Method and device for assessing remaining life of heat transfer tube

Abstract

To provide a remaining lifetime evaluation method for a heat transfer tube and a remaining lifetime evaluation device for the heat transfer tube, capable of accurately predicting a creep remaining lifetime of the heat transfer tube.SOLUTION: In order to estimate a creep remaining lifetime with high accuracy, temperature rise accompanied by growth of thickness of steam oxidation scale, and acceleration of the scale growth are taken into consideration, and temperature gradients of inner and outer faces of a heat transfer tube are also taken into consideration. In considering the temperature gradients of the inner and outer surfaces of the heat transfer tube, thermal load (thermal flux) flowing from the heat transfer tube to a steam side is estimated from the thickness of the steam oxidation scale, and growth of future steam oxidation scale and an elevation curve of a tube wall center temperature in accompany therewith is estimated. Thereby, the creep remaining lifetime of high chrome steel, low alloy steel and the like can be estimated with high accuracy.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for evaluating the remaining life of a heat transfer tube, and more particularly to a method and an apparatus for evaluating the remaining life of a heat transfer tube, which are capable of accurately predicting the remaining creep life of the heat transfer tube. [Background technology]

[0002] Conventionally, in thermal power plants, metal materials with excellent corrosion resistance and heat resistance, such as low alloy steel and high chromium steel, are widely used for heat transfer tubes such as steam piping for power generation. During operation of thermal power plants, such metal materials are exposed to high temperatures and pressures for a long time, so creep gradually progresses in the metal materials, and they break when the creep life is reached. Therefore, in order to operate thermal power plants safely and economically, there is a strong demand for a technology that can accurately predict the remaining life against creep.

[0003] One known remaining life prediction technique for creep is to cut out a portion of the heat transfer tube for which the remaining life is to be predicted from an actual device, process it into a specified creep test piece, and then test this creep test piece in an experimental facility under accelerated conditions of temperature and load stress until it breaks, thereby evaluating creep damage.

[0004] However, the conventional method of using test pieces taken from actual equipment not only requires several thousand hours to obtain test results, but also has the problem that when the temperatures at which the material being evaluated has been used are unknown, it is difficult to determine a reliable remaining life unless the material is evaluated at an appropriate temperature.

[0005] It is known that as the time of use of a heat transfer tube increases, the thermal conductivity characteristics of the heat transfer tube deteriorate due to the growth of steam oxide scale on the inner surface of the heat transfer tube, resulting in an increase in the tube wall temperature. Taking this into consideration, a technique for non-destructive prediction of the tube wall temperature based on the thickness of steam oxide scale is publicly known.

[0006] For example, in Patent Document 1, the metal temperature of the gas downstream heat transfer tube is estimated, and the estimated metal temperature is estimated as the initial metal temperature of the gas upstream heat transfer tube. Then, the metal temperature of the gas upstream heat transfer tube is estimated after determining the metal temperature rise due to the oxide scale of the gas upstream heat transfer tube from a predetermined relational expression based on the estimated initial metal temperature, the measured thickness of the oxide scale of the gas upstream heat transfer tube, and the measured boiler operation time. Furthermore, the remaining creep life of the gas upstream heat transfer tube is estimated from the metal temperature, stress, and measured boiler operation time of the gas upstream heat transfer tube. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2011-64381 A Summary of the Invention [Problem to be solved by the invention]

[0008] 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 consideration 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. However, the temperature gradient in the wall thickness direction is not taken into consideration at all in the process of calculating the metal temperature.

[0009] That is, since the temperature of the outer surface of a heat transfer tube is generally higher than that 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, in order 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, in order 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.

[0010] The present invention has been devised in consideration of the above points, and has an object to provide a method and an apparatus for assessing the remaining life of a heat transfer tube, which are capable of accurately predicting the remaining creep life of a heat transfer tube. [Means for solving the problem]

[0011] In order 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 at which the growth time required for the growth of an actual thickness of steam oxide scale in an evaluation portion of the heat transfer tube approximately coincides with the actual operation time of a boiler, based on a calculation table that can calculate the relationship between the thickness of a 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 a temperature at a central portion of the tube wall of the evaluation portion based on the heat flux; and estimating the remaining creep life of the evaluation portion from the temperature and stress at the central portion of the tube wall.

