Loop Folder.

A loop folder is the complete dossier for a single control loop, the place where every document that proves that loop was specified, installed, calibrated, tuned, and, for safety loops proof-tested is gathered under one tag. It typically holds the relevant P&ID extract, the loop diagram that traces the field-to-system signal path, the transmitter and final-element datasheets, the calibration certificate, the commissioning sign-off, the tuning record, and, for an SIS loop, the proof-test procedure and the accumulating proof-test reports. It is built by the commissioning team as the as-commissioned record, maintained by the operating company for the life of the loop, updated through management of change, and pulled by auditors, which is why an SIS loop folder grows over time into a stack of proof-test reports while a regulatory-control folder stays close to its commissioning state. The shift from paper folders to electronic ones held in asset-management systems is now standard on greenfield projects, though operating plants commonly run a hybrid with legacy loops still on paper. The spine of the whole structure is the I/O list. Every loop on the list should have a folder, and every folder should answer to a loop on the list, so a clean, loop-numbered reading of the set is the precondition for a folder index that has no gaps and no orphans. Reconstructing that index on an inherited plant whose records have drifted is one of the recurring brownfield jobs.