Inerting for Pressure Vessel ?

[pramep]

This time, I must calculate the N2 supplied for pressure vessel.
Can you tell me how to calculate it ?
( I confused with API-2000. It seem to use to supply the inerting gas with Asmospheric Pressure Vessel but in the case of
pressure vessel (the vessel which has pressure more than 15 psig), how can to calculate amount of inert gas ??)

[gtaa]

If you know the volume of the vessel, and you know the pressure you want to inert it at then a simple P1V1 = P2V2 tells you how much volume you need to do it. Common practice for purging hydrocarbons from a vessel is to fill it and empty it 3 times.