The correct answer is A because applying vacuum first is more efficient and reduces the amount of inert gas required , as explained in CCPS guidance on inerting and purging methods.
In a vacuum-first approach , a portion of the vessel’s original gas (containing oxygen) is removed before introducing nitrogen. This immediately lowers the oxygen concentration without consuming inert gas. When nitrogen is subsequently added, it starts from a lower oxygen baseline, meaning fewer purge cycles and less total nitrogen are required to reach the target oxygen concentration.
In contrast, if pressure purging is applied first , nitrogen is added to a vessel still filled with air, and oxygen reduction occurs through dilution. This requires more nitrogen , since each cycle only partially reduces oxygen concentration.
Option C is incorrect because the sequence does affect efficiency, and B is incorrect because pressure-first is less efficient.
CCPS emphasizes that combined vacuum/pressure purging is the most efficient inerting method , but the sequence matters. Starting with vacuum maximizes oxygen removal before dilution, minimizing inert gas consumption and improving operational efficiency, especially for large vessels.
