Inspection and Mechanical Integrity Evaluation of Corroded Digesters, 1997 Engineering Conference Proceedings

Digesters are large pressure vessels used in the paper industry and are generally constructed in accordance with the ASME Boiler and Pressure Vessel Code. Digesters are commonly fabricated from mild carbon steel and include a large corrosion allowance because they experience high rates of corrosion and erosion during operation. A widely used industry practice has been to weld overlay corroded digesters to prevent future corrosion.

This paper investigates issues regarding the inspection and mechanical integrity evaluation of corroded digesters. The conditions under which the added weld overlay can be included as part of the code design thickness are examined. The capabilities and accuracy of inspection methods to measure the carbon steel and clad thickness are investigated. Case studies of corroded digesters are used to evaluate various evaluation methods of locally thin areas. It is concluded that in many situations expensive weld overlay repairs or the down rating of pressure and temperature design conditions of digesters can be avoided. Other issues for complete mechanical integrity evaluations for continuing operation of corroded digesters are enumerated.