JP3549980B2 - Radiant tube mounting method - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for mounting a W-shaped or U-shaped radiant tube used in a continuous annealing furnace, a heating furnace such as continuous galvanization, and more specifically, to reduce thermal stress during operation and extend the life. And a method of mounting a radiant tube.
[0002]
[Prior art]
FIG. 6 shows the structure of a typical conventional W-shaped radiant tube. In the radiant tube 1, the burner-side end 2 of the burner 22 and the recuperator-side end 10 of the recuperator 23 are both welded to the bank 15 in order to prevent oxygen from entering the furnace. Are welded to the heating furnace wall 24 and fixed respectively. In the furnace, the third bend tip support receiving portion 13 at the tip of the third bend 8 is supported by a furnace wall support jig 18 from a heating furnace wall 19 on the opposite side of the burner 22. Furthermore, a saddle 11 between the radiant tubes is provided between the lower part of the second straight pipe 5 on the first bend side and the upper part of the third straight pipe 7, and the lower part of the third straight pipe 7 on the second bend side and the upper part of the fourth straight pipe 9. Between the radiant tube saddle 12 is provided.
[0003]
By the way, the radiant tube 1 is constrained by the above-mentioned support and mounting structure against thermal expansion due to high-temperature heating, and thermal stress and deformation occur. Since it becomes unusable due to the deformation that comes into contact with the device, it must be replaced each time. This thermal stress and deformation will be described in detail with reference to FIGS. In addition, as a heat load state on the radiant tube, two states, that is, a time when the furnace is heated and a time when the furnace is in normal operation are considered.
[0004]
FIG. 7 shows a state when the furnace is heated from normal temperature to a normal operation state. Due to the combustion of the burner 22, the temperature of the first straight pipe 3 rises sharply, and the temperature difference from the part after the first bend 4 greatly increases. This temperature difference may temporarily reach several hundred degrees Celsius in the first straight pipe 3 and the fourth straight pipe 9. In this case, the first straight pipe 3 suddenly starts to thermally expand in the axial direction (horizontal direction) 25. For example, the representative temperature of each straight pipe from the first straight pipe 3 to the fourth straight pipe 9 of the radiant tube is 600 ° C., 400 ° C., 300 ° C., 200 ° C., and the length is 2000 mm, 1600 mm, 1600 mm, 2000 mm, Assuming that the linear expansion coefficient of the tube is 17 × 10 ⁻⁶ , when the recuperator-side end 10 is fixed, the burner-side end 2 tends to move about 11 mm to the outside of the furnace.
[0005]
When the burner side end 2 is fixed to the bank 15 against this thermal expansion, thermal expansion is restricted between the bank 15 and the first bend 4 and thereafter. A first straight pipe axial force 26, which is a compressive force, is generated, and the first straight pipe bending moment (at the time of temperature rise) 27 is generated by the bend portion. As a result, a large stress is generated in the first straight pipe 3. Further, the first bend 4 tends to be deformed downward, and the first bend bending moment (when the temperature is raised) 28 is generated on the side surface of the first bend due to the restraint by the saddle 11, and the straight pipe portion near the saddle has a large vertical direction. , A vertical load (at the time of temperature rise) 29 on the straight pipe near the saddle portion. Further, the downward deformation of the first bend 4 causes the downward deformation of the second bend 6, and the straight pipe in the vicinity of the saddle is restrained by the saddle 12 so that the straight pipe in the vicinity of the saddle, which is a large vertical force. A vertical load (at the time of temperature rise) 30 is applied to the tube.
[0006]
Usually, the temperature of the furnace is repeatedly raised and lowered periodically to repair the furnace. However, temperature deviation due to aging deterioration of the material, oxidation thinning, accumulation of thermal fatigue, scale generation, etc. is added. A first straight pipe crack (at the time of temperature rise) 31 occurs on the straight pipe burner side, or a first bend crack (at the time of temperature rise) 32 occurs at the side surface of the first bend. In addition, buckling may occur in the second straight pipe 5 and the third straight pipe 7 near the saddle 11, or in the third straight pipe 7 and the fourth straight pipe 9 near the saddle 12.
[0007]
FIG. 8 shows a state during normal operation. The temperature in the furnace reaches around 900 ° C., and the temperature of the radiant tube 1 increases by several tens of degrees or more. At this time, the temperature of the bank 15 is normally 100 ° C. to 200 ° C., whereas the temperature of the radiant tube 1 in the furnace is higher than 950 ° C., so the starting point is the support receiving portion 13 at the tip of the third bend 8. Due to thermal expansion in the height direction (vertical direction) 33 reaching the third bend 8, the saddle 11, and the first bend 4, the distal end of the first straight pipe 3 is forced to be considerably upwardly deformed. For example, in the case of a radiant tube in which the distance between the axes of the first straight tube 3 and the fourth straight tube 9 is 900 mm, the temperature of the bank is 150 ° C., the temperature of the radiant tube in the furnace is 950 ° C., and the coefficient of thermal expansion of the radiant tube is Is assumed to be 17 × 10 ⁻⁶ , the vertical extension of the bank portion is about 2.3 mm, and the vertical extension of the tip portion is about 14.5 mm, and there is a difference of 10 mm or more.
