Using tools inside a fume hood that may have a flammable gas mixture should be prohibited. A properly operating hood of the right capacity should keep the mixture of hydrogen in air inside the hood below the Lower Flammability Limit (LFL) of hydrogen further reducing any risk. 

If the use of tools is necessary, the source of hydrogen should be isolated before the work begins even if the concentration of hydrogen is expected to be below the LFL. It is best practice to leak test equipment before introducing hydrogen to minimize the probability of leaks. If spark resistant tools are suitable for a specific task for working with hydrogen systems, use of such tools will lower the probability of producing a spark. 

The key concern with any hydrogen release is the risk of creating a flammable mixture. There should be no environmental issues if you properly vent hydrogen to a safe area where it is diluted in air below the flammability limit before contacting an ignition source. Very small quantities of hydrogen are frequently releasing into a fume hood. Releases have to be small enough so that the vent air is sufficient to dilute to below the lower flammability limit. The fume hood face velocity should be in excess of 100 ft/min (30 m/min). 

Larger quantities of hydrogen should be released through a properly designed and constructed hydrogen vent stack. Standards and codes such as CGA G 5.5 and NFPA 2 provide guidance for vent stack design and installation.

If the concentration of hydrogen is less than the Lower Flammability Limit (LFL) of 4% in an inert gas, it is unlikely that a leak of this gas mix will form a flammable mixture as it dilutes into air. For example, industry uses ‘forming gas’, a mixture of 4 to 5% H2 in nitrogen, as an oxide reducing agent in materials processing furnaces and soldering operations. This mixture can also be used in conjunction with a hydrogen detector for leak testing gaseous hydrogen equipment.

In laboratories, 316 stainless steel tubing is frequently the first choice for small flow and pressures less than 2800 psi (19 MPa). See Best Practices: Material Compatibility for hydrogen compatibility with various materials. Always work within manufacturer’s pressure ratings adjusted for temperature. Read and follow manufacturer’s instructions on making up tubing fittings, avoid threaded fittings, and properly purge with nitrogen, helium or other inert gas to remove air before introducing hydrogen.

We are not aware of a study for blended NG/H2. However, for high concentrations of NG, the vent system should be similar to NG, which still recommends a vent system as NG is less dense than air. For nearly pure hydrogen the recommendations of this presentation are in effect.

This is an interesting question because of the special considerations associated with the H2-air mixture flashback scenario during turbine startup and shutdown. In-line deflagration arresters and detonation arresters for H2-air mixtures are different than those for most other fuels and need to be certified for use with hydrogen. They are commercially available, and NFPA 69 has a good chapter on the different types and ratings of flame/deflagration/detonation arresters. Some companies and universities are working on hydrogen diffusion flame burners for turbines. 

The issue in this case is the stability of the hydrogen diffusion flame on the particular burner configuration, and the possible need for flame holders to prevent flame blowoff, particularly if the same burner is used for different fuels. Recent research literature indicates that the Germans are doing a lot of research in this area. It seems that starting with a very lean mixture and ramping it up slowly after combustion would be the best method. 

NFPA provides procedures for furnaces that may help. Some of these furnaces use pure hydrogen and startup/shutdown procedures may be useful in beginning the investigation. There is an article by Siemens that may help (Advanced Hydrogen Turbine Development, Ed Bancalari, Pedy Chan and Ihor S. Diakunchak, Siemens Power Generation Inc.) This article is for a blended hydrogen gas (from coal) not pure hydrogen. For pure hydrogen, there is a way to safely start the combustion chamber up, but the Panel is not aware of sequence, as many items, such as combustion chamber geometry, burner, air flow, etc., all effect the startup. Start with NFPA 2 furnaces and work from there.