Hello Kailash Bharadwaj,
Non Return Valve (NRV) allows flow of fluid ( Gas or Liquid) in only one direction.
When a fluid leaves a nozzle it prevents the reverse flow.
NRV's are common in pipe lines carrying Gas or Liquid.
Interestingly human heart also has valves that are unidirectional.
It is an amazing creation of Nature.
During welding flame is required only at the end of a welding torch.
The flash back's ( flame arrester's) purpose is to see that flame does not go back to the gas source.
If the flame goes back it will create an explosion.
The enclosed attachment has two slides and illustrates all the above.
V.Raghunathan