Can you offer guidance on the circumstances (such as H2 mass flow, outlet velocities, etc.) under which a thorough assessment of vapor cloud explosions above vents is deemed necessary?

Several organizations published a paper together on this topic in 2017 (see attached). Based on comparisons with tests and CFD simulations, the following conclusions were drawn:

1. The gas concentration for vapor cloud explosion blast load calculations for H2 jets can be limited to approximately 10% to 75%. Note that testing for H2-air VCEs in congested environments has been performed by organizations such as Baker Risk and concluded that 10% is the lowest H2 concentration that needs to be considered. This published this as well.
2. For ignition of the H2-air jet at 30%H2, a mass release rate of about 0.5 kg/s is needed to get above a TNO Muli-Energy Severity Level of 4 (i.e., where VCE blast load perspective starts getting significant with a maximum overpressure of 0.1 bar). This corresponds to a flame speed of about 100 m/s and is shown in Figure 13 of the attached paper.
3. Ignition of the H2-air jet at 60%H2 (worst-case ignition location) requires a mass release rate of about 0.1 kg/s (100 g/s) to get above a TNO Muli-Energy Severity Level of 4. More testing on this has been done and is being done, so these might get refined in the future, but it is not expected that there will be major changes in the “threshold” mass release rate
   needed to produce a jet that (if ignited) can represent a VCE hazard. Of course, the blast loads from a hydrogen jet won’t extend a long distance because the explosion energy (i.e., flammable cloud size) is limited compared to traditional VCE cases (e.g. where a large flammable cloud fills all of a refinery process unit). Lastly, if a facility owner defined a hazard level of concern (e.g., greater than 0.5 psig at 100 feet), then a mass release rate of concern could be calculated.