[0008]
Due to the upward deformation of the distal end of the first straight pipe 3 due to the thermal expansion and the deformation restraint by the saddle 11, an extremely large first straight pipe bending moment (during normal operation) 34 is generated in the first straight pipe 3. In addition, a large first bend bending moment (during normal operation) 35 is also generated in the first bend 4, and the straight pipe near the saddle 11 has a large vertical load on the straight pipe near the saddle, which is a large vertical force. (During normal operation) Receive 36. Further, at high temperatures, the saddles 11 and 12 bear part of the weight of the tube, and the saddles of the third straight pipe 7 and the fourth straight pipe 9 constantly apply vertical loads 36 and 37 which are vertical forces. receive. At this time, the difference in thermal expansion between the straight pipes in the axial direction 25 is small, and is not as problematic as when the temperature is raised.
[0009]
The radiant tube is exposed to high temperature for a long time, and it undergoes creep deformation due to the above-mentioned stress and further the influence of its own weight.In addition, the temperature deviation due to the deterioration of the material over time, oxidation thinning, and the generation of scale is added, The second straight pipe buckling 38 and the third straight pipe buckling 39 are applied to the second straight pipe 5 and the third straight pipe 7 near the saddle 11, and the second straight pipe buckling 39 is applied to the third straight pipe 7 and the fourth straight pipe 9 near the saddle 12. The third straight pipe buckling 40 and the fourth straight pipe buckling 41 occur. Further, the buckling 38 and the extreme decrease in the cross-sectional rigidity of the tube due to the buckling 38 lead to the second straight pipe sag 42, and the end of the service life of the radiant tube.
[0010]
As described above, the radiant tube, in particular, the first straight tube and the first bend are restrained by the support section and the saddle from the furnace wall, and the axial force and the bending moment during the temperature rise and the bending moment during the normal operation. Greatly affected. Further, a large compressive force acts on the straight pipe portion near the saddle due to the restraint of the saddle. As a result, the radiant tube is cracked, deformed, and buckled, and its life is extended. Therefore, reducing the bending moment due to the expansion and relaxing the restraining force by the saddle to prevent buckling will greatly extend the life of the radiant tube.
[0011]
As means for preventing the deformation and the occurrence of cracks, Japanese Patent Application Laid-Open No. 5-272708 discloses a structure in which a bellows for absorbing the axial elongation of a burner-side straight pipe is attached to a burner-side end. Japanese Patent No. 38415 discloses that a bellows is provided at a burner side end in an axial direction of a tube, and further, a bellows provided with a spring supporting a part of a weight of the tube and a burner is provided in a height direction to make thermal expansion free. The structure is presented. Japanese Patent Application Laid-Open No. 3-226519 discloses a method in which the diameter of the first straight pipe or the first and second straight pipes is made larger than that of other parts, and the tip of the first bend is supported from the furnace wall.
[0012]
[Problems to be solved by the invention]
However, in the structures disclosed in JP-A-5-272708 and JP-A-1-38415, although the bellows is effective in relieving stress, the installation of the bellows increases the cost and has a problem in durability of the bellows. Was. In Japanese Patent Laid-Open Publication No. Hei 3-226519, the method of supporting the tip of the first bend from the furnace wall involves remodeling the furnace wall when newly installing the furnace in an existing furnace. It is not realistic because it requires remodeling costs.
[0013]
The present invention has been made to solve the above problems, and significantly reduces the thermal stress of the tube at the time of temperature rise and during normal operation, and prevents cracks, creep deformation, and buckling, thereby improving the life of the tube. Provided is a method of mounting a radiant tube having an advantageous effect.
[0014]
[Means for Solving the Problems]
The gist of the present invention is to provide a method of mounting a W-shaped or U-shaped radiant tube attached to a furnace wall such that a burner side end and an exhaust gas side end of a radiant tube are positioned vertically, wherein the radiant tube has a burner side end. Characterized in that an initial stress is given by pre-fixing the part and the exhaust side end to a vertically open position in accordance with the amount of thermal expansion in the height direction of the tube in the furnace after the operation, and applying an initial stress. It is. It is preferable that the amount δ in which the burner side end and the exhaust gas side end of the radiant tube are opened in advance in the vertical direction is determined by the following equation (2).
