Boiler and furnace inspection: seeing the whole furnace before the scaffold goes up

The main actor in boiler deterioration is the tube. On the fire side: ash erosion, high-temperature corrosion, and thinning where sootblower steam keeps striking. On the water and steam side: scale build-up and the overheating it causes. Superheater and reheater tubes that run hot for years slowly swell through creep. Tube ruptures are a major cause of unplanned outages.

The same applies beyond power stations: waste incinerators, refinery fired heaters and industrial boilers all centre their inspection on tubes and refractory inside the furnace. A different fuel means different ash and different corrosion.

Boilers are protected by a combination of statutory inspection and the owner’s own inspections during planned outages. Inspectors enter the furnace, light up the tubes, and take ultrasonic thickness readings at set locations. To reach high areas they build scaffolding inside the furnace, hang suspended access platforms from the roof, or use rope access.

There are three pains: time (waiting to cool, cleaning ash, building scaffolds), danger (height, falling objects, dust, heat, confined space) and missed defects. With thousands of tubes and a few days, no team can look closely at everything, so inspection tends to focus on “where it failed before” and the fixed measurement points.

The drone’s job is to see the entire furnace once, before any scaffolding goes up. That separates the places that need scaffolding from those that do not — and changes the order of the whole inspection.

The basic pattern is top-down bands, wall by wall. Starting low means the downwash lifts ash off the floor and visibility vanishes, so start high and work down, letting the dust settle. Agree tube numbering markers or a furnace coordinate system first, so every image can later be tied to a specific wall and tube.

A boiler interior collects nearly every drone weakness: total darkness, airborne ash, and a steel box that blocks GNSS. Tube walls are a repeating pattern, so camera-based localisation has few distinctive features. Even after cool-down, heat can linger high up and deep between tubes.

• Dust & smoke: start high, move slowly, angle the lights to reduce glare from drifting ash, and coordinate the order of ash cleaning and flights. Dust & smoke
• Darkness: onboard lighting is the only light. Fix exposure and keep a constant stand-off so brightness stays consistent. Darkness
• GNSS-denied: localise with LiDAR and cameras. Repeating tube patterns confuse localisation, so placing markers helps. GNSS-denied
• Confined spaces: gaps between superheater tube banks are often too narrow for any aircraft. Do not force a caged drone in. Confined spaces
• Heat: wait until the furnace is cool enough for batteries and cameras, and measure the temperature before entry. Heat

The hardest part of furnace inspection is knowing which tube you are looking at. Thousands of identical tubes stand side by side, and unless each defect is tied to a tube number and elevation, no repair order can be issued. AI is good at proposing defects and comparing with last time; deciding whether to replace a tube or keep running remains the engineer’s call.

In Japan, industrial boilers undergo periodic performance inspections under the Industrial Safety and Health Act and its Ordinance on Safety of Boilers and Pressure Vessels, renewing their inspection certificate (whose validity can be shortened or extended depending on the results). Power-station boilers fall under the Electricity Business Act, which requires periodic inspections carried out by the operator. Either way, drones make inspection easier; they do not change its content or who is accountable.

For the flight itself, flying inside an indoor space such as a furnace is generally outside Japan’s Civil Aeronautics Act (MLIT’s interpretation treats a space clearly separated from outside as indoor even with openings; leaving through an opening counts as outdoor flight). Instead, rules on oxygen deficiency, dust and confined-space work, plus the site’s permit-to-work system, apply. Flights outside around ducts and stacks do fall under aviation law, so treat them separately. The plant drone guidelines issued by METI, FDMA and MHLW are also useful.

Abroad, the US relies on the ASME Boiler and Pressure Vessel Code for construction and the National Board Inspection Code (NBIC) for in-service inspection; the UK’s Pressure Systems Safety Regulations 2000 require a written scheme of examination by a competent person. Compare national flight rules in world regulations.

• Tube wall thickness: photos cannot tell how many millimetres remain. Ultrasonic or EMAT readings in contact are needed.
• Deep inside tube banks: gaps between superheater and reheater tubes are too narrow to enter; often only the outer rows are visible.
• Creep and waterside damage: microstructural degradation and internal tube damage are invisible from outside. Tube sampling or other tests are required.
• Inside drums and headers: small openings call for borescopes and dedicated tools rather than drones.
• An operating furnace: nothing flies through flame. In service, furnace cameras and temperature monitoring take over.
• Decisions and accountability: tube replacement, remaining-life assessment and statutory pass/fail stay with people.

The morning after the boiler comes offline, once it has cooled, a drone goes in first. It flies the same route as last time, captures every wall, and alongside the 3D point cloud flags tube swelling and erosion progress. By noon, a plan saying “scaffolding needed in these three places” is ready and the materials are on order.

People no longer spend days searching in the ash. They measure the tubes that need measuring and replace the ones that need replacing. Inspection shifts from going into the furnace to designing the boiler’s remaining life. That is our editorial view — and we expect contact thickness measurement will still be needed then.