I want to pump LOX into a pressure vessel at 2 bar (say). So for pumping it what should be safest flow rate?
I probably shouldn't answer this question, then again I can't vote to close because it is to broad and ambiguous, so I will try and give an suitable answer to help the OP out.
First aspect: What is the propellant?
Different propellants can be piped at different rates mostly related to their chemical volatility. If your propellant was water for instance, you could push that as fast as it isn't sawing your tank in half.
Second aspect: What are diameters of your hose and ports?
If you have a wide enough port you can fill your tank in 5 secs.
Third aspect: What is the overall engineering of your system?
This sort of relates back to the first two aspects but is meant more encapsulate all the other factors that could result in explosions. Say you are piping liquid O2. You could pipe it as fast as you wanted provided you kept it cold, the pipe from exploding, and eliminated any chances for it reacting with something like hydro-carbons. Usually though, extremes of this aren't pursued as they become economically infeasible and simply not worth the effort.