Digester thinning: Erosion-corrosion of internal flow channel headers

A softwood continuous digester has experienced severe erosion-corrosion of the shell wall inside the internal flow channel headers of the extraction and modified continuous cooking (MCC) zones. Although erosioncorrosion is a common form, it is not typically looked for inside digester flow channels. The worst damage was located where high velocity liquor exits the screen orifices and enters the collection headers. With erosion-corrosion rates as high as 200 m/year, the damage has effectively reduced the wall thickness almost by half in the worst areas. Also affected were the horizontal backing rings that form the bottom of the flow channels. An API-579/ASME FFS-1 Part 5, Level 2 analysis was performed to allow the mill to continue operating the digester until the next scheduled outage. Owner-users are encouraged to inspect these locations in their digesters to ensure that erosion-corrosion has not caused accelerated and/or unexpected shell thinning. The internal flow channels represent locations that are not readily accessible for internal visual inspection without removal of the flow channel header cover plates and removal of the residual black liquor typically retained in these flow channels. External, on-stream ultrasonic thickness testing (UT) at the proper collection header elevations has been determined to be an effective way to detect the presence of this erosion-corrosion phenomena. Also discussed are considerations to make on-stream UT as accurate as possible. Corrosion rates determined via electrochemical corrosion probe monitoring have correlated very well with the corrosion rates determined by on-stream UT data.