A method for detecting the coating condition of a large oil storage tank bottom combined protection

Abstract

The application discloses a kind of coating condition detection methods of large oil storage tank bottom joint protection, for detecting the coating condition of the outer wall of the bottom of large oil storage tank according to national standard GB / T 50393-2017 "steel oil storage tank anticorrosion engineering technical standard" protection design.The detection method comprises the following steps: (1) four or more tank circumference detection points are equidistantly arranged on the periphery of the bottom of the large oil storage tank, and the tank circumference detection point is used for coating impedance detection;(2) determine whether the coating has defects;(3) determine the position of coating defect.The method of the application determines the coating state and determines the position of coating defect by analyzing the coating impedance and its change: whether the coating has defects is determined by analyzing whether the coating impedance decreases significantly;The radial position of the coating defect is determined by the radial change of the coating impedance, and the azimuth angle of the coating defect is determined by the fan change of the coating impedance.The results can provide technical support for optimizing tank bottom cathodic protection and tank bottom maintenance.

Description

Technical Field

[0001] This invention relates to the field of detection methods for protective coatings on the bottom of large storage tanks, and more particularly to a method for detecting the coating condition of a combined protection system for the bottom of large oil storage tanks. This method is used to detect and analyze the coating condition of the outer wall of the bottom of large oil storage tanks that are designed for protection according to the national standard GB / T 50393-2017 "Technical Standard for Corrosion Protection Engineering of Steel Oil Storage Tanks". Background Technology

[0002] Oil storage tanks are essential facilities for petrochemical enterprises. Located on the surface, corrosion of their bottom plates can allow leaked oil and gas to directly enter the soil, posing a risk of explosion and combustion. Therefore, the safety of oil storage tanks is of paramount importance. Large oil storage tanks are costly to construct and have long service lives, requiring long-term protection of their bottoms. Large oil storage tanks are typically cylindrical with a circular bottom. According to GB / T 50393-2017 "Technical Standard for Corrosion Protection Engineering of Steel Oil Storage Tanks," large oil storage tanks with an inner diameter of 30 meters or more must employ an independently installed impressed current cathodic protection system. Impressed current cathodic protection on the outer wall of the tank bottom utilizes two designs: deep-well anode and shallow-anode ground bed. Due to the need to ensure cathodic protection potential balance at the tank bottom and to retain soil moisture, large oil storage tanks generally employ a shallow-anode ground bed method with a waterproof layer at the bottom, and their impressed current cathodic protection often uses a concentric circle auxiliary anode design.

[0003] The outer wall of the bottom of oil storage tanks is protected against corrosion using a combination of coating and impressed current cathodic protection. The coating, as a crucial component of the tank bottom protection, not only prevents corrosion but also reduces wasted cathodic protection current; the integrity of the coating is extremely important for effective protection.

[0004] However, coatings inevitably age during use due to factors such as polymer degradation, making coating failure unavoidable. Furthermore, during tank construction, mechanical damage, weld burn-out, and irreparable weld defects can leave behind coating flaws. Areas with coating failure, weak points, and damaged sections inevitably become key corrosion zones.

[0005] Typical coating defects can be identified through surface observation or by electrochemical testing. However, the bottom plate of an oil storage tank is sealed after the tank construction is completed, and personnel and testing equipment cannot enter the bottom of the crude oil storage tank. Conventional coating testing methods cannot be directly used to detect and determine the condition of the coating on the outer wall of the tank bottom, let alone confirm the location and extent of coating defects.

[0006] Cathodic protection plays a certain backup role in combined protection systems, but due to the unique characteristics of the lower part of the tank bottom, cathodic protection has weaknesses and shortcomings, requiring optimization based on specific circumstances. Furthermore, during tank bottom maintenance, it is necessary to pinpoint the exact location of corrosion. While methods such as bottom plate thickness measurement can locate corrosion, the workload of cleaning the oil and performing comprehensive bottom plate thickness measurements is extremely large. A simple and easy-to-implement method is needed to inspect the coating condition and determine the location of coating defects.

