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1. What issues should be considered when installing or removing plug-type elbow bends? Plug-type elbow bends are usually installed on the radiant furnace tubes of heating furnaces where coking is likely to occur. During medium or major repairs, in order to check for coking inside the furnace tubes, the plugs on the return elbows must be removed; after checking for coking inside the tubes, those plugs are put back in place. When removing or installing plugs, the following points must be taken into consideration: (1) Elbows and plugs should be numbered in sequence during removal, so as to ensure they are installed correctly. (2) The plug should be removed when the temperature at the elbow drops to 100–150°C, to avoid difficulty in removal due to too low a temperature. (3) When removing the set screw of the elbow, do not use heat. (4) After removing the plug, gently remove any carbon deposits from the contact surface between the plug and the elbow using a fine wire brush or sandpaper, and then grind it with an abrasive. It is necessary to ensure that there is a continuous contact surface with a width of at least 3 mm. (5) When installing the seal surface of the plug and the threads of the set screw, a uniform coating of flake graphite powder mixed with machine oil should be applied. (6) When removing the pipe end from the elbow, mechanical methods are preferred, but oxygen cutting can also be used. During cutting, the pipe wall must not be punctured; the remaining portion should still be removed by mechanical means. 2. What are the conditions for replacing an elbow? (1) It should be replaced when the wall thickness of the elbow is less than the allowable value calculated using the following formula. S=0.75pD/σ] Where: S – minimum allowable wall thickness of the elbow, mm ； p – operating pressure, MPa ； D – Outer diameter of the elbow (i.e., twice the distance from the center of the pipe hole to the outer surface of the elbow), mm ； σ] – Allowable stress, MPa, see Table 14–2. (2) If the threading on the plug of the elbow fitting is cracked, missing teeth, or has slipped, it should be replaced. (3) Plugs with deep corrosion pits at their tips, with a diameter greater than 2 mm and a depth of 0.4–0.6 mm, should be replaced. 3. Under what circumstances should the furnace tubes be replaced? After being in use for a certain period of time, the furnace tubes of a heating furnace should be replaced if any of the following conditions are observed. (1) Due to severe corrosion, erosion, or tube wall failure, the wall thickness of the furnace tube is less than the calculated allowable value. (2) Due to bulges, cracks, or network-like cracks. (3) The curvature between two adjacent supports of the horizontal furnace tube is greater than twice the outer diameter of the furnace tube. (4) The outer diameter of the furnace tube is 4%–5% larger than the original outer diameter. (5) Swelling gap: Repeated crimping during use results in an expansion exceeding the specified limit. (6) Swelling corrosion, peeling; the exposed part of the swelling is less than 2–3 mm. Table 14-2 Allowable stress σ] of elbow materials, Operating temperature/°C, Elbow material σ]/MPa, Operating temperature/°C, Elbow material σ]/MPa: Carbon steel, Alloy steel, Carbon steel, Alloy steel – 275, 300, 325, 350, 375, 400; 68.7, 67.2, 65.7, 63.8, 59.8, 54.9; 64.7, 42.5, 450, 475, 500, 525, 550, 504; 44.1, 38.3, 314; 59.8, 54.9, 494, 323.2, 34.3, 27. Whether furnace tubes need to undergo a hydrostatic test one by one before installation depends on the specific circumstances. If the furnace tubes in use come with complete certificates of conformity, it is not necessary to conduct hydraulic tests on each tube on-site. However, if the furnace tubes have been in use for many years or are of unknown origin without a certificate of conformity, they must undergo a hydraulic test one by one before use. The hydrostatic test pressure is calculated using the following formula: p = 2sσ / D, where p is the test pressure in Mpa ； D – Pipe diameter ； cm ； s – wall thickness, cm ； σ] – Allowable stress, 73.6 MPa for carbon steel and 88.3 MPa for alloy steel. The maximum test pressure for the furnace tube shall not exceed 15.7 MPa. The furnace tube is held at the test pressure for 5 minutes, then the pressure is reduced to the operating pressure for inspection; the tube is considered qualified if it shows no leaks, no cracks, and no deformation. 5. What is the allowable error for the external dimensions of the furnace tubes? What requirements must their appearance meet? Before installation, the external dimensions of the furnace tubes should be carefully checked; the allowable error for these dimensions is: length ≤ ±15mm ； Outer diameter ≤ +0.5 -1.75 % ； Wall thickness ≤ +20 -10 % ； Curvature ≤ 1.5 mm/m ； Total length curvature < 8mm. The outer surface of the furnace tube should be smooth, without defects such as thick scales, inclusions, or cracks. 6. What are the requirements for cutting off the old furnace tube when replacing it? When replacing the furnace tube, there are two scenarios for cutting off the old one: (1) If what is connected to the furnace tube is an elbow, then when replacing the furnace tube, the tube is first cut off using an oxygen-acetylene flame outside the elbow. When removing the inner pipe end of the elbow, mechanical methods are preferred; oxygen cutting can also be used, but the pipe wall must not be pierced. The remaining portion should still be removed using mechanical methods to avoid damaging the elbow. (2) If what is connected to the furnace tube is an elbow, then when replacing the furnace tube, oxygen cutting should be used outside the elbow; when both the furnace tube and the elbow are made of alloy steel, mechanical cutting is preferred. 