scope:

This Technical Information Paper (TIP) provides guidelines for inspecting digester vessels in alkaline pulping service. (The term 'digester vessels' refers to continuous digesters or impregnation vessels in one- and twovessel systems.) This TIP addresses inspection of the code-defined pressure-containing 'envelope' and internal static and mechanical components, including screens, pipe supports and the bottom scraper. The TIP discusses preparing for inspection, inspecting for cracking and corrosion, inspecting internal components, and how to record the inspection findings. The TIP does not cover batch and M&D digesters.

These guidelines do not supersede jurisdictional and regulatory requirements. Regular inspections of associated external piping, auxiliary systems (such as liquor heaters, flash tanks, steaming vessel, sand separators, and level tanks), and pressure limiting or protective devices are also part of a condition monitoring program but are outside the scope of this TIP. This TIP does not discuss interpreting inspection data or assessing the continuing "fitness for service" of a 'digester vessel' - modern practice involves using API 579-1/ASME FFS-1, Fitness for service¹ for this purpose.

Most digester vessels operated in North America are of welded steel construction. Some digester vessels, especially impregnation vessels, are constructed entirely of composite plate (e.g., 304L stainless-clad) or of duplex stainless steel. Stainless steel is widely used for nozzles, internal screens and blank plates and the central pipe. Carbon steel surfaces in digester vessels may be protected against corrosion with corrosion-resistant weld overlay and thermal spray coatings. Anodic protection systems mitigate cracking and corrosion mechanisms.