[0015]
(Equation 2)
_{δ = kL 0 (T R ·} α R / T B · α B -1) ( however, 0 <k ≦ 1)
[0016]
Here, T _R and α _R are the average temperature and linear expansion coefficient of the radiant tube inside the furnace, T _B and α _B are the average temperature and linear expansion coefficient of the bank, and L ₀ is the distance between the burner center and the recuperator center of the radiant tube. The distance and k are constants.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the drawings.
[0018]
As shown in FIG. 1, in the present invention, the mounting position of the radiant tube 1 to the bank 15 is vertically changed with respect to the conventional burner side end mounting portion 16 to the bank and the recuperator side end mounting portion 17 to the bank. By offsetting so as to open, and fixing the burner side end 2 and the recuperator side end 10 to the burner side end mounting portion 16 ′ to the bank and the recuperator side end mounting portion 17 ′ to the bank, respectively, the first An initial bending moment at the time of offset (first straight pipe side) 20 and an initial bending moment at the time of offset (fourth straight pipe side) 21 are applied to the straight pipe 3 and the fourth straight pipe 9 to reduce stress during hot working.
[0019]
FIG. 2A shows the mounting position of the conventional radiant tube before the temperature rise, as viewed from the outside of the bank 15. Burner end attaching portion 16 of the burner-side end 2 and the recuperator end 10 to the bank, it is attached to the recuperator end mounting portion 17 to the bank, the during this period distance L _0. Assuming that the temperature of the bank 15 during normal operation is T _B and the coefficient of linear expansion of the bank 15 is α _B , as shown in FIG. the distance between the recuperator end mounting portion 17 becomes _{L 0 · T B · α B} . On the other hand, as shown in the figure, in the furnace, the radiant tube 1 also thermally expands in the vertical direction, the amount of the radiant tube 1 becomes maximum near the first bend 4 and the third bend 8, and the first straight tube 3 and the first 4 between the center axis distance of the straight pipe 9, when the average temperature of the tube _{T R,} the linear expansion coefficient and alpha _R, the _{L 0 · T R · α R} . The temperature inside the furnace is several hundred degrees higher than the bank,
L ₀ · T _B · α _B <L ₀ · T _R · α _R
This difference in thermal expansion causes various damages.
[0020]
In the present invention, as shown in FIG. 2 (c), the burner-side end mounting portion 16 'is attached to a bank in which the burner-side end 2 and the recuperator-side end 10 are spread up and down by δ before the temperature is raised. Is offset to the recuperator side end mounting portion 17 '. As shown in FIG. 2 (d), the offset amount δ is the thermal expansion amount between the burner side end mounting portion 16 ′ to the bank and the recuperator side end mounting portion 17 ′ to the bank during normal operation. but as it will become less radiant maximum value L of the vertical thermal expansion of the tube 1 _{0 ·} T _R · α R in the furnace, i.e., taking so as to satisfy the equation number 3 below.
[0021]
(Equation 3)
(L ₀ + δ) · T _B · α _B ≦ L ₀ · T _R · α _R
[0022]
As described above, the initial offset amount δ can be defined by the following equation (4).
[0023]
(Equation 4)
_{δ = kL 0 (T R ·} α R / T B · α B -1) ( however, 0 <k ≦ 1)
[0024]
In the above equation, when the offset amount δ is determined with k = 1, during normal operation, as shown in FIG. 2D, the vertical thermal expansion displacement of the radiant tube 1 and the burner side to the bank as shown in FIG. The thermal expansion displacements of the end mounting portion 16 'and the end mounting portion 17' on the recuperator side to the bank are substantially equal, and the bending deformation of the tube as shown in FIG. 8 is almost eliminated, and various damages caused by this are reduced. Can be reduced. The value of k does not necessarily need to be 1, but may be between 0 and 1. However, in order to exhibit the effect of reducing the thermal stress, 0.5 or more is desirable. For example, k may be around 0.5 in order to reduce the thermal stress by half. Note that if k = 0, it becomes equal to the conventional mounting method.
[0025]
Further, at the time of temperature rise, a bending moment 27 as shown in FIG. 3 is temporarily applied due to a difference between a high temperature portion on the burner side and a low temperature portion on the recuperator side of the radiant tube. Also at this time, in the mounting method of the present invention, since the opposite initial bending moment 20 is applied at the initial stage, the bending moment can be reduced by canceling.
[0026]
【Example】
A trial calculation will be made on the effect of reducing the axial force and bending moment generated in the radiant tube attached according to the present invention. The target is the W-shaped radiant tube 1 shown in FIG. 1, the length of the first straight pipe 3 and the fourth straight pipe 9 is 2500 mm, the length of the second straight pipe 5 and the third straight pipe 7 is 1650 mm. is there. The outer diameter and the inner diameter are 194 mm and 177 mm, respectively, and the material is SCH22 described in JIS G 5122. Axial distance between the centers of the straight pipe is 300mm, respectively, the axial center distance _{L 0} between the first straight pipe 3 fourth straight pipe 9 is 900 mm.
