JP2002061803A - Structure of boiler furnace wall and method for preventing occurrence of crack at welded part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a membrane bar structure which reduces occurrence and development of cracks at a welded part between a tube and a membrane bar, in a membrane panel structure of a boiler furnace wall which is constituted with a plurality of tubes and membrane bars. SOLUTION: The membrane panel of the boiler furnace wall is constituted with an airtight integrated tube bank wherein a plurality of waterwall tubes (6) are connected to membrane bars (9) each made of a narrow strip of steel and disposed along the longitudinal direction between the tubes through welded parts (8) that continue in the axial direction of the tubes. The thickness of the membrane bar at the central portion of its cross section is made larger than the thickness at both ends of the membrane bar, i.e., at the joints of the welded parts to the tubes, respectively, to constitute the membrane structure of the boiler furnace wall. In this way, occurrence and development of cracks at the welded parts between the tubes and the membrane bar are reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structure of a membrane panel of a boiler furnace wall composed of a plurality of tubes and a membrane bar (a plate-shaped fin made of strip-shaped steel material), and more particularly to a welding of a tube and a membrane bar. The present invention relates to a membrane bar structure capable of reducing crack generation and propagation at a welded portion and a method for preventing crack generation at a welded portion thereof.

[0002]

2. Description of the Related Art FIG. 5 shows a schematic structure of a combustion chamber and a furnace of a power plant boiler. This type of boiler combines a membrane panel in which a membrane bar is welded between pipes to form a highly airtight box-shaped combustion chamber and a furnace.

FIG. 6 shows details of the connecting portion 5. The plurality of water wall pipes 6, cage wall pipes 7, and cage bottom wall pipes 10 are each provided with a plate-like fin (fin-shaped) called a membrane bar 9 on both sides of the pipe in order to enhance the heat transfer effect. In the direction of the tube axis and joined by a welded portion 8 continuous in the direction of the tube axis to form an integral hermetic tube group. In addition,
Reference numeral 11 denotes a cage wall header.

The connecting portion 5 has a complicated structure in which three types of tube groups (water wall tube group 1, cage wall tube 7, and cage bottom wall tube 10) are three-dimensionally united and have a complicated structure. As a structure, it constitutes a shape discontinuity that requires the most attention in terms of strength.

[0005]

In such a combustion chamber and a furnace, the temperature difference between the pipe walls of the water wall tube group 1, the cage side wall tube group 2, the cage bottom wall tube group 3, and the ceiling wall tube group 4. This causes a problem that thermal deformation occurs, and cracks are generated particularly at the joint portion 5 between the water wall tube group 1 and the cage side wall tube group 2. The cracks found are repaired and reinforced as needed, but every time a regular inspection is performed after repair and reinforcement,
There has been a problem that a new crack is found in the joint portion 5 between the same water wall tube group 1 and the cage side wall tube group 2, and the generation of the crack cannot be prevented beforehand.

[0006] Originally, the water wall tube group 1 and the cage side wall tube group 2
Has a shape in which stress tends to concentrate due to discontinuity of the shape, and a tensile force for opening the joint 5 due to a difference in elongation between when the boiler is stopped and during operation (that is, a crotch phenomenon).
It is considered that the shearing force generated in the joint portion 5 due to the difference in elongation due to the temperature difference between the water wall tube group 1 and the cage side wall tube group 2 works in a complicated manner, and a crack occurs in the joint portion 5. I have.

As described above, the membrane bar 9 is a portion where the metal temperature is higher than that of a water wall tube and the like, and stress concentration is apt to occur due to discontinuity of the shape and the like. Combustion gas leaks from the boiler furnace wall to the outside, or cracks propagate to the heat transfer tubes, causing steam leakage,
There was a problem that the boiler had to be stopped.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems in the prior art, and in a structure of a membrane panel of a boiler furnace wall composed of a plurality of tubes and a membrane bar, a crack at a weld between the tube and the membrane bar. An object of the present invention is to provide a membrane bar structure capable of reducing generation and development.

