Flammable hydrogen releases can result in deflagration and transition to a detonation. Whether the
deflagration transitions to a detonation depends on numerous parameters such as cloud size, hydrogen
concentration, confinement, and congestion. Releases into confined or congested areas are more
susceptible to generating significant deflagration over-pressures and more likely to transition to
detonation.

The potential of an explosive atmosphere is inherent with any vent system and must be addressed through adequate design. Purging for most vent stacks is impractical due to availability or cost. In addition, and particularly for LH2 systems, the purge gas can cause potential safety issues. The primary way that explosive atmospheres are addressed is through ensuring that the design of the vent system can withstand an internal deflagration or detonation. This is not that difficult for smaller systems (less than 6”) but can be challenging when vent systems are larger and/or operate more as ducting than pipe. Where the vent system can’t be built strong enough for the potential internal overpressure, purging can be a necessary and prudent safeguard.

Additionally, the amount of O2 in the vent stacks is typically small (i.e. 1.22 scf /.1 lbs. in a 3” dia/25 ft tall vent stack). As hydrogen flows into the stack the time that there is a flammable (between 4 and 74%) region within the vent stack is also small.

For a detonation there must be the correct amount of hydrogen and oxygen. In a 3” vent stack, 25 ft tall there is ~ 1.25 cu ft of oxygen at atmospheric pressure. (=.1 lbs/.0032 lbmoles). The flammable range of H2 is 4-74% H2. At the stochiometric ratio, there is ~.0064 lbMoles of H2 that can react with the O2 in the vent stack. This represents ~.013 lb of h2 that can react. This is quite small amount energy release.

Calculations

Radius – 1.5”
Piping Volume = (1.5/12)^2*3.14*25 ft = 1.22 scf
Weight – 1.22 scf/12.08 scf/lb =.1 lb
Moles - .1 lb/32 lb/lbmole =.0032 lbmoles

H2 + ½ O2 = H20
.0064+.0032 = .0064
.0064 lb moles H2 X 2lb/lb mole = .0128 lb H2

All vent stacks/systems should be bonded and grounded to minimize ignition sources. Higher pressure streams from higher velocities have a greater risk of igniting for several reasons, including particle impingement. Adding mesh could create more impact points for particulate, which would increase the potential for ignition, but would not increase the probability of a DDT. Similarly, high flow releases at high pressure can create supersonic flow leading to a shock wave. While this shock wave may be sufficient to ignite the hydrogen, mesh on the outlet will not create the conditions for a DDT.

Mesh must also be designed for substantial flow area to minimize backpressure on the venting system.

Instead of mesh, the outlet of the vent systems can include small diameter stainless steel bar (for example 1/8” – 1/4”), that does not add much turbulence and applied backpressure.

Vent systems are typically open to the atmosphere, so it’s easy to overlook that they must be designed to withstand significant internal pressure. The two primary sources of pressure within vent systems are: 1) backpressure from the flowing gas, and 2) internal deflagration/detonation.

The large flows of gas exiting relief devices and vents will create backpressure within the vent system. The backpressure often must be limited to 10% to meet code requirements for reclosing relief devices such as relief valves, but can be significantly higher for non-reclosing relief devices such as rupture discs or TPRD’s. For high pressure systems such as fuel stations that might have relief devices set as high as 1000 bar, the back pressure can be substantial. Even 10% would be 100 bar, but there might be situations with rupture discs where backpressure as high as 400-500 bar is possible immediately after activation of the device. The maximum backpressure must be calculated during the design and the vent system appropriately
designed for that pressure.

Vent systems are also subject to internal deflagrations or detonations. The vent system may be full of air prior to a vent from a relief device or manual vent. For a short period of time, a flammable mixture may form until the air is swept out by the hydrogen. Due to the high length/diameter ratio of piping, this can lead to an internal deflagration and even a detonation.

The developed overpressure is a function of a number of variables but CGA S1.3 recommends to design the vent system to withstand 300 psig and comparable EIGA documents recommend 40 bar (~600 psig). These pressure ratings are fairly reasonable for typical vent systems below 6” size, but can be challenging for larger vent headers. 

