Chemical Symbol:    H2
DOT Proper Shipping Name:     Hydrogen, Compressed
Hydrogen, Refrigerated Liquid
Classification: 2.1 (Flammable gas)

  U.S. Units      SI Units      
Chemical formula: H2 H2
Molecular weight: 28.01 28.01
Specific gravity of the gas at 70oF (21.1oC)and 1 atm (air=1): 0.967 0.967
Density of the liquid at boiling point and 1 atm 50.47 lb/ft3 808.5 kg/m3 :    -232.4oF -146.9oC
Triple point at 1.81 psia or 12.5 kPa, abs: -346.0oF -210.0oC



Hydrogen is colourless, odourless, tasteless, flammable, and non-toxic. It exists as a gas at ambient temperatures and atmospheric pressures. It is the lightest gas known, with a density approximation of 0.07% that of air. Hydrogen is present in the atmosphere occurring in only concentrations of about 0.5 ppm by volume at lower altitudes. Hydrogen burns in air with a pale blue, almost invisible flame. Its ignition temperature will not vary greatly from the range 1,050oF to 1,074oF (566oC to 579oC) in mixtures with either air or oxygen at atmospheric pressure. The flammable limits of hydrogen in dry air is at atmospheric pressure are 4.0% to 75.0% hydrogen by volume. In dry oxygen at atmospheric pressure, the flammable limits are 4.6% to 93.9% hydrogen by volume. Its flammable limits in air or oxygen vary somewhat with pressure, temperature, and water vapour content. When cooled to its normal boiling point of -423oF (-253oC) and condensed, hydrogen becomes a colourless liquid only one-fourteenth as heavy as water. All gases except helium become solids at the temperature of liquid hydrogen. Because of its extremely low temperature, it can make ductile or pliable material with which it comes on contact brittle and easily breakable (an effect that must be considered whenever liquid hydrogen is handled). Liquid hydrogen has a relatively high thermal coefficient of expansion compared with other cryogenic liquids.





Large quantities of hydrogen are produced on site or pipelined for use by refineries, petrochemical and bulk chemical facilities for hydrocracking. Smaller quantities of hydrogen are produced on site or pipelined for use in the chemical, metallurgical, fats and oils, glass, and electronic industries.

Some of these smaller users have hydrogen delivered to their manufacturing location as gaseous hydrogen in cylinders or tube trailers, or by cascade into on-site storage cylinders. Certain smaller users have liquid hydrogen delivered into an on-site liquid storage system.




Hydrogen is used in the production of a wide variety of chemicals:


Dyes- in the manufacture of derivatives of aniline produced from nitrobenzene and derivatives of toluidine produced from nitrotoluene.


Catalyst-to produce aluminium alkyls, cobalt compounds, metals hydrides, and nickel, as well as a wide variety of others.


Flavours and fragrances- used to make many flavours and fragrances, which are generally compounded from a few chemical intermediates. Pesticides-used in manufacture of complex, proprietary chemicals, several of which are known to require hydrogenation.


Halogen organics- to manufacture a variety of chemicals used in organic synthesis such as dibromobutane, pentachlorobenzene, or fluorocarbons.


Plastic and synthetic fibres used for the production of high-density polyethylene and polypropylene and also used in the production of intermediates for polyurethane, nylon, and polyamide fibres.


Specialty chemicals- used in the production of a variety of specialty chemicals including intermediates in the production of high-energy rocket fuels and other unique applications.


Petroleum- used to hydrogenate various unsaturated petroleum products.





Metallurgical companies use hydrogen in the production of their products:




Heat Treating- Ferrous metals are treated under controlled atmospheres to change their physical properties. Protective atmospheres are used primarily to exclude oxygen and prevent oxidation of the metal at elevated temperatures in the treatment furnace.


Stainless Steel are usually blanketed with the enriched hydrogen atmosphere because the contain chromium, titanium, and other metals, which are reactive with nitrogen at high temperature. Metal production-Basic metal refiners of tungsten, molybdenum, and magnesium using hydrometallurgical processes require large volumes of hydrogen to reduce oxides and prevent oxidation of the metal. It is also used in processing nuclear fuels.


Welding and Cutting- Hydrogen is used with oxygen in oxyhydrogen welding and cutting being used in certain brazing operations. It is also used for welding aluminium and magnesium (especially in thin sections) and for welding lead. The oxyhydrogen flame has a temperature of about 4,000oF (2,204oC) and is well suited for such comparatively low-temperature welding and brazing. It is also used to some extent in cutting metals, particularly in underwater cutting because hydrogen can be safely compressed to the pressures necessary to overcome water pressures at the depths involved in salvage operations. The oxyhydrogen flame is also applied in the working and fabrication of quartz and glass.


