Primary Process Piping
Process piping in normal, active service that cannot be valved-off or, if it were valved off, would significantly affect unit operability. Primary process piping typically does not include small bore or auxiliary process piping (see also secondary process piping).
Source: API 570, Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems, Fourth Edition, February 2016, with Addendum May 2017. Global Standards