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.

1.    As of January 2024, we are not aware of any public data on incidents or investigations where a hydrogen fired steam boiler exploded.

2.    The potential for detonations within a boiler tube would depend on both the equivalence ratio of the hydrogen present and the diameter of the boiler tube.
a.    At a minimum, if the circumference of the tube is smaller than the detonation cell size, then a detonation cannot propagate in the tube (experimentally, the critical diameter might be significantly larger).
b.    If you have a very large diameter tube, but the concentration of hydrogen is below the limit for fast flame acceleration (something like ~10-12% vol. H2), then the hydrogen-air mixture cannot run-up to detonation.

3.    There’s a good database of detonation cell sizes and critical tube diameters at: https://shepherd.caltech.edu/detn_db/html/db.html
a.    Here’s an example of critical tube diameter data for hydrogen-air mixtures: https://shepherd.caltech.edu/detn_db/html/H2-Air11.html
b.    Here’s an example of detonation cell size for hydrogen-air mixtures: https://shepherd.caltech.edu/detn_db/html/H2-Air1.html

4.    For information on pressure loads in tubes resulting from a detonation, there’s information in NFPA 67.
a.    The peak pressure would be related to the CJ detonation pressure of the mixture that forms.  Not applicable to a fire tube boiler, but for other geometries there could be regions where pressures significantly higher than the CJ detonation pressure could develop due to shock reflection at end caps/elbows.
b.    The pressures would be significantly higher in the region where the deflagration transitions to a detonation.
c.    The CJ detonation pressure of a mixture can be calculated with tools like the Shock and Detonation Toolbox: https://shepherd.caltech.edu/EDL/PublicResources/sdt/
d.    Even without a detonation, a fast flame propagating within a tube can generate maximum pressures on the order of the constant volume explosion pressure of the mixture, which can be estimated by a chemical equilibrium solver like Cantera or NASA CEA.

5.      For the pressures where a DDT occurs (i.e., where the pressure can be significantly higher than the CJ pressure), we have seen this in incident investigations, and put out a paper illustrating this. These loads extend over several pipe diameters and have significant associated impulse (i.e., the structure containing the mixture is likely to respond to the peak pressure).
Geng, J. and J.K. Thomas (2012) “Pressure Distribution Inside Pipes Due to DDT,” PVP2012-78590, ASME 2012 Pressure Vessels and Piping Conference, Toronto, July 15-19, 2012.

6.      If you fill the boiler with an H2-air mixture, a DDT can occur.  A fairly applicable example would be a test we ran at very low congestion, which may be representative of the congestion in a fire tube boiler, within our DLG test rig 48 ft long x 24 ft deep x 12 ft high (15 m long, x 7 m deep x  4 m high), with one long face open as a vent.  We got a relatively strong deflagration at 20%H2.  We got a DDT at 22.5%H2.  A paper describing these tests:
Horn, B.J., O.A. Rodriquez, D.R. Malik and J.K. Thomas (2018) “Deflagration-to-Detonation Transition (DDT) in a Vented Hydrogen Explosion,” 14th Global Congress on Process Safety (52st Loss Prevention Symposium), AIChE Annual Meeting, Orlando, FL, April 22-25, 2018.

7.    The DLG tests described above were performed with the entire test rig filled with a relatively uniform and quiescent mixture.  In an accidental scenario, the boiler could have a non-uniform concentration and, depending on the scenario, only a portion of the boiler may be filled with a flammable mixture.  In this case, we would normally turn to computational fluid dynamics (CFD) analysis using the FLACS code.  We have developed a criterion for evaluating the FLACS results to determine if a DDT would occur.  An example of the application of this approach for a H2-air explosion within a vaporizer set is described in:
Thomas, J.K., J. Geng, O.A. Rodriquez, et al. (2018) “Potential for Hydrogen DDT with Ambient Vaporizers,” Mary Kay O’Connor Process Safety International Symposium, College Station, TX, October 2018.
Relative to the point above, please note that some experts do not concur with using FLACS for DDT analysis. That being said, we have gotten a reasonable match to our VCE test data using this approach.

8.    Relative to natural gas fired steam boiler failures due to internal explosions, some work we did relative to reformers is somewhat applicable, although we did not establish the type of frequency information he is looking for:
Maxwell-Shaffer, D.F., A.G. Sarrack and J.K. Thomas (2014) “Unusual Reformer Events and Modeling,” 2014 AIChE Safety in Ammonia Plants and Related Facilities Symposium, Vancouver, September 2014.

See attached files for several references.

Yes, these would be ignitable mixtures.  In this case, it does not appear complicated geometry is involved, so 1200 psig pipe should be more than adequate to protect against internal deflagration. The most likely scenario is a "backfire," similar to a car, where ignition occurs too soon and shoots out the open end of the pipe. Consider using an inline deflagration flash arrestor on the supply line to protect upstream piping and equipment. Also, make sure pipe welds, fittings, and instruments have a comparable rating. The pipe needs to withstand an over-pressure resulting from an ignition at the mixing point of H2-O2. This is the issue that necessitates a high pipe pressure rating, preferably high enough to withstand a detonation.

The nominal H2-air detonation peak pressure is about 10-20 times the initial pressure, but there are also possible reflected pressures, a comfortable margin is needed to withstand shock wave pressures above the nominal detonation peak pressure. It’s possible for reflected pressures to get a little higher, but these generally remain within 20X. If a pipe is at atmospheric pressure, 300 psig will result in 20X the pressure, so that is the basis for the 1200 psig pipe having sufficient margin. 

