HOIS09RP1 HOIS RP for Computed Radiography

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Recommended Practice for the in-service

inspection of wall loss in pipes by computedradiography

HOIS(09)RP1 Issue 1

A Report prepared for HOIS

By

S F Burch, ESR [email protected]

December 2009

mailto:[email protected]





HOIS(09)RP1 Issue 1

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HOIS(09)RP1 Issue 1

Authorisation Sheet

Report Title: 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 BurchESR Technology Ltd16 North Central 127Milton Park Abingdon

OxfordshireOX14 4SAUK

Phone: +44(0)1235 213402Email: [email protected]

© COPYRIGHT ESR Technology Ltd

This report is the Copyright of ESR Technology Ltd and has been prepared by ESR Technology Ltd for the HOISJIP. ESR Technology Ltd accepts no liability whatsoever to any third party for any loss or damage arising fromany interpretation or use of the information contained in this report, or reliance on any views expressed therein.

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Executive Summary

This recommended practice gives guidance for the in-service inspection of pipes for wall losstype flaws (e.g. corrosion pitting, generalised corrosion and erosion) using computed (digital)

radiography (CR). Weld inspection for typical welding process induced flaws is not covered,but weld inspection is included for corrosion/erosion type flaws (e.g. weld rooterosion/corrosion).

The pipes may be insulated or not, and can be assessed where loss of material due, forexample, to corrosion or erosion is suspected either internally or externally.

This document covers the following inspection techniques:

1. Double-wall single image (DWSI) radiography for the inspection of discrete wall lossflaws by their effects on image grey level.

2. Double-wall double-image (DWDI) radiography for the inspection of discrete wall lossflaws by their effects on image grey level.

3. Tangential inspection techniques for detection and through-wall sizing of wall loss,including with the source on the pipe centre line, and offset from it by the pipe radius.This technique is sometimes referred to as profile radiography, but this term is notused in this document.

Note that DWDI is often combined with tangential radiography with the source on the pipecentre line.

The 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. Theuse of portable X-ray sources is also included.

Two different qualities of radiography are considered in this document:

A standard quality of computed radiography for wall loss inspection. This has lessdemanding quality requirements than those defined for weld inspection, since in general wallloss flaws are easier to detect radiographically than welding flaws which can include cracks.

A higher quality of computed radiography for wall loss inspection is also included in thisdocument. This is for CR inspections requiring higher quality (e.g. inspection of small pitting

flaws).

Recommendations are given for these two different quality classes on source to detectordistances, axial and circumferential coverage, CR image quality measures and indicators,and exposure times. In addition to these parameters, for tangential radiography, differentmethods for dimensional measurements are covered.



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Contents

1 INTRODUCTION ..........................................................................................10

2 REFERENCES .............................................................................................11

3 DEFINITIONS...............................................................................................12

4 PERSONNEL QUALIFICATIONS ................................................................13

5 GENERAL ....................................................................................................13

5.1 Protection against ionising radiation...........................................................13

5.2 Size and Strength Of Sources....................................................................13

5.3 Source Containers and Collimation............................................................ 13

5.4 In-Situ Inspection of Plant...........................................................................14

5.5 Identification of Radiographs......................................................................14

6 OVERVIEW OF INSPECTION TECHNIQUES .............................................15

6.1 Double wall single image (DWSI) inspection technique .............................15

6.2 Double wall double image (DWDI) inspection technique............................ 15

6.3 Tangential inspection techniques............................................................... 17

6.3.1 Source on pipe centre line ....................................................... 17

6.3.2 Offset source position tangential radiography..........................18

6.4 Both Tangential and Double wall techniques combined.............................19

7 RADIATION SOURCES ...............................................................................20

7.1 Type of source............................................................................................20

7.2 Source selection.........................................................................................21

7.2.1 Double wall techniques............................................................21 7.2.2 Tangential technique................................................................22

