JP2007205692A - Thermal fatigue crack damage diagnosis method of boiler heat transfer tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To explain behavior of crack damage in an actual machine by increasing the accuracy of its development and behavior for the crack damage generated by the combination of repetition of thermal stress generated on the outside surface of a boiler heat transfer tube with high-temperature corrosion or high-temperature oxidation. <P>SOLUTION: In this diagnosis method for the degree of damage by an elephant skin-like thermal fatigue crack generated from the outside surface of the heat transfer tube, a stress range coefficient range is calculated based on a thermal stress range and crack depth calculated based on the dimension, temperature variation and the like of the heat transfer tube; a thermal fatigue crack development amount is analyzed based on a thermal fatigue crack development diagram expressing a relationship between the stress range coefficient range and a crack development speed and a thermal stress repeat count; a generation amount of high-temperature scales on the tube outside surface is obtained to evaluate the crack of the scales; the generation amount of oxidized scales on the tube inside surface side is obtained; and the degree of damage of the thermal fatigue crack is diagnosed by incorporating acceleration of the thermal fatigue crack development amount and the scale generation amount on the tube outside surface by the rise of transfer tube metal temperature caused by the scale generation amount on the tube inside surface side. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for diagnosing elephant skin-like thermal fatigue crack damage that occurs in high-temperature heat transfer tubes such as water wall tubes, superheater tubes, and reheater tubes of thermal power generation boilers.

  In coal-fired boilers and heavy oil fired boilers, heat transfer efficiency decreases when combustion ash adheres to and accumulates on heat transfer tubes such as water wall tubes, superheater tubes, and reheater tubes, so wall blowers (hereinafter abbreviated as WB) A soot blower (hereinafter abbreviated as SB) periodically sprays high-pressure steam to remove adhering ash.

FIG. 6 is a schematic diagram of the heat transfer tube section when SB is applied. FIG. 7 is a schematic diagram of a change in metal temperature during SB operation. In FIG. 6, 1 is a boiler heat transfer tube, 2 is attached ash, 3 is a steam flow by a soot blower (SB) or a wall blower (WB), 4 is a thermal stress, 5 is an internal scale, and 6 is an elephant skin-like crack. Represent each. Since the superheated steam 3 of 300 to 500 ° C. is blown onto the heat transfer tube 1 in the furnace condition of 600 to 1200 ° C., the metal temperature is lowered by the operation (operation) of WB or SB → The metal temperature is raised by removing the attached ash 2 → The metal temperature is repeatedly lowered by the reattachment of ash, and a thermal stress 4 of 50 to 200 N / mm ² is generated due to a change in the metal temperature and a temperature difference between the inner and outer surfaces of the pipe. The number of WB and SB operations is on the order of 50 to 500 times / year, although it depends on the ash deposition rate, ash properties, and site. Cracks 6 are generated and propagated by repetition of such thermal stress, leading to damage.

FIG. 8 is a fatigue crack growth diagram of a known typical boiler heat transfer tube. The fatigue crack growth rate varies depending on the steel type and temperature, but the growth rate of the ferritic CrMo steel for boilers is almost the same. The stress intensity factor range ΔK on the horizontal axis in FIG. 8 is a numerical value calculated by ΔK = D · σ√ (πa) (D: shape factor, σ: stress range, a: crack depth). It is a function of range and crack depth. When the crack a is 1 mm or less, even if the stress range σ is large, ΔK is 50 kg / mm ^3/2 or less, the crack growth rate is on the order of 10 ⁻⁵ mm / time, and even if it is repeated 100,000 times, 1 mm This is a development and the actual crack situation cannot be explained.

  Therefore, in general, according to the description of the flow shown in FIG. 9, the conventional technology takes into consideration the influence of the high-temperature combustion gas on the fatigue crack growth rate, the generation of high-temperature oxidized scale on the surface, and the crack and high-temperature oxidation under the scale crack. Crack growth calculation is performed (see, for example, Patent Document 1). Details of the calculation flow of the crack growth are as illustrated and described in FIG.

