This Technical Information Paper provides guidelines and procedures for the inspection of continuous digesters and impregnation vessels in alkaline pulping service. This document addresses inspection planning, vessel cleaning and preparation, nondestructive testing (NDT) methods, inspection personnel qualifications, inspection frequency, critical inspection locations or features, and what to do with inspection findings.
This TIP does not discuss the interpretation of inspection data and these guidelines are not intended to supersede jurisdictional and regulatory requirements.
The primary focus in inspecting a single or two-vessel continuous digester system (two vessel systems have a separate impregnation vessel) is the integrity of the primary pressure- containing elements. The identification of cracking or corrosion of nozzles and the soundness of various internal components with their welds and welded attachments inside the vessels also is important. Inspections of associated external piping, auxiliary systems (heaters, flash tanks, steaming vessels, or level tanks) and pressure limiting or protective devices are not addressed.
Essentially all continuous digester pressure vessels in North America at the time of this TIP are of welded carbon steel construction and all but a few are commonly known as Kamyr digesters. The carbon steel surfaces inside these vessels may be protected against corrosion using stainless steel cladding, corrosion-resistant weld overlay, thermal spray coatings or anodic protection systems. These guidelines address all of these systems.
Some digester vessels have recently been constructed entirely of composite plate or of solid, duplex stainless steel. Guidelines for inspecting vessels of solid stainless steel construction are not specifically provided herein, although the main concepts still apply.
The term "digester vessels" in this TIP is used to refer to either digesters or impregnation vessels.
