New tuckstone refractory solution for long life glass furnace superstructure

Date
May 19th, 2021
Description

An optimized refractory solution with an improved design to increase insulation and reduce corrosion inside glass furnaces.

 

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.

 

issues-triggered-by-tuckstone-breakage

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.

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typical-thermal-gradient-on-tuckstone
Description

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.

stress-deformation-tuckstone
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).
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stress-deformation-tuckstone
Description

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).

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.

drawing-and-picture-tuckpro-tuckstone-with-sefpro-shield

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 0,5 W.m-1.K-1 reduces the stress on the sitting face of the tuckstone by 30% to 60% depending on the area being considered.

  With SEFPRO shield Without SEFPRO shield
Radius 16Mpa 40Mpa
Sitting face 87Mpa 120Mpa

Table 1. Maximum of first principal stress on two areas of tuckstone

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).

Low thermal conductivity  λ < 0.8 W/(m.K) up to 800°C
High mechanical resistance MOR > 15 Mpa up to 900°C

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.

compressive-stresses-and-thermal-shock-resistance

*SEFPRO, Le Pontet, France
**Saint-Gobain Research Provence, Cavaillon, France