This European Standard applies to in-service double wall radiographic inspection using industrial radiographic film techniques, computed digital radiography CR and digital detector array
Protection against ionizing radiation
Exposure to X-rays or gamma-rays poses significant health risks It is essential to adhere to legal requirements when using X-ray equipment or radioactive sources Strict compliance with local, national, and international safety precautions is necessary when handling ionizing radiation.
Personnel qualification
Testing must be conducted by skilled and qualified personnel, supervised by competent individuals designated by the employer or the inspection company It is advisable for personnel to be certified under EN ISO 9712 or a similar recognized system to prove their qualifications The employer shall issue operating authorization for qualified individuals following a documented procedure.
NDT operations, unless otherwise agreed, shall be authorized by a competent and qualified NDT supervisory individual (Level 3 or equivalent) approved by the employer
The personnel shall prove additional training and qualification in digital industrial radiology if digital detectors are being used.
Identification of radiographs
Each section of the object being radiographed must have symbols attached, ensuring that these symbols are visible in the radiograph outside the region of interest whenever possible This practice guarantees clear and unambiguous identification of each section.
Marking
Permanent markings on the object to be examined should be made in order to accurately locate the position of each radiograph
Where the nature of the material and/or its service conditions do not permit permanent marking, the location may be recorded by means of accurate sketches.
Overlap of films or digital images
When conducting radiography with multiple films or detectors, it is essential that they overlap adequately to cover the entire area of interest This overlap must be confirmed using a high-density marker placed on the object's surface, which should be visible on every film or detector In cases where radiographs are taken sequentially, the high-density marker must also be clearly visible on each individual radiograph.
Types and positions of image quality indicators (IQI)
Single wire IQI
The quality of image shall be verified by use of IQIs in accordance with EN ISO 19232-1
For DWDI, the single wire IQI should ideally be positioned on the source side of the test object, centered within the area of interest and in close contact with the object's surface In cases where this placement is not feasible, such as with insulated pipes, the IQIs will be positioned on the detector side To assess image quality, a comparison exposure must be conducted, utilizing one IQI on the source side and another on the detector side under identical conditions.
For DWSI, the single wire IQI must be positioned at the center of the area of interest on the detector side of the test object Ideally, the IQI should be in direct contact with the object's surface; however, if insulation prevents this, it should be placed in contact with the film or detector instead.
For both DWDI and DWSI, the wire IQIs must be positioned across the pipe, with their long axis tilted at an angle of 2° to 5° relative to the pipe's orthogonal axis It is essential that the IQIs are placed in a section of uniform thickness, close to the pipe's centerline.
For DWDI, where the IQI's are placed at the detector side, the letter “F” shall be placed near the IQI and it shall be noted in the test report
The extent of image quality verification for repeat exposures of closely similar objects under identical conditions shall be subject to agreement between the contracting parties.
Duplex wire IQI (digital radiographs)
According to EN ISO 19232-5, IQIs should be utilized to measure the basic spatial resolution of the CR/DDA system in a reference radiograph The duplex wire IQI must be positioned next to the imaging plate or detector array, tilted at an angle of 2° to 5° relative to the digital rows or columns of the digital image.
6 Recommended techniques for making radiographs
Test arrangements
General
Normally radiographic techniques in accordance with 6.1.2 and 6.1.3 shall be used
Technique 6.1.2 is normally used for larger diameter pipes Technique 6.1.3 is generally used for smaller diameter pipes (less than typically about 150 mm outside diameter)
For both techniques, the film or digital detector shall be placed as close to the pipe as possible.
Double wall single image (DWSI)
In this setup featuring curved detectors or film, the source is positioned close to the pipe, with the film or detector placed on the opposite side, as illustrated in Figure 1 a) for the non-insulated pipe and Figure 1 b) for the insulated pipe The critical distances necessary for calculating the source to detector distance (SDD) are also indicated.
Figure 1 — Test arrangement for double wall single image radiography (DWSI) using a curved detector
Wall loss can occur on the inner diameter, outer diameter, or both surfaces of the pipe wall near the detector It is important to note that wall loss on the source side of the pipe is not captured in the imaging results.
DWSI can be utilized for rigid planar detectors, as illustrated in Figures 2a and 2b However, this configuration allows for the inspection of a smaller portion of the pipe circumference at each position, with Figure 2a depicting a non-insulated pipe and Figure 2b showing an insulated pipe.
