms.