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Recommended Practice for the in-servi inspection of wall loss in pipes by comput radiograp
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HOIS(09)RP1 Issue 1
Authorisation Sheet Report Title:
Recommended Recommended Practice for the in-service inspection of wall loss in pipes by computed radiography
Customer Reference:
HOIS
Project Reference:
D7888219
Report Number:
HOIS(09)RP1
Issue:
Issue 1
Distribution List:
Open publication
Author:
S F Burch
12.12.09
Checked:
B A Stow
15.12.09
Address for correspondence Dr S F Burch ESR Technology Ltd 16 North Central 127 Milton Park Abingdon
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HOIS(09)RP1 Issue 1
Executive Summary
This recommended practice gives guidance for the in-service inspection of pipes for wall type flaws (e.g. corrosion pitting, generalised corrosion and erosion) using computed (dig radiography (CR). Weld inspection for typical welding process induced flaws is not covere but weld inspection is included for corrosion/erosion type flaws (e.g. weld root erosion/corrosion). The pipes may be insulated or not, and can be assessed where loss of material due, for example, to corrosion or erosion is suspected either internally or externally. This document covers the following inspection techniques: techniques:
1. Double-wall Double-wall single image image (DWSI) radiography radiography for the inspection of of discrete wall flaws by their effects on image grey level.
2. Double-wall Double-wall double-image double-image (DWDI) radiography radiography for the inspection inspection of discrete discrete wall flaws by their effects on image grey level.
3. Tangential inspection inspection techniques techniques for detection and through-wall through-wall sizing of wa including with the source on the pipe centre line, and offset from it by the pipe rad This technique is sometimes referred to as profile radiography, but this term is used in this document.
Note that DWDI is often combined with tangential radiography radiography with the source on the pipe centre line.
The recommendations recommendations cover the main radiation sources used for in-service inspection, i.e Iridium 192, Selenium 75, and for some specialised thick wall applications, Cobalt 60. Th use of portable X-ray sources is also included. Two different qualities of radiography are considered in this document:
A standard quality of quality of computed radiography radiography for wall loss inspection. This has less demanding quality requirements requirements than those defined for weld inspection, since in general w loss flaws are easier to detect radiographically radiographically than welding flaws which can Sign up to vote on this titleinclude crac
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A higher quality of quality of computed radiography radiography for wall loss inspection is also included in this document. This is for CR inspections requiring requiring higher quality (e.g. inspection of small pitti flaws).
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HOIS(09)RP1 Issue 1
Contents 1
INTRODUCTI INTRODUCTION ON ....................... .................................. ...................... ...................... ....................... ....................... .................. .......
2
REFERENCE REFERENCES S ...................... ................................. ...................... ...................... ...................... ...................... ...................... ...........
3
DEFINITIONS DEFINITIONS ...................... ................................. ....................... ....................... ...................... ....................... ....................... ...........
4
PERSONNEL PERSONNEL QUALIFICATI QUALIFICATIONS ONS ...................... ................................. ...................... ...................... ................ .....
5
GENERAL GENERAL ..................... ................................ ...................... ...................... ....................... ....................... ...................... ................... ........
6
5.1
Protection against ionising radiation.......................................................
5.2
Size and Strength Of Sources................................................................
5.3
Source Containers and Collimation............................... Collimation... ............................ .........................
5.4
In-Situ Inspection of Plant.......................................................................
5.5
Identification of Radiographs..................................................................
OVERVIEW OF INSPECTION TECHNIQUES .......................... ............ ............................ .............. 6.1
Double wall wall single image (DWSI) (DWSI) inspection technique ..........................
6.2
Double wall double double image (DWDI) inspection technique................. technique................. ........
6.3
Tangential inspection techniques .......................... ............................. 6.3.1 6.3.2
6.4
7
Source on pipe centre line .......................... ......................... Offset source position tangential radiography......................
Both Tangential and and Double wall techniques techniques combined combined ..........................
RADIATION RADIATION SOURCES SOURCES ...................... ................................. ....................... ....................... ...................... ................... ........ Sign up to vote on this title
7.1
Type of source....................................................................................... Useful Not useful
7.2
Source selection.................................................................................... 7.2.1
Double wall techniques
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HOIS(09)RP1 Issue 1
9
CIRCUMFERENTIAL AND AXIAL COVERAGE AND OVERLAP............ 9.1
10
Circumferential coverage........................................................................ 9.1.1
DWSI.....................................................................................
9.1.2
DWDI .............. ...................... ................ ............... ............... ............... ............... ............... ............... ............... .......
9.1.3
Penetrated Thickness measurement ........................... ........
9.2
Axial coverage coverage ............................. .............................. .............................
9.3
Overlap of images ............................ ............................. .........................
CR IMAGE QUALITY INDICATORS .......................... ............. ......................... ......................... .................. ..... 10.1
Background .......................... ............................. .............................. .......
10.2
Image Quality Measures for Double Wall Techniques Techniques (DWDI & DWSI).. 10.2.1
Target Grey level range .............................. .........................
10.2.2
Signal to Noise Ratio (SNR) ............................ ....................
10.2.3 10.2.4 10.2.5
BSR Measurement............................................................... Wire IQIs ......................... .............................. ....................... Image quality for tangential tangential techniques ......................... ......
11
SCREENS/FILTERS...................................................................................
12
EXPOSURE EXPOS URE TIME ...................... .................................. ....................... ...................... ....................... ....................... ................ .....
13
12.1
DWSI and DWDI....................................................................................
12.2
Tangential................................................................................................
PENETRATED THICKNESS MEASUREMENTS...................................... 13.1
Introduction..............................................................................................
13.2
Principle of method................................................................................
13.3
Effects of scattered radiation............................ radiation ............................ ............................. .........
13.4
Sign up to vote on this title Calibration using step wedge ........................... ............................ ..........
13.5
.............................. Key Points ............................... ...............................
13.6
Limitations ........................... ............................. .............................. ........
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HOIS(09)RP1 Issue 1 14.5.1 14.5.2
15
Interactive on-screen measurements...................................
Grey-level profile analysis methods ............................ .........
SCANNER PARAMETERS, PARAMETERS, IMAGE RECORDING AND PROCESSING 15.1
Scanner parameters...............................................................................
15.2
Image recording and storage..................................................................
15.3
Image processing ........................... ............................ ............................
15.4
Monitor viewing conditions ........................... .............................. ............
16
ACKNOWLEDGMENTS.............................................................................
17
REFERENCE REFERENCES S ...................... ................................. ...................... ...................... ...................... ...................... ...................... ...........
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HOIS(09)RP1 Issue 1
1
Introduction
The scope of this recommended recommended practice covers the in-service inspection of pipes for wa loss type flaws (e.g. corrosion pitting, generalised corrosion and erosion) using computed (digital) radiography radiography (CR). Weld inspection for typical welding process induced flaws is no covered, but weld inspection is included for corrosion/erosion type flaws (e.g. weld root erosion/corrosion). The pipes may be insulated or not, and can be assessed where loss of material due, for example, to corrosion or erosion is suspected either internally or externally. This document covers the following inspection techniques: techniques:
1. Double-wall Double-wall single image image (DWSI) radiography radiography for the inspection of of discrete wall flaws by their effects on image grey level.
2. Double-wall Double-wall double-image double-image (DWDI) radiography radiography for the inspection inspection of discrete discrete wall flaws by their effects on image grey level.
3. Tangential inspection inspection techniques techniques for detection and through-wall through-wall sizing of wa including (a) with the source on the pipe centre line, and (b) offset from it by the radius. This latter technique is sometimes referred to as profile radiography, but term is not used in this document.
Note that DWDI is often combined with tangential radiography radiography with the source on the pipe centre line.
The recommendations recommendations cover the main radiation sources used for in-service inspection, i.e Iridium 192, Selenium 75, and for some specialised thick wall applications, Cobalt 60. Th use of portable X-ray sources is also included. Two different qualities of radiography are considered in this document.
A standard quality of quality of computed radiography radiography for wall loss inspection. This has less demanding demanding quality requirements requirements than those defined for weld inspection, inspection, in EN 1435 for example, since, in general, wall loss flaws are easier to Sign detect than welding flaws. up to votetypical on this title tangential radiography, radiography, standard quality is sufficient when the wall loss approximatel Notisuseful Useful uniform, not isolated pitting. of
y for wall loss inspection is also specified in this
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HOIS(09)RP1 Issue 1
2
References
EN 444, 444, Non-destructive Non-destructive testing - General General principles principles for radiographic radiographic examination metallic materials using X-rays and gamma-rays. EN 473, 473, Non-destructive Non-destructive testing - Qualification and certification of NDT personnel personne General principles EN 584-1, 584-1, Non-destructive Non-destructive testing - Industrial radiographic radiographic film - Part 1: Classifica Classifica of film systems for industrial radiography radiography EN 462-1 462-1 to EN 462-5, 462-5, Non-destructive Non-destructive testing – Parts 1 to 5: Image quality of radiographs. EN 1435, 1435, Non-destructive examination of welds welds – Radiographic Radiographic examination examination of welded joints EN 14784-1, 14784-1, Non-destructive Non-destructive testing testing - Industrial computed radiography radiography with phos imaging plates - Part 1: Classification of systems EN 14784-2, 14784-2, Non-destructive Non-destructive testing testing - Industrial computed radiography radiography with stora phosphor imaging plates - Part 2: General principles for testing of metallic materia using X-rays and gamma rays E 1647-98a, 1647-98a, ASTM Standard Practice for Determining Determining Contrast Sensitivity in Radioscopy. ISO 3999-1, 3999-1, Radiation Radiation protection – Apparatus Apparatus for industrial industrial gamma radiography ISO 11699-1, 11699-1, Non-destructive Non-destructive testing testing - Industrial radiographic radiographic film - Part Part 1: Classification Classification of film systems for industrial radiography. radiography.