Figure 2 — Test arrangement for double wall single image radiography (DWSI) using a planar detector
Double wall double image (DWDI)
In this setup, the radiation source is positioned in front of the pipe, while the planar film or detector is placed on the opposite side, as illustrated in Figures 3a and 3b, which depict a non-insulated pipe and an insulated pipe, respectively.
Figure 3 — Test arrangement for double wall double image radiography (DWDI)
DWDI technology enables the detection of wall loss on both the inner and outer diameters of a pipe, as well as on either the source or detector side.
If DWDI and tangential radiographic techniques are combined, the requirements of EN 16407-1 shall also be met.
Alignment of beam and film/detector
The beam of radiation shall be directed at the centre of the area being examined and should be perpendicular to the pipe axis
For DWDI, the film or detector should be aligned to be orthogonal to the centre of the radiation beam
Modifications to these alignments and the test arrangements given in 6.1.2 and 6.1.3 may be needed in special cases, due for example to the presence of obstructions
Other ways of radiographing may be agreed between contracting parties.
Choice of radiation source
Penetrated thickness ranges for X-ray and gamma ray sources are given in Table 1 and Figure 4 By agreement between contracting parties, these ranges can be extended
The optimal X-ray voltages for film radiography of welds, as illustrated in Figure 4, represent best practice values When utilizing DDAs with precise calibration, it is possible to achieve adequate image quality with higher X-ray voltages than those indicated In contrast, for CR applications, it is advisable to lower the X-ray voltages by at least 20% compared to the values in Figure 4.
In cases where radiographs are produced using gamma rays, the total travel-time to position and rewind the source shall not exceed 10 % of the total exposure time
By agreement between the contracting parties the penetrated thickness minimum value for Ir 192 and Se 75 may be reduced to 5 mm of steel
Table 1 — Total effective penetrated thickness ranges for gamma-ray and high energy X-ray sources for steel pipes
Radiation source Total effective penetrated thickness w tot mm basic technique
X-ray equipment with energy from 1 MeV to 4 MeV 30 ≤ wtot ≤ 200
X-ray equipment with energy from 4 MeV to 12 MeV wtot ≥ 50
X-ray equipment with energy above 12 MeV wtot ≥ 80 a For aluminium and titanium the penetrated material thickness is 35 mm ≤ wtot ≤ 120 mm for class
4 aluminium and alloys w penetrated thickness in mm
Figure 4 — Maximum X-ray voltage U for X-ray devices up to 1 000 kV as a function of penetrated thickness w and material
When selecting sources for product-filled pipes, it is essential to account for the additional radiation attenuation caused by the product For water-filled pipes, the penetrated thickness, \( w \), for steel tested with Ir 192 should be increased by about one-ninth of the path length in the water to determine \( w_{tot} \) In the case of oil-filled pipes, the penetrated thickness, \( w \), must be increased by approximately one-eleventh of the path length in the oil to calculate \( w_{tot} \).
For insulated pipes the additional radiation attenuation caused by the insulation shall be allowed for in the selection of sources.
Film systems and screens
For radiographic examination, film system classes shall be used in accordance with EN ISO 11699-1
Table 2 presents the classification of radiographic film systems and metal screens for various radiation sources To ensure optimal performance with metal screens, it is essential to maintain good contact between the films and the screens, which can be accomplished through vacuum packing or by applying pressure.
Table 2 — Film system classes and metal screens for double wall radiography of steel, copper and nickel based alloy pipes
Radiation source Film system class a Type and thickness of metal screens
C 5 C 4 0,02 mm to 0,15 mm front and back screens of lead
0,1 mm to 0,2 mm front screens of lead b 0,02 mm to 0,2 mm back screens of lead
1 000 kV C 5 C 4 0,25 mm to 0,7 mm front and back screens of steel or copper c
Ir 192 C 6 C 5 0,02 mm to 0,2 mm front and back screens of lead b
Co 60 C 6 C 5 0,25 mm to 0,7 mm front and back screens of steel or copper c
X-ray equipment with energy from 1 MeV to 4 MeV C 6 C 5 0,25 mm to 0,7 mm front and back screens of steel or copper c
X-ray equipment with energy above 4 MeV C 6 C 5
Up to 1 mm front screen of copper, steel or tantalum d Back screen of copper or steel up to
For optimal film system performance, classes utilizing 1 mm and tantalum up to 0.5 mm are recommended Additionally, ready-packed films featuring a front screen thickness of up to 0.03 mm can be employed, provided an extra lead screen of 0.1 mm is positioned between the object and the film In class DWA, lead screens ranging from 0.5 mm to 2.0 mm are permissible, while screens between 0.5 mm and 1 mm may be used upon mutual agreement between the contracting parties.