Atomic Hydrogen Welding, another important application, is particularly suitable for thin stock and can be used with practically all nonferrous metals and alloys, as well as with ferrous alloys. Food companies hydrogenate fats, oils, and fatty acids to control various physical and chemical properties. Both edible and inedible fats and oils are hydrogenated, resulting in an increase in melting temperature and in improvement in colour, odour, and the stability of the material. Hydrogenated edible oils are used in oleomargarine, shortening, and other food products. Inedible oils and fatty acids that are hydrogenated are used in the production of soap, industrial greases and oils, surfactants, and plasticisers. Edible oils are reacted with low pressure (20 psig to 50 psig; 138 kPa to 345 kPa) hydrogen, while inedible oils are reacted under high pressure (300 psig to 500 psig; 2,070 kPa to 3,450 kPa). Both reactions involve bubbling hydrogen through oil in the presence of a nickel catalyst.






Regulator and Control Valve


Contrary to general practice with other gas cylinders, it is inadvisable to partially open, that is, “crack” hydrogen cylinder valves before connecting them to a regulator or manifold since self-ignition of the issuing hydrogen may occur.






All the general rules for compressed gas cylinders apply, but extra precautions are necessary in the handling hydrogen gas. The cylinder valve should be cracked before fitting the regulator (the issuing gas match fire). After attaching regulator and before the cylinder valve is opened, see that the regulator is closed and the adjusting screw is turned out. Then open the cylinder valve slowly with the valve outlet pointing away from you. Fully open the valve (to minimize self-ignition) when the cylinder is in use (connected into a piping system).






Specific requirements for storage of hydrogen are contained in NFPA 50A, Standard for Gaseous Hydrogen Systems at Consumer Sites, and NFPA 50B, Standard for Liquefied Hydrogen Systems at Consumer Sites.


Storage banks, both portable and stationary, are to be inspected periodically for corrosion of containers and supports, condition of pressure relief devices, proper operation of shut-off valves, and condition of manifold piping. A leak test with soap solution or other appropriate leak detection equipment should be performed periodically at maximum operating pressure. Storage banks of DOT.TC cylinders should be checked for the last hydrostatic test date to ascertain that the cylinders are within the allowable time period between tests. Cylinders in these banks must be retested every 5 years. DOT/TC cylinders must be filled and periodically retested in accordance with appropriate regulations. The conditions of pigtails should be checked, and worn or damaged ones replaced.






As with other cryogenic liquid, cryogenic hydrogen is not to be used or handled by any personnel unless they are familiar with its properties and skilled in the procedures necessary for its safe use. Cryogenic liquids other than oxygen, which vaporise in enclosed areas, may reduce the oxygen content in the atmosphere below the level necessary to sustain life. Adequate ventilation must be provided in areas where the materials are being vaporised. The oxygen should be checked at regular intervals for oxygen content.


The following are several precautions that need to be observed during transfer operations:


1. Only personnel thoroughly acquainted with liquid hydrogen properties, equipment, and operating procedures should be permitted to perform transfer operations.

Transfer hoses in liquid hydrogen service must be purged with helium or gaseous hydrogen before using.


2. Hydrogen tankers must be adequately grounded during loading and unloading operations.


3. Hydrogen transfer operations should be discontinued during thunderstorms.


4. Containers for cryogenic liquid must be kept clean and restricted to this service. These containers must e made of Type 300 series stainless steel, copper, brass, aluminium, etc., except for special laboratory equipment made of glass.







Leaking hydrogen cylinders should be handled with special care. If hydrogen leaks from the cylinder valve even when the valve is closed, or if a leak occurs at the pressure relief device, carefully remove the cylinder to an open space well away from any possible source of ignition. Plainly tag the cylinder as having an unserviceable valve or pressure relief device, and immediately notify the cylinder supplier and ask for instructions.


Extreme care is recommended in protecting access to the defective cylinder because the leaking hydrogen may even ignite in the absence of any normally apparent source of ignition. It will burn with an almost invisible flame that can instantly injure anyone coming into contact with it. The presence of a hydrogen flame can be detected by approaching with a straw broom outstretched in front to make the flame visible.


Should hydrogen ignite, let it burn, keeping the surrounding cool with water spray to prevent the spread of fire, but do not spray water on the vent stack. By doing this, an accumulation of the gas that might lead to an explosion can be avoided.



Caution: Water should never be directed on the vent system of a bulk liquid facility because of potential plugging as a result of ice formation.