[0007] A review of relevant domestic and international literature reveals the following coating detection methods related to this invention:

[0008] CN108562619A discloses a device and method for detecting the quality of the inner coating of downhole casing. This method involves filling the target interior with electrolyte, connecting the instrument in series and parallel to the bottom of the well, and using electrodes to detect the corresponding voltage and current. The resistance of the inner coating is calculated based on Ohm's law to determine the quality of the inner coating. However, this method is not feasible in the sealed environment of a storage tank bottom, and its principle is fundamentally flawed due to the complex environment at the bottom of crude oil storage tanks, making accurate calculations impossible.

[0009] CN115901605A discloses a method for evaluating the protective properties of anti-corrosion coatings. This method requires an electrochemical workstation and a dual-electrolysis cell system for electrochemical impedance spectroscopy (EIS) testing. The impedance of the anti-corrosion coating is obtained from the test results, and its protective properties are evaluated based on the impedance. While coating impedance analysis is a fundamental aspect of corrosion electrochemistry, this method cannot be implemented in the closed environment of a tank bottom.

[0010] CN114428052A discloses an equivalent evaluation method for seawater corrosion of coatings in ship ballast water piping systems. This method, after environmental analysis, uses a three-electrode system to determine the electrochemical corrosivity, and evaluates the coating based on the results of the electrochemical corrosion determination. However, environmental sampling and testing using this method cannot be performed in the closed environment of a tank bottom.

[0011] CN111257213A discloses a device and method for in-situ monitoring of underwater anti-corrosion coatings on marine structures. This method requires an electrochemical workstation equipped with a reference electrode, a counter electrode, and two working electrodes to monitor the damage rate of the underwater anti-corrosion coating in situ. This method cannot be implemented in a closed environment at the bottom of a tank.

[0012] CN112881479B discloses a coating monitoring device and method, which provides equipment, coating impedance measurement, data analysis, and coating evaluation. Its coating impedance sensing module needs to be installed on the coating surface, which is not feasible in a closed environment at the bottom of a tank.

[0013] CN114609028A discloses a portable device for in-situ testing of the corrosion resistance of organic coatings, comprising a portable electrochemical probe, a testing system, and software, for in-situ non-destructive testing of the corrosion resistance of organic coatings on metal surfaces. However, its mobile dual electrochemical probe cannot be used in the closed environment at the bottom of the tank.

[0014] In summary, no applicable methods for detecting coating condition or locating coating defects have been found for use on the outer wall of the bottom of oil storage tanks. Summary of the Invention

[0015] The purpose of this invention is to overcome the shortcomings of the prior art and provide a simple and easy-to-implement method for detecting the coating condition of the outer wall of the bottom of a large oil storage tank. This method is used to determine whether there are defects in the coating of the outer wall of the bottom of a large oil storage tank and to determine the approximate location of the defect.

[0016] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0017] A method for detecting the coating condition of a combined bottom protection system for large oil storage tanks, the method being used to detect the coating condition of the outer wall of the bottom of large oil storage tanks designed for corrosion protection according to the national standard GB / T 50393-2017 "Technical Standard for Corrosion Protection Engineering of Steel Oil Storage Tanks", the method comprising the following steps:

[0018] (1) Four or more detection points are equidistantly arranged around the bottom of the large oil storage tank. The detection points are used for coating impedance detection. The angle between any two adjacent detection points and the center of the bottom of the tank is equal.

[0019] (2) Determine if there are defects in the coating: By detecting the impedance between the detection points around the tank and the auxiliary anode, analyze the difference between these impedance detection values ​​and the design value or the detection value of the intact coating to determine if there are defects in the coating. If there are defects, proceed to step (3).

[0020] (3) Determine the location of coating defects: By detecting the impedance between the detection points around the tank and the auxiliary anode, and analyzing the changes in the coating impedance detection value in the radial and sector directions according to the center of the tank bottom, the location of coating defects can be determined.

[0021] Furthermore, the impedance between the detection point around the detection tank and the auxiliary anode is the impedance between the detection point around the detection tank and the concentric anode designed for cathodic protection according to the national standard.

[0022] Furthermore, in step (2), the presence of defects in the coating is determined by whether the impedance detection value decreases significantly by more than two orders of magnitude compared with the design value or the intact coating detection value. The two orders of magnitude are 10 squared.

[0023] Furthermore, in step (3), the radial position of the coating defect is determined by measuring and analyzing the impedance change between a detection point around the tank and each anode; and the azimuth angle of the coating defect is determined by measuring and analyzing the impedance change between each detection point around the tank and an anode. The radial position and the azimuth angle are combined to confirm the number and location of the coating defects.

