Paper360 - November/December 2015 - (Page 26)

techlink | BOILER SAfETy & EffICIENCy Higher Alloyed Composite tubes make recovery boilers Safer and more efficient To achieve superior efficiency and safety in recovery boilers, composite tubes should be capable of superior corrosion and cracking resistance. URBAN FORSBERG Composite tubes, which consist of two components metallurgically bonded together, were first introduced to the market in the early 1970s and have since contributed significantly toward greater safety and efficiency in Black Liquor Recovery Boilers (BLRB). To achieve these performance improvements, the tubes must be capable of superior corrosion and cracking resistance, i.e., with higher alloyed outer components. This is especially the case if they are used in the more exposed areas of the boiler such as the floors, air ports, smelt spouts and steam superheaters. Tubes in these positions are subjected more extensively to thermal shocks and corrosive environments than in other areas of the boiler. A typical composite tube consists of an outer corrosion-resistant component and an inner load-bearing component, generally of carbon or a low alloyed steel. The metallurgical bond is formed by co-extrusion of the two components at high temperature and high pressure. In other words, this is an atomic diffusion between the two different materials; a migration of elements between the two Figure 1. The bonding zone in a composite tube is 0.1 - 0.2 mm thick. The stainless layer is at the top of the picture. 26 Paper360º different grades that results in a mix of the two materials in the so-called "bonding zone," as illustrated in Figure 1. The first composite tubes in the early 1970s consisted of a corrosion-resistant outer component in type 304 stainless steel. However, by the beginning of the 1980s, several cases of cracking were observed in tubes in the most vulnerable positions in the boiler, including the floors, smelt spouts and air port openings. The cracking was caused mainly by thermal fatigue and stress corrosion cracking (SCC). While 304L has, for more than 40 years, built a remarkable track record in withstanding the tough conditions in BLRBs, continued instances of SCC have meant that other composite tube options have also been developed. Today, these are available with different types of inner components and outer components. All are designed to cope with higher pressures and temperatures in the boiler and, ultimately, better satisfy customers' unique operational requirements. The nominal chemical compositions of these different options are listed in Table 1, including the traditional type 304L alongside advanced high-alloyed materials like UNS N08825Mod and UNS N06690. NICKEL-BASE ALLOYS Nickel (Ni) is a key chemical element in an enhanced corrosion and cracking resistant composite tube. Ni-base alloys are more corrosion-resistant and less susceptible to SCC due specifically to the high nickel content. Based on this knowledge, a new replacement for 304L tube was developed in the mid-1980s. The new tube had a high alloyed NickelChromium-Iron (NiCrFe) alloy similar to UNS N08825Mod (Alloy 825Mod) on the outside. As anticipated, the advanced grade exhibited increased corrosion resistance and decreased susceptibility to cracking with resistance against SCC and thermal fatigue that was superior to the previous type 304 composite tube. The first full furnace floor installation with UNS N08825Mod was in a Metsä Fibre recovery boiler in Rauma, designed as the world's first totally chlorine-free (TCF) pulp production factory established on the west coast of Finland in 1996. The floor was later subjected to a complete inspection in 2011, which found Table 1. Chemical composition of outer and inner components. NOVEMBER/DECEMBER 2015

Table of Contents for the Digital Edition of Paper360 - November/December 2015

Over the Wire
Proud to be a Papermaker
Solenis’ Total Mill Perspective Helps Mills Stay Competitive
Maintaining Knife Gate Valves
An Automated Solution to the Kappa Number Test by Titration
Higher Alloyed Composite Tubes Make Recovery Boilers Safer and More Efficient
TAPPI Journal Summaries
New Leadership Model Focuses on Caring, Compassion
Consolidation Watch
Association News
Online Exclusives
Index of Advertisers

Paper360 - November/December 2015