Plant piping inspection: finding corrosion on top of pipe racks and under supports

Piping deterioration falls into external corrosion (coating breakdown, corrosion at supports, corrosion under insulation (CUI)), internal wall loss (erosion, corrosive fluids) and mechanical damage (vibration fatigue, cracked small-bore branches, shifted or bent supports). The difficulty is that only part of the outside can be seen.

In Japan, piping carrying high-pressure gas or hazardous materials is subject to periodic inspection under the High Pressure Gas Safety Act and the Fire Service Act. On top of visual checks, the basic method is repeated ultrasonic thickness measurement at fixed points, estimating remaining life from the rate of wall loss.

The pain is volume and height. Piping runs are long, racks are stacked several tiers high and the upper tiers are metres above ground. Every measurement point needs scaffolding or an aerial lift, and insulated lines must be stripped, measured and reinsulated. Piping scaffolds and insulation work can take up much of a turnaround schedule.

The other pain is choosing where to look. Measuring everything equally is impossible, so you pick high-risk locations (risk-based inspection, RBI). But the *overall external picture* needed to check whether that selection is right has always been hard to get.

The first job for drones on piping is looking along the rack tops and around supports from end to end. With a full external picture, you can narrow down where to bring the gauges. Drones that press an ultrasonic thickness probe against the pipe have also appeared.

The basic pattern is a route parallel to the rack. Image the top tier from above and look up at supports from an angle below, capturing each pipe from several directions. Pipes are round: from one direction you see only half.

A pipe rack is a jungle gym of structural steel and pipe: tight, GNSS-weak, compass-confusing, possibly flammable, and threaded with hot lines. For a drone, tightness and hazard arrive together.

• Confined spaces: gaps between pipes are narrow, and thin instrument lines and cables are hard for obstacle sensors to see. Use caged aircraft and make zooming from outside the rack the default. Confined spaces
• GNSS-denied: under racks and between buildings GNSS is weak. Choose aircraft that localise visually or with LiDAR. GNSS-denied environments
• Magnetic disturbance: surrounded by steel pipe and frames, the compass lies. Take off away from steel and be ready to fly without magnetic heading. Magnetic disturbance
• Explosive atmospheres: areas around live piping are often classified hazardous zones. Off-the-shelf drones are not explosion-proof, so image from outside the zone or approach only when gas-free is confirmed. Explosive atmospheres
• Heat: steam lines and hot process lines warm the aircraft as it approaches, and hot pipes reflect into thermal images. Heat

Piping photos all look alike. Unless each image is tied to which line, which pipe and which location, it is useless. Link images to piping drawings (P&IDs and isometrics) or point clouds and to the thickness measurement register. Only then can they inform where to measure.

In Japan, high-pressure gas piping is subject to safety inspection and periodic self-inspection under the High Pressure Gas Safety Act, and hazardous-materials piping to inspection under the Fire Service Act. For drone use inside plants, the guidelines for safe drone operation in plants issued by METI, the Fire and Disaster Management Agency and the Ministry of Health, Labour and Welfare are the reference. How drone imagery or readings count toward statutory inspection must be confirmed with the relevant authority for each inspection type.

Abroad, in-service piping inspection widely follows the American Petroleum Institute’s API 570 piping inspection code and the practices in API 574. API 570 sorts piping into classes with maximum intervals for thickness measurement and external visual inspection (five years for both in Class 1), and gives specific attention to CUI. In the US, OSHA Process Safety Management (PSM) requires mechanical integrity inspection.

Flight rules apply separately. Outdoor pipe racks fall under aviation law, and flights within 30 m of property need permission or approval. Compare countries in world regulations.

• Under insulation and internal wall loss: photos show only the cladding surface. Thermal images are clues; thickness must be measured with ultrasonics or other gauges.
• The support contact face: where a pipe rests on a support is invisible from any angle. It needs lifting or dedicated inspection methods.
• Contact readings at every point: contact drones cannot reach some points, depending on pipe orientation and congestion.
• Close flight in live hazardous areas: never fly an ordinary drone where flammable gas may be present.
• Rating and accountability: judging remaining life and timing replacement remains a human job.

At the end of the factory’s pipe rack sits a drone box. Once a month the drone flies along the rack, images each pipe from the same pose as last time and presses a thickness probe against the pipe wherever something looks wrong. Results land automatically on a 3D model of the piping, and only the fast-thinning spots change colour on the engineer’s screen. That is the picture our editors expect.

Nobody needs to scaffold the rack and walk it. People work out why pipes are thinning, revisit operating conditions and materials, and set replacement order. Inspection shifts from walking pipe by pipe to caring for the whole life of the piping.