Table 3 — Film system classes and metal screens for double wall radiography of aluminium and titanium pipes
Radiation source Film system class a Type and thickness of intensifying screens
Class DWA Class DWB X-ray potentials ≤ 150 kV
None or up to 0,03 mm front and up to 0,15 mm back screens of lead X-ray potentials > 150 kV to
0,02 mm to 0,2 mm front and back screens of lead b
For optimal film system performance, lead screens ranging from 0.02 mm to 0.2 mm can be utilized on both the front and back Alternatively, two 0.1 mm lead screens can replace a single 0.2 mm lead screen Additionally, the use of Ir 192 is permissible upon mutual agreement between the contracting parties.
Different film system classes may be used by agreement of the contracting parties, provided the required optical densities defined in 7.2 are achieved.
Screens and shielding for imaging plates (computed radiography only)
For optimal performance with metal front screens, it is essential to ensure proper contact between the sensitive detector layer and the screens This can be accomplished through vacuum-packed imaging plates (IPs) or by applying pressure Insufficient contact between lead screens and IPs can lead to image unsharpness Additionally, the intensification achieved with lead screens in contact with imaging plates is considerably less than that obtained in traditional film radiography.
Low energy backscatter and X-ray fluorescence from lead can significantly affect image quality by causing edge unsharpness and reducing signal-to-noise ratio (SNR) in imaging plates (IPs) To minimize these effects, it is advisable to use steel or copper shielding directly behind the IPs Additionally, placing steel or copper shielding between a backscatter lead plate and the IP may further enhance image quality Modern cassette and detector designs can address this issue, potentially eliminating the need for extra steel or copper shielding outside the cassette.
The protective layer between the lead and the sensitive layer of an IP significantly diminishes the intensification effect of electrons, which becomes noticeable only at higher energies The degree of intensification varies based on the radiation energy and the design of the protection layer, ranging from 20% to 100% compared to the absence of a screen.
The small intensification effect from a lead screen in contact with an imaging plate (IP) can be balanced by extending exposure time or milliampere minutes when lead screens are not utilized However, since lead screens can cause scratches on IPs if not properly separated during scanning, they should be employed for intermediate filtering of scattered radiation outside cassettes It is important to note that no intermediate filtering is advised for inspecting steel specimens with a thickness of less than 12 mm.
Tables 4 and 5 outline the suggested screen materials and thicknesses for various radiation sources Contracting parties may agree on alternative screen thicknesses as long as the necessary image quality is maintained It is advisable to use metal screens in front of image plates (IPs), as they can also help minimize the effects of scattered radiation when paired with digital detectors (DDAs).
Table 4 —Metal front screens for CR for double wall radiography for pipes of steel, copper and nickel based alloys
Radiation source Type and thickness of metal front screens mm
X-ray potentials b ≤ 250 kV 0 to 0,1 (lead)
X-ray potentials b > 250 kV to 1000 kV 0 to 0,3 (lead) c
Ir 192, Se 75 b Class DWA: 0 to 0,3 (lead) c
Class DWB: 0,3 to 0,8 (steel or copper)
For Co-60, the recommended thickness for steel or copper is between 0.3 to 0.8 cm, while for lead, it ranges from 0.6 to 2.0 cm In applications involving X-ray potentials greater than 1 MV, the same thicknesses apply When using multiple screens, the steel screen should be positioned between the image receptor (IP) and the lead screen Alternatives to steel or combined steel and lead screens include materials such as copper, tantalum, or tungsten, provided that the image quality is demonstrably maintained Additionally, lead screens can be fully or partially substituted with iron (Fe) or copper (Cu) screens, with appropriate equivalent thickness considerations.
Cu is three times the Pb thickness c For total penetrated thickness above 50 mm the front screen thickness should be larger than 0,1 mm Pb.
Table 5 — Metal front screens for CR for the double wall radiography of aluminium and titanium
Radiation source Type and thickness of metal front screens mm X-ray potentials < 150 kV ≤ 0,03 (lead) a, b X-ray potentials ≥ 150 to
For optimal radiation protection, a lead thickness of ≤ 0.3 mm is recommended For instance, instead of using a 0.2 mm lead screen, a 0.1 mm lead screen combined with an additional 0.1 mm filter can be utilized outside the cassette Additionally, lead screens can be fully or partially substituted with iron (Fe) or copper (Cu) screens, with the equivalent thickness for Fe or Cu being three times that of lead Furthermore, the use of Iridium-192 (Ir-192) is permissible upon mutual agreement between contracting parties.
