Digester Thinning, 2013 PEERS Conference

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A softwood continuous digester has experienced severe erosion-corrosion of the shell wall inside the internal flow channel headers of the extraction and MCC zones. Not surprisingly, 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 mpy, the damage has effectively reduced the wall thickness almost in half in the worst areas. Also affected were the horizontal backing rings that form the bottom of the flow channels. 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. 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 has correlated very well with the corrosion rates determined by on-stream UT data.