Ducts and passages: the part nobody looks into

An inspection usually ends where you can stand comfortably. The furnace chamber is searched, photographed, assessed. The ducts behind it get a glance from the opening.

That is understandable and still the wrong order. A connecting duct has no reserve: if it blocks or collapses, the plant stops — regardless of the condition of the furnace chamber.

Why ducts wear differently

• They are colder. At the end of a flue the temperature is often hundreds of degrees below that in the furnace chamber. Constituents of the flue gas that stay gaseous in the furnace condense there — and attack the joint, not the brick.
• They have changes of direction. At every bend the flow meets a surface. What is flow in a straight duct is abrasion in a curve.
• They are long and held at only two points. Expansion that a joint absorbs in the furnace chamber looks for the weakest point over ten metres in a duct.

Three check points are enough

First the cross-section. Do not estimate, measure — at the same points as last time. A single figure says little; two figures from two overhauls say everything.

Second the bends. That is where both erosion and build-up begin. Both show up first on the outside of the curve, just after the bend in the direction of flow.

Third the roof. Seen from below, a duct arch is rarely perceived as a load-bearing component. But that is exactly what it is — and it does not fail slowly, it fails all at once.

Cleaning is part of the inspection, not something that follows it

A duct cannot be assessed while build-up is lying on it. Conversely, removing the build-up is what first reveals what is missing underneath. That is why cleaning and inspection belong in the same work step and the same shift — otherwise the record describes a surface that never existed.

In practice that means: clean, photograph, measure, then decide. Not: clean, close up and look again at the next standstill.

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