The lesson learned (LL) article referenced in the question cites an incident that occurred in December 1969. While there may have been other accidents, the HSP does not have any other LL articles on alkaline water electrolysis explosions. In the LL article that was updated in 2017, the technology described employs a potassium hydroxide (KOH) electrolyte solution. The KOH electrolyte is held by surface tension within a fibrous mat used as a cell separator. A 30 weight % KOH solution is typically circulated through each of many electrolysis cells internally manifolded together within a bipolar electrolysis cell stack. Product hydrogen with KOH solution rises into an overhead degassing tank (drum in the LL article); product oxygen with KOH solution rises into a separate overhead degassing tank. 

The article describes a breakdown of the separator allowing H2 and O2 gases to mix. Slight differences in pressure between the oxygen and hydrogen sides of the cell are enough to permit crossover of gases. A recent Gangwon electrolysis incident involved a semipermeable alkaline membrane technology instead of a fibrous cell separator. Conditions at near idle current may have allowed sufficient cross cell diffusion to occur, permitting an explosive mixture to collect in the storage tanks with the result reported. This event has not yet been published in our Hydrogen Tools LL database.

There is limited published research on the effect of water sprays on hydrogen deflagrations and deflagration-to-detonation transition, and more extensive data on water spray effects on hydrocarbon gas explosions. The results show the benefits, where there are benefits, to be highly scenario dependent. For example, Carlson et al. (Atomics International report, 1973) described hydrogen detonation tube testing with and without a 500-micrometer drop diameter water spray. They found the water spray prevented detonations at hydrogen concentrations of 20% to 24%, but not at 28%. Drop size is critical since the benefits of sprays depend on the droplets breaking up and partially vaporizing in advance of the propagating flame front. If the droplets do not break up, they induce turbulence effects that can significantly increase flame speeds and exacerbate hydrogen deflagrations. Both the detonation ability and deflagration scenarios are discussed in Technical Aspects of Hydrogen Control and Combustion in Severe Light-Water Reactor Accidents, National Academy Press, 1987. 

The effects of water deluge nozzle spray on methane and propane deflagrations in vented simulated offshore platform enclosures were investigated in Phase 2 (1998 report) of a European test program called Blast and Fire Engineering for Topside Structures. Comparisons of test results in highly obstructed enclosures with and without the deluge spray showed lower pressures with the deluge spray. However, the sprays did increase the initial flame speeds, which can be either good, if the flame induced velocities are large enough to cause droplet breakup, or bad, if the droplets remain intact. The authors conclude that deluge sprays prevent runaway flame acceleration in situations with inherently high flame speeds but will not be effective in applications where only low flame speeds are generated. Thus, the spray would be expected to be effective for near-stoichiometric H2-air mixtures, but detrimental for concentrations in the range of 8% to perhaps 13%.

No, but it is always necessary to determine the possibility of an adverse chemical reaction with the particular material being used for the mesh.

I am communicating with a company that is exploring this technology for an application involving a mixture of flammable gases, including hydrogen.

The Panel has not received such inquiries. Section 14.2 of NFPA 69 Standard for Explosion Prevention Systems covers foam and mesh requirements. NFPA 69 states in 14.3.4 that the tests shall be conducted with a flammable gas/air mixture with a fundamental burning velocity representative of the burning velocities of flammable vapors expected in the intended applications.

Explosion testing with hydrogen should be utilized only where there is not an established alternative and then only by personnel experienced in such testing. 
Testing with hydrogen is always a challenge and needs to be approached carefully due to significant differences in properties between hydrogen and propane. Hydrogen can develop significantly higher overpressures and preliminary testing with leaner mixtures and possibly smaller containers should precede full-scale application tests. Documents such as NFPA 2, NFPA 68, and NFPA 60 provide additional guidance on the potential explosion hazards and properties of hydrogen.

There are two parts for such a system to be effective. First, the system would have to activate quickly enough to establish a water mist throughout the region of interest (i.e., region occupied by a flammable gas mixture) before it could be ignited. This is challenging in terms of timing, and the impact of spraying water inside an enclosure filled with equipment not designed to get wet can be an issue. Second, such a system has to provide a sufficient density of water droplets in the right size range to have the desired effect. For conventional vapor cloud explosions (e.g., large vapor cloud occupying a portion of a process unit at a refinery or chemical plant), experimental work has examined the effect of water sprays. Getting a high enough density of the right sized water droplets has been shown to be effective at slowing the flame front, and hence reducing the resulting blast load. The goal has not been to put the flame out (i.e., quench the flame), as this would be much more difficult, but rather to absorb some of the combustion energy and hence slow the flame. 

The same type of research has been done for mine explosions in the U.S. For a hydrogen-air mixture inside an enclosure, it is theoretically possible to use a water spray (from a fast-acting system triggered on detection) to slow down the flame. This could reduce the demand on an explosion venting system or reduce the structural capacity required for a given vent system. There are a number of recent publications on this topic (i.e., slowing the flame front propagation velocity, reducing the maximum blast pressure, and/or extinguishing the flame for hydrogen or mixtures containing hydrogen). However, the HSP is not aware of any such system for hydrogen explosion prevention or mitigation in industrial applications being available for purchase.