aanita:
This is a vessel purging application. It can be continuous or batch operated. It does not involve calculation of the gas quantity needed.
What is needed is an experienced indication - either empirical or recommended - regarding the quantity of vessel volumes or nitrogen needed to purge out the vapors or gas originally in the vessel down to the level you require. We don't know what the parameters are.
As Saml has indicated, you can use a 3:1 ratio of purge volume to vessel volume - but that is no guarantee that you will achieve what you need or want (which you haven't defined). A lot depends on the location of the injected purge, the method used, and the the particular gas or vapor in the vessel as well as the vessel's geometry and configuration. If this is a homework assignment or class problem, the nature and specifics of the purging operation should have been explained by your instructor or professor. If not, shame on them. As a student, you should always question any operation specified or mentioned by your instructor that hasn't been studied or fully explained.