[0024] Furthermore, the radial location and azimuth of the coating defects are determined using a graphical method, wherein:

[0025] The radial location of coating defects is determined as follows: set the abscissa point with the radius of each anode and the ordinate with the measured impedance value; draw a trend line based on the measured impedance value of each anode, and the abscissa value corresponding to the trough of the trend line is the radial location of the coating defect.

[0026] The specific method for determining the azimuth angle of coating defects is as follows: set the abscissa point of each detection point around the tank as the abscissa point, and the ordinate point as the measured impedance value; draw a trend line based on the impedance values ​​measured at each detection point around the tank, and the abscissa value corresponding to the trough of the trend line is the azimuth angle of the coating defect.

[0027] The beneficial effects of this invention are as follows: The method of this invention determines the coating condition and the location of coating defects by analyzing the coating impedance and its changes; it determines whether the coating has defects by analyzing whether the coating impedance drops significantly; it determines the radial location of coating defects by the radial change of coating impedance, and determines the azimuth angle of coating defects by the sectoral change of coating impedance. The results can provide technical support for optimizing tank bottom cathodic protection and tank bottom maintenance. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the bottom structure of an oil storage tank according to an embodiment of the present invention;

[0029] Figure 2 This is a schematic cross-sectional view of the coating defects at the bottom of the tank and the location of the auxiliary anode in an embodiment of the present invention;

[0030] Figure 3 This is a radial impedance variation diagram according to an embodiment of the present invention;

[0031] Figure 4 This is a diagram showing the sector impedance variation according to an embodiment of the present invention.

[0032] The labels in the diagram are as follows: A, B, C, D, E, F, G, and H are the inspection points around each tank in sequence; ①, ②, ③, ④, ⑤, ⑥, ⑦, and ⑧ are eight concentric circles of anodes from the inside out; X is the coating defect location; 11 is the bottom plate of the oil tank; and 12 is the anode cable connection point. Detailed Implementation

[0033] The invention will be further described in detail below with reference to a specific large-scale oil storage tank bottom protection structure, namely, a method for detecting the coating condition of a large-scale oil storage tank bottom protection structure. Figure 1 As shown, eight detection points are equidistantly set on the steel structure around the bottom of the oil storage tank, with each pair of adjacent detection points having an equal angle with respect to the center of the tank bottom. Figure 1 The auxiliary anode of the oil storage tank shown is a linear anode with an eight-concentric-circle design. The cables of the eight anode rings are connected to an explosion-proof junction box. The connecting lugs of the eight detection points around the tank form a reliable electrical connection with the steel structure around the bottom of the tank, with a contact resistance of less than 0.02Ω.

[0034] The method for determining whether a coating has defects is as follows:

[0035] A high-resistance impedance meter is used, connected to a testing cable, and in low-frequency mode at the same set frequency, to measure the impedance between each test point around the tank and each anode test terminal. If the testing requirements are not high, a high-internal-resistance resistor meter can be used instead of an impedance meter. When using a resistor meter, the testing time needs to be carefully controlled to ensure that the testing time is approximately the same, for example, controlled to 3 seconds, to reduce the deviation caused by the capacitive testing current of the coating. The obtained resistance value can be used to calculate the approximate impedance value, and the same method is used for analysis.

[0036] Analyze the differences between these impedance test values ​​and the design values ​​to determine if there is a significant decrease of two orders of magnitude or more, thus identifying any defects in the coating. If the original design data is unavailable, the same test can be performed on a complete coating prepared with the same material and process to obtain comparative results.

[0037] The bottom coating system for oil storage tanks typically consists of an epoxy primer coating followed by a polyurethane topcoat coating. According to GB / T24596—2021, the impedance of a complete polyurethane coating is 10... 8 Ω·m 2 The above. Related experiments show that the complete tank bottom coating, when used in a soil environment, can achieve a low-frequency impedance of 10. 8 Ω·m 2 A well-coated system can achieve impedances as high as 10. 10 Ω·m 2 Unlike soil impedance variations and coating thickness differences, which generally do not exceed one order of magnitude, coating impedance can drop significantly, sometimes by up to two orders of magnitude, if the coating is damaged or the steel substrate corrodes. In some cases, the coating impedance will even drop below 10. 6 Ω·m 2 Impedance analysis is a commonly used method for assessing the condition of coatings. A significant drop in impedance, with a difference of two orders of magnitude, indicates a defect in the coating. Otherwise, the coating can be considered intact.

