Liquid Hydrogen

40Most of what appears below was taken from the Air Products’ website with its permission.

Information Specific to Liquid Hygrogen


Hydrogen is colorless as a liquid. Its vapors are colorless, odorless, tasteless, and highly flammable. Liquid hydrogen is non-corrosive. Special materials of construction are not required to prevent corrosion. However, because of its extremely cold temperature, equipment must be designed and manufactured of material, which is suitable for extremely low temperature operation. Vessels and piping should be designed to the American Society of Mechanical Engineers (ASME) Code and the American National Standards Institute (ANSI) Pressure Piping Code or Department of Transportation (DOT) Codes for the pressure and temperatures involved.

Health Effects

Hydrogen gas is odorless and nontoxic but may produce suffocation by diluting the concentration of oxygen in air below levels necessary to support life. Caution: The amount of hydrogen gas necessary to produce an oxygen-deficient atmosphere is well within the flammable range, making fire and explosion the primary hazards associated with hydrogen and air atmospheres.


The wide flammability range, 4% to 74% in air, and the small amount of energy required for ignition necessitate special handling to prevent the inadvertent mixing of hydrogen with air. Care should be taken to eliminate sources of ignition, such as sparks from electrical equipment, static electricity sparks, open flames or any extremely hot objects. Hydrogen and air mixtures within the flammable range can explode and may burn with a pale blue, almost invisible flame.

Physical Properties

  • Molecular Weight: 2.016
  • Boiling Point @ 1 atm: -423.0°F (-252.8°C, 20oK)
  • Freezing Point @ 1 atm: -434.5°F (-259.2°C, 14oK)
  • Critical Temperature: -399.8°F (-239.9°C)
  • Critical Pressure: 188 psia (12.9 atm)
  • Density, Liquid @ B.P., 1 atm: 4.23 lb./cu.ft.
  • Density, Gas @ 68°F (20°C), 1 atm: 0.005229 lb./cu.ft.
  • Specific Gravity, Gas (Air = 1) @ 68°F (20°C), 1 atm: 0.0696
  • Specific Gravity, Liquid @ B.P., 1 atm: 0.0710
  • Specific Volume @ 68°F (20°C), 1 atm: 192 cu. ft./lb.
  • Latent Heat of Vaporization: 389 Btu/lb. mole
  • Flammable Limits @ 1 atm in air 4.00%: -74.2% (by Volume)
  • Flammable Limits @ 1 atm in oxygen 4.65%: -93.9% (by Volume)
  • Detonable Limits @ 1 atm in air 18.2%: -58.9% (by Volume)
  • Detonable Limits @ 1 atm in oxygen 15%: -90% (by Volume)
  • Autoignition Temperature @ 1 atm: 1060°F (571°C)
  • Expansion Ratio, Liquid to Gas, B.P. to 68°F (20°C): 1 to 848

Transferring Liquid

Two persons should be present when liquid hydrogen is being used or transferred, or a container is moved. This does not apply where specially trained employees of the liquid hydrogen supplier, who routinely handle liquid hydrogen, are involved. Hydrogen is normally vaporized and used as a gas. Withdrawal of liquid from a tanker, tank, or liquid cylinder requires the use of a closed system, with proper safety relief devices, which can be evacuated and/or purged to eliminate the possibility of creating a flammable atmosphere or explosive mixture of liquid air and liquid hydrogen. Purging should be done with helium since liquid hydrogen can solidify other gases, such as nitrogen, and cause plugging and possible rupture of the transfer line or storage vessel. Liquid transfer lines must be vacuum insulated to minimize product loss through vaporization or the formation of liquid air on the lines with subsequent oxygen enrichment. All equipment must be electrically grounded and bonded before transferring liquid.

Safety Considerations

The hazards associated with handling liquid hydrogen are fire, explosion, asphyxiation, and exposure to extremely low temperatures. Consult the Air Products Material Safety Data Sheet (MSDS) for safety information on the gases and equipment you will be using. The potential for forming and igniting flammable mixtures containing hydrogen may be higher than for other flammable gases because: 

Hydrogen migrates quickly through small openings.