[0009]

Means for Solving the Problems In order to achieve the above object, the present invention is configured as described in the claims. That is, as described in claim 1, a plurality of pipes and a membrane bar made of a band-shaped steel material are joined in the longitudinal direction between the pipes by continuous welding in the pipe axis direction to have airtightness. In a membrane panel of a boiler furnace wall composed of an integral pipe group, a boiler in which a thickness of a central portion of a cross section of the membrane bar is thicker than a thickness of a welded root portion of both ends of the membrane bar with the pipe. The furnace wall has a membrane bar structure. here,
The membrane bar means a fin (fin) made of a plate-like steel material, and the fin is air-tightly welded to both sides of a pipe constituting a boiler water wall pipe by continuous welding in the pipe axis direction of the pipe. The combination is called a membrane bar structure.

[0010] Further, as described in claim 2, claim 1
In the above, the ratio of the thickness of the central portion of the membrane bar to the thickness of the welded root portion is 1.25 to 1.5.
The membrane wall structure of the boiler furnace wall described above is adopted.

[0011] Further, as described in claim 3, claim 1
Wherein the ratio of the thickness of the central part of the membrane bar to the thickness of the welded root part is 1.5 to 2.0 or more, so that the boiler furnace wall has a membrane bar structure.

Further, as described in claim 4, claim 1 is
In any one of the third to third aspects, the cross-sectional shape of the membrane bar is a membrane wall structure of a boiler furnace wall in which one surface is linear and the other surface is arc or triangular.

According to a fifth aspect of the present invention, a plurality of pipes and a membrane member made of a strip-shaped steel material are joined in a longitudinal direction between the pipes by continuous welding in the pipe axis direction to be airtight. In the membrane panel of the boiler furnace wall composed of an integral pipe group having a property, the thickness of the central part of the cross section of the membrane bar is set to be thicker than the thickness of the membrane bar at the root portion welded to the pipe. By increasing the thickness of the membrane bar, the cross-sectional area of the heat propagation is increased, so that heat is easily transmitted to both ends of the membrane bar, so that the temperature of the membrane bar decreases and the temperature gradient generated at the welding root portion. A method for preventing cracks in the welded part of the membrane-bar structure of the boiler furnace wall, in which the thermal stress generated in the welded part is reduced and fatigue or creep strength of the welded part is increased. A.

As shown in FIGS. 1 (a), 1 (b) and 1 (c), the membrane wall structure of the boiler furnace wall of the present invention is such that the thickness of the central portion of the membrane bar 9 is smaller than the thickness of both ends. Also to make it thicker.

When the thickness of the conventional membrane 9 shown in FIG. 4 is constant, the heat quantity at the center of the membrane has a small cross-sectional area and the heat hardly flows to both ends. Part) rises. Further, a large thermal stress is generated in the welded portion 8 between the water wall tube 6 and the membrane 9 whose cross-sectional area changes rapidly due to a large temperature gradient. This thermal stress causes the welded portion 8 of the membrane 9
Cracks in the

On the other hand, as in the membrane bar structure of the present invention, by increasing the thickness of the central portion of the membrane bar 9, the cross-sectional area largely related to the propagation of heat is increased and heat is applied to both ends of the membrane bar 9. Make it easy to escape. Therefore, the metal temperature of the membrane 9 is relatively lower than that of the conventional structure, and the metal temperature of the welded portion 8 between the heat transfer tube 6 and the membrane 9 is relatively higher. Therefore, since the temperature gradient generated in the welded portion 8 is smaller than that in the related art, the thermal stress is greatly reduced. From the above, by increasing the thickness of the central portion of the membrane 9, the thermal stress generated in the welded portion 8 of the membrane 9 can be greatly reduced, and the temperature of the portion also decreases. Therefore, the fatigue or creep life of the welded portion 8 can be further improved.