Reduction of scattered radiation
Filters and collimators
In order to reduce the effect of back scattered radiation, direct radiation shall be collimated as much as possible to the section under examination
For computed radiography and radiography utilizing digital detectors and sources like Ir-192 and Co-60, the thickness of the protective sheet between the pipe and the digital detector or computed radiography cassette ranges from 0.5 mm to 2.0 mm, depending on the required penetration thickness.
Alternative materials like tin, copper, tungsten, or steel can effectively serve as filters For lead filters, it is advisable to incorporate an additional steel or copper filter with a thickness ranging from 0.3 mm to 1.0 mm, positioned between the lead and the detector Ensuring that the filter is placed as close as possible to the sensitive plate enhances its effectiveness.
Interception of back scattered radiation
Each new test arrangement must be verified for back scattered radiation by placing a lead letter "B" behind each film, ensuring it has a minimum height of 10 mm and a thickness of at least 1.5 mm.
When using a CR cassette or DDA, a radiograph that displays a lighter image of the symbol indicates a negative presentation and should be rejected Conversely, if the symbol appears darker or is not visible, the radiograph is deemed acceptable, demonstrating effective protection against scattered radiation.
In digital radiography, it is essential to shield the detector from back scattered radiation using a minimum of 1 mm of lead or 1.5 mm of tin positioned behind the detector Depending on the configuration, up to 6 mm of lead may be required Additionally, a layer of steel or copper, approximately 0.5 mm thick, should be placed between the lead shield and the detector to mitigate the effects of lead X-ray fluorescence radiation It is important to note that lead screens must not be in direct contact with the back side of the detector for energies exceeding 80 keV.
Source-to-detector distance
Double wall single image
The dimensions involved for source to detector determination for the DWSI technique are shown in Figure 1
For the basic technique, DWA, the source to detector distance SDD (in millimetres) shall be, where practicable: d b⋅
0,6 mm (1) where b is the distance between the source side of the pipe and the detector in millimetres; d is the source size in millimetres
For the improved technique, DWB, the source to detector distance SDD (in millimetres) shall be, where practicable: d b⋅
Formulas (1) and (2) indicate geometric unsharpness values of 0.6 mm and 0.3 mm, respectively, as projected onto the plane of the source side of the pipe wall closest to the detector However, the unsharpness values recorded at the detector are slightly higher due to the influence of projective magnification.
NOTE The outside diameter of the pipe often means that the achievable source to detector distances will be greater than the values given in Formula (1) and Formula (2).
Double wall double image
The distances for determining the source to detector in the DWDI technique are illustrated in Figures 3a) and 3b), with the object plane representing the source side of the pipe wall that is closer to the detector.
In the basic technique, DWA, the source to detector distance (SDD) should ideally be determined by Formula (1) In contrast, for the enhanced technique, DWB, the SDD is defined by Formula (2).
Note however that the dimension b is measured differently for DWSI and DWDI, and is shown in Figure 1, Figure 2 and Figure 3 respectively for these techniques
When combining the double wall double image technique with tangential radiography, it is essential to determine the source to pipe center distance while considering the criteria outlined for the tangential technique in Part 1 of the standard Whenever feasible, the larger of the two calculated values should be utilized.
Axial coverage and overlap
The maximum axial coverage of a pipe in a single image or film is determined by a 20% increase in the penetrated thickness at the edge of the inspection area, as shown in Figure 5.
The axial cross section illustrated in Figure 5 depicts the maximum allowable axial length of the assessed area for a single source position This includes measurements on the film/detector, denoted as \(L_d\), and along the pipe, referred to as \(L_p\), specifically on the source side of the pipe.
The total axial extent of the evaluated area on the detector, Ld, should be no greater than:
The total axial extent of the evaluated area on the source side of the pipe, Lp, should be no greater than:
For DWSI, f shall be measured as shown in Figure 5
To determine the interval between exposures along a pipe, the Lp formula is essential When the collimator of gamma sources or the window collimation of X-ray sources is less than ± 35°, it is necessary to adjust Lp and Ld in accordance with the maximum available opening angle of the radiation cone beam.
To ensure comprehensive examination, films or digital images must overlap adequately, leaving no part of the component unexamined Unless stated otherwise, a minimum axial overlap of 25 mm on either side of the diagnostic area is required, measured from the source side.
Circumference coverage
General
When using the DWDI and DWSI techniques, then full circumferential coverage of a pipe is achieved by taking a number of different exposures around the pipe circumference.
