Chimney and stack inspection: looking at the path of heat and smoke, inside and out

Chimneys fall broadly into reinforced-concrete stacks (often with a steel or brick inner flue), steel stacks (self-supporting, guyed or tower-supported) and older masonry stacks. They share the same enemies: acidic condensate as flue gas cools (sulfuric-acid dew-point corrosion and similar), thermal cycling, and wind and earthquakes.

Outside, steeplejacks — specialist high-access trades — have long climbed ladders or ropes to look. Inside, the plant is shut down, cooled and ventilated, then scaffolding or a gondola is rigged inside the flue for people to enter. Power plants and incinerators usually fit this into an outage planned years ahead.

The heaviest cost is downtime. Shutting a power plant or incinerator to inspect its stack stops power or waste processing. If scaffolding alone takes days, inspection happens only when the plant can stop, not when it should. And the inside of a flue is an enclosed space with risks of oxygen deficiency and toxic gas, so every human entry needs heavy safety management.

Drones split chimney inspection in two. While running, from outside: look at heat and shape. Once shut down, from inside: capture the whole flue without scaffolding. The approaches break down like this.

Outside, the drone works column by column or in a spiral at a constant stand-off. Inside, it enters through the access door at the base and climbs slowly with its lights on. You still wait for the stack to cool, but the days of scaffolding disappear entirely.

The inside of a stack combines everything a drone dislikes: no GNSS, total darkness, a narrow tube, residual heat and updraft, and soot and ash. Outside, the top sees far stronger wind than the ground.

• GNSS-denied: no satellites inside the flue. Use aircraft that hold position by measuring distance to the wall with LiDAR or cameras. GNSS-denied
• Confined & dark: use caged aircraft that survive wall contact, plus strong lighting. Fix exposure to keep images consistent. Confined spaces · Darkness
• Heat: a recently stopped stack is hot. Check the operating temperature range of aircraft and batteries, and wait for it to cool. Temperature differences also drive updraft, so expect the aircraft to be lifted. Heat
• Dust & smoke: rotor wash lifts soot and ash from the base and visibility goes white in seconds. Plan flight order, cleaning and ventilation in advance. Dust & smoke
• Oxygen deficiency & toxic gas: keeping people out is the drone’s biggest advantage. But do not skip gas checks for the people who work at the access door or recover the aircraft. Toxic gas
• Wind at height: the top is windy and a wake forms behind the stack. Keep extra stand-off when working on the leeward side. Height

Chimney data is useful only as a map along the height: which defect, at what elevation, facing which direction. Overlaying interior images and exterior thermography at the same height lets you confirm that “bricks are missing inside, right where the outside runs hot.”

In Japan, chimneys over 6 m are treated as structures (kōsakubutsu) under the Building Standards Act, subject to permit and structural requirements. How they are inspected depends on the owner: plant safety rules under the Electricity Business Act for power stations, the operator’s own maintenance programme for factories. Stacks 60 m or taller need aviation obstruction lights or daytime markings (such as red-and-white bands) under the Civil Aeronautics Act, and keeping those working is part of inspection too.

Human entry is governed by occupational safety rules, including Japan’s ordinance on prevention of oxygen deficiency. For the drone, exterior work usually counts as a specified flight (within 30 m of people or property, among others) requiring permission or approval. Inside, Japan’s official interpretation can treat a chimney, whose inside is clearly separated from the outside, as indoor — but you must stop the drone straying out through openings, and the treatment depends on the situation, so check beforehand. Internationally, the model codes of CICIND (the international committee on industrial chimneys) and US standards ACI 307 (concrete chimneys) and ASME STS-1 (steel stacks) are widely referenced. Compare flight rules in world regulations.

• Inside a running stack: no flying in a tube full of hot flue gas. Seeing inside still needs an outage.
• Inside the concrete, and the gap between shell and flue: photos show surfaces only. Rebar corrosion and concrete strength need cores or NDT.
• Surfaces under soot: nothing shows beneath heavy deposits. Clean first.
• Sounding and touch: delamination is traditionally found by tapping. Ultrasonic contact readings are possible, but sounding substitutes are not yet routine.
• Repairs: re-laying bricks and fixing the top are human work.
• Safety rating: seismic adequacy and fitness to operate belong to structural engineers and the plant’s responsible staff.

What follows is the editors’ projection. On the incinerator roof sits a small drone port. Once a month, a drone circles the running stack on a fixed route. Only the elevations where hot spots have grown since last time light up on the engineer’s screen. The next outage is planned from that record: which heights to repair, and in how many days.

On shutdown day, workers open the access door and the drone goes in first. Within hours there is a 3D model of the flue and a thickness map. Nobody spends days inside a stack any more. The job shifts from climbing to planning how a landmark of the town can stand safely for decades to come.