A method for diagnosing the size of a working fluid leakage hole in a tube seat weld at the bottom of a steam drum

Abstract

This invention discloses a diagnostic method for the size of the working fluid leakage hole in the weld of the bottom tube seat of a steam drum. The method includes a data acquisition stage, a calculation stage for the theoretical size of the leakage hole, a calculation stage for the actual average area, and a calculation stage for the actual average diameter. Based on the principles of differentiation and mass conservation, the leakage of the working fluid in the weld of the bottom tube seat of the steam drum is approximated as a constant free outflow from a thin-walled orifice. By monitoring the boiler reference point steam drum water level, the time interval of steam drum water level changes, and the steam drum water level changes, the method can accurately diagnose the size of the working fluid leakage hole in the weld of the bottom tube seat of the boiler steam drum after water is added to the steam drum. Furthermore, it requires fewer measurement points, and the calculation method is simple, accurate, and scientific, overcoming the limitation of not being able to diagnose leakage holes in the weld of the bottom tube seat of the boiler steam drum.

Description

Technical Field

[0001] This invention relates to the field of performance condition monitoring and diagnosis of thermal equipment, and in particular to a method for diagnosing the size of the working fluid leakage hole in the weld seam of the bottom pipe seat of a steam drum. Background Technology

[0002] During actual overhauls of in-service power plant boilers, cracks and even pores are found in the bottom tube seat welds of the steam drum. Frequent boiler start-ups and shutdowns causing thermal stress are the main factors leading to these cracks and pores in the steam drum tube seat welds. During these frequent start-ups and shutdowns, the steam drum is subjected to periodic heating and cooling. Under alternating stress, cracks and pores appear in the bottom tube seat welds, posing a significant safety hazard to boiler operation. In the operation of in-service power plant boilers, these cracks and pores in the bottom tube seat welds are difficult to detect. Once discovered, the cracks and pores have already enlarged, significantly increasing the boiler maintenance cycle and costs, and placing a heavy workload on maintenance personnel. During major and minor overhauls, magnetic particle and ultrasonic testing are used to inspect the bottom tube seat welds of the power plant boiler steam drum. Cracked areas are repaired by welding or patching to eliminate this potential hazard. However, magnetic particle and ultrasonic testing instruments are expensive, have long testing cycles, and can only be tested during major and minor overhauls. This leads to the failure to detect defects in the weld seams of the bottom tube seat of the steam drum in a timely manner, causing cracks and pores to enlarge and missing the best repair period, which is extremely detrimental to the safe operation of the boiler.

[0003] Currently, there is no effective solution for preventing and diagnosing cracks in the weld seams of the bottom tube seats of boiler drums. Leakage of the working fluid in the weld seams of the bottom tube seats seriously affects the safe operation of boilers. To extend the service life of power plant boilers, ensure timely, rapid, and correct maintenance of boiler drums, guarantee the timely, safe, and normal commissioning of inspected boilers, and improve the safety and reliability of boiler operation, researching the early diagnosis of the size of working fluid leakage holes in the weld seams of the bottom tube seats of boiler drums has significant practical engineering implications. Summary of the Invention

[0004] This invention addresses the problems existing in the prior art by creatively devising a diagnostic method for the size of the working medium leakage hole in the weld seam of the bottom tube seat of the steam drum. Based on the principles of differentiation and mass conservation, the leakage of the working medium in the weld seam of the bottom tube seat of the steam drum is approximated as a constant free outflow from a thin-walled orifice. By monitoring the boiler reference point steam drum water level, the time interval of steam drum water level change, and the change in steam drum water level, the size of the working medium leakage hole in the weld seam of the bottom tube seat of the boiler can be accurately diagnosed in advance after the steam drum is filled with water, thereby improving the safety and reliability of boiler operation.

