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1. What are the common thicknesses for furnace tubes? After calculating the wall thickness of the furnace tubes in a heating furnace, the appropriate thickness is selected according to Table 5–1. Table 5-1 Common wall thicknesses for furnace tubes. Outer diameter of furnace tube/m, Wall thickness of furnace tube/m, Outer diameter of furnace tube/m, Wall thickness of furnace tube/m: ϕ60, ϕ89, ϕ102, ϕ114; 5, 6, 8; 6, 8, 10; 6, 8, 10, 8, 10; ϕ127, ϕ152, ϕ219; 6, 8, 10; 6, 8, 10, 16, 20; 8, 10, 12, 14. 2. What regulations should be followed for the selection of furnace tube materials and the calculation of their wall thicknesses? The selection of furnace tube material and the calculation of wall thickness shall comply with the provisions of \"Method for Calculating Wall Thickness of Furnace Tubes in Refineries\" SHT 3037. 3. Is it required that the material grades of the radiant furnace tubes and the convective furnace tubes be identical? Since the operating temperatures in the convective section and the radiant section differ, as well as the heat intensity on the surface of the furnace tubes, it is possible to use two different materials if necessary ； The material of the shielding tube should be the same as that of the radiation tube. If there is sufficient data on the heat transfer characteristics, temperatures, and medium corrosion in various areas of the radiation chamber, the furnace tubes within it should also be made of different materials in different sections. Sometimes, the operating conditions inside the tube remain unchanged, but the tube material may need to be upgraded due to higher heat intensity; in such cases, a lower heat intensity can also be chosen. Although this results in more furnace tubes, the material cost is lower, making it still economical. 4. How is the diameter of the radiant furnace tube determined? The cross-sectional area of the furnace tube is chosen based on the mass flow rate inside the tube. But once the flow rate is determined, either one large pipe or several small pipes can be used. The larger the pipe diameter, the fewer the channels; the thicker the pipe wall ； The smaller the pipe diameter, the more the number of paths, and the thinner the pipe wall. Fewer pathways make control easier, but thicker pipe walls increase costs. From the perspective of strength calculations, the thickness of the furnace tube is proportional to its outer diameter. With unchanged operating conditions, the thickness of the φ219 furnace tube is almost twice that of the φ114 furnace tube. Therefore, unless there are mandatory or other special requirements, from an economic perspective, the use of ϕ219 furnace tubes is generally not recommended. If too small tubes are used, it becomes difficult to distribute the paths evenly when there are many of them ； Although the furnace tubes are eliminated, it increases the investment in the control system. Therefore, the size of the radiant furnace tube should be considered comprehensively. The selection of the diameter and number of tubes in high-pressure furnace tubes requires even greater care. 5. How is the diameter of convection furnace tubes determined? A larger diameter for the convection furnace tubes results in a thicker tube wall, fewer channels, and easier flow control ； A smaller diameter of the furnace tube results in thinner walls, more tubes, and greater difficulty in flow control ； These features are the same as those of the radiation tube. When the diameter of the radiant furnace tubes varies, as long as the ratio of \"tube center distance to tube diameter\" remains constant, the area occupied by the tubes stays the same. In other words, regardless of the size of the furnace tube used, the size of the radiation chamber remains largely unchanged at the same heat transfer area. The convective chamber is completely different; assuming that the width of the convective chamber and the \"tube center-to-center distance/tube diameter\" remain essentially constant, the area occupied by the furnace tubes in each row of convective tubes is the same. However, the distance between adjacent furnace tubes varies depending on the tube diameter; the larger the diameter of the furnace tubes, the greater the spacing between them and the higher the convection chamber becomes. Therefore, using smaller-diameter tubes for the convection chamber can reduce its height. Furthermore, the heat transfer coefficient of the outer membrane of the convective tube also increases as the tube diameter decreases, and it is inversely proportional to the 0.333 power of the tube’s outer diameter. Convection tubes with a smaller diameter offer more advantages compared