Hi Ask. There's two basic ways this happens (that I can think of off the top of my head):
- Impact: Particles in the flow stream impact a stationary part internal to the system, especially where there is a change in direction such as inside valves or at T's or elbows. The impact creates a localized high temperature that can ignite.
- Adiabatic compression: Sudden pressurization of an oxygen pipe line will cause the oxygen to be compressed isentropically anywhere there is a 'dead end' inside the piping system. The very end of that dead end is where the highest temperature will be, and it is there that a fire can ignite.
There are methods to mitigate these risks such as using materials that either can't ignite or are highly resistant to ignition. Obviously keeping the system as clean as possible and removing any kind of hydrocarbon is very important as well.
The Compressed Gas Association (CGA) produces standards that are used for these types of systems. The one you might want to get your hands on is CGA G-4.4 "Industrial Practices for Gaseous Oxygen Transmission and Distribution Piping Systems". Check the CGA web site.