[0005] The technical solution adopted to achieve the present invention is: a method for diagnosing the size of the working fluid leakage hole in the weld of the bottom pipe seat of a steam drum, characterized in that it includes the following steps:

[0006] 1) Data collection stage:

[0007] During the actual operation of the steam drum, after the water level in the steam drum stabilizes for 1 hour, the steam drum water level h at the reference point, the steam drum water level change interval dt, and the steam drum water level change dh are measured, and the length b and the cross-sectional radius R of the steam drum are collected.

[0008] 2) Calculation of the theoretical size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum:

[0009] Establish a two-dimensional coordinate system based on the half-section of the steam drum described in step 1). Let the area S of the working fluid leakage hole at the bottom pipe seat weld of the steam drum be denoted as S, and consider the leakage flow as a constant outflow from the working fluid leakage hole. The working fluid leakage hole orifice is a constant free outflow. The flow rate of water from the working fluid leakage hole orifice, i.e., the rate of change Q of the water volume V through the cross-section of the orifice with respect to time t, can be calculated using equation (1):

[0010]

[0011] In the formula: K is the flow coefficient, ranging from 0.6 to 0.7; S is the orifice area, in m². 2 g is the acceleration due to gravity, in m / s². 2 h represents the water level in the steam drum, in meters (m); V represents the volumetric flow rate of the steam drum water, in cubic meters per second (m³). 3 / s; t represents time, in seconds;

[0012] Within a small time interval [t, t+dt], the water level drops from h to h+dh, where dh<0. The minute change in the steam drum water volume, dV, can be obtained as follows:

[0013] dV=2rbdh (2)

[0014] In the formula: r is the half-width of the water surface side section of the steam drum, i.e., the water surface radius; 2r is the width of the water surface side section of the steam drum, in meters; b is the total length of the steam drum, in meters; dh is the change in water level in the steam drum, in meters.

[0015] Since r is the radius of the water surface at time t, the negative sign on the right is due to dh < 0 and dV > 0, according to... Figure 1 The formula for calculating the water surface radius can be derived as follows:

[0016]

[0017] In the formula: R is the radius of the circular cross-section of the side of the steam drum, in meters;

[0018] Substituting equation (3) into equation (2), we get:

[0019]

[0020] Combining equations (1) and (4), we can obtain:

[0021]

[0022] Equation (5) is the differential equation for the change of water level in the steam drum with time. The theoretical area of ​​the working fluid leakage hole in the weld seam at the bottom of the steam drum can be obtained from equation (5), i.e.

[0023]

[0024] Equation (6) is the differential equation for solving the theoretical size of the working fluid leakage hole in the weld of the bottom pipe seat of the steam drum;

[0025] 3) Calculation of the actual average area of ​​the working fluid leakage hole at the bottom pipe seat weld of the steam drum:

[0026] Due to errors in the data measurement results, the theoretical area size needs to be corrected.

[0027] S sj =Sβ (7)

[0028] In the formula: S sj The actual average size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum, in meters. 2 β is a correction factor, with a value ranging from 0.92 to 0.98.

[0029] 4) Calculation of the actual average diameter of the working fluid leakage hole at the bottom pipe seat weld of the steam drum:

[0030] Assuming the working fluid leakage hole in the weld of the steam drum base is circular, its diameter can be calculated.

[0031]

[0032] In the formula: r0 is the diameter of the working fluid leakage hole in the weld of the bottom pipe seat of the steam drum, in meters.

[0033] Preferably, in step 2), K is the flow coefficient, with a value of 0.62.

[0034] Preferably, in step 3), β is a correction coefficient with a value of 0.96.

[0035] The beneficial effects of the present invention regarding the diagnostic method for the size of the working fluid leakage hole in the weld of the bottom pipe seat of a steam drum are as follows:

[0036] A diagnostic method for the size of the working medium leakage hole in the bottom pipe seat weld of a steam drum is proposed. Based on hydraulic principles and the differential concept of water level change over time, a calculation equation for the average size of the leakage hole in the bottom pipe seat weld of a steam drum is constructed theoretically. The theoretical average size of the leakage hole in the bottom pipe seat weld of a steam drum is obtained by numerical method. This method overcomes the defect of not being able to diagnose leakage holes in the bottom pipe seat weld of a boiler steam drum. It requires fewer measurement points and the calculation method is simple, accurate and scientific, which can achieve the purpose of accurately diagnosing the size of the leakage hole in the bottom pipe seat weld of a steam drum. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the water level inside the boiler drum;

