Guidelines for inspecting batch digesters

This Technical Information Paper (TIP) provides guidelines and procedures for inspecting batch digesters in pulp mills. It describes damage mechanisms and addresses inspection frequency, planning, methods, damage or inspection locations, cleaning and preparation, technician qualifications, test equipment and documentation.

The primary focus of this TIP is stationary, vertical, cylindrical welded, carbon steel digesters that make up the vast majority of the batch digesters in service. Batch digesters may be made from carbon steel or stainless steel. Carbon steel vessels may be wholly or partially internally protected with corrosion resistant alloy (CRA) weld overlay or clad layer. Thermal spray coatings also have been used to prevent steel corrosion. Vertical cylindrical digesters constructed of solid stainless steel are covered in this revision of the TIP.

Digesters of riveted or forge welded construction are not covered in this TIP. Inspection of batch digesters in acidic (sulfite) pulping service, which are stainless steel or have a brick-lining, is not specifically covered by this TIP, although the general guidelines herein apply. This TIP does not provide guidance on interpreting inspection findings, assessing fitness for continued service or designing acceptable repairs. The National Board Inspection Code (1) and API 579-1/ASME FFS-1 Post-Construction Code provide such guidance (2).