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HOIS(09)RP1 Issue 1
3
Definitions
Terms used in this document are as follows: b b’ CR d DWDI DWSI Ew E0 f technique) ID OD R r S SDD SPD T Ug Ug’ w wt wmax WT WT’
The distance between the source side of the pipe and the detector. The distance between the centre of the pipe and the detector. Computed radiography Source size for calculation of geometric unsharpness unsharpness Double wall double image radiographic technique Double wall single image radiographic technique The exposure time in sec for penetrated thickness w The exposure needed to achieve the required SNR_N value for zero penetrated thickness in units of Ci.min @ 500mm. Distance from source to relevant position in component (depends on
Pipe inside diameter Pipe outside diameter Distance from detector to pipe axis (centre line) Radius of pipe OD (=OD/2) Source strength Source to detector (originally film) distance Source to pipe axis distance (i.e. source to centre line of pipe) Exposure time Geometric unsharpness at detector Geometric unsharpness in plane of interest within component Penetrated thickness Total steel equivalent penetrated thickness, including an allowance for any product in the pipe Maximum penetrated thickness for a pipe which occurs for a tangent to surface Actual wall thickness Measured wall thickness Effective material attenuation coefficient Sign up to vote on this title
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HOIS(09)RP1 Issue 1
4
Personnel Qualifications
Personnel qualifications qualifications should be in accordance with EN 473 or ISO 9712. Training in computed radiography is also recommended, if available.
5
General
5.1
Protection against ionising radiation
WARNING – Exposure of any part of the human body to X-rays or gamma-rays can be h injurious to health. Wherever X-ray equipment or radioactive sources are in use, appropr legal requirements must be applied.
Local or national or international international safety precautions when using ionizing radiation shall be strictly applied.
5.2
Size and Strength Of Sources
The strengths (activities) of isotope sources used for CR need to comply with local regulations. Radiography Radiography contractors should state the maximum strength isotopes within their Local Rules as required by IRR 1999.
For in-service inspection applications, applications, a typical size of Ir 192 source is a 2 x 1mm disc (2 740Gbq), whereas for a similar strength Se 75 source, the size is often 2.5 mm (near spherical).
The effective source size for geometric sharpness calculations calculations should be used to calcula the required source to detector distance, as given in Section 8. Sign up to vote on this title
5.3
Source Containers and Collimation Useful Not useful
The source containers should conform to the requirements for source containers given by
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HOIS(09)RP1 Issue 1
The ratings of a source container must be checked for compliance with the type and stren of the isotope to be used.
The selection of an appropriate source container and deployment system depends on the balance of the above factors for each individual site and inspection application, application, together w economic considerations. considerations.
Careful collimation of the sources is recommended to minimise unwanted radiation, and t reduce the effects of scatter on the radiograph.
5.4
In-Situ Inspection of Plant
Use of gamma-ray radiography radiography equipment for in-situ inspection of plant involves significa safety issues associated with the use of ionising radiation. radiation. The appropriate mandatory sa regulations appropriate appropriate to the plant must be adhered to (IRR 1999 in the UK). These inclu the construction and maintenance of radiation controlled areas, by means of appropriate barriers.
Pre-planning Pre-planning of the inspection work to be carried out on a plant is required, to include bot risk assessment and a practical assessment of how the source container and shielding w be placed (IRR 1999).
5.5
Identification of Radiographs
Symbols shall be affixed to each section of the object being radiographed. radiographed. The images of these symbols shall appear on the CR image outside the region of interest where possibl and shall ensure unequivocal identification of the section.
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HOIS(09)RP1 Issue 1
6
Overview of inspection techniques
6.1
Double wall single image (DWSI) inspection technique
In the double wall single image inspection technique, illustrated in Figure 6.1, the radiography radiography is usually carried out with the source in contact with or close to the pipe wall. The detector is placed adjacent to the opposite pipe wall and wrapped around the pipe O
X or gamma-ray source
Pipe Corrosion
Image of wall loss on detector Detector
Figure 6.1
Double wall single image (DWSI) inspection technique for the inspec of wall loss in one wall of the pipe by image grey level variations.
External or internal wall loss pitting type flaws in the pipe wall adjacent to the detector are detected by the changes in image grey level they produce. Wall loss f laws in the opposite Sign up to vote on this title pipe wall nearer the source are either outside the radiation beam, or highly blurred and no generally detectable. detectabl e. Large scale loss of wall, due for example erosion, which produce Useful to Not useful near uniform loss of wall will not be readily detectable with this technique.
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X or gamma-ray source
Pipe Corrosion pits
Detector Detector
Figure 6.2
Images of wall loss on detector
Double wall double image DWDI inspection technique for detection wall loss in both walls of the pipe by image grey level variations.
This method decreases in effectiveness towards the edges of the pipe, since the sensitiv will be reduced as the amount of metal penetrated by the radiation beam increases. See Section 9 for more information on circumferential coverage.
With this method, the size of any pitting type wall loss flaws in the circumferential and axi directions can be measured directly from the CR image, provided methods are used for calibration of distances – see Section 14.3. Sign up to vote on this title
Useful Not useful On a single DWDI image, it is generally not possible to determine which side of the pipe t flaws are located on, and flaws from both sides are superimposed on the same image.
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HOIS(09)RP1 Issue 1
6.3
Tangential inspection techniques
6.3.1
Source on pipe centre line
The tangential inspection technique is used for the inspection of the portion of the pipe w running tangentially tangentially to the radiation beam, as illustrated in Figure 6.3 for a source positio the pipe centre line. The CR image then shows a direct image of the pipe wall.
X or gamma-ray source
Extended area of corrosion
Detector
Image of normal thickness pipe wall
Figure 6.3
Image of reduced thickness pipe wall
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Principle of tangential radiography with the source on the pipe centr Useful Not useful line
The through-wall extent of extended areas of either internal or external wall loss can be
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The tangential radiography method is recommended for the detection and through-wall si of extended areas of wall loss. For small isolated wall loss pits care should be taken to av underestimation underestimation of the maximum loss of wall caused by incorrect geometric alignment alignment (th pit needs to be as close as possible to the tangent position).
Small isolated pits can also be difficult to detect and size using the tangential technique, particularly if the maximum penetrated thickness at the tangent position is close to the lim recommended for the pipe and radiation source (see Section 7.2.2 for 7.2.2 for further information For this reason, use of the higher quality standard for tangential t angential radiography radiography is recommended for applications involving sizing of pitting flaws.
6.3.2
Offset source position tangential radiography
The tangential inspection technique with an offset source position is illustrated in Figure The offset is usually equal to the mean pipe radius, so that the centre of the X-ray beam passes through the tangential position on the pipe wall and is then incident at an angle approximately approximately perpendicular to the detector.
This technique is generally used for the inspection of insulated pipes, or larger diameter n insulated pipes.
X or gamma-ray source
Extended area of corrosion
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As with the tangential tangential technique on on the pipe centre centre line, with the offset offset source technique technique through-wall through-wall extent of extended areas of either internal or external wall loss can be measured directly from the CR images, provided appropriate calibration techniques are u to allow for the enlargement enlargement (“blow-up”) of the CR image – see Section 14.3 for further details.
The tangential (offset) method is often used to measure directly any wall loss identified b the DWDI method given in Section 6.2.
6.4
Both Tangential and Double wall techniques combined
For relatively thin-walled, small diameter pipes, a single CR image has sufficient dynamic range and size to show the presence of wall loss by both the tangential and double wall double image (DWDI) techniques, as illustrated in Figure 6.5.
X or gamma-ray source
Detector
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Figure 6.5
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Radiography combining both the tangential and double wall double image (DWDI) techniques in a single radiograph
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HOIS(09)RP1 Issue 1
7
Radiation Sources
7.1
Type of source
The majority of in-situ on-site radiography is carried out using gamma-ray emitting isotop sources, although portable, light-weight light-weight X-ray sources can also be used in cases where t need for electrical power and high voltages do not cause significant safety issues.
Iridium 192 is a commonly used isotope source for inspection of medium steel thicknesse The gamma-ray spectrum is complex, containing at least 24 spectral lines: those with rela intensities of at least 30% are at 296, 310, 320 and 470 keV. The half-life is 74 days. Iridi 192 is the most commonly used isotope source for in service inspection in the oil and gas industry. Source strengths are available up to 200 Curies or more with physical sizes ran from about 1x1 mm to about 4x4 mm. A typical source size used for pipe inspection is 2 x 1 mm.