[0038] Figure 2 This is a physical model diagram of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum;

[0039] Figure 3 A simplified flowchart of a diagnostic method for the size of a working fluid leakage hole in the weld seam of a bottom pipe seat in a steam drum;

[0040] Figure 4 This is a flowchart of a diagnostic method for the size of a working fluid leakage hole in the weld seam of a bottom pipe seat in a steam drum.

[0041] Figure 5 This is a schematic diagram of a diagnostic system for the size of a working fluid leakage hole in the weld seam of a steam drum bottom pipe seat. Detailed Implementation

[0042] The following is in conjunction with the appendix Figure 1-5 The present invention will be further described in detail with reference to specific embodiments. The specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0043] A diagnostic method for the size of a working fluid leakage hole in the weld seam of a steam drum bottom pipe seat is based on the following concept:

[0044] 1. The weld shape is approximated as a small hole, and the leakage flow is regarded as a constant outflow of a thin-walled orifice. The flow rate of the orifice is determined by using the theoretical method of constant free outflow of thin-walled orifices in hydraulic theory, which makes it possible to calculate the size of the leakage hole of the steam drum weld.

[0045] 2. Based on the concept of differential equations, a differential equation between the steam drum water level and time is established. The equation for calculating the size of the weld leakage orifice is obtained by combining the constant free outflow calculation equation for the thin-walled orifice and the steam drum water level calculation equation.

[0046] 3. After obtaining the theoretical average size of the working fluid leakage hole in the steam drum base weld, it is further corrected by a correction factor to finally obtain the actual average size of the working fluid leakage hole in the steam drum base weld.

[0047] Based on the above conception, and considering the characteristics of boiler drum welds, combined with parameters monitored during actual operation and hydraulic theory, the principles governing the implementation of this invention are further proposed, namely:

[0048] 1. First, after the water in the steam drum has stabilized for 1 hour, measure the steam drum water level h at the reference point, the steam drum water level change interval dt, and the steam drum water level change dh.

[0049] 2. Based on hydraulic principles and the differential concept of water level change over time, a calculation equation for the average size of the leakage hole in the bottom pipe seat weld of the steam drum is jointly constructed.

[0050] 3. The theoretical average size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum is obtained by numerical method;

[0051] 4. By multiplying the theoretically calculated average size of the working fluid leakage hole in the bottom pipe seat weld of the steam drum by a correction factor, the actual average size of the working fluid leakage hole in the bottom pipe seat weld of the steam drum can be obtained.

[0052] As attached Figure 3 , 4 As shown, a method for diagnosing the size of a working fluid leakage hole in the weld seam of a steam drum bottom pipe seat includes the following steps: a. Data acquisition stage

[0053] As attached Figure 1 , 2 As shown, the diagnostic process selects the boiler after water is added and before startup. The data acquisition system collects the reference point steam drum water level h, steam drum water level change interval dt, and steam drum water level change dh. The theoretical size of the working fluid leakage hole in the bottom pipe seat weld of the steam drum is calculated by collecting the above field data.

[0054] List of operating parameters to be monitored for the boiler in this invention

[0055] Reference point steam drum water level h (m) Steam drum water level change dh(s) Steam drum water level change interval dt(s) The correction factor β Flow coefficient K Steam drum length b Steam drum cross-section diameter 2R

[0056] b. Calculation of the theoretical size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum:

[0057] Taking the boiler drum half-section as the research object, a system is established as follows: Figure 2 The two-dimensional coordinates shown assume the bottom pipe seat weld is as follows: Figure 2 As shown, and assuming the weld shape is approximately a small hole, and the leakage flow can be considered as a constant outflow from a thin-walled orifice, hydraulics tells us that for a constant free outflow from a thin-walled small orifice, the flow rate of water flowing out of the orifice (i.e., the rate of change of the volume V of water passing through the orifice cross-section with respect to time t) Q can be calculated using the following formula:

[0058]

[0059] In the formula: K is the flow coefficient, which can be approximated by an empirical parameter obtained experimentally, with a value ranging from 0.6 to 0.7; S is the cross-sectional area of ​​the orifice, in meters. 2 g is the acceleration due to gravity, in m / s². 2 h is the water level in the steam drum, in meters; V is the volumetric flow rate of the steam drum, in cubic meters per second. 3 / s; t is time, s,

[0060] On the other hand, suppose that within a small time interval [t, t+dt], the water level drops from h to h+dh (dh<0), then the small change in the volume of water in the steam drum, dV, can be obtained as follows:

[0061] dV=2rbdh (2)

[0062] In the formula: r is the half-width of the water surface side section of the steam drum, i.e., the water surface radius; 2r is the width of the water surface side section of the steam drum, in meters; b is the total length of the steam drum, in meters; dh is the change in water level in the steam drum, in meters.

[0063] Since r is the radius of the water surface at time t, the negative sign on the right is due to dh < 0 and dV > 0, according to... Figure 2 The formula for calculating the water surface radius can be derived as follows:

[0064]

[0065] In the formula: R is the radius of the circular cross-section of the side of the steam drum, in meters (m).

[0066] Substituting equation (3) into equation (2), we get:

[0067]

[0068] Combining equations (1) and (4), we can obtain:

[0069]

[0070] Equation (5) is the differential equation for the change of water level in the steam drum over time. Equation (5) can be further modified to obtain the size of the leakage hole in the weld of the bottom pipe seat of the steam drum, i.e.

[0071]

[0072] Equation (6) is the differential equation for solving the size of the leakage hole in the weld of the bottom pipe seat of the steam drum;

[0073] C. Calculation of the actual average size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum:

[0074] S sj =Sβ (7)

[0075] In the formula: S sjThe actual average size of the working fluid leakage hole in the weld of the steam drum base, in meters. 2 β is a correction factor, which can be obtained through leakage tests during actual boiler operation, and its value ranges from 0.92 to 0.98.

[0076] D. Calculation of the actual average diameter of the working fluid leakage hole in the weld seam of the steam drum base.

[0077] By approximating the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum as a circular hole, its diameter can be calculated.

[0078]

[0079] In the formula: r0 is the diameter of the working fluid leakage hole in the weld of the bottom pipe seat of the steam drum, in meters.

[0080] As attached Figure 5 As shown, the computer software program of the present invention is developed based on automation control and computer processing technology, which is a technology familiar to those skilled in the art.

[0081] Example:

[0082] There is an electric boiler that produces superheated steam. The boiler has an evaporation capacity of 1025 t / h, a steam drum diameter of 1.79 m, and a steam drum length of 26.7 m. This case study describes the diagnosis of the diameter of the working fluid leakage hole in the weld seam of the bottom tube seat of the steam drum.

[0083] a. Data collection stage:

[0084] The data acquisition system collects the original reference point steam drum water level h, steam drum water level change interval dt, and steam drum water level change dh. Additionally, boiler steam drum design parameters such as steam drum length and cross-sectional diameter are required. The theoretical size of the working fluid leakage hole at the bottom pipe seat weld of the steam drum is calculated using the collected field data. The real-time operating measurement data and collected design data of the boiler steam drum are shown in the table below.

[0085] Boiler drum operation measurement data, test and design data

[0086]

[0087] b. Calculation of the theoretical size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum:

[0088] The theoretical size of the working fluid leakage hole in the bottom pipe seat weld of the steam drum is calculated using formula (6) as follows:

[0089]

[0090] c. Calculation of the actual average size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum:

[0091] The theoretical average area of ​​the working fluid leakage hole in the bottom pipe seat weld of the steam drum is corrected using Equation (7) as follows:

[0092] S sj =Sβ=0.7446×0.96=0.7148mm 2

[0093] β is a correction factor with a value of 0.96;

[0094] d. Calculation of the actual average diameter of the working fluid leakage hole in the weld of the steam drum base:

[0095] The actual average diameter of the working fluid leakage hole in the weld of the steam drum base is calculated using formula (8) as follows:

[0096]

[0097] Diagnosis: The diameter of the working fluid leakage hole in the weld of the boiler drum base is about 0.4771mm, which requires minor repair welding.

