A newly designed retaining ring concept, the RHI Thrust Lock System (TLS), significantly improves the refractory lining service in highly stressed kiln section. Andreas Wiry highlights its advantages as compared to existing concepts and shows first results of this system installed at Zementwerk Leube GmbH (Austria).
Despite numerous different lining designs and refractory materials installed and tested in the past (e.g., magnesia and SiC bricks, castables, and preshaped blocks), until now no comprehensive solution has been available to solve all the severe lining problems that often occur in the kiln outlet zone (fig 1).
Due to the excessive axial lining thrust of modern high-performance kilns and the resulting thermo-mechanically overstressed lining, conventional designs, namely brick and/or castable lining combined with rectangular retaining rings (fig 2), often do not achieve an acceptable service life or stop the axial lining thrust entirely.
By means of RHI Thrust Lock System the axial lining thrust, which causes most refractory lining failures at the kiln outlet, can be stopped due to the special skew brick set. The challenging target was to design a new system that provides additional benefits compared to already existing concepts as well as the possibility to upgrade current retaining designs. Instead of the common brick and rectangular retaining ring design, a special skew brick set is used in the RHI Thrust Lock System (fig 3). Due to the skew brick inclination (>>20 degree), the contact face between retaining elements and brick lining is much higher and therefore the resulting contact pressure is considerably reduced. In addition, the axial lining thrust is diverted into radial and circumferential forces and consequently mechanical stress in the bricks is further reduced.
A simulation of the brick movement and load distribution using the Abaqus, Version S.12, finite element method (FEM) program showed that the new Thrust Lock System can reduce the maximum axial stress level with the Thrust Lock System by at least 30 per cent. In spite of the favourable load reduction with this new system, the thrust bearing bricks are highly stressed and therefore particular material characteristics are required to withstand the complex thermo-mechanical load conditions. Based on the knowledge gained during development of the unique ANKRAL Q-series technology, focused on outstanding structural flexibility combined with high mechanical strength at high temperatures, a new top brand termed ANKRAL TLS was created for this special application. Moreover, due to the raw material basis comprising the purest synthetic sintered magnesia and a special spinel concept, ANKRAL TLS offers excellent resistance against corrosion and clinker melt infiltration. Depending on the installation position and working conditions, high alumina bricks or SiC bricks can also be used.
Over the last years, frequent lining failures have occurred at the nose ring and retaining ring of the Zementwerk Leube GmbH (Austria) precalciner kiln (3.5 m diameter). In spite of several different designs and lining concepts, the achieved service life was only six months and in the worst case the lining had to be replaced after three months (Fig 4).
Failure analysis and lining concepts
After a detailed analysis of the mechanical kiln conditions (e.g., shell test measurements), process conditions, wear mechanisms, and postmortem investigation of brick samples the following lining concept was selected for the customer (Fig 5). A staggered lining at the outlet (600 mm long) was installed to avoid brick twisting in this section. Due to the established high thermo-mechanical stress and positive experiences previously gained with ANKRAL Q1 in the lower transition zone, this brand was chosen for the outlet. ANKRAL TLS used for the Thrust Lock System is characterised by excellent fracture mechanical properties and high thermochemical resistance
Installation and outlet lining
During the winter shutdown period 2013/2014, the entire outlet zone (0û2.4 running metre) was newly installed. After overhauling the retaining ring and performing necessary grinding of kiln shell imperfections the first ring of skew bricks (fig 6a) was installed using a two-component adhesive. Subsequently, the second shape of the Thrust Lock System skew brick set was installed (fig 6b).
In order to achieve a uniform load distribution on the entire skew brick ring and to counteract spiralling of adjacent rings a staggered lining (500 mm) is part of the Thrust Lock System. This design helps divert the axial lining thrust into radial and circumferential forces, consequently reducing the mechanical load on the bricks and retaining elements.
While the outlet (ANKRAL Q1) was installed using the gluing method, the Thrust Lock System and the upper part of outlet zone were installed by means of a bricking rig (DAT rig). The Thrust Lock System rings were closed using special key bricks and shims. Ceramic felt was inserted in the defined expansion gap between the Thrust Lock System and the brick ring at the retaining ring (figs 6 and 7).
Inspection after a full campaign
The lining could be inspected in January 2015, after being in service since March 2014 (fig 8). The lining shows a very even surface without spalling and twisting. The residual thickness is still between 230-250 mm from original 250mm. A remarkable result in view of previous lifetimes of three to maximum six months. Due to the excellent appearance of the lining the TLS will remain in place for a further campaign.
This first Thrust Lock System installation provided evidence that it can be easily installed and that the concept works as expected providing a remarkable improvement of refractory life time. A skilled installation company can manage the installation within a typical time frame. However, special attention needs to be paid to the condition and appropriate maintenance of the retaining ring, especially a precise right angle of the retainer. However, in order to ensure trouble-free and correct installation, RHI provides a supervisory service.
Due to the various design options and combination possibilities with the Thrust Lock System, tailor-made concepts for every individual case can be developed. To select the most appropriate concept, prevailing operating conditions, existing design, lining history, gained experiences, and wear patterns need to be jointly evaluated with the customer. Following RHI´s recommendation, another three Thrust Lock Systems have been installed in Austria, Ecuador and Canada.
Andreas Wiry, RHI AG, Industrial Division, Vienna, Austria.
For further information, contact VMC Refractory Solutions Pvt. Ltd., Mumbai, E mail: email@example.com