[0038] The method for determining the radial location of coating defects is as follows:

[0039] The method of determining coating defects and their distance from the center by radial change in impedance is as follows: determine the detection point, change the anode, and analyze the change in coating impedance value. Figure 2 This is a cross-sectional schematic diagram showing the coating defects at the bottom of the tank and the location of the auxiliary anode. (For tanks exceeding 50,000 m...) 3 For large storage tanks, the distance between adjacent anodes is about 5 meters, while the distance between the anode and the bottom plate is generally no more than 0.4 meters. The anode and the tank bottom plate can be simply considered as... Figure 1 On a plane, the impedance between a determined detection point around the tank and each anode, and the distance from each anode to the center of the circle, become a univariate relationship in a Cartesian coordinate system. The analysis is as follows:

[0040] Total impedance Z:

[0041] Z = Z 电缆 +Z A－X +Z 涂层 +Z (X－①～⑧) +Z 阳极

[0042] Z 电缆 : Test the cable impedance; the impedance is relatively low. The same cable is used for testing, and the impedance is a fixed value.

[0043] Z A－X The resistance between the detection point and the damage point is the impedance of the steel structure of the tank bottom plate, which is a fixed value for a certain detection point and a specific coating defect point.

[0044] Z 涂层 The coating impedance is determined by the fact that the impedance of intact parts of the coating is relatively high, while the interface impedance at coating defects drops by more than two orders of magnitude. Therefore, during measurement, the current can be assumed to pass only through the defects, and the coating impedance can be considered as the impedance of the coating defect points. The coating impedance is a fixed value.

[0045] Z (X－①～⑧) The resistance between each anode and the damage point is actually the impedance of the soil. The resistivity of the soil is relatively fixed, and the closer the distance, the smaller the impedance. Therefore, the resistance between different anodes and the damage point is not the same.

[0046] Z 阳极 The impedance of the anode interface, cable, and the anode itself is generally low. The resistivity of the oxide coating on the anode is typically around 10 μΩ·cm, and the resistivity of titanium in the anode is less than 10 mΩ·m. The total resistance is below 1 Ω and can be ignored. The difference between different anodes is even smaller, so different anodes can be treated as fixed values.

[0047] It can be seen that, for a single detection point, the total impedance Z and Z0(X－①～⑧) The only variable relationship is that the total impedance varies with the distance between the coating defect point and the anode. Therefore, by measuring the impedance between a test point and each anode and analyzing the impedance changes at different anode points, the radial location of the coating defect point can be determined.

[0048] The method for determining the azimuth angle of coating defects is as follows:

[0049] The azimuth of the coating defect area is determined by the change in impedance along the periphery of the tank. The method is as follows: determine the anode, change the detection point, and analyze the change in the coating impedance value.

[0050] As analyzed above, the anode and the bottom plate of the oil tank can be simply regarded as being in Figure 1 On a plane, the impedance between a given anode and each detection point around the tank, and the azimuth angle of each detection point around the tank, form a univariate relationship in a Cartesian coordinate system. The analysis is as follows:

[0051] Total impedance Z:

[0052] Z = Z 电缆 +Z (A～H－X) +Z 涂层 +Z X－① +Z 阳极

[0053] Z 电缆 : To test the cable impedance, the impedance is relatively low. The same cable is used for testing, and the impedance is a fixed value.

[0054] Z (A～H－X) The resistance between the detection point and the damage point is the impedance of the steel structure of the tank bottom plate. The resistance between different detection points and the coating defect points is not exactly the same.

[0055] Z 涂层 The coating impedance is relatively high in intact areas and decreases significantly at defective areas. Therefore, the detection current can be considered to pass only through these defective areas. The coating impedance can be considered a fixed value.

[0056] Z X－① The resistance between a specific anode and a damage point is actually the impedance of the soil. For a given anode, the impedance between it and a given coating defect point is a fixed value.