Furthermore, in addition to the above-described conventional techniques, conventional techniques for diagnosing thermal fatigue crack damage in a boiler such as an elephant skin-like crack have been proposed in, for example, Patent Document 2 and Patent Document 3. This Patent Document 2 is intended to consider the influence of H ₂ S in the high-temperature gas on the fatigue life diagram, and Patent Document 3 is intended to consider the corrosive action while the boiler is stopped. is there.
JP 2002-156325 A JP 2003-351323 A Japanese Patent Laid-Open No. 2003-202101

  By the way, FIG. 10 is a crack growth prediction diagram diagnosed based on the measurement result of the elephant skin-like crack of the actual boiler heat transfer tube investigated by the present inventors, the data at that time, the operation history and the stress condition. . As shown in this diagram, it was predicted that a critical crack would be reached in about 170,000 hours of operation, but in fact, the heat transfer tube was blown out at about 150,000 hours, and this was an evaluation prediction on the unsafe side. Yes.

  And the calculation in the said patent document 1 is accumulatively calculated by making the sum of the amount of fatigue crack growth accelerated by the high temperature gas for every certain period and the amount of high temperature oxidation growth into a progress amount. However, even in such a cumulative calculation method, there is an event in which crack growth in an actual machine cannot be evaluated, and this tendency is particularly noticeable in boilers that have been used for years.

As described above, Patent Document 1 relates to a method for analyzing the fatigue crack propagation on the outer surface of a boiler heat transfer tube, and Patent Document 2 tries to consider the influence of H ₂ S in a high-temperature gas on a fatigue life diagram. The above-mentioned Patent Document 3 considers the corrosive action while the boiler is stopped, and is caused by a combination of repeated elephant skin-like thermal stress and high temperature corrosion (oxidation), which is the subject of the present invention. No consideration has been given to crack damage.

  The purpose of the present invention is to improve the accuracy of the progress of crack damage caused by the combination of elephant skin-like thermal stress and high temperature corrosion (or high temperature oxidation) generated on the outer surface of the boiler heat transfer tube. It is an object of the present invention to provide a diagnostic method that can explain and evaluate the behavior of the above.

In order to solve the above problems, the present invention mainly adopts the following configuration.
A diagnostic method for diagnosing the degree of damage due to an elephant skin-like thermal fatigue crack generated from the outer surface of a boiler heat transfer tube,
Calculating a stress range coefficient range based on the thermal stress range and crack depth calculated based on the dimensions, thermal expansion coefficient, and temperature change of the heat transfer tube;
Analyzing the amount of thermal fatigue crack propagation based on the thermal fatigue crack growth diagram representing the relationship between the stress range coefficient range and crack growth rate and the number of thermal stress repetitions;
Determining the amount of high temperature oxidation scale or high temperature corrosion scale formed on the outer surface of the heat transfer tube, and evaluating cracks due to the generated scale;
Obtaining a production amount of oxide scale on the inner surface side of the heat transfer tube,
Incorporating the increase in heat transfer tube metal temperature due to the obtained tube inner surface side scale generation amount accelerates the thermal stress range, the thermal fatigue crack growth amount and the scale generation amount of the tube outer surface, and the thermal fatigue The degree of damage of the thermal fatigue crack is diagnosed by analyzing the crack progress and evaluating the crack.

  According to the present invention, the elephant skin-like crack propagation behavior of a boiler heat transfer tube, which is caused by the combination of repeated thermal stress associated with the operation of SB or WB or repeated thermal stress associated with start / stop and the effect of high-temperature gas, is highly accurate. Therefore, it can contribute to the safe and stable operation of boiler equipment.

  In the diagnostic method of the prior art, it becomes a diagnosis on the non-safe side, and it causes a situation where the thermal power plant is shut down in advance when it encounters a heat transfer tube blowout trouble, but according to the diagnosis method of the present invention, the heat transfer tube blowout It is possible to avoid the thermal power plant from being stopped by.

