JP7623902B2 - Method and device for assessing remaining life of heat transfer tube - Google Patents

Description

The present invention relates to a method and device for assessing the remaining life of a heat transfer tube. More specifically, the present invention relates to a method and device for assessing the remaining life of a heat transfer tube that can accurately predict the remaining creep life of the heat transfer tube.

Traditionally, in thermal power generation facilities, metal materials with excellent corrosion resistance and heat resistance, such as low alloy steel and high chromium steel, have been widely used for heat transfer tubes such as power steam piping. Furthermore, such metal materials are exposed to high temperatures and pressures for long periods of time during operation of the thermal power generation facility, so creep gradually progresses in these materials, and they break when their creep life is reached. Therefore, in order to operate thermal power generation facilities safely and economically, there is a strong demand for technology that can accurately predict the remaining life against creep.

One known remaining life prediction technique for this creep is to cut out a portion of the heat transfer tube for which the remaining life is to be predicted from the actual equipment, process it into a specified creep test piece, and then test this creep test piece in an experimental facility under conditions of accelerated temperature and load stress until it breaks, thereby evaluating creep damage.

However, the conventional method of using test specimens taken from actual equipment not only requires several thousand hours to obtain test results, but also has the problem that if the temperature at which the material being evaluated has been used is unknown, it is difficult to determine a reliable remaining life unless the evaluation is performed at an appropriate temperature.

That is, it is known that as the time of use of a heat transfer tube increases, the thermal conductivity characteristics of the tube decrease due to the growth of steam oxide scale on the inner surface of the tube, resulting in an increase in the tube wall temperature. Technology is publicly known that takes this into account in relation to a non-destructive prediction method based on the thickness of steam oxide scale.

For example, in Patent Document 1, the metal temperature of the downstream gas heat transfer tube is estimated, and the estimated metal temperature is estimated as the initial metal temperature of the upstream gas heat transfer tube. Then, the metal temperature rise due to the oxide scale on the upstream gas heat transfer tube is calculated from a predetermined relational equation based on the estimated initial metal temperature, the measured thickness of the oxide scale on the upstream gas heat transfer tube, and the measured boiler operation time, and the metal temperature of the upstream gas heat transfer tube is estimated. Furthermore, the remaining creep life of the upstream gas heat transfer tube is estimated from the metal temperature, stress, and measured boiler operation time of the upstream gas heat transfer tube.

JP 2011-64381 A

However, according to the creep remaining life assessment method disclosed in the above-mentioned Patent Document 1, the growth of steam oxide scale thickness is taken into account and the creep remaining life is assessed by estimating the metal temperature of the upstream gas heat transfer tube, which has a high thermal load, but the temperature gradient in the wall thickness direction is not taken into account at all in the process of calculating the metal temperature.

That is, since the temperature of the outer surface of a heat transfer tube is generally higher than the temperature of the inner surface of the heat transfer tube, a temperature gradient occurs between the inner and outer surfaces of the heat transfer tube. Therefore, to perform a more accurate evaluation of the remaining creep life, it is necessary to perform calculations based on the temperature of the center of the wall thickness of the heat transfer tube. However, to calculate the temperature of the center of the wall thickness of the heat transfer tube, information on the heat flow rate passing through the heat transfer tube is required. In this regard, as can be seen from the fact that Patent Document 1 estimates the temperature calculated from the downstream steam oxide scale thickness as the initial metal temperature of the heat transfer tube upstream of the gas, it does not take into account the temperature gradient between the inner and outer surfaces of the heat transfer tube. Therefore, it cannot be said that the creep remaining life is necessarily reliable.

The present invention was devised in consideration of the above points, and aims to provide a method for evaluating the remaining life of a heat transfer tube, and an apparatus for evaluating the remaining life of a heat transfer tube, which can accurately predict the remaining creep life of the heat transfer tube.

To achieve the above-mentioned object, the method for estimating the remaining life of a heat transfer tube of the present invention includes the steps of: determining, as an actual heat flux, a heat flux that approximately matches the actual operating time of the boiler for the growth time required for the growth of the actual thickness of steam oxide scale in the evaluation target portion of the heat transfer tube based on a calculation table that can calculate the relationship between the thickness of steam oxide scale formed on the inner surface of the heat transfer tube and the growth time of the steam oxide scale thickness by applying a predetermined heat flux; determining the temperature of the center of the tube wall of the evaluation target portion based on the heat flux; and estimating the remaining creep life of the evaluation target portion from the temperature and stress of the center of the tube wall.