[0057] Z 阳极 The impedance of the anode interface, cable, and the anode itself is generally low. The resistivity of the oxide coating on the anode is typically around 10 μΩ·cm, and the resistivity of titanium in the anode is less than 10 mΩ·m. The total resistance is below 1 Ω and can be ignored. This is a fixed value for a specific anode.

[0058] It can be seen that, for a given anode, the total impedance is related to Z.(A～H－X) This is the only variable relationship. The total impedance varies with the distance between the detection point and the coating defect point. Therefore, by separately detecting the impedance between each detection point and a certain anode, and analyzing the impedance changes at different detection points, the location of the damage point in the middle of the tank bottom circumference can be determined.

[0059] The specific location of the coating defect can be determined by using its radial position and azimuth angle. If there are multiple coating defects on the bottom of the tank, they will exhibit multiple variations in their radial position or azimuth angle.

[0060] Determining the location of coating defects using graphical methods:

[0061] The measured impedance data is plotted into an impedance variation graph. The location of coating defects is determined by the low and high values. Specifically, the horizontal axis is set with the radius of each concentric anode, and the vertical axis is the measured impedance value. A trend line is plotted based on the measured impedance values ​​of each anode. The horizontal axis value corresponding to the trough of the trend line is the radial position. Figure 3 As shown; the azimuth angle of the coating defect is marked by the impedance change relationship around the tank. Specifically: the azimuth angle of each detection point around the tank is set as the horizontal axis point, and the measured impedance value is the vertical axis; a trend line is drawn based on the impedance values ​​measured at each detection point around the tank, and the horizontal axis value corresponding to the trough of the trend line is the azimuth angle, as shown. Figure 4 As shown. The two methods combined are used to confirm the number and location of coating defects.

[0062] Multiple peaks indicate multiple coating defect areas. The total number of coating defects = radial coating defects * fan-shaped coating defects. The number and location of coating defects can be shown in the figure below. Figure 1 Marked above.

[0063] Specific procedures for inspecting the condition of the combined protective coating on the bottom of this large oil storage tank:

[0064] The testing points can use silver-plated copper lugs on the connector plates. The testing points are welded to the perimeter of the oil tank or connected with conductive adhesive to ensure a reliable electrical connection. Prepare testing cables that can connect the explosion-proof junction box to each testing point around the tank. Install a silver-plated copper lug matching the connector plate lug at one end of the testing cable, and install a connection terminal for connecting to the testing equipment at the other end. During testing, connect one end of the testing cable to the testing point around the tank with a bolt, and connect the other end to the testing equipment at the explosion-proof junction box. Using the same cable for testing ensures the same resistance value, eliminating deviations caused by cable resistance.

[0065] Detection method:

[0066] First, disconnect the power to the cathodic protection system, including the potentiostat. Then, open the explosion-proof junction box, remove each terminal of the anode, mark the terminal corresponding to each anode ring, and then connect the corresponding anode terminals together to form the anode detection terminal.

[0067] With the ability to detect impedance 10 8 For impedance testers with impedances greater than Ω, connect the test cable and the anode test terminal with wires respectively. Use the low-frequency mode of the impedance tester and, at the same set frequency, measure the impedance of each test point around the tank to each anode test terminal, and record these impedance data values.

[0068] Analysis method:

[0069] (1) Determine if there are coating defects

[0070] Compare all the impedance test values ​​with the theoretical values ​​of the original design, i.e., the impedance test values ​​of a complete coated sample prepared with the same materials and process, to see if there is a significant decrease. If the decrease is more than two orders of magnitude, it can be determined that there is a defect in the coating, and further steps to locate the coating defect are required. If all impedance test values ​​do not decrease significantly compared with the theoretical values ​​of the original design, the coating condition can be considered good, and no further location work is needed.

[0071] (2) Determine the radial location of coating defects

[0072] By consulting the design drawings and construction completion data of the crude oil storage tank, the spacing of each linear anode and its distance from the center and the perimeter of the tank were obtained. A plan view of the tank bottom and the concentric circles of each linear anode was then drawn. Figure 1 .

[0073] Extract the impedance between a specific tank perimeter testing point (e.g., testing point A) and each anode from the above test data. Use common plotting software, such as Microsoft Excel or Origin, to plot the impedance variation. Figure 3 . Figure 3 The horizontal axis represents the center, anodes ① to ⑧, and the tank perimeter, with units of length for the distance between the center and anodes, the distance between each anode, and the distance around the anode perimeter, respectively. The vertical axis represents the measured impedance, with the impedance value for each anode corresponding to the cross "×" point on the curve in the graph. The portion extending from the data points to the center and the tank perimeter is the outer extension line.

