Thermocouple Protection Tubes Made of Silicon Carbide Ceramic Using Pressureless Sintering and CIP Technology for Durability

Manufactured by cold isostatic pressing (wet-bag method) followed by pressureless sintering, this SSiC thermocouple protection tube has an outer diameter of 30mm and inner diameter of 14mm (wall thickness 8mm). It is designed for large industrial furnaces, high-temperature high-pressure reactors, and severe erosion environments. With increased wall thickness and greater safety margin, this specification is a reliable temperature measurement protection solution for demanding conditions such as coal gasifiers, large hot blast stoves, sulfuric acid fluidized bed furnaces, and non-ferrous metal smelting.

The 31.8*19.1mm OD/ID specification is commonly found in imported pressureless sintered submicron SiC tubes for ultra-high-temperature applications such as coal gasifiers. The 30*14mm size features an 8mm wall thickness, providing significantly higher mechanical load capacity and erosion resistance compared to the 25*20mm (2.5mm wall thickness). The 14mm ID accommodates larger-diameter thermocouple elements, making it suitable for long-term continuous temperature measurement in large industrial installations.

The following table presents typical technical data for pressureless sintered silicon carbide (SSiC). The 30*14mm product fully meets these values.

With flexural strength of 380–550 MPa and compressive strength >2500 MPa, SSiC exhibits the highest level of mechanical performance among all silicon carbide materials.

Cold isostatic pressing is the key technology for producing 30*14mm thick-walled long tubes. The specific process parameters and advantages are as follows:

Unique advantages of isostatic pressing for 30*14mm thick-walled tubes:

Isostatic uniform pressing ensures highly consistent density distribution across the 8mm thick wall section, resulting in uniform sintering shrinkage, effectively preventing wall thickness eccentricity and stress concentration. It eliminates common defects such as density gradients and microcracks found in conventional dry pressing or extrusion, fundamentally improving the tube's resistance to cracking under high temperature and high pressure.

SSiC contains no free silicon phase and significantly outperforms RBSiC above 1400°C. The fully dense structure of SSiC withstands gas pressures up to 31 MPa, meeting the temperature measurement requirements of high-pressure applications such as coal gasifiers.

1. Ultra-high temperature stability: Long-term service up to 1750°C in oxidizing atmospheres; up to 2000°C in protective atmospheres or vacuum – far exceeding the limits of metals and RBSiC.
2. Very high thermal conductivity: 120–270 W/(m·K) – 5 to 10 times that of stainless steel and 4 to 5 times that of alumina – ensuring fast thermal response even with an 8mm wall thickness.
3. Excellent thermal shock resistance: Low coefficient of thermal expansion (3.8*10⁻⁶/K) combined with high thermal conductivity enables resistance to temperature differences up to 500°C; especially suitable for cyclic furnace operations.
4. Comprehensive corrosion resistance: Long-term stable use in hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid (unique to SSiC), and molten aluminum, zinc, tin, lead, etc.
5. Ultra-high mechanical strength: Flexural strength ≥380 MPa, compressive strength >2500 MPa, hardness next only to diamond – withstands long-term erosion by high-temperature, high-pressure gas flows and fly ash.
6. Fully dense and gas-tight: Zero porosity, gas permeability <10⁻¹⁰ atm·cc/s – maintains seal integrity at pressures up to 31 MPa, protecting the internal thermocouple from external corrosive atmospheres.
7. Isostatic-pressing-specific precision: Uniform wall thickness, smooth inner bore, excellent straightness – even stress distribution in the 8mm thick wall effectively avoids thermal stress cracking.
8. Long service life: Design life of 2–5 years under continuous use at 1600°C in oxidizing atmospheres; in high-temperature dust-laden gas flow conditions, service life is 5–10 times longer than metal alloy tubes or RBSiC tubes.

• Sealing method: Recommended loose flange + flexible graphite gasket; avoid rigid mechanical seals. For high-pressure applications, mechanical seal leak-proof arrangements can withstand pressures up to 12 MPa.
• Expansion clearance: Maintain a 3–5 mm expansion gap between the tube wall and the furnace wall; keep at least 10 mm safety distance between the tube bottom and internal furnace components to prevent thermal expansion contact and stress cracking.
• Fit tolerance: For the 30*14mm specification, ID tolerance is ±0.15 mm (as-isostatically-formed) or ±0.05 mm (after fine grinding); recommended clearance with sheathed thermocouple is 0.2–0.4 mm.
• Handling protection: SSiC is a brittle ceramic material with limited impact toughness; avoid collisions during installation and transportation. Protect the tube end face from hard impacts that could cause edge chipping.