DWSI
For DWSI, circumferential exposures are determined by a 20% increase in penetrated thickness resulting from inclined penetration at the diagnostic area's edges The number of exposures depends on the source to pipe center distance (SPD), the pipe's outside diameter (De), and the wall thickness (t).
Figure 6 shows the number of exposures needed, as a function of two dimensionless variables – t/De and
De/SPD This figure is applicable if the detector is offset from the pipe due to the presence of insulation
Figure 6 — Minimum number of DWSI exposures circumferentially around a pipe, as a function of the ratios t / D e and D e/SPD, where SPD is the distance from the source to the pipe axis (centre)
To obtain the circumferential angular difference, Δθ, (degrees) between exposures, the following formula should be used for DWSI: θ N °
N is the number of exposures given in Figure 6 Alternative values may be used by agreement between contracting parties.
DWDI
For a comprehensive inspection of a pipe's full circumference using DWDI, a minimum of two exposures is required, ideally positioned at least 45° apart, with 90° being optimal for accuracy However, if only a small section of the circumference needs evaluation, a single exposure may suffice.
Alternative values may be used by agreement between contracting parties.
Selection of digital radiographic equipment
General
The detector's basic spatial resolution must not exceed 200 µm for class DWA and 130 µm for class DWB, and it should not surpass 5% of the nominal wall thickness \( t \) Alternative values may be negotiated between the contracting parties.
CR systems
The pixel size of the CR scanner must not exceed 100 µm While increasing the scanner's gain or sensitivity can enhance grey levels for a specific radiographic exposure, it has little impact on image quality as indicated by the normalized signal to noise ratio (SNR N) To effectively improve SNR N, it is essential to increase the exposure rather than the scanner gain.
Using low gain or sensitivity in scanners with linear responses between radiation dose and grey level minimizes the risk of image saturation Conversely, higher scanner gains can lead to image saturation, particularly in free beam areas during short exposures, which may not provide adequate image signal-to-noise ratio (SNR) values to satisfy the image quality standards outlined in section 7.1.
DDA systems
The detector pixel size shall not exceed 200 àm for class DWA and 130 àm for class DWB Different values can be agreed by contracting parties
7 Radiograph/digital image sensitivity, quality and evaluation
Minimum image quality values
Wire image quality indicators
The IQI value requirements for Ir 192 and Se 75 in testing specific thickness ranges of steel pipes are outlined in Annex A For additional thickness ranges, radiation sources, pipe materials, and highly absorbing insulation, refer to EN ISO 19232-3, class A, or derive the requirements accordingly.
In cases where IQIs cannot be positioned on the source side of the object due to insulation, they should be placed on the detector side For DWSI, refer to Table A.2 and Table A.4 to determine the minimum quality values required.
Reference radiographs should be taken to qualify the technique depending on the inspected material, the material thickness, the used radiation quality, filters, screens and the used detector.
Duplex wire IQIs (digital radiographs)
Duplex wire IQIs (EN ISO 19232-5) shall be used for determination of the basic spatial resolution of the digital detector from a reference image according to Annex C, see 7.1.3.
Minimum normalized signal to noise ratio (digital radiographs)
Digital radiographic images can exhibit increased noise when captured under less-than-ideal conditions, such as during short exposure times This excessive noise can significantly hinder the ability to achieve acceptable detection sensitivity.
To maintain acceptable noise levels in digital images from CR and DDA systems, it is essential to measure the normalized signal-to-noise ratio (SNR N) using suitable software and methods.
EN 14784-1 using an area of at least 55 (vertically) × 20 (horizontally) pixels SNR N values shall be measured at a minimum of four separate positions, and the average value taken
To calculate the normalized SNR N value, the basic spatial resolution (SR b) of the imaging system must be measured using the duplex wire IQI method outlined in EN ISO 19232-5 or a comparable method If including a duplex wire IQI for each exposure is not feasible, the basic spatial resolution can be pre-determined for the same imaging system, ensuring that identical system settings are utilized, which for a CR system includes the same CR scanner, imaging plate, pixel size, and radiation source.
The average Signal-to-Noise Ratio (SNR) values measured along the pipe center line must be a minimum of 50 for the Basic Technique (DWA) and at least 80 for the Improved Technique (DWB) These SNR measurements should be conducted in areas where the wall thickness and grey values are consistent.