By making it possible to obtain the relationship between the thickness of the steam oxide scale formed on the inner surface of the heat transfer tube and the growth time of the steam oxide scale, using the heat flux as a variable, it is possible to obtain table data showing the relationship between the steam oxide scale thickness according to the heat flux and the growth time of the steam oxide scale thickness, or a calculation table consisting of a scale growth estimation curve on which the steam oxide scale thickness according to the heat flux and the growth time of the steam oxide scale thickness are plotted.

In addition, by providing a step of determining, as the actual heat flux, the heat flux for which the growth time required for the growth of the measured thickness of steam oxide scale in the evaluation target portion of the heat transfer tube approximately coincides with the actual operating time of the boiler, based on a calculation table prepared in advance, it is possible to determine, as the actual heat flux, the heat flux that coincides with the actual measured value of steam oxide scale in the evaluation target portion and the actual operating time of the boiler.

In addition, by including a step of determining the temperature of the tube wall center of the evaluation target area based on the heat flux, the temperature gradient of the inner and outer surfaces of the heat transfer tube can be estimated, and the temperature of the tube wall center can be calculated based on the temperature gradient.

In addition, by including a step of estimating the remaining creep life in the evaluation target portion from the temperature and stress at the center of the pipe wall, the accuracy of estimating the remaining creep life can be improved by taking into account the temperature at the center of the pipe wall.

In addition, when the calculation table is created based on the steps of acquiring the temperature of the steam flowing through the heat transfer tube and calculating a first temperature, which is the temperature rise on the inner surface of the heat transfer tube caused by the steam flowing through the heat transfer tube when the steam oxide scale thickness is zero, it is possible to calculate the inner surface temperature of the heat transfer tube when the steam oxide scale thickness is zero.

In addition, when the calculation table is created by repeating the calculation of the growth time for the steam oxide scale thickness to grow from zero to a predetermined scale thickness up to the nth time (n is an integer of 2 or more) in which the first calculation is performed using the temperature obtained by adding the first temperature to the steam temperature as the initial temperature of the inner surface of the heat transfer tube, the growth time for each predetermined scale thickness unit is calculated. From the second calculation onwards, if the calculation is created by repeating the calculation of the growth time up to the nth time (n is an integer of 2 or more), the growth time can be calculated taking into account the temperature rise due to the scale thickness of the grown steam oxide scale, so that the growth time can be calculated with higher accuracy.

The time required for the total growth of the steam oxide scale thickness can be calculated by repeatedly calculating the growth time required for each 1 μm of scale thickness up to the nth time and accumulating the results. The scale thickness unit is not limited to 1 μm, and the growth time may be calculated for each 0.1 μm, for example, to obtain more detailed data.

In addition, when the nth calculation calculates the growth time of the steam oxide scale based on the temperature obtained by adding the second temperature, which is the temperature rise on the inner surface of the heat transfer tube due to the steam oxide scale thickness that has grown up to the (n-1)th calculation, to the initial temperature, as described above, the growth time of the steam oxide scale can be calculated taking into account the temperature rise due to the scale thickness of the steam oxide scale that has grown up to a specific point in time, making it possible to calculate the growth time with higher accuracy.

In addition, when the first temperature is a temperature that takes into account the heat transfer coefficient of the water vapor to the heat transfer tube and the actual heat flux calculated from the heat flux, the temperature rise of the heat transfer tube due to the water vapor can be accurately estimated by taking into account the heat transfer coefficient of the water vapor and the heat flux.

In addition, when the second temperature is the temperature rise of the heat transfer tube according to the scale thickness and is a temperature taking into account the thermal conductivity and heat flux of the scale, the temperature rise due to steam oxide scale on the heat transfer tube can be accurately estimated by taking into account the thermal conductivity and heat flux of the steam oxide scale.

In addition, if the step of estimating the remaining creep life includes a step of calculating a creep life consumption rate, which is the consumption ratio of the creep life in a specified scale thickness unit of steam oxide scale, the creep life consumption rate can be calculated for each specified scale thickness unit (e.g., every 1 μm).

In addition, when there is a step of calculating the relationship between the boiler operating time and the creep life consumption rate by integrating the creep life consumption rate from zero to a predetermined scale thickness of the steam oxide scale, the creep life consumption rate for each scale thickness unit described above can be integrated to calculate the relationship between the creep life consumption rate and the boiler operating time, and an estimated curve of the creep life consumption rate versus the boiler operating time can be obtained, for example.