[0017]

Embodiment 1 FIG. 1A shows an example of an embodiment of the present invention. The temperature change and the thermal stress change due to the change in the thickness of the membrane 9 were obtained by the finite element method. The analysis conditions assume a steady operation of the boiler. The details of the analysis conditions are as follows. (1) Dimension of water wall pipe: outer diameter 50.8mm, thickness 10.8m
m (2) Material of water wall tube: 2.25Cr-1Mo steel (% by weight)
(Steel containing 2.25% of Cr and 1% of Mo) (3) Dimensions of membrane bar: width 50 mm, thickness 10 m
m The plate thickness (mm) at the center of the membrane bar is 10, 12.
5, 15, 17.5, 20, 22.5 and 25 types are used. (4) Material of membrane bar: Rolled steel for boiler SB4
10 (5) Fluid temperature inside water wall tube: 300 ° C (6) Heat transfer coefficient inside water wall tube: 14100 × 1.1627
9 W / (m ² · K) [14100 kcal / m ² h ° C.] (7) Combustion gas temperature: 1000 ° C. (8) Combustion gas side heat transfer coefficient: 300 × 1.1627 W
/ (M ² · K) [300 kcal / m ² h ° C.] FIG. 2 shows the temperature change at the center of the membrane bar 9 at the highest temperature relative to the thickness at the center of the membrane (normalized by the thickness at both ends). And the temperature change of the welded portion 8 between the water wall pipe 6 and the membrane 9. Here, a membrane bar having a ratio of the thickness of the center portion to the thickness of the end portion of 1 is a conventionally used membrane bar having a constant thickness.

In FIG. 2, part A shows the center of the membrane bar, and part B shows the base of the membrane bar (the welded portion 8 in FIG. 1). It can be seen that the temperature of the membrane decreases in both the portions A and B as the thickness of the central portion of the membrane increases. The rate of temperature decrease was higher at the center of the membrane (A).

Table 1 shows the details of the data relating to the temperature (° C.) at the center of the membrane and the ratio of the thickness at the center to the thickness at the end shown in FIG.

[0020]

[Table 1] FIG. 3 shows a change in generated stress with respect to the thickness at the center of the membrane (normalized by the thickness at both ends). In both the central part of the membrane bar (part A) and the root part of the membrane bar (part B), the generated stress decreased as the wall thickness increased. The degree of decrease in the generated stress was greater at the root (B) of the membrane bar. For these reasons, by increasing the thickness of the central portion of the membrane bar, the thermal stress generated at the root portion (B portion) of the membrane bar can be greatly reduced, and the temperature of the portion also decreases. Generally, when the temperature decreases, the fatigue strength and the creep strength are improved.
Fatigue and creep life can be greatly improved.

Table 2 shows details of data relating to the stress (MPa) generated in the membrane shown in FIG. 3 and the ratio of the thickness at the center to the thickness at the end shown in FIG.

[0022]

[Table 2] In FIG. 2, the rate of decrease in the temperature of the membrane 9 is such that the ratio of the thickness at the center of the membrane to the thickness at the end is 1.
The temperature starts to decrease from about 25 (decrease rate: 1.9 to 2.5%), and when the ratio becomes 1.5 or more, the temperature decrease rate becomes 3.8.
When the ratio of the wall thickness becomes 2.0 or more, the temperature reduction rate becomes 6.3 to 7.4% or more. Should be at least 1.25 times the wall thickness, preferably 1.5
It is required to be at least twice, more preferably at least 2.0 times.

In FIG. 3, the rate of decrease in the stress generated in the membrane 9 is determined by the thickness of the central part of the membrane:
The stress starts to decrease (the stress reduction rate is 5.7 to 9.7%) when the thickness ratio of the end portion is about 1.25, and when the ratio becomes 1.5 or more, the stress reduction rate becomes 11.5 to 19. When the ratio of the wall thickness increases to about 2.0% or more, the stress reduction rate becomes 25.3 to 31.0%, and the wall thickness at the center of the membrane is the wall thickness at both ends. Is required to be at least 1.25 times or more, preferably 1.5 times or more, more preferably 2.0 or more, so that the stress can be reduced.

In this embodiment, the thermal stress analysis is performed on one side of the membrane 9 as shown in FIG. 1A with an arc shape, but the one side shown in FIG. Similar thermal stress analysis results have been obtained for the shape-like material.