  A thermal fatigue crack damage diagnosis method for a boiler heat transfer tube according to an embodiment of the present invention will be described in detail below mainly with reference to FIGS. FIG. 1 is a diagram showing a flow of a thermal fatigue crack damage diagnosis method for a boiler heat transfer tube according to an embodiment of the present invention. FIG. 2 is a diagram showing the results of crack growth analysis and life diagnosis using the thermal fatigue crack damage diagnosis method according to this embodiment. FIG. 3 is a diagram showing input items and a diagnostic screen example of the thermal fatigue crack damage diagnostic method according to the present embodiment. FIG. 4 is a diagram showing specific results analyzed and diagnosed under the conditions shown in FIG. FIG. 5 is a diagram showing changes in metal temperature when a scale is present on the inner surface of the boiler heat transfer tube according to the present embodiment and when it is not present.

First, the outline | summary of the thermal fatigue crack damage diagnostic method of the boiler heat exchanger tube which concerns on embodiment of this invention is demonstrated. In the first place, the boiler heat transfer tube heats high-temperature water or high-temperature superheated steam, and a deposition adhesion scale from high-temperature water or a steam oxidation scale due to superheated steam is generated on the inner surface side of the tube. Most of the scale that precipitates or forms on the inner surface of the pipe is iron oxide (Fe ₂ O ₃ or Fe ₃ O ₄ ), and its thermal conductivity is 1/20 to 1/40 of that of the heat transfer pipe steel. The metal temperature (tube surface temperature) rises due to heat load from the outer surface of the heat tube.

  Fig. 5 shows a schematic diagram of changes in the metal temperature with and without the pipe inner scale. When the metal temperature rises, the fatigue crack growth rate increases as shown in Fig. 8, and high temperature oxidation and high temperature corrosion also occur. To increase. Furthermore, the metal temperature difference ΔT due to the operation of SB or WB increases due to the internal scale (see FIG. 5), and the thermal stress becomes higher. The main gist of the present invention is to evaluate the elephant skin-like crack growth on the outer surface of the heat transfer tube by incorporating the influence of the heat transfer tube inner surface scale into the diagnostic method. That is, in the evolution analysis of the elephant skin-like crack generated on the outer surface of the boiler heat transfer tube, the conventional technology (for example, the above-mentioned Patent Document 1) considers high temperature oxidation or high temperature corrosion from the outer surface of the tube and scale regeneration under the crack due to cracking of the scale. However, as described above, there is a situation where it is impossible to diagnose crack propagation in the actual heat transfer tube.In this embodiment, the influence of the metal temperature due to the inner surface scale is found to be important, and the inner surface scale of the tube is determined. It is intended to be incorporated into a crack damage diagnostic method.

  In FIG. 1, the method for calculating the fatigue crack growth analysis on the outer surface of the pipe and the high temperature oxidation or the high temperature corrosion amount on the outer surface of the pipe is the same as in the prior art, and the temperature difference, temperature change and linear expansion coefficient of the material Calculate the stress range (σR).

σR = ΔT × Lec × E (1)
Here, σR: thermal stress range (N / mm ² ), ΔT: temperature difference (K), Lec: linear expansion coefficient (1 / K), E: Young's modulus (N / mm ² ).

  Further, the stress intensity factor range ΔK is calculated from σR, crack shape, and structural material shape by the equation (2), and the fatigue crack growth diagram (ΔK−da / dN) of the material (crack growth rate da / shown in FIG. 8). dN)) and the amount of crack growth is calculated from the number of repetitions.

ΔK = D × σR√ (πa) = 112 × (σR / (t ^0.5 × W) × (tan (π × a / t))) ^0.5 (2)
Here, D: shape factor, t: tube plate thickness (mm), W: tube half circumference width (mm), a: crack depth (mm).

