February 19th, 2021
Michel Gaubil*, Thibaut Chuffart*, Isabelle Cabodi** and Pierrick Vespa** explain the main causes for tuckstone breakages and discuss how a long-life solution has been developed.
Tuckstones have several essential functions in the glass melting furnace, such as superstructure stability, steel frame protection and shielding the top face of the soldier block from heavy heat radiation. Tuckstone breakage is a frequent issue in most glass furnaces and often encountered after few years of operation. The premature loss of tuckstone’s nose often leads to trouble. As seen in Fig 1, without protection from tuckstone’s nose, a glass furnace may experience higher glass contamination from superstructure rundowns, increased metal line corrosion, and superstructure destabilisation due to potential creeping of the steel frame.
These issues generally require complex repairs such as ceramic welding, additional brickwork or even tuckstone hot replacement where possible. In spite of these repairs, the furnace continues to operate in a degraded mode. Hence, to extend superstructure and furnace lifetime, SEFPRO has researched the reasons behind premature tuckstone breakage in order to provide an innovative redesign of the tuckstone system with improved performance.
Understanding of tuckstone breakage
In addition to typical corrosion processes in the furnace atmosphere, tuckstones may break due to the tremendous thermomechanical stress they encounter. These stresses come from both thermal gradient (Fig 2) and thermal shocks induced by cold air blowing and thermal cycling during tank over-coat.
SEFPRO performed analysis on post mortem tuckstones to assess the impact of these different stresses, to better understand the mechanism of rupture that was occurring. It highlighted that the cracks were typically initiated on the cold face and propagate towards the hot face until they met a viscoplastic area, otherwise it generally led to tuckstone rupture. This argues for a thermomechanical cause for tuckstone breakage rather than singularly chemical or thermal reasons.
A thermomechanical model was developed (Finite Element Analysis) to better understand the rupture process. Thanks to the model, high levels of stresses were spotted on the cold side of the tuckstone linked to the strong thermal gradient between cold and hot faces. The tuckstone cold face is placed on the air-cooled steel frame, with high thermal exchange. The resulting stresses are worsened by the thermal shock induced during furnace special events. This is the case especially when the air blowing is stopped and restarted for tank over-coating operations or other unexpected shut downs with interrupted cooling. Fig 3 describes the stress evolution in the tuckstone in these conditions.
This model has been validated by thermal measurements and cracks survey conducted in several furnaces: most notably affected area (tuckstone sitting face and radius area) matches with the most cracked areas, as it was observed on tuckstone samples taken during furnace dismantling (Fig 4).
Cracks initiated on tuckstone bottom/cold side by thermal gradient or thermal shocks, may then propagate until rupture of the block because of further thermal events. This dependence on consecutive events can also explain differences of performances from one furnace to another.
Towards highest performance tuckstone
With major reasons for tuckstone rupture being identified, SEFPRO designed a new tuckstone solution, named TuckPro, which resulted in the combination of three essential factors, detailed below:
1. Thermal protection: a composite system integrating the SefproShield, a newly developed rigid ceramic insulating board for consistent insulating power over time.
This high-performance thermal protection significantly reduces thermal stresses responsible for the rupture. As illustrated in table 1, from our numerical model, a thermal insulation material with a conductivity of
The insulating layer must have considerable life expectancy, under high compression stress at high temperatures, and facing thermal shock with potentially corrosive vapor present. In in these conditions, commonly used ceramic fibers, felts or calcium silicate boards do show insufficient performance over time; under compressive stress, at high temperature, the effect of sagging and compression leads to loss of insulating power.SefproShield, a moulded ceramic rigid board, made of alumina and silica, without any ceramic fibers content, perfectly fits the application. It has low thermal conductivity and high mechanical resistance (Table 2).
As illustrated in Fig. 6, under compressive stress corresponding to the superstructure weight (< 1MPa), it is observed that the maximum deformation of SefproShield remains very low whereas ceramic fibres felt may lose up to 70% of initial thickness. Furthermore, this innovative insulating board can also withstand high thermal shock from 1000°C to room temperature without damage.
In addition to its good physical properties, the SefproShield shows a very high corrosion resistance when exposed to alkaline vapours (much better than ceramic fibres, Fig 7)
In this test, SefproShield kept its initial thickness, and only a slight vitrification was observed on the face exposed to vapours, whereas ceramic fibres board felt was totally destroyed.
By combining the fused cast tuckstone with this optimised insulation of the entire sitting face, including the whole radius area, SEFPRO developed an “all-in-one” tuckstone solution (Fig 8).
The performance of this solution has been checked in field testing, at a float furnace location. Thermal evolution inside the tuckstones has been monitored and compared with and without SefproShield insulation. It has shown that SefproShield efficiently reduces thermal gradient and thermal shock amplitude during air blowing start up, and that this insulation remains consistent after several years (Fig 9 and Table 3).
2. Material Improvement
Today, most tuckstones in soda lime glass furnaces are made of AZS materials, typically containing around 32 % of Zirconia. Nevertheless, it has been proven that an increase in Zirconia content has a strong benefit on the tuckstone lifetime.
When submitted to harsh running conditions, High Zirconia fused cast material (95 % ZrO2) with very low glassy phase content compared to fused cast AZS, presents better corrosion resistance. As illustrated, inside a glass furnace, after several years, remaining thickness of High Zirconia Materials is significantly higher than AZS one (32 % ZrO2) (Fig 10).
Even within the AZS family products, the increase in Zirconia content from 32 % ZrO2 content to 41 % ZrO2 may contribute to limit corrosion level. Thus, switching from AZS 32% ZrO2 to AZS 41% ZrO2 or even to High Zirconia tuckstones is also a way to optimize the tuckstone corrosion resistance and lifetime.
3. Improved Design
Sharp angles on tuckstones, especially on cold face, may contribute to crack initiation due to stress concentration. Moreover, because of boundary conditions of the system, a sufficiently thick tuckstone is ideal to limit thermal gradients in the block. From these observations, rounded tuckstones with constant section of more than 200 mm thickness should be preferred. An obtuse angle to connect the rounded part of the tuckstone to the bottom part, will limit the stresses.
All these suggestions were gathered in one tuckstone “ideal case” design Fig 11.
Tuckstones are critical to the efficient operation of the furnace and can break prematurely due to the thermal shock and high thermal gradients they face during the campaign. This leads to a degradation in performance during the continued furnace operation. In many cases it is required to implement dedicated maintenance operations. To avoid these issues and all consequent damages it could generate in the furnace, SEFPRO developed an improved tuckstone refractory solution named Tuckpro, combining several improvements in terms of insulation, design and material properties. This rounded and sufficiently thick tuckstone (>200mm) is insulated with a high performance innovative board, named SefproShield. For an optimised lifetime, the tuckstone material may also be upgraded from AZS 32% ZrO2, to AZS 41% ZrO2 or even High Zirconia Fused Cast refractory.
Authors : Michel GAUBIL, Thibaut CHUFFART – SEFPRO, Le Pontet, France
Isabelle CABODI, Pierrick VESPA - SGR Provence, Cavaillon, France