<Embodiment 2> FIG. 1C shows another embodiment of the present invention. In the embodiment shown in FIG. 1A, the cross-sectional shape of the membrane bar is shown as the membrane bar 9 in which one surface is linear and the other surface is arc-shaped. 2), a membrane bar 9 having an arc shape on both sides welded to the water wall tube 6 was used. Also in the second embodiment, the same thermal stress analysis results as in the case of FIG. 1A exemplified in the first embodiment are obtained. However, if the wall thickness at the center of the membrane bar and the wall thickness at the end of the membrane bar are equal in both FIG. 1 (a) and FIG. 1 (c), FIG.
As shown in (a), the result that the effect of reducing the generated stress was greater when only one side had a thickness was obtained.

[0026]

According to the present invention, the thermal stress generated at the welded portion between the membrane bar and the water wall tube is greatly reduced by making the thickness of the central portion of the membrane bar larger than the thickness of the end portion. And the temperature of the portion can be reduced, so that the fatigue life or creep life of the welded portion between the water wall tube and the membrane bar can be improved. Therefore, the life of the welded portion until crack generation can be extended, and the maintenance cycle can be extended. Therefore, the maintenance cost can be significantly reduced, and the industrial use value is extremely large.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a membrane bar structure of a boiler furnace wall exemplified in an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a wall thickness and a temperature of a central portion of a membrane bar exemplified in the embodiment of the present invention.

FIG. 3 is a graph showing a relationship between a thickness of a central portion of a membrane bar and a generated stress exemplified in the embodiment of the present invention.

FIG. 4 is a schematic diagram showing a conventional membrane bar structure.

FIG. 5 is a schematic diagram showing a schematic structure of a combustion chamber and a furnace of a conventional power plant boiler.

FIG. 6 is a schematic view showing details of a tube wall connecting portion of a conventional boiler furnace wall.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Water wall tube group 2 ... Cage side wall tube group 3 ... Cage bottom wall tube group 4 ... Ceiling wall tube group 5 ... Connection part 6 ... Water wall tube 7 ... Cage wall tube 8 ... Welded part 9 ... Membrane bar 10 ... Cage Bottom wall pipe 11… Cage wall pipe

Claims (5)

[Claims] 1. A plurality of tubes and a longitudinal direction between the tubes,
In a membrane panel of a boiler furnace wall composed of an integral pipe group having airtightness by connecting a membrane bar made of a strip-shaped steel material by continuous welding in the pipe axis direction, a thickness of a central portion of a cross section of the membrane bar is provided. The thickness of the wall of the boiler furnace wall is larger than the thickness of the both ends of the membrane bar at the welding root portion with the pipe. 2. The boiler furnace wall according to claim 1, wherein the ratio of the thickness of the central portion of the membrane bar to the thickness of the welded root portion is 1.25 to 1.5 or more. Structure. 3. The boiler furnace wall according to claim 1, wherein the ratio of the thickness at the center of the membrane bar to the thickness at the welding root is 1.5 to 2.0 or more. Structure. 4. A boiler according to claim 1, wherein the cross-sectional shape of the membrane is such that one surface is linear and the other surface is arc-shaped or triangular. Furnace wall structure. 5. A plurality of tubes and a longitudinal direction between the tubes,
In a membrane panel of a boiler furnace wall composed of an integral pipe group having airtightness by connecting a membrane bar made of a strip-shaped steel material by continuous welding in the pipe axis direction, a thickness of a central portion of a cross section of the membrane bar is provided. Is thicker than the thickness of the membrane bar at the root of the weld to the pipe, thereby increasing the cross-sectional area of heat propagation due to the increase in the thickness of the membrane bar and transmitting heat to both ends of the membrane bar. And reducing the temperature of the membrane and reducing the temperature gradient generated at the weld root, reducing the thermal stress generated at the weld, and increasing the fatigue or creep strength of the weld. To prevent cracking of welds on boiler furnace walls.