The minimum ignition energy for flammable mixtures containing hydrogen is extremely low. Burns may result from unknowingly walking into a hydrogen fire. The fire and explosion hazards can be controlled by appropriate design and operating procedures. Preventing the formation of combustible fuel-oxidant mixtures and removing or otherwise inerting potential sources of ignition (electric spark, static electricity, open flames, etc.) in areas where the hydrogen will be used is essential. Careful evacuation and purge operations should be used to prevent the formation of flammable or explosive mixtures. Adequate ventilation will help reduce the possible formation of flammable mixtures in the event of a hydrogen leak or spill and will also eliminate the potential hazard of asphyxiation. Protective clothing should be worn to prevent exposure to extremely cold liquid and cold hydrogen vapors.

Cold burns may occur from short contact with frosted lines, liquid air that may be dripping from cold lines or vent stacks, vaporizer fins, and vapor leaks. Air will condense at liquid hydrogen temperatures and can become an oxygen-enriched liquid due to the vaporization of nitrogen. Oxygen-enriched air increases the combustion rate of flammable and combustible materials.


Gaseous and liquid hydrogen systems must be purged of air, oxygen, or other oxidizers prior to admitting hydrogen to the systems, and purged of hydrogen before opening the system to the atmosphere. Purging should be done to prevent the formation of flammable mixtures and can be accomplished in several ways. Piping systems and vessels intended for gaseous hydrogen service should be inerted by suitable purging or evacuation procedures. If the piping systems are extensive or complicated, successive evacuations broken first by an inert gas and finally with hydrogen are most reliable. Evacuating and purging of equipment in gaseous hydrogen service should include the following considerations:

Evacuate the equipment and break vacuum with an inert gas, such as nitrogen. Purge with an inert gas if equipment design does not permit evacuation.

Repeat step 1 at least three times. If analytical equipment is available, purge system until oxygen content of residual gas is either less than or meets the process specification impurity level.

Hydrogen may now be introduced to the equipment.

Flush system with hydrogen until required purity is reached. Vent all waste hydrogen through a flue or flare stack. Any purge method should be repeated as often as required to be certain a flammable mixture cannot be formed upon introducing hydrogen or air to the system.

Fire Fighting

Heat, open flames, electrical sparks, and static electricity easily ignite hydrogen. It will burn with a pale blue, almost invisible flame. Most hydrogen fires will have the flame characteristic of a torch or jet and will originate at the point where the hydrogen is discharging. If a leak is suspected in any part of a system, a hydrogen flame can be detected by cautiously approaching with an outstretched broom, lifting it up and down. The most effective way to fight a hydrogen fire is to shut off the flow of gas. If it is necessary to extinguish the flame in order to get to a place where the flow of hydrogen can be shut off, a dry powder extinguisher is recommended. However, if the fire is extinguished without stopping the flow of gas, an explosive mixture may form, creating a more serious hazard than the fire itself should re-ignition occur from the hot surfaces or other sources. The usual fire fighting practice is to prevent the fire from spreading and let it burn until the hydrogen is consumed. Dry powder fire extinguishers should be available in the area. A fire blanket should be conveniently located. An adequate water supply should be available to keep surrounding equipment cool in the event of a hydrogen fire. The local fire department should be advised of the nature of the products handled and made aware of the best known methods for combating hydrogen fires.

Personal Protective Equipment (PPE) 

One must be thoroughly familiar with the properties and safety precautions before being allowed to handle hydrogen and/or associated equipment.

Full face shield, safety glasses, insulated or leather gloves, long-sleeved shirts, and pants without cuffs should be worn when working on liquid hydrogen systems. Pant legs should be worn outside of boots.

In the event of emergency situations, a fire-resistant suit and gloves should be worn. SCBA is also recommended, but remember atmospheres that are oxygen-deficient are within the flammable range and should not be entered.