High-precision method for detecting residual wall thickness of pipeline under thermal insulation layer

Abstract

The invention discloses a high-precision method for detecting the residual wall thickness of a pipeline under a thermal insulation layer, and relates to the technical field of nondestructive testing. The method based on the eddy current nondestructive testing technology is designed aiming at the defects of the oil and gas transportation pipeline containing the heat preservation layer, on the basis that the target is detected, the detection deviation caused by magnetic loss in the detection process can be effectively avoided, and the precision of the detection result is improved. Moreover, the method can effectively inhibit the temperature sensitivity of the sensor, and enables the detection device to be adaptive to a wide temperature range. The method can meet the use requirement for online detection of the pipeline containing the heat preservation layer, and the detection efficiency in industrial production is improved.

Description

Technical Field

[0001] The invention relates to the technical field of nondestructive testing, and in particular to a high-precision method for detecting the remaining wall thickness of a pipeline under a thermal insulation layer. Technical Background

[0002] In industrial applications, in order to optimize heat management and protect pipelines from long-term direct contact with air, an insulation layer and a metal protective layer are usually wrapped around the outer layer of the pipeline, which means that the measurement of the pipeline must be through the insulation layer. Currently, in most scenarios, the insulation layer needs to be removed first, and then the pipeline is inspected, and then reinstalled after the inspection is completed. This method is labor-intensive and inefficient. There is an urgent need for a method that can directly inspect pipelines with insulation layers.

[0003] Eddy current testing (ECT) technology is a non-contact testing method based on the principle of electromagnetic induction. The primary magnetic field generated by the excitation coil acts on the object to be tested, forming eddy currents on the surface of the object to be tested, and then generating a secondary magnetic field. The information of the secondary magnetic field can characterize the defects of the object to be tested. This method can well achieve the purpose of directly testing pipes with insulation layers.

[0004] In exploring new paths for pipeline wall thickness detection technology, Gao Peng and others from Tianjin Special Equipment Supervision and Inspection Technology Research Institute have pioneered the development of dual-mode ferromagnetic coated pipeline wall thickness detection technology. This technology combines pulse signals and sinusoidal excitations, injects these two excitations into the coil of the detection probe at the same time, and then uses the magnetic sensor to accurately capture the dynamic information of the magnetic field changes, thereby achieving dual measurement of the pipeline wall thickness and the thickness of its coating layer, opening up a new perspective for pipeline structural integrity assessment.

[0005] Chen Xingle and others from the Beijing University of Aeronautics and Astronautics proposed an advanced solution based on pulsed eddy current testing to address the wall thickness corrosion problem of ferromagnetic pipelines with coatings. Based on the time domain analysis theory of the pulsed eddy current testing model for ferromagnetic pipelines, the induced voltage response curve is innovatively used to reversely infer pipeline parameters. By setting a reference point and using its wall thickness inversion result as a benchmark, the wall thickness changes of other test points relative to this reference point are accurately calculated, providing a scientific basis for quantitatively evaluating pipeline corrosion conditions.

[0006] In addition, Han Yang and others from Suzhou Thermal Engineering Research Institute Co., Ltd. also proposed a pulsed eddy current detection technology for pipes with coatings. The system generates a pulse square wave signal from the host, driving the excitation coil to generate transient current, thereby inducing an eddy current effect in the pipe to be tested. The time-domain induced voltage data generated by the magnetic field fluctuation of the detection coil is captured by the data acquisition card, and then the host computer is used to deeply process and analyze these data, and finally the wall thickness information of the pipe is intuitively displayed, providing an efficient tool for pipeline maintenance and inspection.

[0007] Guan Xin and others from China Machinery Productivity Promotion Center Co., Ltd. proposed a system and method for detecting the wall thickness of a pipe containing a thermal insulation layer. Based on the pulse eddy current nondestructive testing technology, they designed a detection probe with an I-shaped iron core, and developed a detection system including a detection module, a main control module, a host computer, and a power supply module. This can realize quantitative nondestructive testing of the wall thickness of the pipe under the thermal insulation layer, meet the needs of rapid scanning of in-service pipes containing thermal insulation layers, and achieve integrity management of the pressure pipeline throughout its life cycle.

