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New concepts for corrosion Inspection of Pipelines by Digital Industrial Radiology (DIR)Author : U. Zscherpel (BAM Berlin, Germany)
Co-Authors : Y. Onel, U. Ewert (BAM Berlin, Germany)
As film we used AGFA Pb Vacupack D7 which was exposed using a portable Ir-192 source. The storage phosphor system was the DPS prototype developed by AGFA . It can handle IP formats up to 35x43 cm² in full daylight. The IP's were used in combination with appropriate Pb-filters (see ). For corrosion assessment we used IP prototypes (IPI) with an inherent unsharpness < 230 µm. This should be compared with the inherent unsharpness of the film for an Ir-192 exposure which amounts 230 µm .
1.2 Example for Comparison of Inspection Results
|Fig 1: Comparison of film radiography (left column) with CR (right column)|
2.1. Mathematical Background
In fig. 2 a measured profile (across the 8 mm step of the test pipe in fig. 8) is shown together with the fitted function of an ideal profile across this pipe. The background found by this simple fit corresponds well with results of Monte Carlo calculations about the scatter contribution at this geometrical set-up .
|Fig 2: Measured profile (black line), Result of a fit (red dot dash line) of the sum of the exponential absorption law and a background (green line) representing the scatter contribution|
The filter according equation (1) allows a practical application of a second derivative, because it has only the noise amplification like a first derivative. This is shown in fig. 3 on a experimental data set. The SNR of the true second derivative is too low for correct edge detection, whereas the CT filter gives reliable results.
|Fig 3: Measured CR wall thickness profile and the influence of noise to the filters|
The idea to use adopted CT algorithms is developed further and discussed in detail in . The detection of the profile edges gives the projected wall thickness in pixels of the image data. The next step of data processing is the compensation of the magnification factor in the used tangential projection method.
To consider the geometrical magnification of the pipe image on film or IP in the tangential exposure technique according to the set-up shown in fig. 4 a correction of the measured wall thickness (w') must be performed. The true wall thickness (w) depends in a rather complicated way on the film-focus-distance (f), the radius of the pipe (r) and the radius of the insulation (R) as shown in equation 2 :
|Fig 4: Geometrical magnification effect inherent to the tangential exposure technique|
In most practical applications this is approximated by a simpler relationship:
The error made by this simplification is shown in fig. 5. The values calculated with equation (3) are greater than the correct values according equation (2). This error is smaller than 1 % if the film focus distance is more than 6 times the diameter of the pipe.
|Fig 5: Systematic error of the simple formula (3) compared to the correct model according equation (2) depending on the ratio of film focus distance to pipe diameter. The wall thickness calculated according to (3) is greater then (2) by the given error.|
2.2 User Interface for Computer Supported Wall Thickness Estimation
In fig.6 an example for the realized user interface is shown. The user draws the profile plot interactively with the mouse across the wall of the pipe. The computer returns the estimated wall thickness according equation (1) and (2) and draws the detected edge positions of the profile into the image (ticks perpendicular to the profile indication). Depending on the look up table used to display the image on the monitor the position of these ticks can be visually on "wrong" positions in the displayed image. That's why it is very important to detect the edges according to equation (2) on the original data set.
The accuracy of the presented methods was verified using test pipes with various diameters and wall thickness of which the wall thickness was measured independently using a slide-rule. Both on film and on the monitor the wall thickness could be determined within an accuracy of about 0.2 mm.
|Fig 6: User interface for automated wall thickness estimation in tangential projection technique|
3.1. Basic Principles
Beer's absorption law (4) for the penetrating radiation is used to correlate the penetrated wall thickness (w) and the radiation dose Iw (I0 - radiation dose at w=0, µ - absorption coefficient):
In practice this simple equation is complicated by the fact that µ depends on the radiation energy and beside the radiation absorption there is also scattered radiation generated by the penetrated object.
Modern NDT film systems (with Pb screens) , IP systems and flat panel detectors are very linear X-ray detectors.
Basically it is only possible to determine a wall thickness change in penetration direction from a density variation of a radiographic film. No absolute wall thickness values can be obtained in this way contrary to the tangential method.