8 SOURCE TO DETECTOR DISTANCES (SDD) ...........................................27

8.1 Double wall single image (DWSI) inspection..............................................27

8.2 Double wall double image (DWDI) inspection ............................................ 28

8.3 Tangential Inspection ................................................................................. 31

8.3.1 Source on pipe centre line ....................................................... 31

8.3.2 Combined tangential/DWDI image radiography....................... 33

8.3.3 Source offset from pipe centre line .......................................... 33



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9 CIRCUMFERENTIAL AND AXIAL COVERAGE AND OVERLAP...............35

9.1 Circumferential coverage............................................................................35

9.1.1 DWSI........................................................................................35

9.1.2 DWDI .......................................................................................36

9.1.3 Penetrated Thickness measurement ....................................... 37 9.2 Axial coverage............................................................................................37

9.3 Overlap of images ...................................................................................... 38

10 CR IMAGE QUALITY INDICATORS............................................................39

10.1 Background ................................................................................................ 39

10.2 Image Quality Measures for Double Wall Techniques (DWDI & DWSI)..... 39

10.2.1 Target Grey level range ........................................................... 39

10.2.2 Signal to Noise Ratio (SNR) .................................................... 40

10.2.3 BSR Measurement...................................................................41

10.2.4 Wire IQIs .................................................................................. 45

10.2.5 Image quality for tangential techniques ................................... 48

11 SCREENS/FILTERS.....................................................................................50

12 EXPOSURE TIME ........................................................................................51

12.1 DWSI and DWDI.........................................................................................51

12.2 Tangential...................................................................................................52

13 PENETRATED THICKNESS MEASUREMENTS.........................................53

13.1 Introduction.................................................................................................53

13.2 Principle of method.....................................................................................53

13.3 Effects of scattered radiation...................................................................... 53

13.4 Calibration using step wedge ..................................................................... 53

13.5 Key Points ..................................................................................................54

13.6 Limitations .................................................................................................. 54

14 TANGENTIAL RADIOGRAPHY...................................................................56

14.1 Recommended SDD...................................................................................56

14.2 Source location relative to pipe centre line.................................................56

14.3 Dimensional calibration .............................................................................. 57

14.3.1 Measurement of distances.......................................................58

14.3.2 Dimensional comparator.......................................................... 61

14.4 Use of lead strips to avoid burn-off.............................................................62

14.5 Wall thickness measurement......................................................................62



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14.5.1 Interactive on-screen measurements.......................................62

14.5.2 Grey-level profile analysis methods......................................... 63

15 SCANNER PARAMETERS, IMAGE RECORDING AND PROCESSING....65

15.1 Scanner parameters...................................................................................65 15.2 Image recording and storage......................................................................65

15.3 Image processing .......................................................................................66

15.4 Monitor viewing conditions ......................................................................... 66

16 ACKNOWLEDGMENTS...............................................................................67

17 REFERENCES .............................................................................................67



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1 Introduction

The scope of this recommended practice covers the in-service inspection of pipes for wallloss type flaws (e.g. corrosion pitting, generalised corrosion and erosion) using computed

(digital) radiography (CR). Weld inspection for typical welding process induced flaws is notcovered, but weld inspection is included for corrosion/erosion type flaws (e.g. weld rooterosion/corrosion).

The pipes may be insulated or not, and can be assessed where loss of material due, forexample, to corrosion or erosion is suspected either internally or externally.

This document covers the following inspection techniques:

1. Double-wall single image (DWSI) radiography for the inspection of discrete wall lossflaws by their effects on image grey level.

2. Double-wall double-image (DWDI) radiography for the inspection of discrete wall lossflaws by their effects on image grey level.

3. Tangential inspection techniques for detection and through-wall sizing of wall loss,including (a) with the source on the pipe centre line, and (b) offset from it by the piperadius. This latter technique is sometimes referred to as profile radiography, but thisterm is not used in this document.

Note that DWDI is often combined with tangential radiography with the source on the pipecentre line.

The 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. Theuse of portable X-ray sources is also included.

Two different qualities of radiography are considered in this document.

A standard quality of computed radiography for wall loss inspection. This has lessdemanding quality requirements than those defined for weld inspection, in EN 1435 forexample, since, in general, wall loss flaws are easier to detect than typical welding flaws. Fortangential radiography, standard quality is sufficient when the wall loss is approximatelyuniform, not isolated pitting.

A higher quality of computed radiography for wall loss inspection is also specified in thisdocument. This is for CR inspections requiring higher quality (e.g. inspection of small pittingflaws). For tangential radiography, higher quality is recommended when there is arequirement to size pitting flaws.