For objects with significant irregular corrosion or variations in wall thickness, homogeneous regions for measuring SNR N may be absent In such instances, the mean grey level along the pipe's center line must be assessed This mean grey level should surpass the calibration-derived minimum grey level that corresponds to the required SNR N values (50 for DWA and 80 for DWB) using the same detector, gain, sensitivity, and user settings as the test image, as outlined in EN ISO 17636-2:2013, Annex D.
When measuring the Signal-to-Noise Ratio (SNR), it is crucial to ensure that the image's grey levels are directly proportional to radiation intensity; otherwise, the resulting values will be inaccurate.
Density of film radiographs
The exposure conditions must ensure that the minimum optical density of the examined radiograph is at least 2.0, with a permissible measuring tolerance of ± 0.1 By mutual agreement between the contracting parties, this tolerance can be lowered to 1.5.
High optical densities can be used with advantage where the viewing light is sufficiently bright in accordance with 7.4
To prevent excessive fog densities due to film ageing, development, or temperature, periodic checks of fog density on a non-exposed sample from the films in use are essential These samples must be handled and processed under the same conditions as the actual radiographs, with a maximum allowable fog density of 0.3 Fog density is defined as the total density (emulsion and base) of a processed, unexposed film In multi-film techniques where single films are interpreted, each film's optical density must adhere to the specified limit Additionally, if double film viewing is required, the optical density of one single film should not fall below 1.3.
Film processing
Films must be processed according to the guidelines provided by the film and chemical manufacturers to achieve the desired film system class Key factors such as temperature, developing time, and washing time require careful monitoring Regular control of the film processing should adhere to EN ISO 11699-2 standards It is essential that the radiographs are devoid of defects caused by processing or other factors that could hinder accurate interpretation.
Film viewing conditions
Radiographs must be analyzed in a dimly lit environment using a viewing screen that allows for adjustable luminance, as specified by EN 25580 Additionally, the viewing screen should be appropriately masked to focus on the area of interest.
8 Measurement of differences in penetrated thickness
Principle of technique
To a first approximation, the radiation intensity transmitted through an object is related to penetrated thickness by:
I(w) is the intensity for penetrated thickness w;
I(0) is the unimpeded radiation intensity incident on the object; μ is the effective linear attenuation coefficient of the object material
Differences in penetrated thickness within a component therefore give rise to corresponding changes in film density or image grey level for digital images
Software can estimate changes in penetrated thickness of digital images by analyzing corresponding grey level values For two different penetrated thickness values, \( w_1 \) and \( w_2 \), and assuming equal incident radiation intensities and attenuation coefficients, the application of the relevant formula yields the relationship between these thickness values and their corresponding intensities.
Formula (7) shows that the difference in penetrated thickness, w 2 – w 1 , can be derived from the ratio of the two radiation intensities and the effective linear attenuation coefficient of the material
The ratio of radiation intensities in film radiography shall be determined from the measured netto optical densities by the following formula:
D0 is the optical density of film base and fog
In applying this method to digital radiographs it is therefore important to ensure that the image grey levels are directly proportional to the detected radiation intensity.
Measurement of attenuation coefficient
The effective linear attenuation coefficient of the tested material can be influenced by scattered radiation, necessitating measurement for each test object using a small step wedge This step wedge should consist of steps approximately 10 mm x 10 mm in area, with each step featuring a precisely machined known thickness, such as 1 mm or 2 mm.
Position the step wedge on the pipe to ensure it is imaged as close as possible to the area of interest In the DWDI method, the step wedge can be placed on either the source or detector side of the pipe For the DWSI method, it must be positioned between the pipe wall and the detector, while avoiding any significant distortion or bending of the imaging plate or film.
Source and detector positioning
To effectively apply this technique, it is crucial to position the source and detector so that the area of interest is as close as possible to the pipe's center line and centered within the radiographic image This positioning is especially vital for smaller diameter pipes, as the penetrated thickness increases significantly with distance from the pipe's center line.
Image grey level profiles
The grey level profile shape of the underlying image between the two measured areas will be evaluated and adjusted if needed, as outlined in Annex B.
Validation
To validate the measurement technique for penetrated thickness changes, exposures will be conducted using calibration objects that closely resemble the test object An appropriate validation object should match the test object's diameter, wall thickness, and material, and include machined flat-bottomed holes with precisely known depths, both less than and greater than the wall loss observed in the test object.
Validation radiographs must be captured under the same radiographic conditions as the test radiographs Additionally, the measurements of wall loss in the holes, obtained using the available software tool, should be shown to align with the known values to the necessary level of accuracy.