The isotope source Selenium 75 has been developed for industrial radiography more recently than Ir 192. It has a lower energy gamma-ray spectrum than Ir192 with main pea at 137keV and 265keV and a longer half life (120 days). Source strengths are available between about 2 and 80 Curies with physical sizes ranging from 1x1 mm to 3x3 mm. Due the lower gamma-ray energies emitted by Selenium 75 compared with Iridium 192, Selenium 75 can give radiographs with higher contrast on components with moderate ste thickness. However, Selenium 75 is less commonly used for tangential radiography, due its reduced penetrating power, compared with Iridium 192.
Ytterbium 169 gives several different energies between 63 keV and 307 keV, with a half l of 32 days. Its mean effective energy is lower than both Selenium 75 and Iridium 192, an occasionally occasionally used for the in-service inspection of thin-walled components (penetrated ste thicknesses between between 1 and 15 mm, f or Test Class A film radiography, see EN 444 and E 1435).
Cobalt 60 is a high energy source (photon energies of 1.17 and 1.33 MeV), with an exten half-life of 5.3 years. This source is sometimes used for the in-service inspection of thick walled components, for which Iridium 192 has insufficient penetration (i.e. steel penetrate thicknesses greater than c. 85 mm). Because of its greater penetrating power, there are Sign up to vote on this title substantial additional additional safety requirements for in-service inspection using Cobalt 60, compared with Iridium 192. Useful Not useful
The highest energy sources used successfully for in-service CR are Betatrons, which are
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HOIS(09)RP1 Issue 1
7.2
Source selection
7.2.1
Double wall techniques
For the double wall techniques (DWSI & DWDI), the penetrated thickness for pipe inspec will be equal to twice the pipe wall thickness, on the centre line of the pipe, and will increa with distance away from the centre line. This increase in penetrated thickness will initially gradual, and then more rapid, as the edge of the pipe is approached, as illustrated for DW in Figure 7.1 below.
70
60 ) m50 m ( s s e n 40 k c i h t d 30 e t a r t e n e 20 P
10
0 -80
-60
-40
-20
0
20
40
60
8
Distance from pipe centre (mm)
Figure 7.1
Penetrated thickness across a typical pipe (OD 150mm, WT 7.1mm), assuming a very distant source.
Generally, the penetrated thickness thickness at the pipe centre (2WT) can be taken as thebasis fo Sign up to vote on this title source selection.
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Recommended lower and upper limits are given in using Table 7.1 for the different isotop sources, for the two different wall loss inspection classes (standard and higher quality).
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Table 7.1 Source selection for the double wall inspection techniques (DWDI & DW Radiation Source
Yb 169 Se 75 Ir 192 Co 60
Standard quality wall class Penetrated thickness, w (mm) 1 w 15 5 w 55 7 w 85 40 w 200
loss inspection
Higher quality wall loss inspection
Pipe WT
Penetrated thickness, w (mm) 1 w 15 10 w 40 20 w 85 40 w 200
(mm) 0.5 WT 7.5 2.5 WT 27 3.5 WT 42 20 WT 100
Pipe W
(mm) 0.5 WT 5 WT 10 WT 20 WT
For very high penetrated thicknesses, Betatron sources have proved effective with CR plates, although as with Co 60, there are additional safety requirements for in-service inspection. For the higher quality wall loss inspection class, for X-rays up to 500 kV, Figure 1 of EN 14784-2 (identical to Figure 20 of EN1435 :1997) should be used.
For product filled pipes, the additional radiation attenuation caused by the product should allowed for in selection of sources. For a fully product filled pipe, the penetrated thicknes in Table 7.1 should be increased by approximately ID/9 for water, as measured by [1 [ 1]. Fo oil, the factor is likely to be larger (estimated as 11 on the basis of relative densities of wa and oil with 0.8 gm/cm3) but no measured values are available.
It is also important to note that the presence of product may increase the scattered radia levels in the CR images. Additional thickness of detector screens may then be required to produce acceptable quality images (see Section 11 for further details).
7.2.2
Tangential technique
For a pipe with wall thickness WT and outside diameter OD, the maximum penetrated thickness, wmax, through the pipe wall occurs for a line forming a tangent with the inner diameter. This maximum path is shown in Figure 7.2 and is given by Sign up to vote on this title
w max 2 WTOD - WT
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(7.1)
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HOIS(09)RP1 Issue 1
wmax
Dete
Source
Figure 7.2
Maximum penetrated thickness, wmax, for the tangential technique
Note that this applies to any line drawn through the pipe, forming a tangent to the inner surface of the pipe. Thus w max is independent of the source position.
Values for the maximum penetrated thickness, w max, through pipes of various diameters a schedules are given in Figure 7.3 and Table 7.2, for ease of reference. Note that these penetrated thickness values are generally much larger than twice the wall thickness of pipe.
Also shown on on Figure 7.3 are the approximate approximate maximum penetrated penetrated thicknesses for for Se7 192 and Co 60, recommended recommended for CR inspection using the standard quality tangential technique [2 [2]. These values are given in Table 7.2, which also gives recommended reduced values for higher quality tangential inspection (for sizing of pitting flaws, which are more difficult to detect and size than generalised wall loss). Sign up to vote on this title
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HOIS(09)RP1 Issue 1
Tangential path lengths 200
Schedule 40 Schedule 80 Schedule 160 Limit for Se 75 Limit for Ir 192 Limit for Co 60
180 160 140
) m m120 ( h t a p l 100 a i t n e g 80 n a T
Co 60
Ir 192
60 40
Se 75
20 0 0
2
4
6
8
10
Pipe nominal bore (inch)
Figure 7.3
Maximum (tangential) path lengths through the walls of pipe of differ diameter. The maximum recommended penetrated thicknesses for different isotope sources are also shown.
Table 7.2 Maximum tangential tangential paths in steel steel for different isotope isotope sources Isotope Source
Se 75 Ir 192 Co 60
Maximum tangential path (mm) Standard quality Higher quality (for generalised wall loss) (for pitting flaws) c. 55 c. 40 c. 85 c. 60 Sign up to vote on this title c. 140 c. 100
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Table 7.3 shows which pipes can be inspected using the standard and higher quality
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HOIS(09)RP1 Issue 1
Table 7.3 Maximum paths through through different schedule pipes pipes of various diameters, diameters, toget with applicable isotope sources for tangential CR.
Nominal Bore
Outside diameter, OD
(inches)
(mm)
1
33.4
1.5
48.3
2
60.3
3
88.9
4
114.3
5
141.3
6
168.3
8
219.1
Schedule
Nominal wall thickness, 1 WT (mm)
Nominal maximum Tangential 1 path (mm)
Isotope applicability applicability
Se 75 40 80 160 XXS 40 80 160 XXS 40 80 160 XXS 40 80 160 XXS 40 80 120 160 XXS 40 80 160 XXS 40 80 120 160 XXS 40 80 120 XXS 160
3.4 4.5 6.4 9.1 3.7 5.1 7.1 10.2 3.9 5.5 8.7 11.1 5.5 7.6 11.1 15.2 6.0 8.6 11.1 13.5 17.1 6.6 9.5 15.9 19.0 7.1 11.0 14.3 18.3 21.9 8.2 12.7 18.3 22.6 23.0
Ir 192
20.2 22.8 26.3 29.7 25.7 29.7 34.2 39.4 29.7 34.7 42.5 46.7 42.8 49.7 X 58.8 66.9 X 51.0 60.3 X X 67.7 X 73.8 81.5 X X 59.6 70.8 X X X 89.3 X X 96.4 67.7 X X 83.2 X X 93.9 X X 104.8 X X 113.2 83.2 X Sign up to vote on this title 102.5 X X Useful Not useful X X 121.2 X X 133.3 X X 134.3
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key to isotope applicability: applicability: Both standard and high quality Only standard quality Neither
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HOIS(09)RP1 Issue 1
8
Source to detector distances distances (SDD)
This section contains recommended minimum minimum source to detector (SDD) distances for the different techniques. If two techniques are combined (e.g. DWDI and tangential) then the larger of the two recommended distances should be used. The terminology follows that used in EN1435 (see Section 3).
8.1
Double wall single image (DWSI) inspection
In the DWSI technique for in-service inspection, inspection, the source is conventionally positioned positioned c to, but outside one wall of the pipe, and the detector is wrapped around the opposite pipe wall, as shown in Figure 8.1.
The distances involved for SDD determination for the DWSI technique are shown in Figu 8.1. The object plane is the source side of the pipe wall nearer the detector.
SDD
b
f
Detector
Source, size d
Figure 8.1 Distances for DWSI DWSI
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The recommended SDD given in EN1435 for basic class A inspection is
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For larger diameter pipes, formula (8.2) allows the source to be placed adjacent to the pip wall opposite the detector (as is conventional conventional practice for DWSI).
This may not be true for pipes with diameters of 4" or less, or those covered by insulation that the detector could not be placed in close contact with the pipe wall. The SDD should then be calculated according to formula (8.2) above, and the source positioned according
For pipes with diameters of less than about 3" to 4", the DWDI technique (see below) ma preferable in some cases as greater axial coverage can be achieved in a single exposure
8.2
Double wall double image (DWDI) inspection
For the DWDI technique, the SDD is increased compared with DWSI inspection, allowing inspection of both pipe walls, as illustrated in Figure 8.2, but with a significant increase in exposure time. The detector is then usually flat, and not wrapped around the pipe wall. In case, the relevant object plane is the external surface of the pipe closest to the source.
