Are the pockets of 20-35% of hydrogen in air potentially ignitable mixtures? The project in question is concerned with predicting the outcome of hydrogen air mixing in a small area (6 inches) of pipe. Process gas flows at a max of 21 scfm and consists of hydrogen primarily and some hydrocarbons that need to be treated in an incinerator. The process flows some 30 feet in a 3-inch pipe. To enable combustion, air is needed to premix with the hydrogen to enable the hydrocarbons to oxidize. Since the air and hydrogen need to be pulled into the burner, downstream of the burner is a large air blower that creates a suction (-6 inches of water) in the air and process line. The air is brought in an 8-inch pipe. The hydrogen transitions from 3 to 4 to 8 inches and mixes with air. That transition area is where the model shows mixing occurs and there are pockets of 20-35% of hydrogen in air. All the pipes are open, with one leading to the burner and the other leading to the process source. The pipes are capable of handling 1200 psi when newly installed.

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.