Key Points
The key points for this technique are:
— The method can only give measurements of the change in penetrated thickness between two different locations in a radiographic image, not an absolute value of penetrated thickness
— The digital image grey levels shall be linearized such that the grey levels are directly proportional to incident radiation intensity
— The effective attenuation coefficient of the object shall be measured by means of a small step wedge located close to the area of thickness change being measured
— The underlying image grey level profile between the two measurement positions needs to be assessed and any variations taken into account
The method's accuracy will be validated through the analysis of radiographs from a validation object that closely matches the dimensions of the test object It is essential that the radiographic conditions and software tools employed for the validation object are identical to those used for the test object.
9 Digital image recording, storage, processing and viewing
Scan and read out of image
Detectors and scanners must be utilized according to the manufacturer's guidelines to achieve optimal image quality It is essential that digital radiographs remain free from artifacts caused by processing, handling, or other factors that could hinder accurate interpretation.
Calibration of DDAs
When using DDAs, it is essential to follow the manufacturer's recommended detector calibration procedure, which includes calibrating with a background image and at least one gain image Multi gain calibration enhances the signal-to-noise ratio (SNR) and linearity but requires more time To minimize noise during calibration, all images should be captured with at least double the exposure dose intended for production radiographs Calibrated images must be treated as unprocessed raw images for quality assurance, provided the procedure is documented Regular calibration and bad pixel interpolation should be conducted periodically or whenever there are significant changes in exposure conditions.
Bad pixel interpolation
Bad pixels are suboptimal detector elements in digital detector arrays (DDAs), as defined by ASTM E2597 When utilizing DDAs, it is crucial to create a bad pixel map following the manufacturer's guidelines, and this map must be documented Interpolating bad pixels is a necessary procedure in radiography with DDAs It is advisable to use detectors that do not contain cluster kernel pixels (CKP) within the region of interest (ROI).
Image processing
The evaluation of digital data from the radiographic detector involves a linearized grey value representation that correlates directly with the radiation dose, essential for determining SNR, SR b, and SNR N To achieve optimal image display, it is crucial that contrast and brightness are interactively adjustable The software should also incorporate optional filter functions, profile plots, and tools for SNR and SNR N to enhance image evaluation For detailed image analysis, operators must be able to zoom in with a factor ranging from 1:1, where one pixel of the digital radiograph corresponds to one monitor pixel, to 1:2, where one pixel of the digital radiograph is represented by four monitor pixels.
Further means of image processing applied on the stored raw data (e.g high pass filtering for image display) shall be documented, be repeatable and be agreed between the contracting parties.
Digital image recording and storage
CR/DDA images should be stored in a file format which supports a minimum of 12-bits/pixel
Original images must be preserved in their full resolution as provided by the detector system Prior to storing the raw data, only image processing related to detector calibration—such as offset correction, gain calibration for equalization, and bad pixel correction (refer to ASTM E2597)—should be applied to ensure the images are free of artifacts.
The data storage shall be redundant and be supported by suitable back-up strategies to ensure “loss-less” data storage
Any data compression techniques used in the storage of these files shall be “loss-less”, i.e it shall be possible to reconstruct the exact original data from the compressed data.
Monitor viewing conditions
The digital radiographs shall be examined in a dimmed room The monitor setup shall be verified with a suitable test image
The display used for image evaluation must meet specific minimum standards, including a brightness of at least 250 cd/m², the ability to show a minimum of 256 shades of grey, a light intensity ratio of at least 1:250, and a resolution of no less than 1 megapixel with a pixel pitch of less than 0.3 mm.
A test report must be generated for each exposure or set of exposures, detailing the radiographic technique employed and any special circumstances that could enhance the understanding of the results.
The test report must include essential information such as a reference to the applicable standard, the name of the examination body, and details about the object and pipe isometric along with pipe content It should specify the material type, outer diameter (De), and nominal wall thickness (t) of the pipe, as well as the insulation's material, thickness, and condition Additionally, the report must outline the examination specifications, including IQI acceptance requirements, the radiographic technique and class, and the test arrangement as per section 6.1 It should detail the marking system used, the detector position plan, and the radiation source, including the type, size of the focal spot, and identification of the equipment Furthermore, the report must include information on the detector, screens, filters, tube voltage, current or source activity, exposure time, SDD, PDD, film type, film system, and processing It should also cover the CR system, IP type, scanner model, scanner parameters such as scan speed and pixel size, DDA type, operating parameters, and the basic spatial resolution of digital detectors, along with the measured image parameters.