SDD
f
Source, size d
b
Detector
Figure 8.2 Distances for DWDI DWDI
In general, a large source to t o detector distance (SDD) will minimise the unsharpness in th radiograph caused by the size of radiation source (known as geometric unsharpness). unsharpness). However, large source to detector distances can lead toSign very exposure uplong to vote on this titletimes, and increased shielding difficulties. difficulties. Thus trade-offs must bemade, ensuring Usefulwhilst useful acceptable Not image quality.
There is no universally accepted method for the choice of the source to t o detector distance
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In [4 [4], it was shown that t he source to detector distance, SDD, needed to achieve a spec geometric unsharpness in the plane of the object (Ug') object (Ug') is given by: SDD
= (d . b) / Ug'
(8.3)
Where d is the effective source size for geometric unsharpne unsharpness ss calculations and b is the distance between the detector and the source side of the external diameter of the pipe (O
For small bore connector inspection, [4 [ 4] recommended a value of Ug' of 0.3mm, and it recommended recommend ed that this value is used for the higher quality quality of wall loss inspection as defi in the present document.
Thus, for higher quality wall quality wall loss inspection, the following equation should should be used for f or S SDDmin
= (d . b)/0.3
(8.4)
For the standard quality wall quality wall loss inspection, a higher unsharpness value of 0.6 mm is acceptable, i.e. SDDmin
= (d . b)/0.6
(8.5)
Where b is the distance between the source side of the pipe and the detector.
The recommended SDD’s derived using equations (8.4) and (8.5) are shown in Figure 8.3 for an assumed source size of 2.3 mm. Tabulated values are given in Table 8.1.
These distances will be increased for larger source sizes, and reduced for smaller source sizes, as given by equations (8.4) and (8.5).
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HOIS(09)RP1 Issue 1
Approximate pipe nominal bore for detector in contact with pipe 1
2000
2
3
4
5
6
8
1800 Standard Quality
1600
Higher Quality
1400
1200
) m m ( 1000 D D S
800
600
400
200
0 0
20
40
60
80
100
120
140
160
180
200
2
Distance from Detector to Source side of pipe OD, b (mm)
Figure 8.3
Recommended Recommended Source to Detector distances (SDD) for DWDI, for an assumed effective source size of 2.3mm.
Table 8.1 Recommended Recommended source to detector distances (SDD) (SDD) for DWDI, for an assumed effective source source size of 2.3mm and for the detector in contact with the p wall. wall.
b
mm 33.4 60.3 88.9 114.3
SDD mm
Pipe Nominal Bore (if detector in contact with Standard Higher pipe wall) Quality Quality in Sign128 up to vote on this 1 256title 2 3 4
Useful 231 341 438
useful Not462 682 876
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HOIS(09)RP1 Issue 1
8.3 8.3.1
Tangential Inspection Source on pipe centre line
For tangential radiography with the source on the pipe centre line, the dimensions and distances are shown in Figure 8.4.
SDD
SPD
b'
Film/Detecto Film/Detect Source, size d
Figure 8.4 Dimensions and distances distances for tangential radiography radiography (source on pipe centre line) For tangential radiography, there are two main factors which affect the accuracy of wall thickness measurements and hence the recommended SDD’s.
Dimensions Dimensions measured measured in the CR image are are progressively progressively distorted away from the source axis due to the finite source to detector distance. This has differing effects the accuracy of the various methods used for calibration of dimensions in the CR images. Sign up to vote on this title
Geometric unsharpness.
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Consideration Consideration of the accuracy of dimensional measurements measurements is given below. It is likely th
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HOIS(09)RP1 Issue 1
beam” geometry calculations assume assume that the radiation beam diverges from a point sourc a thin, two-dimensional fan beam, as shown in Figure 14.3, and that the actual wall thickn WT is related to the measured value WT’ by equation (14.2).
In reality, for all usual forms of radiography, radiography, the radiation beam diverges as a threedimensional dimensional cone from the source. However, if the line joining the source to the centre of detector is perpendicular to the detector plane, then the fan-beam geometry applies to an line drawn through the centre point of the CR image.
For the purposes of the calculations shown in Figure 8.5, the 3-D cone beam complicatio ignored, and it is assumed that the “fan-beam” geometry calculation method gives no erro in the derived wall thicknesses (provided the distances are measured accurately). For the other calibration methods, the wall thickness measurement accuracy decreases with decreasing SDD. 10
9 Centre line Magnification
8
Pipe Diameter measurement Ball bearing comparator
) 7 % ( t n e m6 e r u s a 5 e m T W n 4 i r o r r E 3
Fan-beam geometry calculations
2
1
0 0
1
2
3
4
5
6
7
8
9
SPD / OD
Figure 8.5
Calculations of effects of different dimensional calibration Sign up to vote on this title technique accuracy of wall thickness measurements using the tangential Useful Not useful technique.
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It should also be verified that the SDD is not smaller than the values given below. These based on geometric unsharpness considerations considerations similar to those used for DWDI (see Se 8.2), 8.2), but adapted for the different plane of interest on the pipe (tangent position instead o source side of the pipe): Standard quality: SDDmin
= (d . b’)/0.6
(8.7)
Higher quality: SDDmin
= (d . b’)/0.3
(8.8)
Where b’ is the distance from the pipe centre to the film/detector.
8.3.2
Combined tangential/DWDI image radiography
For smaller diameter pipes (< 4" - 6" diameter) , the double-wall double image (DWDI) technique is often combined with the tangential technique with the source on the pipe cen line.
The SDD’s for this combined technique should be those for DWDI, i.e. as given in equatio (8.4) and (8.5), unless the value given by equation (8.6) is larger, in which case that shou be used where practicable.
8.3.3
Source offset from pipe centre line
For tangential radiography radiography with the source offset from the pipe centre line, the effects of t different dimensional calibration techniques techniques on measured wall thickness accuracy are mu less significant than with the source on the pipe centre line. It is therefore appropriate to use recommended SDD’s based solely on an unsharpness criterion. In this case, the plane of the object of interest is the pipe tangent position, as illustrated in Figure 8.6 below. Sign up to vote on this title
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SDD
b’
SPD
Source, size d
Detector
Figure 8.6 Distances for offset offset source tangential tangential radiography The recommended SDD is then given by equations (8.7) and (8.8) for the higher and standard qualities of inspection.
These recommended SDD values for offset tangential radiography are plotted in Figure 8 Approximate Approximate pipe nominal bore for detector in contact with pipe (in) 1200
1
2
3
4
5
6
8
1000
800
) m m ( 600 D D S 400
Standard Quality Higher Quality
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9
Circumferential and axial coverage and overla
9.1
Circumferential coverage
When using the DWDI and DWSI techniques, then full circumferential coverage of a pipe achieved by taking a number of different exposures around the pipe circumference.
9.1.1
DWSI
For DWSI, the approach used in EN 1435 : 1997, is for the number of circumferential exposures to be calculated on the basis of a maximum permissible increase in penetrate thickness due to inclined penetration at the edges of the diagnostic area. This increase in penetrated thickness is a function of:
SDD Wall thickness, WT Pipe OD
Following EN1435 EN1435 class A for DWSI, with the standard and higher higher quality classes of CR loss inspection, it is recommended that for wall loss CR, the maximum permissible increa in penetrated thickness should be 20%. The minimum number of exposures is then give Figure A.4 of EN 1435 :1997 (page 17). For the source positioned outside the pipe, Figure 9.1 shows the number of exposures needed, as a function of two dimensionless variables variables – WT/OD and OD/SDD. Minimum number of exposures DWSI 1.05 1 0.95
3
0.9 0.85 0.8
4
0.75
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0.7 0.65 0.6 D D0.55 S / D 0.5 O
5
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If the detector is offset from f rom the pipe wall due for example to the presence of insulation, t Figure 9.1 is not applicable, applicable, and the values given in Figure 9.2 should be used instead. In Figure 9.2 note that the vertical axis is the pipe OD divided by the distance from the sour to the pipe axis (SPD). To obtain the circumferential angular difference (in degrees) between exposures, the following formula should be used for DWSI: Angular difference difference
= 360 / (Number (Number of exposures)
Minimum number of exposures DWSI 2.1
3
2 1.9 1.8 1.7
4
1.6 1.5 1.4 1.3
5
1.2 D P 1.1 S / D 1 O 0.9
6
0.8 0.7
7
0.6 0.5
8
0.4 0.3
9
0.2 0.1 0 0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0.22
0.24
0.26
0.28
0.3
WT/OD
Figure 9.2
Minimum number of DWSI exposures circumferentially circumferentially around a pipe a function of the ratios WT/OD and OD/SPD, where SPD is the distan from the source to the pipe axis (centre). This figure should be used instead of Figure 9.1 if the detector is not in contact with the pipe wa Sign up to vote on this title (e.g. due to the presence of insulation).
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For DWDI (see Section 9.1.2), 9.1.2), recent trials [5] have shown that the maximum permissible increase in penetrated thickness can be greater than 20%, without significant loss of
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pitting flaws at or very close to the tangent position was generally reduced compared with that obtained when the flaws were on the pipe centre line, especially with pipes having lar maximum penetrated thickness values, w max. However rotation rotation of the source/detector source/detector the flaws were 15-20º from the tangent position then gave detectability similar to that achieved with the flaw on the pipe centre line.