to those with a larger diameter; therefore, boilers’ convection sections use multiple small-diameter tubes, and uneven water distribution does not lead to serious consequences. In the heating furnaces of petrochemical plants, convection tubes and radiation tubes are generally connected in series. Whether the feed oil is distributed evenly among the various coils is a key factor affecting the lifespan of the furnace tubes, the operating cycle, and the quality of the oil; therefore, the convection tubes used are larger than those in boilers. However, under normal conditions, φ219 furnace tubes are not recommended for use in convection chambers. The superheated steam pipes in the convection chamber are supposed to have a small diameter, but sometimes, in order to standardize the pipe diameter, and to facilitate interchange with the oil pipes as well as to simplify design and component manufacturing after changes in heat load, the same specifications as those of the oil pipes are often used for the relatively small number of steam pipes as well. 6. How to consider the service life of the furnace tube wall thickness? The service life of the furnace tube wall thickness is determined based on the following factors. (1) For long-term operation of more than 8,000 hours per year, it is designed for 100,000 hours. (2) Operating hours of 6,000–8,000 hours per year, designed for 80,000 hours. (3) Start up with a gap, designed for 20,000 to 40,000 hours. 7. How to consider the corrosion margin of the furnace tubes? In the absence of relevant corrosion data, the minimum corrosion margin for the furnace tubes shall be selected as follows. (1) For hydrocarbon processing, it is 3 mm for carbon steel and ferritic steel. (2) For hydrocarbon processing, the thickness of austenitic alloy steel is 1.5–3 mm. (3) For steam, the value for ferritic alloy steel is 1.5 mm. 8. What are the advantages and disadvantages of thickening the wall thickness of furnace tubes? Since oil flows inside the furnace tubes while they are heated by an open flame on the outside, the temperature distribution is uneven, which can lead to localized overheating. An explosion resulting from this can cause severe fires and substantial losses; therefore, the design of furnace tubes must be handled with care to ensure safety. But don’t make it thicker arbitrarily either. The furnace tubes being too thick does increase their service life against certain types of corrosion, but it offers no benefits for intergranular corrosion and stress corrosion, which are not related to thickness; instead, it increases thermal stress. Furthermore, the metal consumption for the furnace tubes accounts for 40% to 50% of the total steel used in the furnace, while the investment associated with them exceeds 60%. If the thickness of these tubes is increased by 2 mm, for a furnace with a heat load of 47 MW, the total amount of steel required will increase by about 40 tons, accounting for 11% of the total steel usage. 9. Why is the outer diameter of the furnace tube different from that of the process pipeline? This is due to historical reasons. In the inclined roof furnaces used in the units imported from abroad in the 1950s, the diameters of most furnace tubes were φ 60, φ 89, φ 102, φ 114, φ 127, φ 152, and φ 219 mm. To facilitate future maintenance, the elbow fittings have also been standardized according to these specifications. Subsequent heating furnaces were also designed according to these tube specifications, and in 1963, the former Ministry of Metallurgy established these tube specifications as ministry standards. The above specifications are not exactly the same as those of the plant’s process pipelines; although this poses difficulties in connecting the pipelines, it has advantages in terms of production, maintenance, and spare parts. Moreover, adjusting and modifying the furnace tubes again would also present many problems. When welding furnace tubes with the same nominal diameter but different outer diameters to process pipelines, elbow fittings should be used ； When using flange connections, there are no issues since the bolt holes and the diameter of the center circle on the flanges designed for furnace tubes are of the same standard as those of ordinary flanges. 10. What are the advantages and disadvantages of 10# and 20# carbon steel furnace tubes? 10# steel has a carbon content of 0.1%; it has lower strength but higher elongation, making it suitable for use in expansion jointed furnace tubes, and it also possesses good weldability. 20# steel has a carbon content of 0.2%; it possesses high strength but low elongation, making it unsuitable for use in expansion joint tubes. Its weldability is fairly good.