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2 References

EN 444, Non-destructive testing - General principles for radiographic examination of

metallic materials using X-rays and gamma-rays. EN 473, Non-destructive testing - Qualification and certification of NDT personnel -

General principles

EN 584-1, Non-destructive testing - Industrial radiographic film - Part 1: Classificationof film systems for industrial radiography

EN 462-1 to EN 462-5, Non-destructive testing – Parts 1 to 5: Image quality ofradiographs.

EN 1435, Non-destructive examination of welds – Radiographic examination ofwelded joints

EN 14784-1, Non-destructive testing - Industrial computed radiography with phosphorimaging plates - Part 1: Classification of systems

EN 14784-2, Non-destructive testing - Industrial computed radiography with storagephosphor imaging plates - Part 2: General principles for testing of metallic materialsusing X-rays and gamma rays

E 1647-98a, ASTM Standard Practice for Determining Contrast Sensitivity inRadioscopy.

ISO 3999-1, Radiation protection – Apparatus for industrial gamma radiography

ISO 11699-1, Non-destructive testing - Industrial radiographic film - Part 1:Classification of film systems for industrial radiography.



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3 Definitions

Terms used in this document are as follows:

b The distance between the source side of the pipe and the detector.b’ The distance between the centre of the pipe and the detector.CR Computed radiographyd Source size for calculation of geometric unsharpnessDWDI Double wall double image radiographic techniqueDWSI Double wall single image radiographic techniqueEw The exposure time in sec for penetrated thickness wE0 The exposure needed to achieve the required SNR_N value for zero

penetrated thickness in units of Ci.min @ 500mm.f Distance from source to relevant position in component (depends ontechnique)

ID Pipe inside diameterOD Pipe outside diameterR Distance from detector to pipe axis (centre line)r Radius of pipe OD (=OD/2)S Source strengthSDD Source to detector (originally film) distanceSPD Source to pipe axis distance (i.e. source to centre line of pipe)T Exposure timeUg Geometric unsharpness at detectorUg’ Geometric unsharpness in plane of interest within componentw Penetrated thicknesswt Total steel equivalent penetrated thickness, including an allowance for any

product in the pipewmax Maximum penetrated thickness for a pipe which occurs for a tangent to the ID

surfaceWT Actual wall thicknessWT’ Measured wall thickness

Effective material attenuation coefficient



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4 Personnel Qualifications

Personnel 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 highlyinjurious to health. Wherever X-ray equipment or radioactive sources are in use, appropriatelegal requirements must be applied.

Local or national or international safety precautions when using ionizing radiation shall bestrictly applied.

5.2 Size and Strength Of Sources

The strengths (activities) of isotope sources used for CR need to comply with localregulations.

Radiography contractors should state the maximum strength isotopes within their LocalRules as required by IRR 1999.

For in-service inspection applications, a typical size of Ir 192 source is a 2 x 1mm disc (20Ci,740Gbq), whereas for a similar strength Se 75 source, the size is often 2.5 mm (nearspherical).

The effective source size for geometric sharpness calculations should be used to calculatethe required source to detector distance, as given in Section 8.

5.3 Source Containers and Collimation

The source containers should conform to the requirements for source containers given by

ISO3999-1:2000 or BS5650:1978 ISO 3999-1977 and any applicable national standards.

Conventional projection equipment can be used, provided the requirements of the currentradiation safety regulations are complied with. For these systems, a large radiation controlledarea is normally needed, which often requires out of hours working, or even shutdown ofplant.

Systems which keep the source within a single container or single container/collimatorassembly are now available that allow a much smaller size of controlled area (of order 1 m -5 m). These systems reduce radiation doses to operators, and the small size of thecontrolled area generally means that plant operation does not need to be interrupted whensite radiography is underway.



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The ratings of a source container must be checked for compliance with the type and strengthof the isotope to be used.

The selection of an appropriate source container and deployment system depends on thebalance of the above factors for each individual site and inspection application, together witheconomic considerations.

Careful collimation of the sources is recommended to minimise unwanted radiation, and toreduce the effects of scatter on the radiograph.

