Guidelines for inspection of continuous digesters and impregnation vessels

This Technical Information Paper (TIP) provides guidelines for inspecting digester vessels in alkaline pulping service. Digester vessel refers to a continuous digester or an impregnation vessel in one- and two-vessel systems. These guidelines and procedures also apply to level tanks.

This TIP covers inspection of the code-defined pressure-containing ‘envelope’ and of internal parts, including screens and their support structure, the center-pipe and its supports and the bottom outlet device/scraper. The TIP discusses preparing for inspection, inspecting the shell and nozzles for cracking and corrosion, inspecting internal components, and how to record the inspection findings and data. The TIP does not cover inspection of M&D and batch digesters – inspection of the latter is covered in a separate TIP.

These guidelines supplement jurisdictional regulations for pressure vessel inspection. This TIP contains no guidance for inspecting associated external piping, auxiliary systems such as liquor heaters, flash tanks, steaming vessel, the sand separator and pressure limiting devices. This TIP does not help the reader interpret inspection data or assess the continuing “fitness for service” of a digester vessel, which can be done as described in API 579-1/ASME FFS-1, Fitness for service (1) post-construction code, which is accepted by the National Board Inspection Code (NBIC).