[0012] Here, by making it possible to obtain the relationship between the steam oxide-scale thickness formed on the inner surface of the heat transfer tube and the growth time of the steam oxide-scale thickness using heat flux as a variable, it is possible to obtain table data showing the relationship between the steam oxide-scale thickness and the growth time of the steam oxide-scale thickness depending on the heat flux, or an operation table consisting of a scale-growth estimation curve on which the steam oxide-scale thickness and the growth time of the steam oxide-scale thickness depending on the heat flux are plotted.

[0013] In addition, by providing a step of determining, as the 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 prepared in advance, it is possible to determine, as the actual heat flux, a heat flux that matches the actual value of the steam oxide scale in the evaluation target portion and the actual operating time of the boiler.

[0014] In addition, by providing a step of determining the temperature of the central part of the tube wall of the evaluation target portion based on the heat flux, the temperature gradient on the inner and outer surfaces of the heat transfer tube can be estimated, and the temperature of the central part of the tube wall can be calculated based on the temperature gradient.

[0015] In addition, by including a step of estimating the remaining creep life in the evaluation target portion from the temperature and stress at the central portion of the pipe wall, the accuracy of estimating the remaining creep life can be improved by taking into account the temperature at the central portion of the pipe wall.

[0016] 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.

[0017] In addition, when the calculation table is created by repeating the calculation of the growth time for the steam oxide scale to grow from zero to a predetermined scale thickness up to the nth calculation (n is an integer of 2 or more) for each predetermined unit of scale thickness, with the temperature obtained by adding the first temperature to the steam temperature being the initial temperature on the inner surface of the heat transfer tube, the growth time for the steam oxide scale to grow from zero to a predetermined scale thickness is calculated as the first calculation, and then the growth time for each predetermined unit of scale thickness is calculated up to the nth calculation (n is an integer of 2 or more). Therefore, the growth time can be calculated more accurately from the second calculation onwards, up to the nth calculation.

[0018] The time required for the total growth of the steam oxide scale can be calculated by repeatedly calculating the growth time required for each 1 μm of the steam oxide scale thickness up to n times 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 to obtain more detailed data.

[0019] Furthermore, 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.

[0020] Furthermore, when the first temperature is a temperature taking 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 and heat flux of the water vapor.

[0021] 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 of the heat transfer tube due to steam oxide scale can be accurately estimated by taking into account the thermal conductivity and heat flux of the steam oxide scale.

[0022] In addition, when 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 predetermined scale thickness unit of steam oxide scale, the creep life consumption rate for each predetermined scale thickness unit (e.g., every 1 μm) can be calculated.

[0023] In addition, when the method includes a step of calculating the relationship between the boiler operation time and the creep life consumption rate by integrating the creep life consumption rate from a steam oxide scale thickness of zero to a predetermined scale thickness, the creep life consumption rate for each scale thickness unit described above is integrated to calculate the relationship between the creep life consumption rate and the boiler operation time, and an estimated curve of the creep life consumption rate versus the boiler operation time can be obtained, for example.

[0024] In addition, when the method includes 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 correlating the current boiler operation time with the estimated curve of the creep life consumption rate versus the boiler operation time described above.

[0025] In order to achieve the above-mentioned object, the remaining life evaluation device for a heat transfer tube of the present invention comprises a calculation unit that calculates the 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, 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 an actual thickness of steam oxide scale in an evaluation target portion of the heat transfer tube is the actual operating time of a boiler, a temperature calculation unit that calculates the temperature of a tube wall central portion 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 central portion.

[0026] 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.

[0027] Furthermore, by providing a heat flux determination unit that determines, based on the calculation unit, an actual heat flux that corresponds to 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 during the actual operation time of the boiler, it is possible to determine the heat flux that corresponds to the actual measured value of steam oxide scale in the evaluation target portion during the actual operation time of the boiler.

[0028] Furthermore, by providing a temperature calculation unit that calculates the temperature of the tube wall central portion of the evaluation target portion based on the actual heat flux, the temperature of the tube wall central portion of the heat transfer tube can be calculated.

