Evaluation of Thinning in Continuous Digesters with Fitness-for-Service Methods, 2023 PEERS Conference

The pulp and paper industry has devoted significant resources to understanding the damage mechanisms affecting continuous digesters in the Kraft pulping process. The detection and monitoring of these damage mechanisms are addressed in industry guidelines such as TAPPI TIP 0402-27 “Guidelines for Inspection of Continuous Digesters and Impregnation Vessels”. This TIP advises that to assess the continuing “fitness for service” of a digester vessel, it can be done as described in API 579-1/ASME FFS-1.

However, once damage is detected, industry practice is generally to perform “in-kind” repairs once varying conservative thresholds are met. Important steps are omitted by taking this approach, namely, fitness-for-service (FFS) evaluations and consideration to the rate of degradation addressed in various post-construction codes (PCC).

This paper serves to give the reader an overview of the options available when evaluating thinning-related damage in continuous digesters. The FFS methods outlined in API 579-1/AMSE FFS-1 for general thinning, local thinning, and pitting are addressed with regard to damage in continuous digesters. Also, consideration of the rate of damage and its effect on an FFS evaluation is discussed.

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