In general, at least two exposures are recommended for DWDI, separated in circumferen angle by at least 30-40º (for best results the angle would be 90º).
However, for those pipes having maximum tangential paths, w max, less than about 50% o maximum recommended values for the standard tangential image quality (see Table 7.2) adequate sensitivity may be achieved using a single exposure.
9.1.3
Penetrated Thickness measurement
The number of circumferential exposures derived from Figures 9.1 – 9.3 can be used for general DWDI and DWSI techniques.
However, if quantitative analysis of the image grey level information is being used to estim penetrated thickness (Section 12), it is important to ensure that the feature being measur on the CR image is as close as possible to the centre of CR image. This is likely to requir additional circumferential circumferential exposures to align the flaw of interest more closely with the sou axis.
9.2
Axial coverage
The maximum axial coverage for a single CR image can be determined in a similar mann to that for DWSI circumferential coverage (see Section 9.1.1). 9.1.1). For axial distances, the geometry is then simpler, as illustrated in Figure 9.4. For a 20% increase in penetrated thickness at the edge of the area to be inspected, then it can be shown the total extent, L this area on the detector is Ld= 1.32 SDD The corresponding axial coverage on the source side ofSign theup pipe, L , is then: to votepon this title Lp = 1.32 f
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where f is the distance between the source and the source side of the pipe. Lp should be
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SDD
Lp
f Ld
Detector Source, size d
Figure 9.4
9.3
Axial cross section for DWDI inspection (also applicable to DWSI), showing the maximum permissible axial length, Ld, of the evaluated a for a single source position, on the detector and along the pipe, Lp
Overlap of images
The separate CR images shall overlap sufficiently to ensure that no portion of the compo remains un-examined. Unless otherwise specified, the minimum overlap shall be 25 mm axially either side of the diagnostic area, measured on the source side.
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HOIS(09)RP1 Issue 1
10
CR image quality indicators
10.1
Background
Experience has shown that some form of objective check on image quality is important fo service computed radiography. radiography. Operators should be aware that very short exposure time with computed radiography radiography can give CR images which may appear to the eye, subjective least, to have adequate quality, when displayed on a computer monitor. However, quantitative analysis can show that such images may have low signal to noise ratios, and hence poor sensitivity for wall loss detection by image grey level variations.
It is strongly recommended that some form of image quality indicator, or objective check on image quality is used on all CR exposures, as described below.
10.2 10.2.1
Image Quality Measures for Double Wall Techniques (DWD DWSI) Target Grey level range
For the standard quality class quality class of wall loss inspection CR, a minimum check on CR imag quality is to ensure that a specified (target) grey level range is achieved in the area of inte of the CR image. The grey level range achieved for a particular radiation exposure also depends on the IP type and the scanner gain (or sensitivity) used, so it is also necessary specify the IP type and all user selectable scanner parameters when setting a target grey level range (pixel size, gain/sensitivity gain/sensitivity etc).
In setting a target grey level range for a particular CR scanner, IP type and scanner gain, measurements of image normalised signal to noise ratio (see Section 10.2.2 below) shou first be made for a range of exposures. The results should be plotted plotted as a function of gre level, as illustrated in Figure 10.1. This allows the grey level corresponding to a normalise signal to noise ratio of 50 to be determined, determined, for a specified scanner/IP combination and th specified scanner scanner user settings including including gain/sensitivity and pixel pixel size.
Sign>up50 to vote title In the example shown in Figure 10.1, for low gain, SNR_N can on bethis achieved for grey levels >3700, whereas for medium gain grey levels must >24000. These target grey le Useful useful be Not ranges are specific to the scanner, the IP and all the scanner settings and the CR analys software. If changes are made to t o any of these variables, a repeat calibration is required t
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HOIS(09)RP1 Issue 1
80
70
CR50P Low gain 60
50
CR50P Medium gain
N _ R40 N S
30
20
10
0 0
5000
10000
15000
20000
25000
30000
35000
40000
45000
Grey Level
Figure 10.1 10.1
Measurements of normalised normalised signal signal to noise ratio ratio as a function function of image grey level, for two different gain/sensitivity settings with Ir 192 and a specific CR scanner/IP and pixel size.
For the higher quality class of wall loss inspection CR, it is required that one of t he more rigorous measures of image quality described below is also applied to ensure the required image quality has been achieved.
10.2.2
Signal to Noise Ratio (SNR)
For the standard and higher quality classes, quality classes, it is recommended that CR image quality i assessed by measuring the normalised signal to noise ratio (SNR_N) in the area of intere using appropriate software if available.
Note that it is important when making SNR measurements for the images to have grey le Sign up to vote on this title directly proportional to radiation intensity (i.e. linear response). For non-linear response C Useful(LUT), scanners it is necessary to select an appropriate look-up to useful achieve a linear table Not relation between radiation radiation intensity and CR image grey level. Different CR scanners can have different characteristics (e.g. logarithmic or square-root, as well as linear), and the
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In all cases, SNR_N measurements should be made at several locations (minimum of fou within the area of interest, and a mean value derived.
Note that for some components it may not be possible to find a homogeneous homogeneous area of the image suitable for SNR_N measurements. An example would be a pipe containing generalised irregular irregular corrosion over the whole area of the image. In such cases, the targe grey level method described in the Section above should be used instead.
The SNR measured on a CR image needs to be normalised using a factor which depend the basic spatial resolution (BSR) of the CR system (see EN 14784-1, equations 2 and 3 The formula to be applied to calculate the normalised signal normalised signal to noise ratio is: SNR_N = SNRmeas (88.6/BSR)
(10.1)
Where SNRnorm SNRmeas BSR 88.6
is the normalised signal to noise ratio is the signal to noise ratio measured on the CR image is the basic spatial resolution of the CR system in microns (depends scanner pixel size and the model of CR plate) is the length of the side of a square having the same area as a circle diameter 100 microns (see EN14784-1, p11 for explanation)
For double wall techniques (DWSI and DWDI), and the standard quality class, the normalised signal-to-noise ratio (SNR_N) as calculated from equation (10.1) in the centre should be at least 50. For double wall techniques (DWSI and DWDI), and the higher quality class, the normalised signal-to-noise ratio (SNR_N) as calculated from equation (10.1) in the centre should be at least 80.
10.2.3
BSR Measurement
The basic spatial resolution (BSR) can be measured according to EN 14784-1, which involves use of orthogonal duplex wire IQIs. An improved BSR Sign up tomeasurement vote on this title technique f isotope sources, to minimise the effects of noise, is described method involve Not useful Usefulin [5]. This the analysis of the responses obtained with a Duplex wire IQI, as shown in Figure 10.2.
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Figure 10.2 10.2
CR image image of the Duplex Duplex wire IQI, rotated so that the wires are as as accurately vertical as possible.
The modulation of the different wire pairs can then be plotted as a function of wire diame The variation of modulation with wire diameter is linear on a log – log plot, which allows ready derivation of the wire diameter for which the modulation is 0.2 (or 20%), as illustrat in Figure 10.3. Derivation Derivation of BSR based on measured wire pair modulations 1
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The advantages of this more quantitative method for BSR determination include improve repeatability, better noise tolerance, less subjectivity, and determination of values to a precision of better than the spacing between available wire pair diameters.
In some cases, the measured BSR values vary with direction in the CR image. In this cas is important to ensure consistency between the directions of the SNR_N and BSR measurements. measurements. For the default SNR measurement direction of horizontal within a CR ima (along the fast scan direction), the wires themselves within the duplex wire IQI should be aligned orthogonal orthogonal to this t his direction (i.e. vertical in the displayed image). This is achieved when the long axis of the duplex wire IQI assembly is horizontal in the image, i.e. aligned with the SNR measurement direction.
Measured values for BSR for some current CR systems are given in Table 10.1, taken fro [5]. Note that the values depend on the radiation source as well as the imaging plate, pixe size and scanner model.
If a value for the t he BSR for the screen/scanner combination in use is not available, a conservative estimate of 200 microns can be used, provided the screen resolution is kno to be similar to those screens given in Table 10.1, and the scanner pixel size does not exceed 100 microns.
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Table 10.1 Basic spatial resolution (BSR) of some CR scanners and imaging plates measured using Duplex wire IQI
CR System Radiation source Scanner
HD CR 35 NDT HD CR 35 NDT HD CR 35 NDT HD CR 35 NDT HD CR 35 NDT HD CR 35 NDT HD CR 35 NDT HD CR 43 NDT HD CR 43 NDT Fuji system DynamiX Fuji system DynamiX GE CR50P GE CR50P GE CR50XP GE CR50XP GE CR50XP
Basic spatial resolutio BSR* (microns)
IP
Pixel size (m)
According to EN14784-1
More quantitat metho [see ref
White, pre April 2008 White, pre April 2008 White, post April 2008 White, post April 2008 White, post April 2008 White, post April 2008 Blue
100
Iridium 192
160
100
Selenium 75
160
100
Iridium 192
145
163
50
Iridium 192
115
133
100
Selenium 192
145
50
Selenium 192
100
50
Iridium 192
100
White, post April 2008 Blue
100
Iridium 192
145
50
Iridium 192
90
ST-VI
100
Iridium 192
160
ST-VI
100
Selenium 75
160
IPC2 IPS IPC2 IPS IPS
100 50 100 50 50
Iridium 192 200 Iridium 192 130 Sign up to vote on this title Iridium 192 Not useful Useful 200 Iridium 192 145 Selenium 75 130
176
236
165
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HOIS(09)RP1 Issue 1
10.2.4
Wire IQIs
Use of physical IQIs (wire or step/hole) is recommended for CR inspection for wall loss to provide a quantifiable measure of image quality. In general, the minimum IQI wire diamet visible on a CR image will be limited by the total unsharpness of the image, which is mad from contributions from the screen unsharpness and geometric unsharpness. The CR im signal to noise ratio and radiographic contrast will also affect the minimum visible IQI wire diameter.
