Fundamental Safety Characteristics of Hydrogen

1. Leakage 

   Hydrogen leakage from containers and pipelines is approximately 1.2 to 2.8 times that of methane and about 4 times that of air. However, leaked hydrogen disperses rapidly in the atmosphere through mass diffusion, turbulent convection, and buoyancy, reducing the persistence of hazardous zones.

   

   Infographic: The properties of hydrogen (https://www.kiwa.com/)

   
   

2. Buoyancy 

   Hydrogen has a density about 1/14 that of air, causing it to rise quickly upon leakage and reducing ignition risk. However, saturated hydrogen vapor is heavier than air and initially spreads near the ground. As temperature increases and density decreases, upward dispersion becomes more likely. In ambient air, hydrogen's buoyant rise speed ranges from 1.2 to 9 m/s, depending on density differences. Cold, dense vapor from liquid hydrogen initially spreads near the ground and rises more slowly than standard gaseous fuels.

1. Flame Visibility 

   Hydrogen-air flames primarily emit in the infrared and ultraviolet spectrum, making them nearly invisible in daylight. Visible flames during the day are due to impurities like moisture or particles. In darkness, hydrogen flames are more visible, and during the day, heat radiation from the flame can be felt on the skin. At low pressure, hydrogen flames appear pale blue or violet. Exposure to hydrogen flames can cause burns.

1. Flame Temperature 

   The flame temperature of hydrogen in air (at 19.6% concentration) is measured at 2318 K. Explosions or detonations may produce even higher flame temperatures.

1. Burning Velocity 

   Burning velocity refers to the laminar flame speed of a fuel-air mixture. Hydrogen's laminar flame speed ranges from 2.65 to 3.46 m/s, depending on pressure, temperature, and equivalence ratio. This is an order of magnitude higher than methane (maximum ~0.45 m/s in air).

1. Thermal Radiation of Flame 

   Exposure to hydrogen flame radiation can cause severe injuries. The intensity of thermal radiation depends largely on atmospheric water vapor. Moist air absorbs radiant heat from the flame and significantly reduces thermal exposure.

1. Limiting Oxygen Index 

   The limiting oxygen index is the minimum oxygen concentration required to sustain flame propagation in a vapor-air mixture. For hydrogen, if the oxygen volume fraction is below 5%, flame propagation cannot be observed under standard conditions.

1. Joule-Thomson Effect (J-T Effect) 

   When gas expands from high to low pressure through porous media, small holes, or nozzles, its temperature typically decreases. However, real gases like hydrogen may increase in temperature if expanded above the critical temperature and pressure defined by the J-T curve. Hydrogen's maximum inversion temperature at zero absolute pressure is 202 K. Above this temperature and pressure, hydrogen heats up upon expansion. For example, expanding hydrogen from 100 MPa to 0.1 MPa raises its temperature from 300 K to 346 K, which is insufficient to ignite hydrogen-air mixtures (autoignition temperature is 858 K at 1 atm, 620 K at low pressure).

1. Minimum Ignition Energy 

   Minimum ignition energy is the smallest spark energy needed to ignite hydrogen-air or hydrogen-oxygen mixtures. At 101.325 kPa, hydrogen-air mixtures require only 0.019 mJ, much lower than methane (0.2 mJ) or gasoline (0.24 mJ). Even weak sparks from static electricity can ignite hydrogen.

Objects at 500–581°C can ignite hydrogen mixtures at 1 atm. At lower pressures, prolonged contact with surfaces at ~317°C can also cause ignition.

10. Hydrogen Flammability Limits 

    At 101.3 kPa and ambient temperature, hydrogen's upward flammability limits in dry air range from 4.1% to 74.8%. In oxygen, the range extends to 4.1%–94%. Lower pressures reduce the flammability range.

    
    

11. Hydrogen Explosion Types

    Unconfined Vapor Cloud Explosion: Occurs in open or outdoor environments.

    Confined Vapor Cloud Explosion: Occurs in enclosed spaces.

    Boiling Liquid Expanding Vapor Explosion (BLEVE): Occurs when high-pressure liquefied hydrogen explodes due to rapid vapor expansion.