In addition, if there is a step of subtracting the boiler operation time at the time of evaluation from the boiler operation time corresponding to a creep life consumption rate of 100%, the remaining time until the creep life consumption rate reaches 100% can be estimated by matching the current boiler operation time with the estimated curve of creep life consumption rate versus boiler operation time described above.

To achieve the above object, the heat transfer tube remaining life evaluation device of the present invention includes a calculation unit that calculates the relationship between the thickness of steam oxide scale formed on the inner surface of the heat transfer tube and the growth time of the steam oxide scale thickness, a heat flux determination unit that determines an actual heat flux based on the calculation unit such that the growth time required for the growth of the actual thickness of steam oxide scale in the evaluation target portion of the heat transfer tube is the actual operating time of the boiler, a temperature calculation unit that calculates the temperature of the tube wall center of the evaluation target portion based on the actual heat flux, and a creep remaining life calculation unit that calculates the creep remaining life of the evaluation target portion from the temperature and stress of the tube wall center.

Here, by providing a calculation unit that calculates the relationship between the thickness of the steam oxide scale formed on the inner surface of the heat transfer tube and the growth time of the steam oxide scale, it is possible to calculate the relationship between the steam oxide scale thickness according to the heat flux and the growth time of the steam oxide scale.

In addition, by providing a heat flux determination unit that determines the actual heat flux that corresponds to the growth time required for the growth of the measured thickness of steam oxide scale in the evaluation target portion of the heat transfer tube based on the calculation unit, it is possible to determine the heat flux that corresponds to the actual measurement value of steam oxide scale in the evaluation target portion during the actual operation time of the boiler.

In addition, by providing a temperature calculation unit that calculates the temperature of the center of the tube wall of the evaluation target area based on the actual heat flux, the temperature of the center of the tube wall of the heat transfer tube can be calculated.

In addition, by providing a creep remaining life calculation unit that calculates the creep remaining life in the evaluation target area from the temperature and stress in the center of the pipe wall, the accuracy of estimating the creep remaining life can be improved by taking into account the temperature in the center of the pipe wall.

The method and device for assessing remaining life of a heat transfer tube according to the present invention make it possible to accurately predict the remaining creep life of a heat transfer tube.

1 is an overall configuration diagram of a remaining life evaluation device for a heat transfer tube according to an embodiment of the present invention. 1 is a diagram showing an overall flow of a remaining life evaluation method for a heat transfer tube according to an embodiment of the present invention. FIG. 1 is an image diagram of calculation of the time required for the growth of steam oxide scale. 1 is a graph (scale growth estimation curve) showing the relationship between scale thickness and operating time. 1 is a graph (metal temperature rise curve) showing the relationship between the metal temperature and the operating time. 1 is a graph (creep life consumption rate curve) showing the relationship between operating time and creep life consumption rate. 2 is an example of a creep remaining life calculation format. 1 is a graph showing verification results of the remaining life assessment method of the present invention.

Below, a method for evaluating the remaining life of a heat transfer tube and an apparatus for evaluating the remaining life of a heat transfer tube according to an embodiment of the present invention will be described with reference to the drawings to facilitate understanding of the present invention.

Figure 1 is an overall configuration diagram of a remaining life evaluation device for a heat transfer tube (hereinafter, sometimes simply referred to as a "tube") according to an embodiment of the present invention. The remaining life evaluation device 1 for a heat transfer tube is mainly composed of a control unit 10, a storage unit 20, an input unit 30, a display unit 40, and a memory unit 50.

The control unit 10 is, for example, a CPU (Central Processing Unit), which controls the entire remaining life evaluation device 1 and calculates the creep remaining life in accordance with a creep remaining life evaluation program stored in the memory unit 20, and is mainly composed of a heat flux determination unit 11, a temperature calculation unit 12, and a creep remaining life calculation unit 13. The calculation methods in the heat flux determination unit 11, the temperature calculation unit 12, and the creep remaining life calculation unit 13 will be described later.

The storage unit 20 is a device capable of storing at least various data and programs, such as a hard disk.

The input unit 30 is an interface for providing at least the operator's commands and various information to the control unit 10, and is, for example, a keyboard, a mouse, or a touch panel. Note that the input unit 30 may be configured with multiple types of interfaces, such as a keyboard and a mouse, or a touch panel.

