Fine-particle erosion in ash lines and fan casings behaves nothing like chute abrasion

Sliding abrasion in a chute removes material broadly and leaves a polished, fairly even surface. Gas-borne fine particles do the opposite: they concentrate wherever the stream cannot follow the wall, cutting grooves and eventually a hole while the surface a hand's width away is untouched. A wall thickness survey that samples evenly will miss it.

The practical consequence is that the inspection plan has to follow the flow. Mark the outside of every bend, the region just downstream of any restriction, the area opposite an inlet and the volute of a fan — then measure those points, not a grid.

In gas-borne service, small changes in velocity produce disproportionate changes in erosion rate. That makes a duct or fan running above its design flow a wear problem before it is a process problem, and it also means a lining specified for the original duty may be undersized after a capacity increase nobody linked to the liner.

Ask whether the operating point has changed since the last reline. A recurring failure that resists every material upgrade is often a flow problem being answered with a lining answer.

A dry ash line is an erosion problem. A wet ash line is an erosion and corrosion problem, and the corrosion attacks the adhesive, the jointing compound and the steel behind the ceramic rather than the ceramic itself. The same wear face can therefore be correct in one line and a poor choice in the other, purely because of the bonding system it depends on.

State it explicitly at the enquiry stage, including lines that run dry normally but wet during upsets or washdowns. An intermittent wet condition is harder on a joint than a permanent one, because it cycles.

The instinct is to line the area that failed. In gas-borne erosion the next failure usually appears immediately downstream, where the disturbed stream re-attaches to the wall — a location that was fine only because the previous section was taking the damage. Extending the protected length is normally cheaper than a second outage.

Where the geometry itself causes the concentration, consider whether a change of shape is available at the same outage. A lining protects a wall; it does not remove the reason the stream hits that wall.

Gas velocity at the failing section and whether it has changed; ash or dust chemistry; operating temperature and any excursions; whether the line runs wet, dry or alternates; and photographs showing where the loss is concentrated relative to the flow direction.

A sketch of the duct run with the failure locations marked is worth more than a thickness table, because it shows the pattern. The pattern is what identifies the mechanism.