  Next, the amount of high-temperature oxidation scale or high-temperature corrosion scale generated on the outer surface of the pipe is calculated from the regression equation based on the database from the material, temperature, gas properties, and attached ash properties as follows. The high temperature oxidation or high temperature scale thickness is as follows:

Y = EI × (Kp × t) ^0.5 (3)
Kp = A × exp (−Q / RT) (4)
Log (Kp) = as × 1 / T + bs (5)
Where Y: oxide scale thickness (mm), Kp: parabolic law rate constant, t: time (h), T: temperature (K), Q: activation energy, R: gas constant, A: material constant, as, bs: Regression coefficient obtained from an experimental value (or actual measurement value) of scale thickness and a regression equation of temperature and time, EI: environmental acceleration coefficient.

Here, the Kp value can be calculated by substituting the as, bs value and temperature (K) of each material into Equation (5), and the oxidation scale can be obtained by substituting the Kp value, time t, and environmental acceleration coefficient EI into Equation (3). Thickness Y can be calculated. If a and b corresponding to each environment are obtained, EI = 1 may be used. However, in general, a and b have many numerical values obtained in the atmospheric environment, and as and bs values under the conditions are used. In this case, the EI is set in accordance with the gas properties (corrosive SO ₂ and HCl concentration) and the properties of the attached ash.

  Next, when the inside of the tube is superheated steam, the amount of scale generated on the tube inner surface side is calculated from the temperature and material using a regression equation (using steam oxidation c and d instead of high temperature corrosion as and bs). In the case of high-temperature water, it is determined from the Fe concentration in water, pH, and past results. The rise in the surface temperature and metal temperature due to the inner scale of the pipe is a function of the thickness of the inner scale, the scale thermal conductivity, the thermal load, and the pipe dimensions, and the heat transfer formula represented by formula (6) Use.

ΔT = DI × Q × ts / λs (6)
Where ΔT: temperature rise due to scale (K), DI: shape factor, Q: thermal load (W / m ² ), ts: scale thickness (mm), λs: scale thermal conductivity (W / mK) is there.

  If the surface temperature, metal temperature, and temperature difference during SB or WB operation increase due to the scale inside the tube, the thermal stress range, fatigue crack growth and high-temperature oxidation and high-temperature corrosion are accelerated, and the development of elephant skin-like thermal fatigue cracks from the outer surface of the tube It will be accelerated.

  Explaining in detail, as shown in FIG. 1, the analysis of the fatigue crack growth on the outer surface of the pipe calculates the thermal stress range σR based on the temperature difference, temperature change, and coefficient of thermal expansion of the heat transfer pipe. The stress intensity factor range ΔK is calculated based on the crack shape and structural material shape, and the fatigue crack growth diagram showing the relationship between this ΔK and the crack growth rate and the number of stress repetitions associated with SB or WB and start / stop Based on this, crack growth is analyzed. Next, in the evaluation of the high temperature oxidation or high temperature corrosion scale on the outer surface of the pipe, the amount of high temperature oxidation or high temperature corrosion scale generated is calculated based on the temperature, material, gas properties, and ash properties, and the generated scale cracks are evaluated.

  The basic thermal fatigue crack damage is diagnosed by the above fatigue crack growth analysis and pipe outer surface scale crack evaluation. In addition to this, in the embodiment of the present invention, the temperature rise effect due to the scale generation on the inner surface of the pipe is intended to be reflected in the diagnosis of the basic thermal fatigue crack damage. Here, the pipe inner scale calculates the amount of steam oxidation scale generated based on temperature, material, time, and heat load when the pipe is steam, and the Fe concentration in water, temperature, and pH when the pipe is hot water. The amount of deposition adhesion scale is calculated based on the flow rate.

  The generation of the pipe inner scale increases the metal and surface temperature due to the inner scale thickness, the thermal conductivity of the scale, the thermal load, and the pipe dimensions, and this temperature rise causes the crack growth and high temperature oxidation of the pipe outer surface. Or hot corrosion accelerates. Therefore, in this embodiment, this temperature rise is intended to be reflected in the diagnosis of the basic thermal fatigue crack damage.

