Tube erosion / tube wastage

Tube erosion and tube wastage describe loss of boiler tube metal. The terms are often used together because the observed result is the same - reduced tube wall thickness - even when the mechanism differs. In boilers, wastage may come from fly-ash impact, sootblower impingement, corrosion under deposits, reducing furnace atmosphere, chloride attack, thermal fatigue or combinations of these mechanisms.

Tube wastage matters because the boiler tube is a pressure part. Local thinning can cause leaks, forced outages, collateral damage and safety risk. A small area of high wastage can be more important than a broad area of light fouling.

Main mechanisms

• Fly-ash erosion - hard particles strike tubes, especially at bends, leading edges and high-velocity lanes.
• Sootblower erosion - steam jets cut tube surfaces when nozzles are misaligned, too close, too frequent or too wet.
• Corrosion under deposits - salts or sulphates create aggressive chemistry against the metal.
• Waterwall wastage - reducing conditions and sulphidation near burners attack furnace wall tubes.
• Thermal fatigue - repeated water cannon or quench events crack protective scales and tube metal.

The dominant mechanism is identified through location, deposit chemistry, tube-metal analysis, thickness mapping and operating history.

Inspection and monitoring

Common inspection tools include ultrasonic thickness testing, grid mapping, visual inspection after cleaning, replica metallography, tube sampling and comparison with previous outage maps. Good programmes track wall loss rate, not only remaining thickness, because an apparently acceptable tube may be on a fast path to failure.

Operational indicators include rising sootblower use, local slag growth, abnormal gas lanes, tube leaks, high fly-ash loading and changes in fuel ash abrasiveness.

Sonic horns and erosion

Sonic horns are non-contact devices and do not create a cutting jet, so they are attractive where steam sootblowing has contributed to wastage. They can reduce the need for aggressive sootblower cycles by keeping deposits mobile in the convective pass. They do not solve furnace-side chemical wastage, hard slagging or ash erosion from poor gas distribution. In Sylio-style decisions, tube-wastage history is used to decide whether acoustic cleaning can replace part of the duty or only supplement the existing system.

Damage mechanisms

Tube wastage is a broad plant term because several mechanisms can remove metal at the same time. Fly-ash erosion cuts tubes where gas velocity is high or flow is concentrated through narrow lanes. Sootblower erosion creates local wastage where steam, air or water jets repeatedly strike the same area. Falling slag can impact lower banks or waterwalls. Fireside corrosion removes metal under ash deposits, especially where chlorine, sulphur, alkalis or reducing conditions are present. Steam-side oxidation and exfoliation can also contribute on high temperature superheater and reheater tubes.

The pattern matters. Smooth thinning on the gas-facing side suggests particle erosion. Grooves aligned with a sootblower path suggest jet cutting. Under-deposit pitting or wastage near burners points toward chemistry and combustion. Local loss around attachments can indicate thermal fatigue, ash packing or disturbed flow. Correct diagnosis is essential because adding more cleaning can make an erosion problem worse.

Inspection and prevention

Maintenance programmes use ultrasonic thickness readings, visual inspection, borescopes, tube samples, deposit analysis and trend maps by elevation and bank. Good records show whether wastage is accelerating, moving with a fuel change or linked to a specific cleaning device. Prevention may involve changing sootblower settings, adding shields, improving gas distribution, correcting burner balance, reducing fly-ash carry-over, upgrading materials or altering fuel blend.

Acoustic cleaning can reduce wastage when it prevents ash bridges that increase local velocity or require aggressive sootblowing. It can also be neutral if the damage is chemical or caused by hard particle erosion elsewhere. Sonic horns should therefore be specified with inspection evidence: the target deposit must be removable, and the expected benefit should be lower blockage, lower direct jet cleaning or more stable gas flow.

Related terms

• Tube fouling
• Steam sootblower
• Waterwall
• Superheater