[0098] The above description is only a preferred embodiment of the present invention. It should be noted that, for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for diagnosing the size of a working fluid leakage hole in the weld seam of a steam drum bottom pipe seat, characterized in that, It includes the following steps: 1) Data collection stage: During the actual operation of the steam drum, after the water level in the steam drum stabilizes for 1 hour, the steam drum water level h at the reference point, the steam drum water level change interval dt, and the steam drum water level change dh are measured, and the length b and the cross-sectional radius R of the steam drum are collected. 2) Calculation of the theoretical size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum: Establish a two-dimensional coordinate system based on the half-section of the steam drum described in step 1). Let the area S of the working fluid leakage hole at the bottom pipe seat weld of the steam drum be denoted as S, and consider the leakage flow as a constant outflow from the working fluid leakage hole. The working fluid leakage hole orifice is a constant free outflow. The flow rate of water from the working fluid leakage hole orifice, i.e., the rate of change Q of the water volume V through the cross-section of the orifice with respect to time t, is calculated using equation (1): （1） In the formula: K is the flow coefficient, ranging from 0.6 to 0.7; S is the orifice area, in m². 2 g is the acceleration due to gravity, in m / s². 2 h represents the water level in the steam drum, in meters (m); V represents the volumetric flow rate of the steam drum water, in cubic meters per second (m³). 3 / s; t represents time, in seconds; Within a small time interval [t, t+dt], the water level drops from h to h+dh, where dh<0. The minute change in the steam drum water volume, dV, can be obtained as follows: （2） In the formula: r is the half-width of the water surface side section of the steam drum, i.e., the water surface radius; 2r is the width of the water surface side section of the steam drum, in meters; b is the total length of the steam drum, in meters; dh is the change in water level in the steam drum, in meters. Since r is the water surface radius at time t, the negative sign on the right is due to dh < 0 and dV > 0. The formula for calculating the water surface radius is derived as follows: （3） In the formula: R is the radius of the circular cross-section of the side of the steam drum, in meters; Substituting equation (3) into equation (2), we get: （4） Combining equations (1) and (4), we can obtain: （5） Equation (5) is the differential equation for the change of water level in the steam drum with time. The theoretical area of ​​the working fluid leakage hole in the weld seam at the bottom of the steam drum can be obtained through equation (5), i.e. （6） Equation (6) is the differential equation for solving the theoretical size of the working fluid leakage hole in the weld of the bottom pipe seat of the steam drum; 3) Calculation of the actual average area of ​​the working fluid leakage hole at the bottom pipe seat weld of the steam drum: Due to errors in the data measurement results, the theoretical area size needs to be corrected. （7） In the formula: The actual average size of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum, in meters. 2 ; This is a correction factor, with a value ranging from 0.92 to 0.98; 4) Calculation of the actual average diameter of the working fluid leakage hole at the bottom pipe seat weld of the steam drum: Assuming the working fluid leakage hole in the weld of the steam drum base is circular, its diameter can be calculated. （8） In the formula: The diameter of the working fluid leakage hole in the weld seam of the bottom pipe seat of the steam drum is in meters (m).

2. The method for diagnosing the size of the working fluid leakage hole in the weld seam of the bottom pipe seat of a steam drum according to claim 1, characterized in that, In step 2), K is the flow coefficient, with a value of 0.

62.

3. The method for diagnosing the size of the working fluid leakage hole in the weld seam of the bottom pipe seat of a steam drum according to claim 1, characterized in that, in In step 3), This is a correction factor, with a value of 0.96.

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