Lubisol Engineering Co. has been engaged closely with the problems of thermal insulation, hermetic sealing and hot repair of silica crowns for more than 30 years, studying case histories of rat holes and gathering information about more than 100 glass furnaces.
Rat holes are caused by condensation corrosion in loose joints of silica crowns. Regardless of the improved quality of the silica bricks they are still a problem for the glass industry worldwide.
The experience of Lubisol shows that the early detection of hot spots in the crown may help in the early detection of rat holes enabling an easy early hot repair.
Condensation corrosion in the silica crown is a very slow process taking at least 6-12 months before the hot spot is converted into a rat hole. The regular inspection of the crown by measuring the cold face temperatures in different areas of the crown may help for early detection of the hot spots. The recommended inspection period is once per month. The cold face temperatures of silica crowns with standard light silica insulation are usually between 180 and 220°C. In cases when this temperature becomes 250-300°C this may be an early indication for a hot spot formation. By removing the thermal insulation in this area one can make a hot repair by patching the crown with Lubisol S-Seal and this will stop the corrosion process. The action is easy and simple and this may reduce to a great extent the formation and further enlargement of the rat-hole.
Lubisol had promoted this simple method among its clients with great success. In all cases the regular monthly inspections and reports have contributed for achieving a long service life of the silica crowns where such inspections have been regularly applied.