[0029] In addition, by providing a creep remaining life calculation unit that calculates the creep remaining life in the evaluation target portion from the temperature and stress at the center of the pipe wall, the accuracy of estimating the creep remaining life can be improved by taking into account the temperature at the center of the pipe wall. Effect of the Invention

[0030] 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. [Brief description of the drawings]

[0031] [Figure 1] 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. [Diagram 2] 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. [Diagram 3] FIG. 1 is an image diagram of calculation of the time required for the growth of steam oxide scale. [Figure 4] 1 is a graph (scale growth estimation curve) showing the relationship between scale thickness and operating time. [Diagram 5] 1 is a graph (metal temperature rise curve) showing the relationship between the metal temperature and the operating time. [Figure 6] 1 is a graph (creep life consumption rate curve) showing the relationship between operating time and creep life consumption rate. [Figure 7] 2 is an example of a creep remaining life calculation format. [Figure 8] 1 is a graph showing verification results of the remaining life assessment method of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] Hereinafter, a method 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 for understanding the present invention.

[0033] 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.

[0034] The control unit 10 is, for example, a CPU (Central Processing Unit), which controls the entire remaining life assessment device 1 and calculates the creep remaining life in accordance with a creep remaining life assessment 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 and the like in the heat flux determination unit 11, the temperature calculation unit 12, and the creep remaining life calculation unit 13 will be described later.

[0035] The storage unit 20 is a device capable of storing at least various data and programs, and is, for example, a hard disk.

[0036] The input unit 30 is an interface for giving at least an operator's command 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 a plurality of types of interfaces, such as a keyboard and a mouse, or a touch panel.

[0037] The display unit 40 draws and displays characters, figures, images, and the like under the control of the control unit 10, and is, for example, a display.

[0038] The memory 50 serves as a memory space that is a working area when the control unit 10 executes various controls and calculations, and is, for example, a RAM (abbreviation of Random Access Memory).

[0039] Then, the creep remaining life assessment program stored in the memory unit 20 is executed, whereby calculations are performed to assess the creep remaining life of the target portion of the tube. Figure 2 shows the overall flow of the method for assessing 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 of the target portion of the tube is acquired as basic data (S1).

[0040] Next, the heat transfer coefficient of water vapor through the tube is calculated from the steam temperature, pressure, and flow rate per tube using the steam property values ​​obtained from the Steam Table (1999 Japan Society of Mechanical Engineers Steam Table) (S2).

[0041] In addition, by applying a specified 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).

[0042] 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, a curve of the growth of steam oxide scale over time and the metal temperature rise is obtained (S5), and the creep life consumption rate is calculated from the creep rupture data of the new material (S6).

[0043] The above is the overall flow of the method for evaluating the remaining life of a heat transfer tube according to the embodiment of the present invention. Each calculation flow will be described in detail below.

[0044] [Acquisition of basic data] Of the basic data, the steam temperature of the evaluation target portion can be estimated based on the metal temperature obtained by a metal thermometer installed at the tube inlet / outlet (unheated inlet section and unheated 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.

[0045] The temperatures inside the furnace are highest in the order of "metal temperature of the inlet unheated part," "steam temperature inside the furnace," and "metal temperature of the outlet unheated part," so the steam temperature also gradually rises during the heat exchange process inside the furnace. Therefore, when estimating the temperature of the steam flowing inside the pipe of the evaluation target part, an error will occur if the metal temperature of the outlet unheated part is used as is. For this reason, it is necessary to confirm at what station of the total length of the pipe extending into the furnace the pipe of the evaluation target part is located (the whole length is considered to be the 10th station), and then estimate the steam temperature after setting a specified proportionate ratio.

[0046] For example, if the part to be evaluated is located downstream from the inlet to the outlet 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 unheated part at the inlet, T out (Metal temperature of the non-heated part at the outlet)

[0047] In addition, when the metal temperature of the inlet unheated part and the outlet unheated part are unknown, the steam temperature can be estimated from the measured thickness of the steam oxide scale at the inlet unheated part and the outlet unheated part. In this case, since the growth of steam oxide scale is generally expressed by a parabolic law, the scale thickness can be calculated by the following equation: s Then, the following equation (2) holds. δ s =K p ×t 0.5 (2) (K p : scale growth rate constant, t: boiler operation time)

[0048] In addition, the scale growth rate constant K p 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, K can be calculated from equation (2). p The steam temperature T 0 can be sought.

