Hello,
"Can someone tell me whether the pressure range is critical for adiabatic compression of oxygen in the pipe to happen?" - Yes.
"I checked the design and the only thing that I can deduce is that the smaller size pipe can create high velocities" - correct, as an example: with 6 mm tubing you get sonic/ near sonic velocities during filling of cylinders. Smaller hose/ pipe diameters gives you a massive velocity increase depending on the pressure differential.
Now for most of other common uses, standards (like CGA G-4.4) dictate velocities based on the pressure range, and whether the piping is an impingement site or not. At 6 barg the CGA G-4.4 states a velocity of 30 m/s at a working pressure of 0.6 Mpa, or 6 bar, at impingement sites. For non-impingement sites the speed is 60 m/s at that pressure rating.
Now when you say "client's hose", is it a metal hose or a flexible hose with reinforced fluoroplastic tubing? PTFE is quite common, as well as PFA. Metal hoses are more prone to ignition. Properly cleaned 316L tubing is safe to use with oxygen, even at sonic velocities at 200 bar.
"So do I have adiabatic compression or exceeding of the impingement velocity with the presence of impurities in the hose (normally we use our own hose but this time it was too short so we took one from the client - probably mistake number 1)" ayayay. Yes that was a mistake. Never use a non-certified hose for oxygen systems.
Now when it comes to oxygen you have many potential ignition mechanisms:
Particle impact - most likely. A minor contaminant is more than enough.
Heat of compression - not likely, but a small contributing factor.
Flow friction - not likely.
Mechanical impact - not likely unless a nonmetal got ignited.
Fresh metal exposure - not likely unless there is aluminium or titanium present in the system.
Static discharge - not enough info to base an opinion, but not that common.
Chemical reaction - not enough info to base an opinion, but not that common.
Thermal runway - not enough info to base an opinion.
Resonance - rare but possible.
External heat - Possible.
There are more but these are the ones often listed. In all of my years working the most common causes is: particle impact and heat of compression acting in unison. A small contaminant is enough to start an ignition, and there you go; a kindling chain. Now, I must state that particle impact is a lot more likely in high velocity systems, and where the impact point is 45 degrees to perpendicular to the path of the particle.
I would look at the materials in the system, possible contaminants and their flammability ratings. Consider the total heat that can accumulate in the system (see ignition mechanisms) and relate that to autoignition temperatures and heat of combustion of the materials (metals and nonmetals) in the system. Note that many ignition mechanisms can exist in the same system, and a kindling chain assessment is therefore needed (the ability of ignition to propagate with a component or system).
Now there could be other reasons as well, depending on factors like oxygen concentration, the temperature of the oxygen, the possibility of LPG backflow into the oxygen distribution system (A client had that once, ugly business.) Not sure if this helps you, but there is, like you say, a certain difficulty in figuring out the reasons for an explosion after it happens.
