TL;DR
Lower furnace corrosion typically appears as thinning or pitting on waterwall tubes. Inspect with UT thickness gauging during planned outages and trend wall thickness over time.
What you might see
- localized tube wall thinning found on UT survey
- visible pitting or cratering on waterwall tubes
- corrosion product deposits on floor tubes
- increasing makeup water consumption suggesting a leak
Likely causes
Reducing atmosphere near the char bed causing sulfide corrosion of tube steel
Smelt accumulation on floor tubes insulating them from cooling water and overheating the steel
Composite tube coating breakdown allowing direct corrodent contact with base metal
Fluctuating air distribution creating localized reducing zones at the lower furnace walls
Required tools
- Ultrasonic thickness gauge (calibrated)
- Soapstone or paint marker for grid layout
- Confined-space gas monitor
- LOTO kit
- Camera for documentation
Safety first
- Furnace entry requires a confined-space permit and continuous atmosphere monitoring for CO and O2.
- All air register, steam, and smelt systems must be locked out before personnel enter the lower furnace.
- Do not use water to clean the lower furnace until steam and smelt systems are fully isolated and cooled.
Procedure
- 1
Plan the inspection during a scheduled boiler outage. Confirm LOTO on all air and steam systems before entering the furnace.
Warning: Confined space entry into the furnace requires a confined-space permit, continuous gas monitoring, and a standby attendant outside. - 2
Clean smelt and char deposits from the lower waterwall and floor tubes using dry tools. Do not use water until steam systems are fully isolated.
- 3
Mark a UT grid on the lower waterwall tubes at 30 cm intervals using soapstone or paint marker.
- 4
Measure tube wall thickness at each grid point with a calibrated ultrasonic thickness gauge. Record all readings.[1]
- 5
Compare readings to the original wall thickness from the fabrication record. Flag any location below the minimum allowable wall thickness specified by the boiler code.
- 6
Photograph all areas of visible pitting or cratering for the inspection record.[1]
- 7
Consult a qualified pressure-vessel inspector before returning the boiler to service if any tube is below minimum wall thickness.
Sources
ANDRITZ ANDRITZ HERB Recovery Boiler general technical documentation, ANDRITZ
Recovery boiler waterwall tube inspection and UT thickness gauging, general pressure vessel inspection practice (general)
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