[0049] [Calculation of heat transfer coefficient] The heat transfer coefficient α of 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 part (a specific calculation method will not be explained). 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.

[0050] [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., steam oxide scale is not taken into consideration). Even when steam oxide scale does not exist, the temperature of the inner surface of the tube rises at the tube-steam interface. In this case, the temperature rise ΔT α can be calculated using the following equation (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)

[0051] And the temperature rise ΔT α Considering the above, the temperature T of the inner surface of the tube when there is no steam oxidation scale is i1 can be calculated using the following equation (5). T i1 =T 0 +ΔT α (5)

[0052] [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 calculated based on the above-mentioned equations (2) and (3). In this case, the scale growth rate constant K p has temperature dependence depending on the tube material. Based on the above formula (5), the tube inner surface temperature T i1 The time required for scale growth can be calculated from the growth rate constant of steam oxide scale for the relevant material.

[0053] [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 caused by the formation of steam oxidation scale, the temperature rise due to steam oxidation scale, ΔT λs can be calculated using the following equation (6). ΔT λs =q i ×δ s / λ s (6) (λs: thermal conductivity of steam oxide scale, δ s : steam oxide scale thickness)

[0054] And the temperature rise ΔT λs Considering this, the steam oxide scale thickness is δ s When the temperature of the inner surface of the tube is T i2 can be calculated using the following equation (7). T i2 =T 0 +ΔT α+ ΔT λs (7)

[0055] [Calculation of time required for steam oxide scale growth] 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 Δt required to grow a given scale thickness (1 μm in this embodiment of the present invention) is 1 Repeatedly calculate Δt 1The growth time to the final scale thickness was calculated by integrating Δt 1 can be calculated using the following equation (8). Δt 1 =t d -t d-1 (8) (t d : Time required for the scale to grow to a thickness of dμm, t d-1 : time required for scale thickness d-1μm to grow)

[0056] 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. However, calculations may also be performed in units of 0.1 μm as the scale thickness unit, and the scale thickness unit used in the calculations can be changed as desired.

[0057] A calculation table capable of calculating the relationship between the time required for steam oxide scale growth and the scale thickness obtained by the above calculation flow according to the heat flux is stored in the memory unit 20, and is used in calculating the tube wall center temperature and estimating the remaining creep life, which will be described later.

[0058] An example of a scale growth estimation curve for a given heat flux obtained from a calculation table is shown in Figure 4. When a given heat flux is given from the calculation table, a scale growth estimation curve can be expressed in which the scale thickness as a function of time is plotted as a curve, with the horizontal axis representing time and the vertical axis representing the thickness of steam oxide scale.

[0059] [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.

[0060] [Calculation of tube wall center temperature] Pipe wall center temperature T m In the calculation of, first, the actual heat flux q determined based on the above calculation table t Calculate the heat flux q t Using the following equation (9), the tube wall center temperature T m Calculate T i2 is the inner surface temperature of the tube taking into account the thickness of the steam oxide scale calculated by the above-mentioned equation (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)

[0061] The tube wall center temperature T m In the calculation of, for example, the temperature of the outer surface of the tube is calculated using a relational expression, and the median value taking into account the temperature gradient of the tube wall is taken as the tube wall central temperature T m It may be calculated as follows.

[0062] Figure 5 shows the relationship between the operating time and the inner surface temperature T i2 , and the pipe wall center temperature T m The metal temperature rise curve was created by plotting the temperature of the steam oxide scale and the temperature of the metal. As shown in Fig. 5, the metal temperature rises as the steam oxide scale grows with the operation time. In addition, the temperature gradient on the inner and outer surfaces of the tube increases the temperature of the inner surface of the tube, T i2 The temperature at the center of the tube wall T m It can be seen that the PV is higher throughout the entire operation period.

[0063] As mentioned above, the inner surface temperature T i2 The temperature at the center of the tube wall T mTherefore, as disclosed in the above-mentioned Patent Document 1, if the inner surface temperature of the pipe is used to calculate the remaining creep life, it may be determined that the remaining creep life is sufficient even though it is not. However, 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.