The display unit 40 draws and displays characters, figures, images, etc. under the control of the control unit 10, and is, for example, a display.

The memory 50 is a memory space that serves as a working area when the control unit 10 executes various controls and calculations, and is, for example, a RAM (short for Random Access Memory).

Then, the creep remaining life evaluation program stored in the memory unit 20 is executed, and calculations are performed to evaluate the creep remaining life in the target portion of the tube. Figure 2 shows the overall flow of the method for evaluating the remaining life of a heat transfer tube based on the growth of steam oxide scale. First, information on the steam temperature, pressure, steam flow rate, and number of tubes in the target portion of the tube is obtained as basic data (S1).

Next, the heat transfer coefficient of water vapor to the tube is calculated using the steam property values obtained from the steam table (1999 Japan Society of Mechanical Engineers Steam Table) based on the steam temperature, pressure, and flow rate per tube (S2).

In addition, by applying a predetermined heat load to the tube-steam interface, the temperature of the tube inner surface is calculated from the heat transfer coefficient of the tube-steam interface (S3), and the steam oxide scale growth thickness (growth rate) is calculated from the steam oxide scale growth rate constant (S4).

Furthermore, by repeatedly adding the temperature rise due to the grown steam oxide scale to the initial temperature of the inner surface of the tube and recalculating the steam oxide scale growth rate, the growth of steam oxide scale over time and the metal temperature rise curve are obtained (S5), and the creep life consumption rate is calculated from the creep rupture data of the new material (S6).

The above is the overall flow of the method for evaluating the remaining life of a heat transfer tube according to an embodiment of the present invention, but each calculation flow will be explained in detail below.

[Acquisition of basic data]
Of the basic data, the steam temperature of the evaluation target portion can be estimated based on the metal temperature measured by a metal thermometer installed at the tube inlet/outlet (non-heated inlet section and non-heated outlet section) outside the boiler furnace. The metal thermometer is attached to the outer surface of the tube that does not involve heat exchange, and the metal temperature of the tube measured by the metal thermometer can be considered as the temperature of the steam flowing inside the tube.

The temperatures inside the furnace are highest in the order of "metal temperature at the inlet unheated section," "steam temperature inside the furnace," and "metal temperature at the outlet unheated section," so the steam temperature also gradually rises as heat exchange takes place inside the furnace. Therefore, when estimating the temperature of the steam flowing inside the pipe of the part being evaluated, an error will occur if the metal temperature of the outlet unheated section is used as is. For this reason, it is necessary to confirm at which station of the total length of the pipe extending into the furnace the pipe of the part being evaluated is located (the whole length being the 10th station), and then estimate the steam temperature after setting a specified allocation rate.

For example, if the part to be evaluated is located downstream from the inlet side to the outlet side of a tube extending inside a furnace, and the steam temperature apportionment rate is set to 0.6, the steam temperature can be estimated using the following formula (1).
T ₀ =T _in +0.6(T _out -T _in ) (1)
(T ₀ : steam temperature, T _in : metal temperature of the inlet non-heated part, T _out : metal temperature of the outlet non-heated part)

In addition, when the metal temperatures of the inlet unheated section and the outlet unheated section are unknown, the steam temperature can be estimated from the measured values of the steam oxide scale thicknesses of the inlet unheated section and the outlet unheated section. In this case, since the growth of steam oxide scale is generally expressed by a parabolic law, the following equation (2) holds when the scale thickness is _δs .
δ _s =K _p ×t ^0.5 (2)
(K _p : scale growth rate constant, t: boiler operation time)

In addition, the scale growth rate constant _Kp can be approximated by the following equation (3).
LogK _p =A(1/T ₀ )+B (3)
Here, A and B are coefficients for each material. If the boiler operation time t and scale thickness are known, _Kp can be obtained from equation (2), and the steam temperature _T0 can be found from equation (3).

[Calculation of heat transfer coefficient]
The heat transfer coefficient α of the water vapor to the pipe is calculated using the steam property values in the steam table (1999 Japan Society of Mechanical Engineers Steam Table) from the steam temperature, pressure, and flow rate per pipe of the evaluation target portion (a specific calculation method will not be described). Note that the heat transfer coefficient may be calculated using a predetermined logical formula based on the steam temperature, pressure, steam flow rate, and steam property values obtained from the boiler operation data.

