Thread Content
XII. Furnace Lining 1. What regulations must the design of the furnace lining comply with? The design of the furnace lining shall meet the following requirements: (1) Under conditions of an external temperature of 25°C and no wind, the design temperature of the outer surfaces of the tubular furnace body and the air preheating system shall not exceed 80°C. (2) The lining material should be selected reasonably through economic comparison, taking into account factors such as the structural shape and size of the tubular furnace wall, operating temperature, material availability, and construction conditions. (3) Expansion joints should be provided in the furnace lining structure according to the properties of the materials used. The expansion joints of different layers in a multi-layer lining shall not be connected to each other. (4) Except for furnace linings with a cast structure, an anti-corrosion protective layer is generally advisable to be applied on the inner side of the furnace wall steel plates. 2. What regulations must be followed in the design of brick structures? The design of brick structures shall comply with the following regulations: (1) On the back side of a segmented load-bearing furnace wall, at least 10% of the bricks must be connected to the steel structure. (2) Upright cylindrical brick furnace beds have no expansion joints along the circumferential direction. When the height is below 10 m, a expansion joint can be left at the top of the furnace wall ； For furnace walls with a height of over 10 m, an expansion joint can be provided every 4–5 m along the vertical direction ； Except for the lowest layer of furnace wall, which is supported by the furnace bottom, the weight of all upper layers should be supported by the backing bricks of each layer. 3. What regulations must the design of cast linings comply with? The design of cast linings shall meet the following requirements: (1) The heat-resistant concrete materials and gradation should be selected appropriately based on the design temperatures of different areas. (2) For the double-liner structure, the minimum thickness of the hot face layer shall be 75 mm. (3) Insulation nails for the cast lining: ① The furnace wall and roof lining of the radiation chamber should be secured using 18Cr–8Ni or 25Cr–20Ni type stainless steel insulation nails. ②The length of the insulation nails must be no less than 70% of the insulation thickness of the corresponding layer, and the distance from the end of the insulation nail to the fire-facing side should be no less than 12 mm. ③The insulation nails are arranged in a square pattern. The maximum spacing between the insulation nails in the furnace lining should be twice the total thickness of the lining; however, on the furnace wall, this maximum spacing must not exceed 300 mm ； It shall not be greater than 230 mm on the furnace top. The directions of the prongs of adjacent insulation nails should be staggered. ④When thermal insulation linings are used for elbow boxes, chimneys, and flues, their thickness shall not be less than 50 mm, and the spacing between insulation nails shall not exceed 200 mm. 4. What regulations must be followed in the design of fire-resistant fiber structures? The design of fire-resistant fiber structures shall comply with the following regulations: (1) The thermal face layer of fire-resistant fibers should be made of fiber mats or needle-punched blankets with a density of not less than 128 kg/m3. The backing material for refractory fibers can be refractory fiber products with lower density or other fibrous insulation materials. (2) Reversible fiber structures without a reliable protective layer are generally not suitable for the convection zone of tubular furnaces. (3) The refractory fiber lining shall be secured using 18Cr-8Ni and 25Cr-20Ni stainless steels or ceramic components. The maximum distance between the fixed point and the edge of the refractory fiber product is 75 mm. 13. Operation and Accident Handling 5. What are the requirements for the overall pressure test of the furnace tube system? The overall pressure test of the furnace tube system shall be carried out in accordance with the following rules: (1) Hydrostatic test: The hydrostatic test pressure is calculated using the following formula: Pr = 1.25σ] / σ]t, where Pr represents the hydrostatic test pressure in MPa ； P – Design pressure, MPa ； σ ] – Allowable stress of the material at the test temperature, MPa ； σ ]t – Allowable stress of the material at the design temperature, in MPa. When the medium being heated inside the pipe is a hydrocarbon, the Pr value must be no less than 3.7 MPa. The stress in the pipe under the test pressure: for high-quality carbon steel and alloy steel, it must not exceed 80% of the yield strength ； For stainless heat-resistant steel, it shall not be greater than 70% of the yield strength. For coiled tubes connected by elbows or flanges, the hydrostatic test pressure Pr shall not be greater than the test pressure of the elbow or flange. Clean water should be used for the hydrostatic test. The water used for pressure testing austenitic stainless steel coils must have a chloride ion content of no more than 25 ppm (1 ppm = 10-6), and the water temperature must be at least 5°C. (2) Pneumatic testing: When hydraulic testing cannot be used due to design requirements or other reasons, pneumatic testing shall be employed. The pneumatic test pressure Pr′ should be calculated using the following formula: Pr′ = 1.15pσ] / σ]t, where Pr′ is the pneumatic test pressure in MPa. When the medium being heated inside the pipe is a hydrocarbon, the Pr′ value must be no less than 3.4 MPa. Stress in the pipe at test pressure: For high-quality carbon steel and alloy steel, it shall not exceed 70% of the yield strength ； For stainless heat-resistant steel, it shall not be greater than 60% of the yield strength. For coiled tubes with elbows and flange connections, the Pr′ value for the pressure test shall not be greater than the test pressure of the elbow or flange. The gas used for pressure testing should be dry and clean air, nitrogen, or other inert gases, with the gas temperature being no lower than 15°C. 6. Which types of heating furnaces require drying? All newly built heating furnaces whose furnace walls are made of refractory bricks or lined with lightweight heat-resistant concrete need to be dried ； If the furnace walls of an old furnace have been repaired over a large area, and the materials used for the repairs are still refractory bricks or a lightweight heat-resistant concrete lining, the furnace still needs to be dried out. For the entire furnace wall of a newly built heating furnace, or the furnace wall of an existing furnace, if ceramic fiber fabric is used as the material, there is no need to carry out furnace drying. 7. What is the purpose of baking the furnace? The purpose of baking the furnace is to slowly remove the moisture accumulated in the furnace walls during construction, and to ensure that the refractory mortar is fully sintered. If this moisture is not removed, as the furnace temperature rises rapidly at the start of operation, this moisture will evaporate quickly, causing the brick joints to expand and cracks to form; in severe cases, this can lead to the collapse of the furnace walls. 8. What medium is passed through the furnace tubes during drying? What is the maximum temperature at the outlet of the furnace tubes? During drying, steam is passed through the furnace tubes. Steam outlet temperature inside the furnace tube: for 10# and 20# steel, it shall not exceed 400℃ ； Cr5Mo furnace tubes should not exceed 500°C. 9. Which type of thermocouple is used to control the furnace temperature during baking? During baking, the thermocouple located at the radiation outlet is used to control the furnace temperature. 10. What curve is used to control the furnace temperature during baking? When a heating furnace is being baked, it is controlled according to the baking curve shown in Figure 13-1.