  FIG. 2 shows the results of crack growth analysis and life diagnosis using the crack damage diagnosis method according to the embodiment of the present invention. The conditions are the same as the conditions shown in FIG. 10, and the STBA24 steel was analyzed under the conditions of SB frequency 250 times / 10 kh, initial average temperature 520 ° C., inner surface: superheated steam, 50,000 h to 150,000 h. The temperature rise effect by the steam oxidation scale in the tube of about 0.2-0.3mm thickness (average metal temperature rises by 12 ° C), the stress range doubles including the crack propagation effect, and reaches the life at 150,000h The figure is obtained, and the diagram is in good agreement with the actual machine measurements.

FIG. 3 is an example of input items and an example of a diagnosis screen for a high-accuracy thermal fatigue crack damage diagnosis method for a boiler heat transfer tube according to an embodiment of the present invention. FIG. 4 shows specific results of the present embodiment analyzed and diagnosed under the conditions shown in FIG. Material: STBA24 (2.25Cr1Mo) steel, temperature 520 ° C., initial thermal stress range σR: 180 N / mm ² , repetition rate: 200 times / year, operation time: 7000 h / year, and operation of about 90,000 h This is a result of analysis based on an elephant skin-like crack having a depth of 1.2 mm.

  As shown in Fig. 4 (see 0, 2, 4, 6, and 8 years described in the upper side of Fig. 4), it will grow into a blowout limit crack in the next 3.9 years. Considering this, it can be diagnosed that replacement is necessary in two years. With the diagnostic method of the prior art, the life is diagnosed after 8 to 10 years, and with the diagnostic method according to the present embodiment, diagnosis with higher accuracy and higher equipment reliability is possible.

  The main point of the diagnostic method according to the embodiment of the present invention is that items that can be used to evaluate scale growth on the inner surface side of the tube and an increase in metal temperature due to the inner surface scale are added to items used for evaluation of thermal fatigue crack propagation on the outer surface side of the tube. . Furthermore, the present embodiment includes a diagnosis by performing crack growth analysis on an input screen taking the input items shown in FIG. 3 as an example.

  As described above, the thermal fatigue crack damage diagnosis method for a boiler heat transfer tube according to an embodiment of the present invention is for solving the following problems and is characterized by having the following configuration. Is. In other words, with the aging of boilers such as coal fired, the elephant skin-like crack damage in the soot blower (SB) or wall blower (WB) working part of boiler heat transfer tubes such as water wall tubes, superheater tubes, reheater tubes, etc. This crack is caused by the combination of the repeated action of thermal stress accompanying the change in metal temperature during SB or WB operation and the high temperature corrosion or high temperature oxidation of the combustion gas. Under the influence of various factors, there was a problem that the degree of damage could not be generally evaluated.

  This embodiment finds that the effect of high temperature oxidation (high temperature corrosion) on the outer surface of the pipe and oxidation scale on the inner surface of the pipe is significant in the development of this elephant skin-like crack, and proposes a thermal fatigue crack damage diagnostic method that takes these effects into account. Specifically, in the method of diagnosing the degree of damage and remaining life due to elephant skin-like thermal fatigue cracks generated from the outer surface of the boiler heat transfer tube, the oxide scale due to high temperature oxidation or high temperature corrosion on the tube outer surface side In addition to generation, the degree of increase in metal temperature and the degree of increase in thermal stress due to the oxide scale on the inner surface of the tube is incorporated into the crack growth damage diagnosis. Also, in the method of diagnosing the degree of damage and remaining life due to elephant skin-like thermal fatigue cracks generated from the outer surface of the boiler heat transfer tube, the high temperature fatigue crack growth diagram, temperature and gas properties of the heat transfer tube material are used as parameters. Thermal fatigue crack damage diagnosis is performed using the oxidation rate, crack strain of high-temperature oxide scale, thermal stress range, crack size, thermal load on the pipe outer surface, pipe inner scale growth rate, pipe inner scale thermal conductivity as input items. .