[Calculation of tube inner surface temperature when steam oxide scale thickness is zero]
Next, calculate the temperature of the inner surface of the tube when there is no steam oxide scale (i.e., when steam oxide scale is not taken into consideration). Even when steam oxide scale is not present, a temperature rise occurs at the tube-steam interface. In this case, the temperature rise ΔT _α at the inner surface of the tube can be calculated using the following formula (4).
ΔT _α =q _i ×α (4)
(q _i : heat flux on the inner surface of the tube, α: heat transfer coefficient at the tube-steam interface)

Taking the above-mentioned temperature rise ΔT _α into consideration, the temperature T _i1 of the tube inner surface when there is no steam oxide scale can be calculated by the following formula (5).
T _i1 =T ₀ +ΔT _α (5)

[Calculation of steam oxide scale growth rate and time required for growth]
The growth rate of steam oxide scale can be approximated by a parabolic curve and can be calculated based on the above-mentioned formulas (2) and (3). In this case, the scale growth rate constant _Kp has temperature dependency depending on the material of the tube. Then, based on the above-mentioned formula (5), the time required for scale growth can be calculated from the growth rate constant of steam oxide scale of the corresponding material at the tube inner surface temperature T _i1 when the amount of steam oxide scale is zero.

[Calculation of tube inner surface temperature taking into account steam oxidation scale]
Next, in order to calculate the temperature of the inner surface of the tube due to the formation of steam oxide scale, the temperature rise ΔT _λs due to steam oxide scale can be calculated using the following equation (6).
ΔT _λs =q _i ×δ _s /λ _s (6)
(λs: thermal conductivity of steam oxide scale, _δs : thickness of steam oxide scale)

Taking the above-mentioned temperature rise ΔT _λs into consideration, the temperature T _i2 of the inner surface of the tube when the thickness of the steam oxide scale is δ _s can be calculated by the following formula (7).
T _i2 =T ₀ +ΔT _α+ ΔT _λs (7)

[Calculation of time required for growth of steam oxide scale]
As the steam oxide scale grows, the tube inner surface temperature also rises, and the growth rate constant of the scale also changes (the growth rate increases as the scale grows). Therefore, the growth time _Δt1 required to grow a predetermined scale thickness (1 μm in this embodiment of the present invention) was repeatedly calculated as the scale thickness to be grown, and the growth time to the final scale thickness was calculated by integrating _Δt1 (see FIG. 3). _Δt1 can be calculated using the following formula (8).
Δt ₁ =t _d -t _d-1 (8)
(t _d : time required for the scale to grow to a thickness of d μm, t _d-1 : time required for the scale to grow to a thickness of d-1 μm)

In an embodiment of the present invention, the time required for growth of each 1 μm of scale thickness is repeatedly calculated and integrated to calculate the total time required for growth, but the calculation may be performed in units of 0.1 μm as the scale thickness unit, for example, and the scale thickness unit used in the calculation can be changed as desired.

A calculation table that can calculate the relationship between the time required for steam oxide scale growth and scale thickness obtained in the above calculation flow according to the heat flux is stored in the memory unit 20, and is used to calculate the tube wall center temperature and estimate the remaining creep life, as described below.

Figure 4 shows an example of a scale growth estimation curve for a given heat flux determined from a calculation table. When a given heat flux is given from the calculation table, the scale growth estimation curve can be expressed as a curve that plots the scale thickness over time, with the horizontal axis representing time and the vertical axis representing the steam oxidation scale thickness.

[Determination of actual heat load]
Next, the actual heat flux in the evaluation target portion is determined using the above-mentioned calculation table. To determine the actual heat flux, first, the actual value of the steam oxide scale thickness of the tube to be evaluated is measured. The actual scale thickness can be measured from the outer surface non-destructively, for example, by ultrasonic method. Then, based on the calculation table, the heat flux determination unit 11 searches for a heat flux that matches the actual value of the steam oxide scale thickness for the current boiler operating time, and the searched heat flux is determined as the actual heat flux.

