Hydrogen is a safe alternative to helium for gas chromatographic applications provided proper safety measures are in place. Gas generation is a viable consideration for carrier gas supply.
Although hydrogen is flammable (flammability limits are 4-74%) and combustible (explosion limits are 18.3-59%) see Fig. 1, a hazardous mixture would be very difficult to achieve in a lab with good ventilation. One reason is that at almost 45 miles per hour (20 m/s), the gas rises twice as fast as helium in air. Furthermore, most up-to-date GC systems include a safety shutoff in case of a sudden change in pressure such as when a column breaks in the GC oven.
In general, there are two main concerns when using hydrogen:
- Improper venting
- Risk of the accidental release of a large volume of gas
As hydrogen is being used as a carrier gas in GC analysis, the question to ask is: What happens to the gas that’s expended? If the detector burns all gas exiting the unit, then there is no gas to vent. However, in units where not all the gas is burned, you should consider adding a venting system.
One source of venting is the septum vent port. The typical septum purge is 1-5 cm3/min, so this flow is easy to vent with the split vent exhaust. The highest vented flow will be the split exhaust port, which can have flows of 50-500 cm3/min. This can result in a good amount of vented hydrogen and will require venting, i.e. a special fume hood. This type of hood is a good idea, both to vent the sample and the hydrogen.
Accidental gas release
Labs obtain their carrier gas supply in one of two ways: either in compressed gas cylinders or from a gas generator. The risk is that if there is a break somewhere then a large amount of gas will be released all at once, potentially resulting in an explosion.
In the case of a gas generator, the explosion risk is very low. Generators only store about 60 cm3 of gas. Furthermore, if a column or supply line were to break, the generator would stop producing gas.
The risk is greater with gas cylinders since they hold large volumes of compressed gas. Cylinders also involve long lines leading to the GC systems with hookups often at the end of benches or in other rooms, increasing the possibility of a line break. If this were to occur, all of the gas in the line would be released (as well as possibly all the gas in cylinders connected to the lines). To mitigate this risk, it’s best to follow all manufacturer-recommended installation and operating procedures — as well as to abide by all codes for safe building and installation practices. Appropriately licensed and accredited third parties should also test installations.
Figure 1: Chart showing Hydrogen Flammability Limits (in air), Explosion Limits (in air), Ignition Energy (mJ), Flame Temperature in air, and Stoichiometric Mixture (in air) as compared to Gas Vapor and Natural Gas.
Information in this post was adapted from an article by Reginald Bartram and Peter Froehlich: Considerations on Switching from Helium to Hydrogen.
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