Maintenance and Repair
Core concepts
Maintenance questions test whether you can keep a loop reliable without creating new hazards. Separate preventive maintenance (scheduled inspection, cleaning, filter changes, valve stroking, battery and surge-protector replacement, calibration interval review) from corrective maintenance (repair after failure). Know the safety chain: work permit, lockout/tagout, isolation and venting of impulse lines, hazardous-area classification and intrinsic safety, PPE, and ESD protection for electronics.
Be able to read a loop drawing to find isolation valves, test points, terminations, and power sources. Understand environmental ratings (NEMA/IP), enclosure sealing, breathers and desiccants, grounding and shielding, and why moisture ingress is a leading cause of transmitter failure. For final elements, know packing adjustment, stem lubrication, stroking to prevent seizure, seat leakage, and air supply quality (dry, filtered instrument air).
Documentation is part of the job: work order, as-found and as-left values, parts replaced, and updated drawings. Repaired equipment must be re-verified, not assumed good.
Traps
- Assuming a repair restores accuracy without recalibration or verification.
- Isolating electrically but not venting trapped process pressure before disassembly.
- Bypassing an interlock without authorization or without restoring it.
- Fitting a substitute part with the wrong rating, wetted material, or area certification.
- Leaving a loop in manual or a bypass in place at handover.
- Skipping the documentation, so the next technician starts blind.
Worked example
A pressure transmitter drifts high every summer. The as-found check is within tolerance in the shop but out in the field. Inspection shows a cracked cover gasket and condensation inside the housing. Correct action: replace the gasket, dry and inspect the electronics, verify insulation resistance, recalibrate, and add housing inspection to the PM schedule, rather than simply trimming the zero again each year.
Rule of thumb
If it is not documented, it did not happen, and a repaired instrument is an uncalibrated instrument until proven otherwise.