5.4 In-Situ Inspection of Plant

Use of gamma-ray radiography equipment for in-situ inspection of plant involves significantsafety issues associated with the use of ionising radiation. The appropriate mandatory safetyregulations appropriate to the plant must be adhered to (IRR 1999 in the UK). These includethe construction and maintenance of radiation controlled areas, by means of appropriatebarriers.

Pre-planning of the inspection work to be carried out on a plant is required, to include both arisk assessment and a practical assessment of how the source container and shielding willbe placed (IRR 1999).

5.5 Identification of Radiographs

Symbols shall be affixed to each section of the object being radiographed. The images ofthese symbols shall appear on the CR image outside the region of interest where possible,and shall ensure unequivocal identification of the section.



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, theradiography 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 OD.

X or gamma-ray source

Pipe

Detector

Corrosion

Image of wall loss on

detector

Figure 6.1 Double wall single image (DWSI) inspection technique for the inspectionof 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 aredetected by the changes in image grey level they produce. Wall loss flaws in the oppositepipe wall nearer the source are either outside the radiation beam, or highly blurred and notgenerally detectable. Large scale loss of wall, due for example to erosion, which produces anear uniform loss of wall will not be readily detectable with this technique.

The amount of wall loss affects the image grey level, and computer analysis of the CR imagegrey levels can be used to estimate the change in penetrated thickness, provided certainprocedures are followed (see Section 13).

6.2 Double wall double image (DWDI) inspection technique

In the double wall double image (DWDI) inspection technique, illustrated in Figure 6.2, theradiography is usually carried out with the source directly in-line with the centre of the pipe.For the double-wall double image method, a sufficiently large SDD is used to ensure thatexternal or internal wall loss can be detected in the portion of the pipe wall nearer the source,as well as that closer to the detector.

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Pipe

Detector

Corrosion pits

Images of wall loss on

detector

Figure 6.2 Double wall double image DWDI inspection technique for detection ofwall 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 sensitivitywill be reduced as the amount of metal penetrated by the radiation beam increases. SeeSection 9 for more information on circumferential coverage.

With this method, the size of any pitting type wall loss flaws in the circumferential and axialdirections can be measured directly from the CR image, provided methods are used forcalibration of distances – see Section 14.3.

On a single DWDI image, it is generally not possible to determine which side of the pipe theflaws are located on, and flaws from both sides are superimposed on the same image.

As with DWSI, large scale loss of wall, due for example to erosion, which produces a nearuniform loss of wall will not be readily detectable with this technique.

The amount of wall loss affects the image grey level, and computer analysis of the CR imagegrey levels can be used to estimate the change in penetrated thickness, provided certainprocedures are followed (see Section 13).

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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 wall

running tangentially to the radiation beam, as illustrated in Figure 6.3 for a source position onthe pipe centre line. The CR image then shows a direct image of the pipe wall.


Detector

Extended area of

corrosion

Image of reducedthickness pipe wall

Image of normal thicknesspipe wall

Figure 6.3 Principle of tangential radiography with the source on the pipe centre-line

The through-wall extent of extended areas of either internal or external wall loss can bemeasured directly from the CR images, provided appropriate calibration techniques are usedto allow for the enlargement (“blow-up”) of the CR image – see Section 14.3 for furtherdetails.

The tangential method inspects only a small extent of the circumference of the pipe for asingle source/detector position, and so full coverage can only be achieved by use of anumber of CR images taken circumferentially around the pipe.

The tangential method is often used to measure directly any wall loss identified by the DWDImethod given in Section 6.2, by rotating the source/detector axis to align the wall loss withthe tangential position.

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The tangential radiography method is recommended for the detection and through-wall sizingof extended areas of wall loss. For small isolated wall loss pits care should be taken to avoidunderestimation of the maximum loss of wall caused by incorrect geometric alignment (thepit 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 limitsrecommended for the pipe and radiation source (see Section 7.2.2 for further information).For this reason, use of the higher quality standard for tangential radiography isrecommended 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 6.4.The offset is usually equal to the mean pipe radius, so that the centre of the X-ray beampasses through the tangential position on the pipe wall and is then incident at an angleapproximately perpendicular to the detector.

This technique is generally used for the inspection of insulated pipes, or larger diameter noninsulated pipes.

