Project Start-up, Commissioning, and Loop Check
Core concepts
This domain covers the path from construction to a running plant. Know the sequence: mechanical completion, then pre-commissioning (inspection, flushing, hydrotest, insulation resistance, continuity, point-to-point wiring checks), then commissioning (calibration verification, loop checks, valve stroking, alarm and interlock testing, DCS graphics and I/O verification), then start-up (introducing process fluid, cold/wet/hot commissioning).
Distinguish the documents: P&ID, loop drawing, instrument index, termination schedule, punch list, and as-built. A loop check verifies the entire path from field device to operator station, including polarity, wiring, tagging, configuration, and alarms. It is not a calibration: the instrument is calibrated separately, then the loop is proven by injecting known signals (commonly 4, 12, and 20 mA, or a five-point check) and confirming the correct value, tag, and engineering units appear at the HMI.
Safety and control are central: permits, bypass and override registers, removing all jumpers, simulators, and temporary wiring, restoring valves to their safe position, and confirming fail-safe action on loss of signal or power. Every bypass must be logged and removed before handover.
Traps
- Treating a loop check as a calibration, or skipping one because the other was done.
- Checking only at the field and never verifying the HMI value, tag, units, and alarms.
- Leaving a simulator, jumper, or forced value in the controller.
- Forgetting to verify fail-safe direction and valve stroke.
- Performing loop checks before the loop is fully installed and terminated.
- Not updating drawings and punch items before turnover.
Worked example
A flow loop is calibrated and terminated. The technician injects 4.00, 12.00, and 20.00 mA at the field terminals, then confirms 0, 50, and 100 percent appear at the operator station with the correct tag and units. The control valve is stroked 0-100 percent and its fail direction is confirmed on air failure. Results are recorded, jumpers removed, and the loop released.
Rule of thumb
Calibrate the device, then prove the loop end to end, and remove every temporary before handover.