[0064] [Calculation of creep life consumption rate] The creep life consumption rate is the time Δt required for a scale thickness of dμm to go from d-1μm to dμm. 1 The tube wall center temperature T m It is calculated by integrating the hoop stress σ applied to the pipe due to internal pressure, and the creep rupture time obtained from the creep rupture data of new materials. The creep rupture data can be obtained from the relationship between stress and the Larson-Miller parameter (hereinafter referred to as "LMP").

[0065] Here, LMP is a parameter for uniformly organizing creep rupture data at different temperatures, and can be organized using the following equation (10). LMP = T × (C + logT r ) (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 approximate formula for stress and creep rupture LMP.

[0066] The hoop stress σ can be calculated using the following equation (11). σ=P(Dw) / 2 / w (11) (P: internal pressure, D: outer diameter of the pipe, w: wall thickness of the pipe)

[0067] From the above, the growth time required for a scale thickness from d-1μm to dμm is Δt 1 Then, the creep life consumption rate φ d can be calculated using 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)

[0068] Based on the above equation (12), the creep life consumption rate φ per unit scale thickness is d By calculating the thickness of the steam oxide scale from zero, the operating time and creep life consumption rate φ d The relationship between these two can be expressed as a creep life consumption rate curve.

[0069] The remaining creep life is calculated by subtracting the current operating time from the time it takes for the integrated value of each creep life consumption rate φd to reach 100%. 。

[0070] [Calculation format for remaining life assessment based on scale growth] An example of a remaining creep life calculation format used to actually calculate the remaining creep life is shown in Fig. 7. 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.

[0071] In the creep remaining life calculation format, the data necessary for calculation is prepared by first entering 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.).

[0072] When items 1 to 11 and 13 to 15 of the creep remaining life calculation format are entered, items 12 and 16 to 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 the scale growth estimation curve (Fig. 4), metal temperature rise curve (Fig. 5), and creep life consumption rate curve (Fig. 6) are obtained, from which the current life consumption rate and remaining life can be estimated.

[0073] As described above, when calculating the remaining creep life, the remaining creep life of the heat transfer tube to be evaluated can be easily calculated by simply inputting the necessary information into the remaining creep life calculation format.

[0074] [Verification of remaining life assessment method] In order to verify the remaining life evaluation method for heat transfer tubes in 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 measured to be approximately 300 μm, and the operation time was approximately 140,000 hours. The results of the trial calculation using the above-mentioned remaining life evaluation calculation format based on the actual data of this blasted tube are shown in Figure 8.

[0075] Figure 8 shows the scale thickness and the scale thickness at the time of approximately 140,000 hours of operation for a blowout tube with an outlet steam temperature of 439°C. Remaining life 1 is a graph showing the relationship between

[0076] As shown in Fig. 8, the remaining life of the blowout tube is short when the scale thickness is 300μm or more. It is less than 50,000 hours. It can be seen that the remaining life is sufficient when the scale thickness is 200 μm or less.

[0077] 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. [Explanation of symbols]

[0078] 1. Remaining life assessment device 10 Control section 11 Heat flux determination section 12 Temperature calculation section 13 Creep remaining life calculation section 20 Memory section 21 Creep remaining life assessment program 30 Input section 40 Display section 50 Memory

Claims

1. 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 in the evaluation target portion from the temperature and stress in the central portion of the pipe wall. A method for assessing remaining life of heat transfer tubes.

2. 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 method for evaluating remaining life of a heat transfer tube according to claim 1 .

3. The n-th calculation is A growth time of the steam oxide scale is calculated based on a temperature obtained by adding a second temperature, which is an increase in temperature of the inner surface of the heat transfer tube caused by the steam oxide scale grown up to the n-1th calculation, to the initial temperature. The method for evaluating remaining life of a heat transfer tube according to claim 2.

4. 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.

5. 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 a time when the steam oxide scale thickness is zero to a predetermined scale thickness; and subtracting the boiler operation time at the evaluation time from the boiler operation time corresponding to a creep life consumption rate of 100%. The method for evaluating a remaining life of a heat transfer tube according to any one of claims 1 to 4.

6. 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 of the pipe wall central portion. A device for assessing remaining life of heat transfer tubes.

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