Cracking of straight co-extruded composite wall tubes in black liquor recovery boilers

Cracking of co-extruded 304L/SA-210 Gr A1 tube is a well-known phenomenon and several studies have been
conducted around this issue. Generally, cracks have been detected in floor tubes and lower furnace opening tubes,
such as smelt spout and primary air-port openings. The cracking mechanisms have been found out to be stress
corrosion cracking or thermal/corrosion fatigue. As there is a general understanding of this phenomena, regular
inspection practices are being followed as a preventing action for catastrophic failures.
However, there have been a few cases where cracking has been discovered – unexpectedly – in non-typical location.
Cracking of straight co-extruded composite wall tubes and membranes have been encountered at and above
secondary and tertiary air level.
In this paper two case studies are presented where cracks were discovered on membrane wall – tubes and
membranes - above secondary air level. The cracks characteristics showed mechanism to be stress corrosion
cracking and fatigue. Though final root causes could not be verified, it was concluded to be related to non-ideal
operation of burners (CNCG or natural gas). 

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