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1. What is the purpose of installing spherical viewing ports at the bottom of a cylindrical furnace? Spherical viewing ports are generally installed at the bottom of cylindrical furnaces; their purpose is to check the degree of bending of the radiant tubes, to observe the combustion process in the burners, and to inspect the condition of the furnace’s roof lining. 2. What is the purpose of an explosion-proof door? Explosion accidents do not occur in heating furnaces during normal operation. Most furnace explosion accidents occur during the startup and ignition phase. Before ignition, if combustible gases remain in the furnace due to a poorly sealed fuel gas valve or repeated attempts at ignition that fail, an explosion is likely to occur during ignition. In this case, the furnace pressure will push open the explosion relief valve to release some of the pressure inside the furnace, thereby reducing the damage to the furnace. However, a furnace equipped with an explosion-proof door cannot completely prevent damage to the furnace body in the event of an explosion inside it. Explosions inside the heating furnaces of domestic petrochemical plants have occurred, causing severe damage to the furnaces. Therefore, it is essential to strictly follow the operating procedures: blow out the furnace chamber with steam in a timely manner before ignition, and also blow it out promptly if ignition does not occur. This is the most fundamental and effective measure to prevent explosions inside the furnace. Furthermore, if the furnace tubes burst during operation and a large amount of oil flows into the furnace chamber, this can also lead to an explosion; therefore, it is very important to regularly check the wall thickness of the furnace tubes and to monitor them on a regular basis during operation. 3. Where should the explosion-proof door be located? It is very important to determine the location of the explosion-proof door when designing a heating furnace; if it is placed in the wrong location, unnecessary losses can occur in the event of an explosion in the furnace. To this end, explosion-proof doors should be installed in explosive areas and places where no one walks. 4. How is the number of explosion-proof doors determined? When designing a heating furnace, the number of explosion-proof doors is specified as follows: (1) If the furnace’s thermal load is less than 5 MW, 1 explosion-proof door is required. (2) When the furnace’s heat load is 5–10 MW, 2 explosion-proof doors are provided. (3) When the furnace’s heat load is greater than 10 MW, 3 explosion-proof doors shall be installed. When a furnace chamber is divided into several sections, each section should have at least one explosion-proof door. For small round furnaces without a convection chamber, an explosion-proof door should be installed when using gas ； For example, when burning oil, a manhole should be installed in the conical section (and used as an explosion-proof door). 5. How is the number of manholes determined? When designing a heating furnace, determining the correct number of manholes is crucial for its maintenance and installation. The number of manholes shall be determined in accordance with the following provisions: (1) A single-chamber furnace shall have at least one manhole. (2) Each chamber of a multi-chamber furnace should be equipped with at least one manhole. (3) In small cylindrical furnaces where there is insufficient space at the bottom, the nozzle openings can be used as manholes. (4) A manhole should be provided at the upper part of the flue or the lower part of the chimney at the outlet of the convection chamber. 6. What is a positioning tube? A positioning tube is a component welded at the cantilevered end elbow of the radiant tubes to prevent lateral movement of the vertical furnace tubes in the radiant chamber of the heating furnace. 7. What are the common specifications for positioning tubes, and how should their materials be selected? The specifications and materials for positioning tubes shall be chosen in accordance with the following rules: (1) When suspension tubes are used for vertical furnace tubes, positioning tubes with diameters of ϕ60, ϕ89, and ϕ102 mm should use tubes with a diameter of ϕ42×4 mm; the material to be used is Cr5Mo or 18-8 ； For the positioning tubes of furnace tubes with diameters of ϕ27, ϕ152, and ϕ219 mm, those with a diameter of ϕ60x5 mm are used, and the material is either Cr5Mo or 18-8. (2) When a socket tube is used for the vertical furnace tube, the positioning tubes with diameters of ϕ60, ϕ89, and ϕ102 mm are made of ϕ42×4 mm tubes, constructed from 10# or 20# steel ； The positioning tubes for furnace tubes with diameters of ϕ27, ϕ152, and ϕ219 mm are of diameter ϕ 60x 5 mm, and are made of 10# or 20# steel. 8. What are the conditions for using guide pipes? The condition for using guide pipes is that when suspension pipes are used in vertical furnace tubes, guide pipes should be installed at the bottom of the furnace. 9. What material is used for the guide tube? The guide tube is made of 10# or 20# seamless steel tubes ； It can also be rolled from Q253-A·F sheets with a thickness of 4 mm. 10. What is the gap between the guide tube and the positioning tube? The gap between them is 10–15 mm.