For DWDI, the IQIs should be placed on the source side of the pipe, where possible. If th pipe is insulated, the IQI will need to be placed on the detector side. For DWSI, the IQIs n to be placed on the detector side (between pipe wall and detector).
For both DWDI and DWSI, the IQIs should be positioned close close to the centre of the result CR image. If the IQIs are close to the edges of the images, a smaller number of wires ma be detected than for centrally placed IQIs.
For DWDI inspection using Ir 192, source-side measured measured and target values for IQI wire numbers (as defined in EN 462-1) are shown in Figure 10.4. These measurements cover pipes with wall thicknesses in the range c. 3 mm to 22 mm. Separate values are shown f the standard and higher image quality classes (SNR_N ≥ 50 and ≥ 80 on the pipe centre respectively). For product filled pipes, the total equivalent equivalent steel penetrated thickness sho be calculated using: wt = ws + ID/ f
(10.2)
where ws is the steel penetrated thickness, ID is the pipe internal diameter and f is a facto representing the lower attenuation of the product compared with steel. Estimates for f are for water and 11 for oil (no measured value is available for oil). DWDI - Ir 192
15 14 13 12 11 W e u l a 10 v I Q I 9
DWDI Bergen Oct 08 IPC2/CR100 DWDI IPC2/CR50P NDT Services DWDI Bergen Feb 08 IPC2/CRX Tower DWDI Bergen May 09 ST6/Fuji DWDI Water filled Bergen Feb 08 IPC2/CRX Tower DWDI Water filled Bergen Oct 08 IPC2/CR100 DWDI Water Filled Bergen May 09 ST6/Fuji & HD CR 35/White DWDI Ir 192 Target values DWDI HIGHER Bergen Feb 08 IPS/CRX Tower DWDI HIGH Bergen Oct 08 IPS/CR50XP DWDI HIGHER Bergen May 09 HD CR 35 White IP DWDI Water filled HIGHER Bergen Feb 08 IPS/CRX Tower DWDI Water filled HIGH Bergen Oct 08 IPS/CR50XP DWDI Ir 192 High Target values EN1435 Class A (DWDI)
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Corresponding Corresponding measured IQI values and target values for DWSI inspection using Iridium are given in Figure 10.5. DWSI - Ir 192
16
DWSI Bergen Oct 08 IPC2/CR100 DWSI IPC2/CR50P NDT Services DWSI Bergen Feb 08 IPC2/CRX Tower DWSI Bergen May 09 ST6/Fuji DWSI Water filled Bergen Feb 08 IPC2/CRX Tower DWSI Water Filled Bergen May 09 HD CR 35/White DWSI Ir 192 Target values
15 14
DWSI Water filled Bergen Oct 08 IPC2/CR100 DWSI HIGHER Bergen Feb 08 IPS/CRX Tower DWSI HIGHER Bergen Oct 08 IPS/CR50XP DWSI HIGHER Bergen May 09 HD CR 35/White IP DWSI Water filled HIGHER Bergen Feb 08 IPS/CRX Towe DWSI Water filled HIGHER Bergen Oct 08 IPS/CR50XP DWSI HIGHER Water Filled Bergen May 09 HD CR 35/Wh
13 12 W e 11 u l a v I 10 Q I
DWSI Ir 192 High Target values EN1435 Class A (DWSI)
9 8 7 6 5 0
10
20
30
40
50
Equivalent steel total penetrated thickness (mm)
Figure 10.5 10.5
Measurements of smallest smallest IQI wires visible visible on DWSI Ir192 Ir192 CR images a function of equivalent steel total penetrated thickness (on pipe cen line). The values from EN1435 for film radiography weld inspection ar shown for comparison purposes.
With Selenium 75, increased radiographic contrast is generally obtainable compared with Iridium 192, within the range of applicability applicability of the source. This results in higher IQI value with Selenium 75 for images having the same SNR_N values. The available measureme for Selenium 75 DWDI and DWSI inspections are given in Figures 10.6 and 10.7 respectively. DWDI : Se 75
15
14
13
12 W11 e u l a v I 10 Q I
DWDI SELENIUM Bergen Feb 08 IPC2/CRX Tower DWDI Bergen Oct 08 IPC2/CR100 DWDI Bergen May 09 HD CR 35/White DWDI Water Filled Bergen Oct 08 IPC2/CR100 DWDI Water Filled Bergen May 09 HD CR 35/White DWDI Ir 192 Target values DWDI HIGHER Bergen Oct 08 IPS/CR50XP DWDI HIGHER Bergen May 09 HD CR 35/White DWDI Ir 192 High Target values EN1435 Class A (DWDI)
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DWSI : Se 75
15
DWSI SELENIUM Bergen Feb 08 IPC2/CRX Tower DWSI Bergen Oct 08 IPC2/CR100 DWSI Water Filled Bergen Oct 08 IPC2/CR100 DWSI Bergen May 09 HD CR 35/White DWSI Water Filled Bergen May 09 HD CR 35/White DWSI Se 75 Target values DWSI SELENIUM HIGHER Bergen Feb 08 IPS/CRX Tower DWSI HIGHER Bergen Oct 08 IPS/CR50XP DWSI HIGHER Bergen May 09 HD CR 35/White IP DWSI Higher Water Filled Bergen Oct 08 IPC2/CR100 DWSI HIGHER Water Filled Bergen May 09 HD CR 35/White IP DWSI Se75 High Target values EN1435 Class A (DWSI)
14
13
12 W11 e u l a v I 10 Q I 9
8
7
6 0
10
20
30
40
50
60
Equivalent steel total penetrated thickness (mm)
Figure 10.7 10.7
Measurements of smallest smallest IQI wires visible visible on DWSI Se 75 CR image a function of equivalent steel total penetrated thickness (on pipe cen line).
The tables below summarise the target IQI values derived from the experimental CR ima collected during the HOIS trials, as given in Figures 10.4 to 10.7 above. Note that the thickness t is the equivalent steel total penetrated thickness on the pipe centre line, w t given by equation 10.2. DWDI Iridium 192 (source side IQIs) Standard Image quality Thickness IQI value, W range (mm) 9 5 wt < 15 8 15 wt < 25 7 25 wt < 40 6 40 wt < 60
Higher Image quality Thickness IQI value, W range (mm) 12 5 wt < 8 11 8 wt < 12 10 12 wt < 15 Sign 9 up to vote on this title 15 wt < 20
DWSI Iridium 192 (detector side IQIs)
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DWDI Selenium 75 (source side IQIs) Standard Image quality Thickness IQI value, W range (mm) 11 5 wt < 12 10 12 wt < 18 9 18 wt < 25 8 25 wt < 35
Higher Image quality Thickness IQI value, W range (mm) 12 5 wt < 10 11 10 wt < 20
DWSI Selenium 75 (detector side IQIs) Standard Image quality Thickness IQI value, W range (mm) 11 5 wt < 15 10 15 wt < 25 9 25 wt < 30 8 60 wt < 60
10.2.5
Higher Image quality Thickness IQI value, W range (mm) 11 5 wt < 25 10 25 wt < 40 9 40 wt < 45
Image quality for tangential techniques
For tangential radiography, conventional wire or step/hole IQIs are not directly applicable because they cannot be positioned near to the tangential pipe position, and the rapid changes in penetrated thickness in this part of a radiographic image makes it impossible assess IQI visibilities in any meaningful way.
However, very noisy CR images will give lower wall t hickness measurement measurement accuracies t less noisy CR images. In addition, the unsharpness of the CR image will influence the WT measurement measurement accuracy.
Hence, some form of quality control for tangential CR radiography is considered necessa Sign up to vote on this title as follows.
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Ensure that the SDD’s recommended recommended in Section 8.3 are used.
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If the pipe centre line is available for measurement of SNR_N, then the DWDI SN values of 50 and 80 for standard and higher qualities respectively can be used a alternative to the above free beam SNR_N values.
Note that in all cases when measuring SNR_N, it is important that the image is in a form having the image grey levels directly proportional to radiation intensity, otherwise the valu can be misleadingly high.
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11
Screens/filters
The filmless plates used in CR are more sensitive than conventional film to scattered radiation, which generally generally has a lower average photon energy than that of the primary radiation beam. It is ttherefore herefore important to reduce scattered radiation by means of lead screens.
The recommended values given in EN 14784-2 are reproduced for convenience in Table 11.1. These screen thickness values are generally greater than those recommende for film radiography and are thicker than standard.
Thinner screens with standard thickness can be used provided the specified image qualit values are achieved (see Section 10).
