Hydrogen, the lightest of all the gases listed in Table 3, is produced for the most part by steam reforming natural gas and light hydrocarbon liquids over a catalyst. Metallurgical hydrogen is refined from excess hydrogen-gas feedstocks from petrochemical steam-reforming plants. The gas supplier removes hydrocarbons and moisture from the raw gas stream and liquefies the partially refined gas at -253 °C (-423 °F). Other impurities such as nitrogen and argon are removed during the liquefaction process.

Hydrogen is delivered either as compressed gas or liquid to volume and purity requirements. Commercial grades of compressed hydrogen gas, with typical pressure of 17 MPa (2500 psig), are delivered by trailer in cylinders with capacities between 1000 and 4000 m3 (35,000 and 140,000 standard ft3). Smaller quantities of compressed hydrogen usually are available in cylinders containing 7 m3 (250 ft3).

Most liquid hydrogen is delivered by trailer to liquid receivers or storage vessels with capacities between 5000 and 75,000 L (1500 and 20,000 gal). Liquid hydrogen usually is vaporized as required. One liter of liquid hydrogen yields 0.85 m3 (113.4 ft3) of hydrogen gas (or one gallon yields 133.4 ft3 of hydrogen gas). Smaller quantities of liquid hydrogen are available in 1150 L (40 gal) cylinders.

Hydrogen specifications vary, depending on the grade. Typical hydrogen is 99.999% pure. Commercial-grade hydrogen meets the following specifications:


99.995% min


8 ppm max at -68 °C (-90 °F)


1 ppm max

Because hydrogen is highly flammable, care must be taken to ground major stationary and mobile equipment to remove static electricity and the potential for spark ignition of the hydrogen. Electrical equipment must meet applicable national electric codes and fire prevention standards.

As indicated in Table 5, hydrogen is the most effective reducing atmosphere available because it has the largest hydrogen-to-water ratio. It is essentially nondecarburizing, like dissociated ammonia, to carbon steels as long as the dew point remains below approximately -40 °C (-40 °F). It is similarly inefficient at burning lubricant vapors because of a low dew point. Of all atmospheres commonly used in the P/M industry, hydrogen is the most heat conductive.

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