1) film densities measured at pipe centre;
2) SNR N , achieved at the pipe centre;
The IQI reading includes measurements of wall thickness variations based on penetration direction, along with any additional observations It also accounts for deviations from the standard as per special agreements Furthermore, the report must include the name, certification, and signature of the operator, as well as the dates of exposure and the test report.
The IQI value requirements for Ir 192 and Se 75 in testing specific thickness ranges of steel pipes are detailed in Tables A.1 to A.4 For additional thickness ranges, radiation sources, pipe materials, and highly absorbing insulation, guidelines can be obtained from EN ISO 19232-4.
Table A.1 — DWDI Iridium 192 – source side wire IQIs
Total effective penetrated thickness mm
IQI value Total effective penetrated thickness mm
Table A.2 — DWSI Iridium 192 – detector side wire IQIs
Total effective penetrated thickness mm
IQI value Total effective penetrated thickness mm
Table A.3 — DWDI Selenium 75 – source side wire IQIs
Total effective penetrated thickness mm
IQI value Total effective penetrated thickness mm
Table A.4 — DWSI Selenium 75 - detector side wire IQIs
Total effective penetrated thickness mm
IQI value Total effective penetrated thickness mm
Penetrated thickness measurements from image grey levels
Spatial variations in the grey level of a radiographic image are influenced by several factors, including the angular dependence of the radiation beam intensity from the source, variations in the thickness of the objects being penetrated, and differences in the distances between the radiation source and various points on the detector.
To accurately measure changes in penetrated thickness, it is essential to maintain consistent unimpeded radiation intensities (I0) for both thickness values, as outlined in Formula (6) This process necessitates the extrapolation or interpolation of grey levels from one location in an image to a neighboring position, as demonstrated in Figures B.1 and B.2.
Figure B.1 illustrates a DWDI digital radiograph of a pipe featuring a clearly defined circular area of wall loss, alongside a small step wedge for measuring the effective attenuation coefficient An extracted profile along the pipe axis reveals that the grey level profile of the base material, unaffected by wall loss, remains relatively constant Consequently, a reference measurement of the grey level for the base material can be accurately obtained near the wall loss area, specifically in a position parallel to the pipe axis, such as above the wall loss region depicted in Figure B.1.
Figure B.1 — CR image of a 3” test pipe containing internal holes and showing a step wedge for calibration of the attenuation coefficient
The grey level profile along the pipe axis, illustrated in Figure B.1, indicates a consistently stable background level, which can be assessed using a reference area located on one side of the flaw.
Figure B.2 presents the same digital radiograph as Figure B.1, but features a grey level profile measured orthogonally across the pipe axis This profile reveals a significant curvature in the grey level corresponding to the base material, which is mainly attributed to the increased thickness penetrated as one moves away from the pipe's center line.
Figure B.2 — CR image of a 3” test pipe containing internal holes and showing a step wedge for calibration of the attenuation coefficient
In Figure B.2, the grey level profile taken in the across pipe direction shows a curved background level which cannot be measured adequately using a single reference area
The curvature in the underlying grey-scale profile, as illustrated in Figure B.2, affects the accuracy of measurements taken near the wall loss area, making them insufficient for determining the grey level of the base material at that specific location To obtain accurate grey level measurements at the indicated image location, advanced interpolation and extrapolation techniques are required.
To accurately estimate the grey level corresponding to the base material at a thickness change, any software method must evaluate the grey level profile shape between defined reference areas and the center of the thickness change Users should analyze this profile shape to understand its impact on the method's accuracy.
Note that a single reference area is insufficient to allow for the curved profile shown in Figure B.2, but may be sufficient for the almost constant profile shown in Figure B.1
It shall also be noted that similar issues apply when measuring the effective attenuation coefficient from the step wedge response (i.e a localized area of known increase in penetrated thickness)
Determination of basic spatial resolution
Linearized grey levels are essential for accurately measuring basic spatial resolution values, as they ensure that grey values correspond proportionally to radiation exposure at specific image locations This process is usually facilitated by the software provided by the manufacturer.
The duplex wire IQI must be placed directly on the surface of the detector or cassette, and its reading should follow the guidelines set by EN ISO 19232-5 to determine the basic spatial resolution (SR b) of the detector.
If the duplex wire IQI is placed on a test object rather than directly on the detector, the measurement obtained will reflect the basic spatial resolution of the image (SR b image) instead of the basic spatial resolution of the detector (SR b detector).
If the first unsharp wire pair cannot be recognised clearly (see EN ISO 19232-5), the 20 % dip method shall be applied as follows: