You will perform PQ of Pure Steam Generator(PSG) only once. You might as well do revalidation and requalification studies after some period of time or incase you have a problem depending up on Validation Schedules you ink in your Validation Master Plan (VMP).
You will always use purified water or even WFI in some casses. Before you start to validate your PSG your water Generation systems must pass atleast 2nd Phase of their validation duration. This would ensure what WATER you feed into your PSG is as per specified standards.
Initially during qualification procedures you will also mention the type of water you feed into PSG & also mention the quality attributes that particular feed must have.
Unless you are sure that Feed(Water) meets the standards you must not start PQ of PSG.