[Calculation of tube wall center temperature]
In calculating the tube wall central temperature _Tm , the actual heat flux _qt determined based on the above-mentioned calculation table is first calculated. Using this heat flux _qt , the tube wall central temperature _Tm is calculated using the following formula (9). Note that T _i2 is the tube inner surface temperature taking into account the steam oxide scale thickness calculated by the above-mentioned formula (7).
T _m = T _i2 + (r _i /λ _t )・q _t・ln (r _m /r _i ) (9)
(r _m : radius from the center of the tube to the center of the tube wall, λ _t : thermal conductivity of the tube)

In addition, in calculating the tube wall central temperature _Tm , for example, the temperature of the tube outer surface may be calculated using a relational expression, and then the median value taking into account the temperature gradient of the tube wall portion may be calculated as the tube wall central temperature _Tm .

Figure 5 shows a metal temperature rise curve created by plotting the tube inner surface temperature T _i2 and the tube wall center temperature T _m against the operating time. As shown in Figure 5, the metal temperature rises as the steam oxide scale grows in thickness over the course of operating time. It can also be seen that due to the temperature gradient on the inner and outer surfaces of the tube, the tube wall center temperature T _m is higher than the tube inner surface temperature T _i2 throughout the entire operating period.

As described above, since the pipe wall center temperature _Tm is relatively higher than the pipe inner surface temperature T _i2 , if the pipe inner surface temperature is used in the calculation of the remaining creep life, as disclosed in the above-mentioned Patent Document 1, for example, it may be determined that the remaining creep life is sufficient when it is actually not sufficient, but in the present invention, it is possible to accurately estimate the usage time of the pipe until the actual remaining creep life is reached. A specific method for estimating the remaining creep life will be described below.

[Calculation of creep life consumption rate]
The creep life consumption rate is calculated by integrating the time Δt ₁ required for a scale thickness dμm to go from d-1μm to dμm divided by the tube wall center temperature T _m , the hoop stress σ applied to the tube due to internal pressure, and the creep rupture time obtained from creep rupture data for new materials. Note that the creep rupture data can be the relationship between stress and the Larson-Miller parameter (hereinafter referred to as "LMP").

Here, LMP is a parameter for uniformly organizing creep rupture data at different temperatures, and can be organized by the following equation (10).
LMP=T×(C+ _logTr ) (10)
(T: temperature, C: material constant, T _r : rupture time)
The creep rupture LMP and material constant C for each material are calculated from an approximation formula for stress and creep rupture LMP.

The hoop stress σ can be calculated by the following formula (11).
σ=P(D-w)/2/w (11)
(P: internal pressure, D: outer diameter of the tube, W: wall thickness of the tube)

From the above, if the growth time required for a scale thickness to grow from d-1 μm to d μm is Δt ₁ , the creep life consumption rate φ _d can be calculated by the following equation (12).
φ _d = Δt ₁ /10 ^{(LMP/(273.15+TM)-C)} ×100 (12)
(TM: tube wall center temperature at scale thickness dμm)

The creep life consumption rate _φd per unit scale thickness is calculated based on the above-mentioned formula (12), and then the steam oxide scale thickness is integrated from zero, whereby the relationship between the operating time and the creep life consumption rate _φd can be expressed as a creep life consumption rate curve shown in FIG. 6 .

The remaining creep life is obtained by subtracting the current operating time from the time required for the integrated value of each creep life consumption rate φd to reach 100% .

[Calculation format for remaining life assessment based on scale growth]
7 shows an example of a remaining creep life calculation format used to actually calculate the remaining creep life. The remaining creep life calculation format is stored in, for example, the storage unit 20, and the remaining creep life can be calculated automatically by an operator inputting information required for calculating the creep life from the input unit 30.

In the creep remaining life calculation format, the data required for the calculation is prepared by first entering the basic information for items No. 1 to No. 11 and No. 13 to No. 15 for the part to be evaluated (e.g., heat transfer tube dimensions, operating time, measurement scale thickness, inlet steam temperature, outlet steam temperature, steam temperature correction value based on height level, etc., steam pressure, and steam flow rate, etc.).

When items No. 1 to No. 11 and No. 13 to No. 15 of the creep remaining life calculation format are entered, items No. 12 and No. 16 to No. 27 are automatically calculated. At this time, the actual heat flux (No. 28) is searched for so that the calculation time required for the growth of the measured scale thickness (No. 31) becomes the operating time, and a scale growth estimation curve (Figure 4), metal temperature rise curve (Figure 5), and creep life consumption rate curve (Figure 6) are obtained, from which the current life consumption rate and remaining life can be estimated.

As described above, when calculating the remaining creep life, the remaining creep life of the heat transfer tube being evaluated can be easily calculated by simply entering the necessary information into the remaining creep life calculation format.