Insulation

Extended area ofcorrosion


Detector

Image of reduced

thickness pipe wall

Figure 6.4 Principle of tangential radiography with the source offset from the pipecentre-line

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As with the tangential technique on the pipe centre line, with the offset source technique thethrough-wall extent of extended areas of either internal or external wall loss can bemeasured directly from the CR images, provided appropriate calibration techniques are usedto allow for the enlargement (“blow-up”) of the CR image – see Section 14.3 for furtherdetails.

The tangential (offset) method is often used to measure directly any wall loss identified bythe 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 dynamicrange and size to show the presence of wall loss by both the tangential and double walldouble image (DWDI) techniques, as illustrated in Figure 6.5.


Detector

Figure 6.5 Radiography combining both the tangential and double wall doubleimage (DWDI) techniques in a single radiograph

For this combined type of radiography, as shown in Figure 6.5, the tangential method can beapplied to both sides of the pipe on the same CR image, and the region in between will showloss of wall by increased transmitted radiation intensity, which cause increased CR imagegrey levels.

The DWDI technique can be used to identify the area of greatest apparent wall loss, and thesource/detector can then be rotated to find the remaining ligament for this area using thetangential technique.

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7 Radiation Sources

7.1 Type of source

The majority of in-situ on-site radiography is carried out using gamma-ray emitting isotopesources, although portable, light-weight X-ray sources can also be used in cases where theneed 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 thicknesses.The gamma-ray spectrum is complex, containing at least 24 spectral lines: those with relativeintensities of at least 30% are at 296, 310, 320 and 470 keV. The half-life is 74 days. Iridium192 is the most commonly used isotope source for in service inspection in the oil and gasindustry. Source strengths are available up to 200 Curies or more with physical sizes rangingfrom about 1x1 mm to about 4x4 mm. A typical source size used for pipe inspection is 2 x1 mm.

The isotope source Selenium 75 has been developed for industrial radiography morerecently than Ir 192. It has a lower energy gamma-ray spectrum than Ir192 with main peaksat 137keV and 265keV and a longer half life (120 days). Source strengths are availablebetween about 2 and 80 Curies with physical sizes ranging from 1x1 mm to 3x3 mm. Due tothe lower gamma-ray energies emitted by Selenium 75 compared with Iridium 192,Selenium 75 can give radiographs with higher contrast on components with moderate steelthickness. However, Selenium 75 is less commonly used for tangential radiography, due toits reduced penetrating power, compared with Iridium 192.

Ytterbium 169 gives several different energies between 63 keV and 307 keV, with a half lifeof 32 days. Its mean effective energy is lower than both Selenium 75 and Iridium 192, and is

occasionally used for the in-service inspection of thin-walled components (penetrated steelthicknesses between 1 and 15 mm, for Test Class A film radiography, see EN 444 and EN1435).

Cobalt 60 is a high energy source (photon energies of 1.17 and 1.33 MeV), with an extendedhalf-life of 5.3 years. This source is sometimes used for the in-service inspection of thickwalled components, for which Iridium 192 has insufficient penetration (i.e. steel penetratedthicknesses greater than c. 85 mm). Because of its greater penetrating power, there aresubstantial additional safety requirements for in-service inspection using Cobalt 60,compared with Iridium 192.

The highest energy sources used successfully for in-service CR are Betatrons, which are

high-energy portable X-ray sources, based on circular beta particle (electron) accelerators.Models are available with output energies of typically 2, 6 and even 7.5 MeV. As withCobalt 60, there are substantial additional safety requirements for in-service inspection usingthese sources, but unlike isotope sources, the radiation beam can be switched off by removalof the supply voltage.



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 inspectionwill be equal to twice the pipe wall thickness, on the centre line of the pipe, and will increasewith distance away from the centre line. This increase in penetrated thickness will initially begradual, and then more rapid, as the edge of the pipe is approached, as illustrated for DWDIin Figure 7.1 below.

0

10

20

30

40

50

60

70

-80 -60 -40 -20 0 20 40 60 8

Distance from pipe centre (mm)

P e n e t r a t e d t h i c k n e s s ( m m )

0

Figure 7.1 Penetrated thickness across a typical pipe (OD 150mm, WT 7.1mm),

assuming a very distant source.