Table 11.1 Recommended metal metal screens for computed radiography radiography taken from Table 2 of EN 14784-2. Thinner (standard) screens can be used provided the speci image quality values are achieved (see Section 10). Radiation Source
Yb 169, Tm 170 Ir 192, Se 75 Co 60
Penetrated WT, w mm w<5 w>5 w < 50 w > 50 w < 100 w > 100
Type and minimum thickness in mm of m screens Front Back Pb 0.1 Pb 0.1 Pb 0.1 Pb 0.1 Pb 0.3 Pb 0.3 Pb 0.4 Pb 0.4 Fe 0.5/Pb 1.5 Fe 0.5/Pb 1.0 Fe 0.5/Pb 2.0 Fe 0.5/Pb 1.0
For the corresponding values for X-ray sources, see EN 14784-2, Table 2.
For tangential radiography, the values given in Table 11.1 should be regarded as minimum values, and thicker screens should be used, where possible, to reduce burn-off effects at outer edges of the pipes, and lower the overall image contrast.
Sign up to vote on this title For pipes containing product, increased levels of scatter may be obtained, depending on useful of scatter a pipe wall thickness, and the penetrated thickness of product. Increased Useful Not levels
likely to be associated with greater penetrated thicknesses of product, and reduced pipe thicknesses.
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12
Exposure time
CR plates have a wide tolerance or latitude to variations in exposure times. However, experience has shown that using very short exposure times will result in poor image qual and hence poor sensitivity to wall loss with the DWDI and DWSI techniques. Correspond lower wall thickness measurement accuracies can be anticipated with the tangential meth for short exposure time images.
To avoid these issues, the exposures times for computed radiography should be sufficien give CR images with the required quality, as described in Section 10.
In all cases, for isotope sources, it is recommended that the time taken to move the sour out of the container into the exposure position and back again should be less than 10% o the exposure time, to avoid effects connected with movement unsharpness.
12.1
DWSI and DWDI
To calculate exposure times, the following approximate formula may be used:
Ew
2.4x10
-4
E 0 exp w tot
SDD S
2
(12.1)
Where Ew E0 S SDD
wtot
is the exposure time in sec for penetrated thickness w. is the exposure needed to achieve the required SNR_N value for zero penetrated thickness in units of Ci.min @ 500mm. is the source strength in Ci. is the source to detector distance in mm. is the measured effective material attenuation coefficient (0.04 /mm for Ir 192 and 0.08 /mm for Se 75). is the total steel equivalent penetrated thickness, including any product in the pipe
Note that if the exposure, E 0, is expressed in units of gBq. sec @ 1000mm, then the cons in equation (12.1) is 2.7 x 10 -8. Sign up to vote on this title
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For pipes fully filled by product, the steel penetrated thickness (2 WT) on the pipe centre should be increased by approximately approximately ID/9 for water and c. ID/11 for oil (density of 0.8 3 gm/cm ) to give the total steel equivalent penetrated penetrated thickness, w
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Significantly longer exposures may be needed if high-resolution high-resolution imaging plates intended weld inspection are used for in-service examination (E 0 30 Ci.Min @ 500mm for standa image quality and E 0 70 Ci.Min @ 500mm for higher image quality).
12.2
Tangential
For tangential radiography, the times given above for DWDI can generally be used as a guide.
It is also important to ensure that the scanner gain/sensitivity setting is adjusted so that th unimpeded radiation radiation beam outside the pipe wall is not saturated, which will cause “burn-o and errors in wall thickness measurement.
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13
Penetrated Thickness measurements measurements
13.1
Introduction
For the DWDI and DWSI techniques, computer analysis of the CR image grey levels can certain conditions, be used to estimate wall thickness changes. It is important to note however that this a relatively new method which has not yet been fully validated.
This method should therefore be used with caution, and validated in advance for the component under inspection using test components with closely similar wall thickness an pipe OD. These test components components should contain areas of wall loss of known through-wal extent, to allow validation of the penetrated thickness analysis method, described described below.
It should be emphasised that this is a relative method for measurement measurement of wall loss, and unlike the tangential method (Section 14), the penetrated thickness method does not prov direct measurements of remaining wall thickness. t hickness.
13.2
Principle of method
The software used for this purpose generally assumes that the detected radiation intensit related to penetrated thickness by: I(w) = I0 exp(- w)
(13.1)
Where I(w) I0
13.3
is the intensity for penetrated thickness w is the unimpeded beam intensity is the effective linear attenuation coefficient of the material
Effects of scattered radiation
In practice, it is important to appreciate that a number ofSign effects, in particular up to vote on this titlethe presenc scattered radiation, complicates the analysis. Thus equation 13.1 is often only approxima Useful Not useful and generally only valid for relatively small percentage changes in wall thickness.
The presence of significant levels of scatter on CR images also makes nominal attenuatio
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For both DWSI and DWDI methods, the penetrated thickness increases away from the pi centre-line, so it is important that the step wedge is positioned so that the penetrated thickness for the step wedge is similar to that for t he wall loss being measured.
The effective attenuation coefficient should be measured from at least the first and secon steps on the step wedge. If the attenuation values values derived from the different steps are in good mutual agreement, an average value can be used, and this provides some confiden in the accuracy of the subsequent measurements of wall loss. However, if the attenuation coefficients derived from the different steps are not in good mutual agreement, the value derived from the first step (smallest increase in penetrated thickness) should be used wh estimating wall loss. Such differences can also indicate the presence of significant levels scatter on the image, which lead to a loss of accuracy in the wall loss measurements, especially for larger percentage wall losses.
13.5
Key Points
The key points for this technique are:
CR image image grey levels must be linearised linearised using using the correct look-up look-up table for the scanner, if a non linear amplifier is built into the CR scanner. A step wedge must be used to measure the effective attenuation coefficient, located such that the local penetrated thickness is close to that for the wall loss b measured. The reference and measurement areas should be as large as possible given the of features in the image to measure. (Small areas are more affected by noise). The reference reference and measurement areas should should be as close together as as poss given the limitations of the image content.
With this technique, initial results suggest that accurate (±1mm) results can only obtained relatively small wall losses of c. 30 – 50%.
If the actual total penetrated thickness is not known accurately, differences from an assum value can be used to measure small percentage wall losses with reasonable accuracy. accuracy. Use of substantially thicker lead screens may preferentially absorb the lower energy scattered radiation and hence increase accuracy in some cases. Sign up to vote on this title
13.6
Limitations
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3. For DWSI, and some DWDI images, the CR CR images can contain contain significant significant variations background grey level, due the rapidly changing SDD and penetrated pipe wall thickn values across the image. These background variations introduce uncertainties into th measurement of an appropriate reference grey level for the area of wall loss, especia a single measurement area on only one side of the indication is used. In the presence of significant variations in background background grey level, analysis methods based on extrapolation of grey-scale profiles extracted from the CR image are recommended to estimate the background image grey level “underneath” the area of loss.
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HOIS(09)RP1 Issue 1
14
Tangential radiography radiogra phy
This Section contains important recommendations recommendations regarding tangential radiography. radiography.
14.1
Recommended SDD
The recommended source to detector distances (SDD) for ttangential angential radiography radiography have already been covered in Section 8.3.
14.2
Source location relative to pipe centre line
For small diameter insulated or non insulated pipes, the radiation source for tangential radiography radiography is generally positioned positioned on the pipe central axis, as illustrated in Figure 14.1. allows both walls, on either side of the pipe, to be inspected on a single CR image.
CR plate
Figure 14.1 14.1
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Tangential radiography with the radiation source source on the pipe centre centre l
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For larger diameter pipes (both insulated and non insulated), the radiation source should offset from the pipe centre line, to be instead in-line with the tangential position on the pip
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CR plat
Figure 14.2
Tangential radiography with the radiation source offset from the pipe centre line by half the pipe diameter, so it is in-line with the tangentia position on the pipe.
There are no specific recommendations recommendations concerning the maximum pipe diameter which should be inspected using the radiation source on the pipe axis. This will in practice depe on the size of the CR plate available, as well as the method used for dimensional calibrat (see Section 14.3 below).
14.3
Dimensional calibration
For tangential radiography, when making dimensional on-screen measurements of wall thickness, it is important to calibrate the distances involved in the radiography, to allow fo the image enlargement or “blow-up”. The geometric magnification effect for tangential radiography radiography is shown in Figure 14.3. Sign up to vote on this title
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WT’
x
Detector
r SPD
PDD SDD
Figure 14.3
Geometric magnification for tangential radiography showing the measured wall thickness WT’.
Two methods can be used for dimensional calibration, calibration, to derive the actual wall thickness from the measured value WT’ as follows.
14.3.1
Measurement of distances
This method involves direct physical measurement of the key distances involved in the radiography.
Note that reliable and accurate physical measurements of distances may be difficult to Sign up to vote on this title achieve in a plant environment, and the measurements may not be available at the time o the analysis. If this is considered to be the case, the alternative Useful comparator Not usefulmethod (Sec 14.3.2) is recommended. recommended.
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For offset tangential radiography radiography (with x ~ r) , the true wall thickness WT at the tangentia pipe position as shown in Figure 14.2 can be calculated from the projected (measured) w thickness WT’ on the CR plate using the approximate equation (14.1):
WT (1
PDD SDD
) WT'
(14.1)
For tangential radiography on the pipe centre line (x =0), equation (14.1) is an approxima which becomes less accurate as the angle, shown in Figure 14.3 increases. The exact calculation then requires application application of the more involved equation (14.2) [ 6].