[0074] Figure 3 The location of the downward-sloping low impedance peak indicates the radial position of the coating defect from the center of the circle, and the area between the inflection points on both sides of the peak is the defect region. Multiple peaks indicate multiple coating defects. A flat peak indicates a large radial area of ​​coating defects.

[0075] 3) Determine the orientation angle of coating defects

[0076] Extract the impedance between a specific anode (e.g., anode ①) and each detection point around the tank from the above test data. Use common plotting software, such as Microsoft Excel or Origin, to plot the impedance variation graph. Figure 4 . Figure 4 The horizontal axis represents the detection points A through H around the tank, with each unit length representing an azimuth angle of 45°, totaling 360°. The vertical axis represents the measured impedance, with the impedance value corresponding to the direction of each detection point marked by an "×" on the curve in the graph.

[0077] Figure 4 The location of the downward impedance low peak indicates the azimuth angle of the coating defect corresponding to the detection point around the tank. The area between the inflection points on both sides of the peak is the defect region. Multiple peaks indicate multiple coating defects. A flat peak indicates a large sector of coating failure.

[0078] (4) Location of coating defects

[0079] Based on the radial location and azimuth angle of the coating defect, it is possible to... Figure 1 Mark the specific location X of the coating defect to complete the location of the coating defect.

[0080] An example of the condition inspection of the combined protective coating on the bottom of a large oil storage tank:

[0081] A 100,000m³ plant of a certain refining and chemical enterprise 3 Among crude oil storage tanks built and in use in the same period, the cathodic protection current was higher than that of other tanks, suggesting possible coating defects. The detection method of this invention was applied to analyze the coating condition of the outer wall of the tank bottom.

[0082] The implementation steps are as follows:

[0083] 1. Investigation

[0084] Reviewing the design drawings and construction completion data of the crude oil storage tank, it was confirmed that the impressed current cathodic protection at the bottom of the tank was designed according to national standards, and that its auxiliary anodes were designed as concentric circles. The diameters of the anode rings from the inside out are 6, 16, 26, 36, 46, 56, 66, and 76 meters, respectively. The diameter of the tank bottom perimeter is 80 meters, and the tank bottom area is approximately 5000 m². 2 The linear auxiliary anodes are arranged at a depth of 30-40 cm below the bottom plate of the storage tank.

[0085] Investigations revealed that the anti-corrosion coating on the outer wall of the tank bottom consists of two layers of epoxy primer followed by two layers of polyurethane topcoat, with a total coating thickness exceeding 400 μm. According to relevant data and verification tests, the impedance of this coating system in its intact state is 10... 10 Ω·m2 In the above cases, the impedance of the coating is even less than 10 after defects occur. 6 Ω·m 2 .

[0086] 2. Setting up testing points and preparing testing cables

[0087] According to the above implementation method, four coating detection points are set on the perimeter of the tank bottom in the east, west, south, and north directions along the tank circumference. Then, one additional detection point is added between each adjacent detection point, resulting in a total of eight detection points (A, B, C, D, E, F, G, H) equidistantly distributed along the tank bottom perimeter. Each pair of adjacent detection points forms a 45° angle with respect to the center. The detection points use silver-plated Φ8mm screw-hole copper connectors, connected and installed around the tank perimeter using conductive adhesive and fasteners to form a reliable electrical connection.

[0088] The testing cable is 160m long and 6mm thick. 2 The cable is a single-core cable with a resistance of 0.487Ω. One end is fitted with a silver-plated copper lug with an 8mm threaded hole using crimping pliers, and the lug joint is soldered. During testing, it is connected to the terminal block lug at the testing point using a 6mm stainless steel fastener. The other end is soldered to the connection terminal of the testing equipment and plugged directly into the testing equipment during testing.