Generally, the penetrated thickness at the pipe centre (2WT) can be taken as the basis forsource selection.

Recommended lower and upper limits are given in using Table 7.1 for the different isotopesources, 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 & DWSI)

Standard quality wall loss inspectionclass

Higher quality wall loss inspection classRadiationSource

Penetratedthickness, w

(mm)

Pipe WT

(mm)

Penetratedthickness, w

(mm)

Pipe WT

(mm)

Yb 169 1 w 15 0.5 WT 7.5 1 w 15 0.5 WT 7.5

Se 75 5 w 55 2.5 WT 27 10 w 40 5 WT 20

Ir 192 7 w 85 3.5 WT 42 20 w 85 10 WT 42

Co 60 40 w 200 20 WT 100 40 w 200 20 WT 100

For very high penetrated thicknesses, Betatron sources have proved effective with CRplates, although as with Co 60, there are additional safety requirements for in-serviceinspection.

For the higher quality wall loss inspection class, for X-rays up to 500 kV, Figure 1 of EN14784-2 (identical to Figure 20 of EN1435 :1997) should be used.

For product filled pipes, the additional radiation attenuation caused by the product should beallowed for in selection of sources. For a fully product filled pipe, the penetrated thickness, w,in Table 7.1 should be increased by approximately ID/9 for water, as measured by [1]. Foroil, the factor is likely to be larger (estimated as 11 on the basis of relative densities of waterand 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 radiationlevels in the CR images. Additional thickness of detector screens may then be required toproduce 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 penetratedthickness, wmax, through the pipe wall occurs for a line forming a tangent with the innerdiameter. This maximum path is shown in Figure 7.2 and is given by

WT-ODWT2wmax (7.1)

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HOIS(09)RP1 Issue 1

Detector

wmax

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 innersurface of the pipe. Thus wmax is independent of the source position.

Values for the maximum penetrated thickness, wmax, through pipes of various diameters andschedules are given in Figure 7.3 and Table 7.2, for ease of reference. Note that thesepenetrated thickness values are generally much larger than twice the wall thickness of thepipe.

Also shown on Figure 7.3 are the approximate maximum penetrated thicknesses for Se75, Ir192 and Co 60, recommended for CR inspection using the standard quality tangentialtechnique [2].

These values are given in Table 7.2, which also gives recommended reduced values forhigher quality tangential inspection (for sizing of pitting flaws, which are more difficult todetect and size than generalised wall loss).

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Tangential path lengths

0

20

40

60

80

100

120

140

160

180

200

0 2 4 6 8 10

Pipe nominal bore (inch)

T a n g e n

t i a l p a

t h ( m m

)

12

Schedule 40

Schedule 80

Schedule 160

Limit for Se 75

Limit for Ir 192

Limit for Co 60

Se 75

Ir 192

Co 60

Figure 7.3 Maximum (tangential) path lengths through the walls of pipe of different

diameter. The maximum recommended penetrated thicknesses fordifferent isotope sources are also shown.

Table 7.2 Maximum tangential paths in steel for different isotope sources

Isotope Source Maximum tangential path(mm)

Standard quality(for generalised wall loss)

Higher quality(for pitting flaws)

Se 75 c. 55 c. 40

Ir 192 c. 85 c. 60

Co 60 c. 140 c. 100

Table 7.3 shows which pipes can be inspected using the standard and higher quality

tangential techniques with the different isotope sources.

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Table 7.3 Maximum paths through different schedule pipes of various diameters, togetherwith applicable isotope sources for tangential CR.

Isotope applicability2

Nominal

Bore

(inches)

Outside

diameter,OD

(mm)

Schedule Nominal

wallthickness,

WT1

(mm)

Nominal

maximumTangential

path1

(mm)

