How can I evaluate the risk of deflagration to detonation transition (DDT) inside equipment? My particular concern is a fire tube boiler. The scenario to be considered is a hydrogen air mixture being fed to the boiler that was not ignited until the mixture contacted an oxygen analyzer in the exhaust stream.
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.