1
SPD
WT r
r SPD
2
r
2
r SPD
2
r
2
WT'
SDD WT'
(14.2)
SDD
Where r is half the pipe outside diameter (=OD/2).
Provided all the distances involved can be measured sufficiently accurately, and that the image pixel size is known reliably, then software which applies equation (14.2) to derive t actual wall thickness WT from the t he measured value WT’ is capable of high wall thickness measurement measurement accuracy, even for values of SDD/OD < 3.
Note however, that equation (14.2) is only valid for “fan-beam” geometries, and will be on approximate for three-dimensional three-dimensional “cone-beam” geometries, i.e. for sections through the pipe, either above or below the source position in the CR image, for which the angle in Figure 14.4 is non zero (see also Section 8.3.1).
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Pipe walls
Detector
f
R SDD
Figure 14.4
Section along the pipe axis axis showing the three-dimensional three-dimensional cone bea effect (for non zero values of the angle ).
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14.3.2
Dimensional comparator
An alternative method method for dimensional dimensional calibration calibration is the use of of ball bearing or or similar dimensional dimensional comparator. This is an effectively radiation opaque object (usually spherical) with a known diameter, which is placed close to the pipe, and in the same plane as the tangent position on the pipe wall, as illustrated in Figure 14.5.
Comparator, diameter c
Figure 14.5
Tangential radiography showing use of comparator for dimensional calibration. The comparator should be placed as close to the pipe wa possible, without overlapping it.
On-screen measurements measurements of the imaged size of the comparator, using the CR system software, then allow the pipe wall thickness measurement to be calibrated in mm, hence allowing for the radiographic magnification magnification or “blow-up”. The actual wall thickness WT is t given by: Sign up to vote on this title
WT WT'
c c'
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(14.3)
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Comparator
Figure 14.6
14.4
Tangential radiography showing use of offset offset source position with comparator for dimensional calibration, for insulated pipes, where th comparator should be placed as close to the outside of the insulation possible.
Use of lead strips to avoid burn-off
Some CR procedures specify the use of lead strips close to, or coincident with edge of th pipe to avoid image burn-off effects at the pipe OD.
However, the use of these strips is not recommended for computed radiography, radiography, as t would adversely affect the accuracy of the preferred wall-thickness wall-thickness measurement method which involve analysis of the image values (profiles) extracted along lines orthogonal to th pipe wall, see Section 14.5 below.
Burn-off should instead be minimised by ensuring the exposure time is adjusted so that th unimpeded beam beyond the pipe wall is not greater than 90% of the dynamic range of Sign up to vote on this title CR plate (not too close to saturation).
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In addition, the use of lead screens, thicker than the minimum values given in Table 10.1 can be useful in avoiding burn-off, by reduction of scattered radiation.
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inner walls difficult to determine by eye. Furthermore, the apparent locations can be affec significantly by the contrast and brightness settings in use on the CR image at the time. Thus, the WT measured using this technique can change appreciably as the contrast and brightness settings are varied on the same image.
If the on-screen measurement method is used, it should, if at all possible, first be checke accuracy using the current contrast and brightness settings of the displayed image, by application to a section of the pipe with known wall thickness (e.g. known to be uncorrode or not eroded).
For pipes having maximum tangential paths which approach the maximum permissible fo the source in use (see Table 6.2), the interactive on-screen measurement method is best applied to CR images having logarithmic response functions to radiation exposure/dose, which reduces the overall dynamic range of the image. This improves the visibility of the position of internal pipe wall (ID).
Logarithmic CR images can be obtained from logarithmic-response logarithmic-response CR scanners (e.g. the th CR100) or by application of an appropriate logarithmic logarithmic look-up table (LUT) to CR images resulting from scanners with linear response functions.
In addition, use of high-frequency spatial filtering (sharpening) is recommended since this improves the accuracy of this measurement measurement method, by emphasizing the positions positions of t edges of the pipe wall in the CR images, and reducing any dependence on the contrast a brightness settings on the image.
It is however recommended recommended that for improved accuracy interactive on-screen measureme are made in combination with grey-level profile analysis methods as described in the following section.
14.5.2
Grey-level profile analysis methods
Many CR systems have software which allows the user to mark lines on the CR image orthogonal to the pipe wall axis. The software extracts a grey-level profile along this line, which is then generally presented on-screen, superimposed on the image. Measurements wall thickness can be obtained by either interactive or automated analysis of these grey-l profiles. Sign up to vote on this title
Automated routines Useful Not useful Automated analysis analysis routines can increase the reliability reliability of the measured measured wall thickness thickness values, unless the maximum maximum tangential penetrated penetrated thickness thickness (w max) is approaching the maximum possible, given the radiation source in use (see Section 7.2). In addition, other
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Figure 14.7 shows an example of interactive measurement of wall thickness, using curso on a grey level profile across the pipe wall, after applying a logarithmic look-up table to th CR image, and high-pass filtering to enhance details. The position of the outer diameter corresponds to a clear peak in the profile, and the location of the inner diameter is given b the minimum and pronounced change in gradient of the profile. This method, combined with a visual assessment of the image, is recommended.
Figure 14.7
Example of interactive interactive wall thickness thickness measurement measurement using cursors superimposed on a grey level profile taken across the pipe wall.
It should be noted that the accuracy of all measurement methods decrease as the tangen isotop penetrated thickness, wmax, approaches the maximum value recommended for the Sign up to vote on this title use (see Section 7.2), 7.2), since the location of the inner wall becomes increasingly difficult to Useful Not useful increased determine with any reliability due to lack of contrast and noise.
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15
Scanner parameters, parameters, image recording and processing
15.1
Scanner parameters
For the standard image quality, the scanner pixel size should be 100 microns or less. For the higher image quality, the scanner pixel size should be 50 microns or less.
For a given radiographic exposure, it is important to note that increasing the scanner gain sensitivity increases the grey levels, but has negligible effect on t he image quality, as measured by the normalised signal noise ratio (SNR_N). To increase SNR_N, the exposu must be increased, not the scanner gain.
For scanners with linear responses between radiation dose and grey level, use of low gain/sensitivity gain/sensitivity reduces the risk of image saturation. For higher scanner gains, image saturation may occur, especially in the free beam areas, for relatively short exposures, w do not give sufficiently high image SNR_N values to meet the quality criteria given in Sec 10.
15.2
Image recording and storage
The CR images from the scanner should be stored in a file format which supports a minim of 12-bits/pixel. 12-bits/pixel.
The images should be stored at full resolution and full dynamic range, as delivered by detector system. Only image processing required for detector calibration should be applie the images prior to storage of these raw data.
Any data compression compression techniques techniques used in the storage storage of these files should be “loss-less” “loss-less”
The following information should be recorded as header information, attached to each CR image, to be used for reporting purposes. Sign up to vote on this title
(a) (b) (c)
Useful Name of company carrying out the inspection Test report number Component Component under test
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15.3
Name and certification of responsible person Average pixel values (grey levels) in the area of interest Measured SNR, if software permits derivation
Image processing
All measures of of image quality (mean (mean image grey grey level in area of of interest, signal to noise noise r if available, IQI wire visibility etc) must be must be made on images which have not been subject any image processing routines which use spatial filtering. filtering.
Spatial filtering is defined as a process in which the value of the current image pixel is alt by an amount which depends on other (usually adjacent) image pixel values. Examples o spatial filtering or spatial image processing include noise smoothing/filtering, smoothing/filtering, edge sharpening, edge contrast, un-sharp masking etc. Spatial filtering such as edge sharpeni may however be beneficial for the interactive measurement of wall thickness using the tangential method (see Section 14.5). Image processing which changes contrast and brightness of the displayed CR image (sometimes referred to as level control) is however permissible when evaluating image quality, and strongly recommended for image evaluation at the monitor. It is recommended that the results of any image processing are saved as a separate file, that the raw data derived from the CR scanner should not be altered by any subsequent processing.
15.4
Monitor viewing conditions
The computed radiographs shall be examined in a darkened room using a monitor, or film hardcopy, as specified in Section 7.10 of EN 14784-2.
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16
Acknowledgments
Several members of the HOIS JIP provided useful comments on earlier drafts of this document.
17
References
1. Burch S F (2009). (2009). Results from HOIS CR CR Trials in Bergen Bergen October 2008. 2008. HOIS(0 Issue 1. January 2009
2. “Development of protocols for corrosion and deposit evaluation in large diameter pipe radiography” radiography” IAEA report, Vienna, 2008.
3. Halmshaw, R (1995) (1995) Industrial Radiology Radiology Theory and and Practice Second Edition, Chap & Hall, p125
4. Burch, S F and Collett N J, (2005) Recommended Recommended Practice Practice for the rapid inspectio inspectio small bore connectors using radiography, HSE Research Report, RR 294.
5. Burch S F (2009). Results from HOIS HOIS CR Trials in Bergen May May 2009. HOIS R HOIS(09)R5 Issue 1. August 2009
6. Willems P, Vaessen B, Hueck W and U Ewert (1999) ‘Applicability of comp radiography for corrosion and wall thickness measurements’. measurements’. Insight, Vol 41, No 10, 637.
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