Reading crack patterns: symptom, cause, need for action

Almost every furnace inspection turns up cracks. The actual work does not consist in finding them, but in classifying them: which crack is normal, which is an early warning sign and which has to be treated immediately? The course of a crack reveals more than its width.

What the course reveals

• A fine network across the surface: typical drying and shrinkage cracks after the first heat-up. As a rule uncritical, provided they do not open up and show no discolouration.
• Straight, continuous cracks running with the component: a missing or blocked expansion joint. Here the lining has created for itself the joint that was missing from the design.
• Cracks along a change of material: differing expansion at the transition. Keep them under observation, but the cause lies in the build-up and cannot be remedied by filling.
• A crack with discolouration or efflorescence: medium has already passed through. This is no longer a crack pattern but an infiltration finding — action is needed regardless of the width.

The three questions for every crack

• Is it changing? Without a comparison photo from the last visit this cannot be answered — which is why every crack pattern should be photographed and dated.
• Does it go all the way through? A surface crack and a through-crack look the same from the inside. Thermography from the outside separates the two reliably.
• Where is it? The same crack is an immediate case in the bath line and a note for the next overhaul in the upper furnace.