7. What issues should be considered when locally replacing furnace tubes? Due to unstable operation or inadequate inspection during maintenance, furnace tubes may suddenly deform or even break during production, which necessitates emergency repairs. During emergency repairs, in order to save time, only simple measures are taken, typically involving the partial replacement of the furnace tubes. When performing a local replacement, the following requirements should be observed: cut off the burned sections from both ends and replace the tube in the middle with a new one. When welding a new tube, it is possible to cut off the guide tube at the bottom of the old tube first, or to remove the hooks holding the furnace tube, so that the entire furnace tube can be moved about 200–300 mm away from the furnace wall to facilitate welding. After welding and passing the inspection, weld on the guide tube or install the furnace tube pull hook. The above methods are applicable to vertical tube-type heating furnaces. 8. What issues should be considered when replacing an entire furnace tube? When replacing an entire furnace tube, first cut off the section of tube from outside the elbow, prepare the groove properly. The furnace tube is inserted from the cover plate of the top elbow; whether to align the lower end first or the upper end first is determined by the on-site workers based on the specific circumstances. After welding the upper and lower openings, an inspection must be carried out; only after it is confirmed to be satisfactory should the guide tube and furnace tube hooks be installed. 9. What are the requirements for welding furnace tubes together, as well as for welding furnace tubes to sharp bends? The quality of furnace tube welding is directly related to whether the heating furnace can operate safely. Therefore, great care must be taken when welding furnace tubes; carelessness is absolutely unacceptable, as the consequences could be unimaginable. The following requirements must be followed when welding furnace tubes: (1) Those who participate in the welding of furnace tubes must pass an examination before they can carry out the welding work. (2) The furnace tube must come with a certificate of conformity; if no such certificate is available, it must undergo a thorough inspection, and only after being found to be satisfactory can it be used. (3) When welding the furnace tubes, it is necessary to strictly follow the construction specifications for furnace tubes as well as the requirements specified in the construction drawings. (4) When welding the furnace tubes, a temporary shed should be set up to protect them from wind, rain, and snow. During winter construction, the temperature inside the shed should be maintained above 10°C. (5) The furnace tubes should preferably be whole; if welding is necessary due to insufficient length, each tube is allowed to have one weld joint. The furnace tube welds should be located in the low-temperature area or outside the furnace wall; it is necessary to avoid placing them at the center of the furnace tube or near the flanges. (6) The pipe end groove shall be cut using mechanical processing methods. For Cr-Mo steel, when high-speed cutting is used, a coolant should be employed to prevent cracking caused by the hardening of the surface layer. Oil and rust spots on the groove surface and within a range of 20 mm must be removed. The groove angle is typically set at 60º–70º±5°, while the root gap and alignment tolerance can be controlled within 1 + 0 – 0.5 mm. (7) When aligning the weld joints of the furnace tubes, an inner diameter misalignment of less than 1 mm is allowed; the degree of bending is checked using a 400 mm ruler, with the gap between the ruler and the tube wall being less than 2 mm. When aligning and welding the furnace tube with the sharp bend elbow, the concentricity error should not exceed 1 mm, and forced centering is not allowed. After welding the radiant furnace tubes of a cylindrical furnace, the error in the pitch diameter should not exceed 0.2%, and it must not be greater than 12 mm. (8) The welding materials used for furnace tube welding shall meet the following requirements: ① The electrodes and wires used for furnace tube welding should be selected in accordance with the chemical composition of the furnace tubes and the welding conditions, in compliance with the following current standards: «Carbon Steel Electrodes» (GB5117), «Low-Alloy Electrodes» (GB5118), «Stainless Steel Electrodes» (GB983), «Stainless Steel Bars and Wires for Tungsten Inert Gas Shielded Welding» (GB4223), «Stainless Steel Coils for Welding» (GIM241), «Stainless Steel Wires for Welding» (GB4242), and «Welding Wires» (GB1300). ② For the welding of chromium-molybdenum heat-resistant steel, electrodes that correspond to the chemical composition and mechanical properties of the base metal should be used. (9) After the furnace tubes are welded, the quality of the welds must meet the requirements specified in the design drawings. 10. What requirements must be met when laying the flue bricks of a burner? The following requirements must be satisfied when laying the flue bricks: (1) The flue bricks must come with a certificate of conformity; if no such certificate is available, they must undergo a thorough inspection before they can be used. (2) If the flue bricks are clay-based refractory bricks, use fine-grained clay-based refractory mortar during construction. (3) When laying the flue bricks, it is necessary to strictly follow the brick-laying specifications and construction drawings. (4) The brick joints must be less than 2mm. (5) The brick joints must be offset from the gas nozzles. (6) The mud filling in the brick joints must be complete and uniform, with a fullness rate of not less than 95%. (7) The fire-facing side of the bricks must not be missing edges or corners, nor shall it be processed. (8) When laying bricks, only a wooden hammer may be used for alignment; the use of an iron hammer is strictly prohibited. (9) The brick joints on the surface should be hooked.