Sanchem,
What my esteemed colleague is saying is that only you can know what flow your process needs/has/uses. Obviously you will need to know this flow so a suitable PRV can be sized. So assuming there is no flow meter on the liquid nitrogen line then you are going to have to work out or at least estimate what the nitrogen flow is. You are the only person that can do that because we know nothing about your process.
You may have to obtain a whole lot of data from your process so that you can get some implied flow value
For example, look at how much nitrogen your whole plant uses over what time frame. This may give you a ball park flow range.
What ever the process is that requires the nitrogen may have some pumps , pressure gauges, or even just design details that may give you some useful information. If you are using the nitrogen for cooling then maybe you can get some rough data from the change in temperature and the heat load on the system.
Then again it may be easier to put in a flow meter or a install valves on a trial and error basis. If you install a valve with known performance you may well be able to reverse engineer the system based on pressure reduction to get a flow value.
Regards
Ashtree
"Any water can be made potable if you filter it through enough money"