[0008] The above methods all use pulsed eddy current nondestructive testing technology to solve the problem of quantitative detection of the wall thickness of the pipe with coating layer. However, when using eddy current nondestructive testing technology, there will inevitably be the problem of magnetic loss. The principle of this method is to use the principle of electromagnetic induction to generate eddy currents on the surface of the object to be measured, and further generate a secondary magnetic field, and use the changes in the two superimposed magnetic fields to characterize the defects of the object to be measured. However, due to the existence of magnetic loss, the intensity of the secondary magnetic field will be inconsistent with the expected intensity, which will lead to deviations in the detection results. In addition, during the use of eddy current sensors, they are highly sensitive to temperature, and the ambient temperature of the industrial production site where the detection is carried out is uncertain, which will also cause distortion of the sensor detection results. Summary of the invention

[0009] The purpose of the present invention is to overcome the influence of magnetic loss and ambient temperature on the detection result of the sensor during the eddy current detection process, and to realize high-precision online detection of the pipeline with the thermal insulation layer.

[0010] The technical solution of the present invention is a high-precision method for detecting the remaining wall thickness of a pipeline under an insulation layer. The method adopts a pulsed eddy current nondestructive testing method, and the steps include:

[0011] S1. At room temperature, place the detection probe in an unloaded area and apply an excitation signal to it;

[0012] S2, simultaneously collecting the probe detection signal output voltage V and the probe temperature T;

[0013] S3, when the temperature T changes by a certain amount ΔT less than the set threshold within a period of time, stop collecting data;

[0014] S4, solving the inversion function of output voltage V and probe temperature;

[0015] S5, solving the voltage inversion function of the remaining thickness D of the pipeline;

[0016] S6. Measure on the pipeline, collect the output signal V, and invert it into the pipeline thickness D.

[0017] Furthermore, the specific method of step 4 is:

[0018] S41, fitting the probe detection signal V and the probe temperature T by mathematical method to obtain a V'-T' curve;

[0019] S42. Perform secondary differentiation on the V'-T' curve. When the differential value is close to zero, the temperature at this point is used as the starting point T of the linear change part. 0 ;

[0020] S43, intercept T>T 0 Part of the data is linearly fitted to obtain the slope k of the voltage V changing with the temperature T;

[0021] S44, T=T 0 As the standard temperature, the voltage value is inverted to obtain the voltage inversion function.

[0022] Furthermore, the specific method of step 5 is:

[0023] S51. Take n sections of standard pipes with different thicknesses, and mark the pipe thickness as D. i , i=1,2,3…n;

[0024] S52. Use the probe to measure each section of the pipeline and record its output voltage V i , probe temperature T i ;

[0025] S53, the output voltage V i Substitute the inversion function obtained in step S4 to obtain the standard voltage value V i0 ;

[0026] S54, V i0 With D i Perform linear fitting, the slope of the straight line is a, the intercept is b, and the standard voltage is inverted into the residual thickness relationship D = aV 0 +b;

[0027] S55. During measurement, the collected voltage is V, and the standard voltage inversion function in S4 is substituted into the function to obtain the thickness inversion equation.

[0028] Furthermore, the voltage inversion function in step S44 is: V 0 =Vk(TT 0 ).

[0029] Furthermore, in step S55, the thickness inversion relation D=a[V+k(TT 0 )]+b

[0030] The method of the present invention can effectively avoid the detection deviation caused by magnetic loss during the detection process and improve the accuracy of the detection result on the basis of realizing the detection of the target. In addition, the method can effectively suppress the temperature sensitivity of the sensor, so that the detection device can adapt to a wide temperature range. The method can meet the use requirements of online detection of pipelines with insulation layers and improve the detection efficiency in industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The probe detection signal changes with temperature.

[0032] Figure 2 Fitting the linear variation of the probe detection signal.

[0033] Figure 3 The output voltage of the Hall sensor changes with temperature.

[0034] Figure 4 This is the temperature change over time after reaching 90°C.

[0035] Figure 5 is the VT curve obtained by fitting.

[0036] Figure 6 is the second differential of the VT curve.

[0037] Figure 7 is a linear fit to the truncated data.

[0038] Figure 8 Measure the voltage inversion result for the probe.