[0027]
The average temperature of the radiant tube 1 during normal operation is 950 ° C., the average temperature of the bank 15 is 200 ° C., the linear expansion coefficient of the radiant tube 1 is 17 × 10 ⁻⁶ , and the linear expansion coefficient of the bank 15 is 13 × 10 ⁻⁶ . Then, the difference in thermal expansion between the two becomes 12.2 mm. The distribution of the axial force and bending moment generated in the tube when the offset amount δ is set to 4 mm, 8 mm, and 12 mm is shown in FIGS. From this, it can be seen that the axial force and the bending moment are greatly reduced by the offset. When the axial force and the bending moment are reduced, the stress generated in the radiant tube is necessarily reduced.
[0028]
According to the method of the present invention, the radiant tube was mounted with an offset amount of 10 mm at the burner side end, and even after 6 years had passed from the actual furnace, it was operated without any abnormality. The life of the conventional structure is 3 to 4 years on average, and a long life can be realized.
[0029]
【The invention's effect】
ADVANTAGE OF THE INVENTION By this invention, the thermal stress of a radiant tube at the time of a temperature rise and normal operation is remarkably eased, a crack, creep deformation, and buckling can be prevented, and the life of a radiant tube can be extended.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of one embodiment of the present invention.
FIG. 2 is a detailed explanatory diagram of one embodiment of the present invention.
FIG. 3 is an explanatory diagram of an effect of the present invention.
FIG. 4 is a view showing a trial calculation result of an axial force distribution of a radiant tube.
FIG. 5 is a diagram showing a trial calculation result of a bending moment distribution of a radiant tube.
FIG. 6 is an explanatory view of a structure of a conventional typical W-shaped radiant tube.
FIG. 7 is an explanatory diagram of a deformation and damage mode of a conventional typical W-shaped radiant tube at the time of temperature rise.
FIG. 8 is an explanatory diagram of a deformation and a damage mode of a conventional typical W-shaped radiant tube during normal operation.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Radiant tube 2 Burner side end part 3 1st straight pipe 4 1st bend 5 2nd straight pipe 6 2nd bend 7 3rd straight pipe 8 3rd bend 9 4th straight pipe 10 Recuperator side end part 11 Saddle between radiant tubes 12 Saddle between radiant tubes 13 Third bend tip support receiving part 14 Refractory 15 Bank 16 Burner side end mounting part 16 'to bank Burner side end mounting part 17 to bank Recuperator side end mounting part 17 to bank '' Recuperator-side end attachment part to bank 18 Furnace wall support jig 19 Heating furnace wall 20 Initial bending moment at offset (first straight pipe side)
21 Initial bending moment at offset (4th straight pipe side)
22 Burner 23 Recuperator 24 Heating furnace wall 25 Axial direction (horizontal direction)
26 First straight pipe axial force 27 First straight pipe bending moment (at elevated temperature)
28 1st bend bending moment (at elevated temperature)
29 Vertical load on the straight pipe near the saddle (when the temperature rises)
30 Vertical load on the straight pipe near the saddle (when heated)
31 First straight pipe crack (when temperature rises)
32 First bend crack (at elevated temperature)
33 Height direction (vertical direction)
34 1st straight tube bending moment (during normal operation)
35 1st bending moment (during normal operation)
36 Vertical load on straight pipe near saddle part (during normal operation)
37 Vertical load on straight pipe near saddle part (normal operation)
38 second straight pipe buckling 39 third straight pipe buckling 40 third straight pipe buckling 41 fourth straight pipe buckling 42 second straight pipe sag

Claims (2)

In a method of mounting a W-shaped or U-shaped radiant tube mounted on a furnace wall such that a burner side end portion and an exhaust gas side end of a radiant tube are positioned vertically, a burner side end portion and an exhaust gas side end portion of a radiant tube are provided. A method of mounting a radiant tube, comprising: preliminarily fixing a tube in a vertically opened position in accordance with a thermal expansion amount of a tube in a furnace after operation to apply initial stress. The method for mounting a radiant tube according to claim 1, wherein the amount δ of opening the burner side end and the exhaust gas side end of the radiant tube in the vertical direction is determined in advance by the following equation (1).
(Equation 1)
_{δ = kL 0 (T R ·} α R / T B · α B -1) ( however, 0 <k ≦ 1)
Here, T _R and α _R are the average temperature and linear expansion coefficient of the radiant tube inside the furnace, T _B and α _B are the average temperature and linear expansion coefficient of the bank, and L ₀ is the distance between the burner center and the recuperator center of the radiant tube. The distance and k are constants.

Images