The resulting relationship for the radiation intensity depending on the wall thickness change Dw is as following :
(Inom - intensity at wall thickness wnom, Iero - intensity at wall thickness wnom + Dw)
To determine the wall thickness change Dw it is necessary to have the effective absorption coefficient µeff for the given radiation energy and nominal wall thickness wnom . µeff can be determined from the digital image, if there a known wall thickness change can be found in the image. As such a wall thickness change DwIQI an IQI according ASTM or a CERL A test wedge could be used. Then the µeff is determined by :
After this calibration step (the effective absorption coefficient is determined from a known wall thickness change and the corresponding variation of the radiation intensity) the evaluation of local wall thickness changes D w (corresponding to Iero) from the nominal wall thickness wnom (corresponding to Inom) can be done according to :
But there are many practical applications where no known wall thickness changes can be obtained from the radiographic film under consideration because at the exposure it was not known that such a feature is important for the evaluation. Our investigation shows that in this case µeff can be determined from a step wedge exposure with the same detector system, the same radiation energy, the same nominal wall thickness and geometrical set-up.
Principally the penetrated wall thickness is changed by 2 influences :
3.2 Application on Test Objects
|Fig 7: User interface for wall thickness evaluation based on density variations on a step wedge, the nominal wall thickness was set to 15 mm|
The user interface is realized by 2 windows on the monitor (beside the window with the image under evaluation):
After the calibration step an effective absorption coefficient according to equation (6) of µeff = 1.48 1/cm was determined (for X-rays at 160 kV).
The profile plot in fig. 7 shows a highly linear relationship between the estimated wall thickness change (derived from the density variation of the digitized film) and the actual wall thickness (given by the step of the step wedge) over a wall thickness range of nearly 15 mm (at 160 kV). This wide wall thickness range is achieved by using film densities less then 0.5. In practice this is not usable caused by the low contrast sensitivity for corrosion detection in this region. Fig. 7 shows also the influence of the scattered radiation. This generates a slope of each step, which is increasing with increasing wall thickness. So the influence of the scattered radiation increases with increasing wall thickness. But the mean value of each step is still in accordance to Beer's absorption law, which gives the straight forward line of the total plot.
The influence of the reduced contrast sensitivity at low optical densities can be seen on the noise level, which increases with the wall thickness. The same highly linear relationship as shown for X-ray penetration can be obtained for Ir192 or Co60 at higher wall thickness and with a lower contrast sensitivity .
The same approach can be used for flat panel detectors or CR with IP's. Here the gray level in the image depend on the characteristics of the Analogue to Digital Converter (ADC) used in the IP reader. Typical characteristic transfer functions are linear, square root or logarithmic. The photo stimulated luminescence signal generated by the IP is proportional over several decades to the radiation dose exposed. By a 16 bit LUT the characteristics of the ADC can be corrected and a digital image generated, where the digital value is directly proportional to the penetrated wall thickness. This is shown in fig. 8 for a test pipe penetrated with Ir 192 and the AGFA DPS prototype CR system. A linear relationship of 20 mm is obtained.
|Fig 8: Test pipe (wall thickness given down left) image (CR system, Ir 192) and profile plot after LUT correction of the ADC response and exponential correction of the absorption law resulting in digital values proportional to the penetrated wall thickness|
3.3 Practical Applications
Practical applications of the presented algorithm have been done on corrosion monitoring in pipe systems of power plants. These examples were obtained from a reducing pipe fitting after an valve on a condensation trap. Corrosion damage was expected prior to the stop valve, but actual it was found after the valve in the reducing pipe fitting.
In fig. 9 local corrosion by erosion is shown in a pipe with a bore of 100 mm behind a welding. In this case only the nominal wall thickness of the pipe is known (6.3 mm). To calibrate the obtained density changes into wall thickness changes a step wedge exposure with a nominal wall thickness of 13 mm (double wall penetration in the pipe exposure) and the same source / film system combination was used. From this a µeff = 1.30 1/cm can be expected which is used for the wall thickness estimation of the pipe image according to equation (7).
The minimum wall thickness found with a manual ultrasonic wall thickness meter was 4.0 mm (»2.3 mm wall thickness loss). This is in good accordance to the estimated wall thickness loss by radiography, which shows for small spots even a higher loss up to 2.8 mm. The accuracy for these measurements is about 0.2 mm.
|Fig 9: Corrosion inside a pipe (bore 100 mm, wall thickness 6.3 mm), projection technique at 160 kV (double wall penetration), profile plot with calibrated wall thickness loss|
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