[0039] Fig. 9 It is the inversion curve of standard voltage and measured thickness. DETAILED DESCRIPTION

[0040] The magnetic loss of eddy current is the loss of the coil itself, but more importantly, it is the loss caused by the eddy current effect in the magnetic core. Reasonable selection of wire and core materials can effectively reduce the loss, but the loss is still inevitable. This part of the lost energy will be presented in the form of heat during the working process, resulting in a significant upward trend in the temperature of the working coil. The Hall sensor is used to detect the superimposed magnetic field strength and the output is a voltage value. It can be found that as the temperature rises, the voltage value of the detection signal shows a corresponding change, such as Figure 1As shown. As the temperature of the probe increases, its output voltage shows a monotonous downward trend, and after multiple measurements, it can be determined that the voltage changes with temperature with good repeatability, so the output of the probe can be corrected in combination with temperature information. At the same time, it can be seen that under different starting temperature conditions, the test results still have good consistency, which means that under different ambient temperatures, the probe output signal changes with temperature in the same way, so this method can simultaneously solve the problem of the adaptability of the test probe to different ambient temperatures.

[0041] In addition, Figure 1 It can be seen that at the beginning of each measurement, the curve has a steep decline process. After reaching a certain temperature, the curve shows a linear change. In the three measurements, the boundary temperatures of linear and nonlinear changes are T 1 、T 2 、T 3 The steep drop process is not consistent, but it has good consistency in the linear change range. The reason for the steep change is that when the probe starts working, the heat generated by the heat source has not yet been transferred to the entire probe, resulting in uneven temperature distribution of the probe. After a period of time, the heat is fully transferred throughout the probe, and the output voltage begins to enter the linear change area with temperature changes. The starting temperature each time is related to the ambient temperature, so the boundary temperature is not consistent.

[0042] The steeply descending part of the curve in the figure cannot accurately reflect the probe detection signal, so a certain method is needed to find the decomposition temperature T 0 , the temperature T <T 0 The part of the curve is removed, and the slope of this part of the curve gradually decreases. When the temperature T>T 0 After that, the slope no longer changes. The slope k is obtained by differentiation, and the starting point of the linear part signal is determined by its change. However, the collected signal will fluctuate due to the influence of noise. At this time, differentiation will obtain a local extreme value and cannot reflect the signal slope. Therefore, it is necessary to first perform cubic spline fitting on the curve and then perform differential calculation.

[0043] Through the above method, T <T 0 After partial elimination, linear fitting is performed on the linear part signal, such as Figure 2 As shown, it can be seen that the output voltage V and the temperature T change show a good linear relationship. According to this linear law, the voltage signals at different temperatures can be collected and inversion calculations can be performed to eliminate the impact of temperature changes on the output voltage.

[0044] Temperature inversion method for pipeline detection probe with insulation layer

[0045] Depend on Figure 2It can be seen that in the linear variation part of the detection signal, the output voltage varies with temperature as shown in formula (1).

[0046] V=kT+d (1)

[0047] Where V is the output voltage value, T is the temperature of the detection probe, k is the slope of the linear change of the output voltage with temperature, and d is the intercept of the linear change of the output voltage of the probe with temperature.

[0048] Let the probe work under no-load conditions. After a heating process, collect its temperature and output voltage data to find the decomposition temperature T of its linear part and nonlinear part. 0 , for T>T 0 The slope k is obtained by fitting the part. 0 As the standard temperature under no-load conditions, the output voltage can be inverted into the standard voltage value V according to formula (2) 0 .

[0049] V 0 =Vk(TT 0 ) (2)

[0050] According to the above method, the target pipeline is tested. First, the probe with the temperature sensor is placed in the no-load position, an excitation signal is applied to it, and the Hall sensor output voltage V and coil temperature T are recorded. The results are as follows: Figure 3 shown.

[0051] When the temperature reaches above 90℃, the temperature changes very slowly, with a change of only 0.15℃ in 1 minute. Figure 4 As shown, the acquisition is stopped.

[0052] The VT curve is obtained by fitting the voltage and temperature. Figure 5 As shown:

[0053] The equation of the curve is:

[0054] f(x)=-9.882×10 -8 x 5 +3.674×10 -5 x 4 -5.365×10 -3 x 3 +0.3917x 2 -18.47x+3529.

[0055] Its quadratic differential curve f”(x) is as follows Figure 6 As shown in Figure 2, when the temperature reaches 45°C, the second derivative value is within 0.05, which is taken as the starting point of the linear part, that is, T 0 =45℃.

[0056] From T 0 =45℃, the data was cut off and the remaining data was linearly fitted. Figure 7 shown.

