Paper360 - November/December 2015 - (Page 26)
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
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.
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
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 (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.
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
Index of Advertisers
Paper360 - November/December 2015