Very different physical situation, but we had to deal with something similar when handling tanks/piping/valves etc. that had previously had a flow of flammable (or worse) hypergolic fuels in them.

We had to open the entire flow path up and flood it with nitrogen, fed from a liquid (LNO2) tank. That was the only job of the LNO2: purge and leave behind a path of nothing but inert gas that didn’t support combustion.

That was great, and “safed” the system from combustion caused by a heat source, but didn’t take care of the threat of chemically-induced combustion. There were still compounds and by-products left over from hypergolic combustion from MMH (monomethylhydrazine — you really don’t want to be around this stuff), and from the squib-fired initiators we used in case spark-fire ignition failed. Those firing charges polluted the entire system with compounds that were very excited about infighting and burning at high temperatures.

So, after a nitrogen LMO2 purge, the entire system got a brief flush of pure O2 from a LOX tank… which would very definitely show (vigorously) that’d we fucked up by burning profusely.

Even then, you couldn’t enter any of these spaces unsupervised or alone. It was called “two-man” or “no-solo” rules. You were equipped for self-preservation and self-rescue, but always had a partner far enough outside of the hazard are to observe you, report an emergency, and do an assist with help on the way.

We had a specific page in our SPM (Safety Procedures Manual): 11(3)(b) that discussed uncapping an enclosed pressurized vessel without an LMO2 purge and rubbing something around in it likely to make the fire gods unhappy. It said, “don’t.”

I just made this up because I’m bored, but for fuck’s sake, don’t do this.