We are going to shut down a circuit and purge the vessels with N2. How to calculate N2 usage for purging? Thank you in advance.

What is the objective? Usually it's to reduce oxygen below a certain concentration.

Hello Latexman, thanks for the response.

This is a turnaround work related issue. So, we are opening up the vessels for inspection.

xyz,
You are not purging with nitrogen to create the right environment for vessel entry. In fact, the resulting atmosphere is lethal since life-supporting oxygen would not be present. Rather, you are purging with nitrogen probably to displace a potentially explosive atmosphere of combustible vapors. Only you (and your coworkers) could know what your objectives are and therefore YOU must establish the design basis for purging. As previously said,

Dear xyz Hello/Good Afternoon,
May I suggest my little bit of experience from previous employer.
It has been practical customary

to have Hydrocarbon system's pulled with vacuum subsequently broken with Nitrogen gas(usually a minimum of three such ) purgings if you like to call that way.

thereafter as per actual explosivity checks further vacuum pulling and breaking the vacuum with nitrogen gas. Aim is to get rid off explosive environment alone;
Wheras for vessel entry*

separate (breathable air purging is required till the internal environment becomes safe for human entry and fit for sustainable stay (i.e. oxygen % age between19.5% plus to 21.5%).Even then initial entry takes place with SCBA or supplied air full masks.

Reciprocal is true when decommissioning

i.e. pull vacuum and break with nitrogen (minimum three times or more) so as to remove the excess oxygen%

bringing it below 2% level to avoid explosive mixture formation possibilities and there after commissioning in live hydrocarbon service.
Best regards
Qalander
* Fatal incidents are reported with nitrogen purged vessels se attached file from CSB