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.

The recognized and generally accepted good engineering practices (RAGAGEP) for employing a nitrogen purge into a hydrogen vent stack is that inerting is not generally used for nitrogen purge in a hydrogen vent stack because most inert gases freeze at liquid hydrogen temperatures. The vent stack should be designed for a fire and the internal overpressure caused by a deflagration. If inerting is used, it should be with helium, although a continuous purge with helium is not practical due to availability and cost. CGA G-5.5, Section 6.2, states that “[h]ydrogen vent systems do not require inerting of the vent stack or flare to ignite gases exiting the vent system. If inerting is chosen, vent stacks connected to a liquid hydrogen source shall not be inerted with any other gas than helium as other gases can solidify at hydrogen temperatures.” CGA G-5.5 Section 5.5 says a reduced L/D (length over diameter ratio) of the vent line reduces the potential for an explosion in the vent stack. 

However, Section 6.2.12 also states that “[h]ydrogen vent systems within the scope of this publication (gaseous and liquid hydrogen at user sites) are unlikely to sustain deflagration or detonations, regardless of the L/D ratios. The relatively simple geometry of the system (few turns, few tie ins) and operating scenarios are not conducive to forming detonable hydrogen-air concentrations within the system and limit potential ignition sources external to the stack discharge. In the unlikely instance that a deflagration or detonation occurs, experience has shown that a system designed for 150 psig (1030 kPa) will sustain the event without bursting.” It is important to note that the vent stack should not have an opening in the vent system that can pull air into the vent stack (e.g., an open drain connection), as this substantially increases the risk of a fire, deflagration, or detonation in the vent stack. 

The recognized and generally accepted good engineering practices (RAGAGEP) for employing a purge into a hydrogen vent stack is that inerting is not generally used. Best practice is that the vent stack should be designed for a fire and the overpressure caused by an internal deflagration. This is typically not an issue for smaller sizes (less than 4”) and when using typical materials for a vent stack (carbon or stainless steel pipe).

CGA G-5.5, Standard for Hydrogen Vent Systems, Section 6.2 states that “Hydrogen vent systems do not require inerting of the vent stack or flare to ignite gases exiting the vent system.” Section 5.5 says a reduced L/D (length over diameter ratio) of the vent line lowers the potential for an explosion in the vent stack. However, CGA G-5.5 also states in Section 6.2.12, “Hydrogen vent systems within the scope of this publication (gaseous and liquid hydrogen at user sites) are unlikely to sustain deflagration or detonations, regardless of the L/D ratios. The relatively simple geometry of the system (few turns, few tie ins) and operating scenarios are not conducive to forming detonable hydrogen-air concentrations within the system and limit potential ignition sources external to the stack discharge. In the unlikely instance that a deflagration or detonation occurs, experience has shown that a system designed for 150 psig (1030 kPa) will sustain the event without bursting.” EIGA Doc 211/17, Hydrogen Vent Systems for Customer Applications, specifically does not cover inerted vent systems in Section 2. It does require the vent stack design to withstand the maximum peak pressure created by detonation unless an inert gas is used for continuous purging. EIGA requires a vent stack design pressure minimum of 40 bar for a maximum detonation pressure of 120 barg (section 5.5). It is important to note that the vent stack should not have an opening in the vent system that can pull air into the vent stack (e.g., an open drain connection), as this substantially increases the risk of a fire, deflagration, or detonation in the vent stack.

Most gases that might be used to purge a vent stack (where it might be applicable) can also potentially freeze and could form a blockage when used on vent systems for LH2 equipment. Care must be taken to prevent this possibility. Purge vent systems can also create back pressure considerations on relief devices and must be adequately designed when used.
 

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 National Fire Protection Association (NFPA), the Compressed Gas Association (CGA), and the Society of Fire Protection Engineers (SFPE)   represent the U.S. fire protection and engineering community, and these organizations publish handbooks and standards/guidelines that describe the properties of hydrogen. There are many other organizations and documents that provide similar information. The Center for Hydrogen Safety within AIChE has training material for the properties and safe handling of hydrogen at the following link:

Fundamental Hydrogen Safety Credential