It is a figure which shows the flow of the thermal fatigue crack damage diagnostic method of the boiler heat exchanger tube which concerns on embodiment of this invention. It is a figure which shows the result of having implemented the crack growth analysis and lifetime diagnosis using the thermal fatigue crack damage diagnostic method regarding this embodiment. It is a figure which shows the input item and example of a diagnostic screen of the thermal fatigue crack damage diagnostic method regarding this embodiment. It is a figure which shows the specific result analyzed and diagnosed on the conditions shown in FIG. It is a figure which shows the metal temperature change in the case where a scale exists in the pipe inner surface of the boiler heat exchanger tube which concerns on this embodiment, and the case where it does not exist. It is explanatory drawing showing the thermal fatigue crack damage at the time of making a soot blower act on a boiler heat exchanger tube. It is a figure which shows the metal temperature change at the time of making a soot blower act on a boiler heat exchanger tube. It is a fatigue crack progress diagram of a boiler heat exchanger tube. It is a flow which shows the calculation and life evaluation of the crack propagation amount of the boiler heat exchanger tube regarding a prior art. It is a crack measurement result and crack growth prediction diagram of an actual boiler heat exchanger tube.

Explanation of symbols

1 Boiler heat transfer tube 2 Adhering ash 3 Steam flow by soot blower (SB) or wall blower (WB) 4 Thermal stress 5 Inner scale 6 Elephant skin crack

Claims (3)

A diagnostic method for diagnosing the degree of damage due to an elephant skin-like thermal fatigue crack generated from the outer surface of a boiler heat transfer tube,
Calculating a stress range coefficient range based on the thermal stress range and crack depth calculated based on the dimensions, thermal expansion coefficient, and temperature change of the heat transfer tube;
Analyzing the amount of thermal fatigue crack propagation based on the thermal fatigue crack growth diagram representing the relationship between the stress range coefficient range and crack growth rate and the number of thermal stress repetitions;
Determining the amount of high temperature oxidation scale or high temperature corrosion scale formed on the outer surface of the heat transfer tube, and evaluating cracks due to the generated scale;
Obtaining a production amount of oxide scale on the inner surface side of the heat transfer tube,
Incorporating the increase in heat transfer tube metal temperature due to the obtained tube inner surface side scale generation amount accelerates the thermal stress range, the thermal fatigue crack growth amount and the scale generation amount of the tube outer surface, and the thermal fatigue A thermal fatigue crack damage diagnostic method for a boiler heat transfer tube, wherein the damage degree of the thermal fatigue crack is diagnosed by analyzing the crack propagation amount and evaluating the crack. In claim 1,
The amount of scale generated on the outer surface of the heat transfer tube is calculated using the tube material, temperature, gas properties and ash properties as parameters,
The amount of oxide scale generated on the inner surface of the heat transfer tube is calculated using the tube material, temperature, steam or high temperature water characteristic values as parameters,
The rise in the heat transfer tube metal temperature caused by the tube inner surface side oxide scale generation amount is calculated using the heat conductivity of the tube inner surface side oxide scale, the thickness of the oxide scale, the tube dimensions, and the heat load as parameters,
In addition to each of the parameters described above, the thermal fatigue crack propagation diagram, crack depth dimension, and thermal stress range of the heat transfer tube material are used as input items to diagnose the degree of damage of the thermal fatigue crack. Thermal fatigue crack damage diagnosis method for heat transfer tubes. In claim 1 or 2,
By the crack damage degree calculated based on the amount of thermal fatigue crack growth and scale cracks due to scale generation on the outer surface of the tube, and the limit crack depth determined based on the material strength of the boiler heat transfer tube, A thermal fatigue crack damage diagnostic method for boiler heat transfer tubes, characterized by diagnosing the remaining life of boiler heat transfer tubes.

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