[0089] 3. Testing

[0090] First, shut off the power to all cathodic protection systems, including the potentiostat, and interrupt the impressed current cathodic protection system for at least 24 hours to completely depolarize the tank bottom plate and eliminate its influence on the detection. Then, open the explosion-proof junction box and use a stray current detector to check the area around the crude oil storage tank to confirm there is no external stray current interference. Next, disconnect each terminal from the anode terminal block and label the corresponding terminals for each anode. Use a stray current detector to check the stray current on the anodes at the tank bottom to confirm there is no external stray current. Finally, connect the corresponding anode terminals together with stainless steel bolts to form a detection terminal for 8 rings of anodes: ①, ②, ③, ④, ⑤, ⑥, ⑦, and ⑧.

[0091] 4. Detection and Analysis

[0092] (1) Determine if the coating has defects

[0093] Using a certain model of impedance meter, in low-frequency mode, the detection frequency was set to 0.01Hz, and the area was set to 5000m². 2 The impedance values ​​between each tank perimeter detection point and each anode detection end are detected and the detection data are recorded.

[0094] Analysis of these test data revealed that the detected coating impedance values ​​were all within 10.5 Ω·m 2 Order of magnitude. Compared to the original complete coating 10 8 Ω·m 2 Compared to the theoretical values ​​above, the coating impedance shows a significant decrease, indicating a defect in the coating on the outer wall of the crude oil tank bottom. The next step is to locate the coating defect.

[0095] (2) Determine the radial location of coating defects

[0096] Extract the impedance values ​​between detection point A and each anode from the above detection data, and plot the radial impedance variation graph using Microsoft Excel. (See figure) Figure 3 The horizontal axis represents the center, anodes ① through ⑧, and the perimeter of the tank, with spacings of 3, 5, 5, 5, 5, 5, 5, 5, and 2 meters respectively. Therefore, the coordinates corresponding to the eight rings of anodes (①, ②, ③, ④, ⑤, ⑥, ⑦, and ⑧) and the perimeter of the tank are 3 meters, 8 meters, 13 meters, 18 meters, 23 meters, 28 meters, 33 meters, 38 meters, and 40 meters respectively. The vertical axis represents the detected impedance value, with the impedance value corresponding to the cross "×" point on the curve in the graph.

[0097] Figure 4 The downward-sloping low-impedance peak in the graph indicates the middle part of the coating defect. The graph shows only one peak, meaning only one circle indicates a coating defect. This circle is located between the ⑤ and ⑥ anode rings, with a spacing of 5 meters. The inflection points on both sides of the peak are between 24.3 meters and 25.5 meters, with a midpoint of 24.9 meters. This indicates that the defective area is within a radial range between the ⑤ and ⑥ anode rings, with a radial width W = outer diameter of the range - inner diameter of the range = 25.5 - 24.3 = 1.2 meters.

[0098] (3) Determine the location of coating defects

[0099] Extract the impedance between anode ① and each detection point from the above detection data, and use Microsoft Excel to plot the impedance versus detection point curve, showing the relationship between the sector impedance and the impedance. Figure 4 . Figure 4 All detection points form a closed loop of 360°. The horizontal axis A to H represents the azimuth angle of each detection point, with the origin at H. The distance between each of the eight points in the sector A to H is 45°. The azimuth angles corresponding to the eight detection points around the tank are 45°, 90°, 135°, 180°, 225°, 270°, 315°, and 360° (0°). The vertical axis represents the detected impedance value, and the impedance value corresponding to the direction of each detection point is marked by the corresponding "×" point on the curve in the graph.

[0100] Figure 4The location of the downward impedance low-value peak in the figure is the azimuth position of the coating defect. A single peak indicates only one sector of the coating defect. The slightly wider peak indicates the coating defect is within a certain sector between detection points D and E. The inflection point interval on both sides of the peak tip in the figure is between the sector angles of 189° and 202°. Based on the sector radius of the coating defect area, the sector length L of the coating defect area is calculated as: sector radius of coating defect area * 2 * 3.14 * (sector angle interval, unit: °) / 360°, L = 24.9 * 2 * 3.14 * (202 - 189) / 360 = 5.6 meters.

[0101] (4) Locating coating defects

[0102] The fact that there is only one coating defect in the radial position and only one coating defect in the fan-shaped position indicates that there is only one coating defect at the bottom of the tank.

