Hi Austin, I’m not too sure about the Nitrogen. These come from tanks, etc that you purchase and are certified so a once off OQ/PQ of the distribution and control systems should suffice. Oil, moisture content, supply pressures, leak tests, blah blah. Most of the “PQ” work would be covered and documented via the Maintenance and Calibration System in any case.
If your compressed air is externally sourced in pressurised tanks from certified suppliers, then I would imagine the same as above applies. Unless you are generating your own? If so, then I would imagine that you would treat it in identical fashion to a typical HVAC installation and use the 3-phase PQ approach, since the impact of changes in prevailing climatic conditions would need to be evaluated, and the system confirmed as being able to meet minimum quality specs throughout. Typically, short period of intense sampling without failures, followed by an extended period of slightly less intense sampling without failures while in use, followed by a 1 year monitoring period.