[Verification of remaining life assessment method]
In order to verify the remaining life evaluation method of the present invention, a heat transfer tube that was blown out of an actual boiler equipment (hereinafter referred to as "blasted tube") was used for the verification. The scale thickness formed on the upstream inner surface of the blasted tube was approximately 300 μm by actual measurement, and the operation time was approximately 140,000 hours. The result of the trial calculation using the above-mentioned remaining life evaluation calculation format based on the actual data of this blasted tube is shown in Figure 8.

FIG. 8 is a graph showing the relationship between scale thickness and remaining life at about 140,000 hours of operation for a blowout tube calculated with an outlet steam temperature of 439° C.

As shown in FIG. 8, the remaining life of the blast tube is less than 50,000 hours when the scale thickness is 300 μm or more, and it can be seen that the remaining life is sufficient when the scale thickness is 200 μm or less.

As described above, the method and device for assessing remaining life of a heat transfer tube according to the present invention make it possible to accurately predict the remaining creep life of a heat transfer tube.

REFERENCE SIGNS LIST 1 Remaining life evaluation device 10 Control unit 11 Heat flux determination unit 12 Temperature calculation unit 13 Creep remaining life calculation unit 20 Storage unit 21 Creep remaining life evaluation program 30 Input unit 40 Display unit 50 Memory

Claims (6)

determining, as an actual heat flux, a heat flux that corresponds to a time required for the growth of an actual thickness of steam oxide scale at a portion of the heat transfer tube to be evaluated, approximately equal to an actual operating time of the boiler, based on a calculation table that allows a relationship between a steam oxide scale thickness formed on the inner surface of the heat transfer tube and a growth time of the steam oxide scale thickness to be calculated by applying a predetermined heat flux;
determining a temperature of the central portion of the pipe wall of the evaluation target portion based on the heat flux;
and estimating a remaining creep life at the evaluation target portion from the temperature and stress at the center of the tube wall. The calculation table is
Obtaining a temperature of steam flowing through the heat transfer tube;
calculating a first temperature, which is a temperature rise on the inner surface of the heat transfer tube caused by steam flowing through the heat transfer tube when a steam oxide scale thickness is zero;
a step of calculating a growth time for a steam oxide scale to grow from zero to a predetermined scale thickness using a temperature obtained by adding the first temperature to the temperature of the steam as an initial temperature on the inner surface of the heat transfer tube, and thereafter repeating the calculation of the growth time for each predetermined scale thickness unit up to an nth time (n is an integer of 2 or more). The n-th calculation is
3. The method for evaluating a remaining life of a heat transfer tube according to claim 2, further comprising the step of calculating a growth time of the steam oxide scale thickness based on a temperature obtained by adding a second temperature, which is a rise in temperature on the inner surface of the heat transfer tube due to the steam oxide scale thickness grown up to an (n-1)th calculation, to the initial temperature. the first temperature is a temperature taking into consideration a heat transfer coefficient of the water vapor to the heat transfer tube and the heat flux,
The second temperature is a temperature taking into consideration the thermal conductivity to the heat transfer tube according to the scale thickness of the steam oxide scale and the heat flux.
The method for evaluating remaining life of a heat transfer tube according to claim 3 . The step of estimating the remaining creep life includes:
calculating a creep life consumption rate, which is a consumption rate of the creep life per unit of a predetermined scale thickness of the steam oxide scale;
calculating a relationship between a boiler operation time and a creep life consumption rate by integrating the creep life consumption rate from zero to a predetermined steam oxide scale thickness;
The method for evaluating a remaining life of a heat transfer tube according to any one of claims 1 to 4, further comprising a step of subtracting a boiler operation time at the time of evaluation from a boiler operation time corresponding to a creep life consumption rate of 100%. a calculation unit that calculates a relationship between a thickness of a steam oxide scale formed on an inner surface of the heat transfer tube and a growth time of the steam oxide scale;
a heat flux determination unit that determines an actual heat flux based on the calculation unit, such that a growth time required for the growth of an actual thickness of steam oxide scale in an evaluation target portion of the heat transfer tube corresponds to an actual operation time of the boiler;
a temperature calculation unit that calculates a temperature of the pipe wall central portion of the evaluation target portion based on the actual heat flux;
a creep remaining life calculation unit that calculates a creep remaining life in the evaluation target portion from the temperature and stress in the tube wall central portion.

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