[0103] Based on the radial position and azimuth angle of the coating defect, mark as follows: Figure 1 The specific location of the coating defect was determined, and the defect was located. Conclusions: First, the coating on the outer wall of the crude oil tank bottom has a defect; second, the defective area of ​​the coating on the outer wall of the crude oil tank bottom is located within... Figure 1 The location of the coating defect area, shown as the south-southwest corner, has a radial radius of 24.3-25.5 meters and an azimuth range of 189° to 202°. The size of the coating defect area is 1.2 meters in radial width and 5.6 meters in fan-shaped length.

[0104] The above description of the embodiments is provided to enable those skilled in the art to understand and use this patent. Those skilled in the art will readily make various modifications to this embodiment and apply the general principles described herein to other embodiments without inventive effort. For example, for a design using a mesh auxiliary anode for impressed current cathodic protection at the bottom of a storage tank, the aforementioned radial and azimuth positioning method can be replaced with a two-dimensional coordinate positioning method using longitudinal and transverse coordinates. Similarly, for a design using a deep-well anode for impressed current cathodic protection at the bottom of a storage tank, since the number of anode points that can directly form a detection circuit is limited, additional circuit detection points can be added at equidistant locations around the storage tank, and these can also be located by analyzing the coating impedance variation. Therefore, this patent is not limited to the above application examples. Any improvements and modifications made by those skilled in the art based on the disclosure of this patent without departing from the scope of this patent should be within the protection scope of this patent.

[0105] The above content is only used to illustrate the technical solution of the present invention. Simple modifications or equivalent substitutions made by those skilled in the art to the technical solution of the present invention do not depart from the essence and scope of the technical solution of the present invention.

Claims

1. A method for detecting the coating condition of a combined bottom protection system for large oil storage tanks, the method being used to detect the coating condition of the outer wall of the bottom of large oil storage tanks designed for protection according to the national standard GB / T50393-2017 "Technical Standard for Corrosion Protection Engineering of Steel Oil Storage Tanks", characterized in that: The detection method includes the following steps: (1) Four or more detection points are equidistantly arranged around the bottom of the large oil storage tank. The detection points are used for coating impedance detection. The angle between any two adjacent detection points and the center of the bottom of the tank is equal. (2) Determine if there are defects in the coating: By detecting the impedance between the detection points around the tank and the auxiliary anode, analyze the difference between these impedance detection values ​​and the design value or the detection value of the intact coating to determine if there are defects in the coating. If there are defects, proceed to step (3). (3) Determine the location of coating defects: By detecting the impedance between the detection points around the tank and the auxiliary anode, and analyzing the changes in the coating impedance detection value in the radial and sector directions according to the center of the tank bottom, the location of coating defects can be determined.

2. The method for detecting the coating condition of the combined bottom protection system for large oil storage tanks according to claim 1, characterized in that: The impedance between the detection point around the test tank and the auxiliary anode is the impedance between the detection point around the test tank and the concentric anode designed for cathodic protection according to the national standard.

3. The method for detecting the coating condition of the combined bottom protection system for large oil storage tanks according to claim 2, characterized in that: In step (2), the presence of defects in the coating is determined by whether the impedance detection value decreases significantly by more than two orders of magnitude compared with the design value or the intact coating detection value. The two orders of magnitude are 10 squared.

4. The method for detecting the coating condition of the combined bottom protection system for large oil storage tanks according to claim 3, characterized in that: In step (3), the radial position of the coating defect is determined by measuring and analyzing the impedance change between a detection point around the tank and each anode; and the azimuth angle of the coating defect is determined by measuring and analyzing the impedance change between each detection point around the tank and an anode. The radial position and azimuth angle are combined to confirm the number and location of the coating defects.

5. The method for detecting the coating condition of the combined bottom protection system for large oil storage tanks according to claim 4, characterized in that: The radial location and azimuth of coating defects are determined using a graphical method, where: The radial location of coating defects is determined as follows: set the abscissa point with the radius of each anode and the ordinate with the measured impedance value; draw a trend line based on the impedance value measured by each anode, and the abscissa value corresponding to the trough of the trend line is the radial location of the coating defect. The specific method for determining the azimuth angle of coating defects is as follows: set the abscissa point of each detection point around the tank as the abscissa point, and the ordinate point as the measured impedance value; draw a trend line based on the impedance values ​​measured at each detection point around the tank, and the abscissa value corresponding to the trough of the trend line is the azimuth angle of the coating defect.

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