Why Thermocouple Protection Tubes Fail Early or Drift in Glass Furnaces

Why Thermocouple Protection Tubes Fail Early or Drift in Glass Furnaces

After twelve years supporting glass melting operations, a repeated issue is that thermocouple readings gradually drift or the protection tube fails well before the scheduled campaign. The furnace temperature is stable and the thermocouple element is the correct type, yet the indicated temperature shifts by tens of degrees or the tube cracks or thins at the glass line or in the combustion space. The underlying factors are the rate of corrosion by alkali vapors or molten glass and the thermal conductivity of the tube material, which together determine both service life and the lag in temperature response.

Alumina tubes (typically 99 % Al₂O₃) resist attack by molten glass and alkali vapors reasonably well because of their chemical stability, but they have moderate thermal conductivity (around 25–30 W/m·K) and limited thermal-shock resistance. Silicon carbide tubes, particularly sintered or recrystallized grades, offer higher thermal conductivity (often 30–50 W/m·K or more) and better shock resistance, which reduces response lag, but some grades can experience surface oxidation or reaction in strongly alkaline atmospheres. Corrosion proceeds by reaction of alkali oxides with the tube surface, forming a low-melting layer that slowly dissolves or spalls. In immersion positions the attack is faster; in the crown or superstructure the attack is driven by vapor condensation. Wall thinning increases the risk of mechanical failure, while a thick or low-conductivity wall increases the time constant of the measurement.

In a container-glass furnace operating at 1450–1500 °C, two closed-end protection tubes were installed side-by-side in the crown for 90 days. The alumina tube (original wall 6 mm) showed 1.2 mm average thinning at the hottest zone and a measured time constant of approximately 45 seconds when the burner load was changed. The sintered silicon carbide tube (original wall 5 mm) showed 0.7 mm thinning under the same conditions and a time constant of approximately 28 seconds. No through-cracks occurred in either tube. After removal, the alumina surface had a thin reaction layer containing sodium and potassium; the silicon carbide surface had a thin silica scale with limited alkali penetration. Temperature readings from the silicon carbide tube tracked the process change more closely, while the alumina tube lagged enough to require a larger control dead-band.

Tube selection therefore balances corrosion resistance against thermal conductivity and wall thickness for the specific location. For thermocouple protection tube glass furnace corrosion control, the practical variables are material purity, density, and exposure to alkali vapor versus direct glass contact. Alumina vs SiC thermocouple sheath glass performance is confirmed by measuring wall loss and response time under the same furnace conditions rather than by laboratory corrosion data alone. Thermocouple tube lifespan molten glass is extended when the material resists both chemical attack and thermal shock at the operating temperature. When readings drift or tubes fail early, the confirming evidence is wall-thickness measurements and the observed time lag during a known temperature change. Choosing a higher-conductivity, adequately corrosion-resistant tube, as shown by the reduction from 1.2 mm to 0.7 mm thinning and from 45 s to 28 s time constant in the crown comparison, improves both measurement responsiveness and service life in glass-furnace service.