[0057] The fitting curve obtained is: f(x) = -3.955x + 3310, its slope k = -3.955, and the standard voltage inversion function is

[0058] V 0 =V+3.955(T-45) (3)

[0059] The new "voltage-temperature" curve obtained after the inversion of the probe is compared with the curve before the inversion. Figure 8 shown.

[0060] On the standard specimen testing platform, objects of different thicknesses are tested respectively, and the output voltage V and temperature T are recorded. The inverted standard voltage V is calculated according to formula (3): 0 , the data are shown in Table 1.

[0061] Table 1 Detection data of objects with different thicknesses

[0062]

[0063] On the standard specimen testing platform, objects of different thicknesses are tested respectively, and the output voltage V and temperature T are recorded. The inverted standard voltage V is calculated according to formula (3): 0 , the data are shown in Table 1.

[0064] The standard output voltage V 0 Fitting with the thickness D of the object being measured, the result is as follows Fig. 9 As shown. The fitting curve can be obtained as:

[0065] D=0.1526V 0 -479 (4)

[0066] Substituting equation (3) into equation (4) for calculation, the final inversion function can be obtained as follows:

[0067] D=0.1526V+0.6035T-506.159 (5)

[0068] Where D is the thickness of the pipe to be measured, V is the sensor output voltage, and T is the sensor output temperature.

[0069] According to the above data, the pipeline thickness is inverted through the measured value. At the same time, without considering the temperature change, the output voltage is directly fitted with the pipeline thickness for inversion. The comparison results are shown in Table 2.

[0070] Table 2 Inversion results of pipeline thickness considering temperature change

[0071]

[0072] Without considering the temperature change, the probe continues to heat up during the measurement process, resulting in large errors in the measurement results. However, in this method, after considering the temperature factor, the error is very well controlled and the measurement accuracy is greatly improved.

Claims

1. A high-precision method for detecting the remaining wall thickness of a pipeline under an insulation layer, the method adopts a pulsed eddy current nondestructive testing method, and the steps include: S1. At room temperature, place the detection probe in an unloaded area and apply an excitation signal to it; S2, simultaneously collecting the probe detection signal output voltage V and the probe temperature T; S3, when the temperature T changes by a certain amount ΔT less than the set threshold within a period of time, stop collecting data; S4, solving the inversion function of output voltage V and probe temperature; S5, solving the voltage inversion function of the remaining thickness D of the pipeline; S6. Measure on the pipeline, collect the output signal V, and invert it into the pipeline thickness D.

2. A high-precision method for detecting the remaining wall thickness of a pipe under an insulation layer as claimed in claim 1, characterized in that: The specific method of step 4 is: S41, fitting the probe detection signal V and the probe temperature T by mathematical method to obtain a V'-T' curve; S42, perform secondary differentiation on the V'-T' curve, and when the differential value is close to zero, the temperature at this point is taken as the starting point T0 of the linear change part; S43, intercepting the data of the portion where T>T0, performing a linear fit on the data, and obtaining a slope k of the change of the voltage V with the temperature T; S44. Taking T=T0 as the standard temperature, invert the voltage value to obtain a voltage inversion function.

3. A high-precision method for detecting the remaining wall thickness of a pipeline under an insulation layer as claimed in claim 1, characterized in that: The specific method of step 5 is: S51. Take n sections of standard pipes with different thicknesses, and mark the pipe thickness as D. i , i=1,2,3…n; S52. Use the probe to measure each section of the pipeline and record its output voltage V i , probe temperature T i ; S53, the output voltage V i Substitute the inversion function obtained in step S4 to obtain the standard voltage value V i0 ; S54, V i0 With D i Perform linear fitting, the slope of the straight line is a, the intercept is b, and the standard voltage inversion is the relationship between the residual thickness D = aV0 + b; S55. During measurement, the collected voltage is V, and the standard voltage inversion function in S4 is substituted into the function to obtain the thickness inversion equation.

4. A high-precision method for detecting the remaining wall thickness of a pipe under an insulation layer as claimed in claim 2, characterized in that: The voltage inversion function in step S44 is: V0=Vk(T-T0).

5. A high-precision method for detecting the remaining wall thickness of a pipeline under an insulation layer as claimed in claim 4, characterized in that: In step S55, the thickness inversion relation D=a[V+k(T-T0)]+b.

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