Se 75 Ir 192 Co 60 1 33.4 40 3.4 20.2

80 4.5 22.8

160 6.4 26.3

XXS 9.1 29.7

1.5 48.3 40 3.7 25.7

80 5.1 29.7

160 7.1 34.2

XXS 10.2 39.4

2 60.3 40 3.9 29.780 5.5 34.7

160 8.7 42.5

XXS 11.1 46.7

3 88.9 40 5.5 42.8

80 7.6 49.7

160 11.1 58.8 X

XXS 15.2 66.9 X

4 114.3 40 6.0 51.080 8.6 60.3 X

120 11.1 67.7 X

160 13.5 73.8 X

XXS 17.1 81.5 X

5 141.3 40 6.6 59.6 X

80 9.5 70.8 X

160 15.9 89.3 X X

XXS 19.0 96.4 X X

6 168.3 40 7.1 67.7 X

80 11.0 83.2 X

120 14.3 93.9 X X

160 18.3 104.8 X X

XXS 21.9 113.2 X X

8 219.1 40 8.2 83.2 X

80 12.7 102.5 X X

120 18.3 121.2 X X

XXS 22.6 133.3 X X

160 23.0 134.3 X X

10 273.0 40 9.3 99.0 X X 80 15.1 124.8 X X

120 21.4 146.8 X X

XXS 25.4 158.6 X X X

160 28.6 167.2 X X X

12 323.8 40 10.3 113.6 X X

80 17.5 146.4 X X X

120, XXS 25.4 174.1 X X X

160 33.3 196.7 X X X

1 Note that if corrosion/erosion is present, the wall thickness and maximum penetratedthickness can be substantially reduced, allowing application beyond the limits implied by thenominal wall thickness shown in this table.



HOIS(09)RP1 Issue 1

26

2 key to isotope applicability:

Both standard and high quality

Only standard quality

Neither



HOIS(09)RP1 Issue 1

8 Source to detector distances (SDD)

This section contains recommended minimum source to detector (SDD) distances for the

different techniques. If two techniques are combined (e.g. DWDI and tangential) then thelarger 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, the source is conventionally positioned closeto, but outside one wall of the pipe, and the detector is wrapped around the opposite pipewall, as shown in Figure 8.1.

The distances involved for SDD determination for the DWSI technique are shown in Figure8.1. The object plane is the source side of the pipe wall nearer the detector.

SDD

b

Source,size d

Detector

f

Figure 8.1 Distances for DWSI

The recommended SDD given in EN1435 for basic class A inspection is

3/2

mm

b5.7

d

f

(8.1)

This can be re-arranged to give:

3/2

mm

b 7.5d bSDD

(8.2)

Formula 8.2 can be used for both the standard and higher qualities of wall loss inspection.

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HOIS(09)RP1 Issue 1

For larger diameter pipes, formula (8.2) allows the source to be placed adjacent to the pipewall opposite the detector (as is conventional practice for DWSI).

This may not be true for pipes with diameters of 4" or less, or those covered by insulation, sothat the detector could not be placed in close contact with the pipe wall. The SDD shouldthen be calculated according to formula (8.2) above, and the source positioned accordingly.

For pipes with diameters of less than about 3" to 4", the DWDI technique (see below) may bepreferable 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, allowinginspection of both pipe walls, as illustrated in Figure 8.2, but with a significant increase inexposure time. The detector is then usually flat, and not wrapped around the pipe wall. In thiscase, the relevant object plane is the external surface of the pipe closest to the source.

SDD

b

Source, size

d

Detector

f

Figure 8.2 Distances for DWDI

In general, a large source to detector distance (SDD) will minimise the unsharpness in theradiograph caused by the size of radiation source (known as geometric unsharpness).However, large source to detector distances can lead to very long exposure times, andincreased shielding difficulties. Thus trade-offs must be made, whilst ensuring acceptableimage quality.

There is no universally accepted method for the choice of the source to detector distancewhich will provide a satisfactory radiographic technique [3] and existing standards fromvarious countries differ widely.

For wall loss inspection, it is recommended that a geometric unsharpness criterion is used toprovide a basis for setting source to detector distances, following the HOIS RecommendedPractice on radiographic inspection of small bore piping [4]. In [4], a specified unsharpnessvalue in the plane of the object was used as opposed to the unsharpness in the detectorplane. The unsharpness in the plane of the object was considered to be more physicallymeaningful than the unsharpness in the detector plane, as it is the unsharpness in the objectplane that principally affects the minimum discernable detail size in the component underinspection.

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HOIS(09)RP1 Issue 1

29

In [4], it was shown that the source to detector distance, SDD, needed to achieve a specifiedgeometric unsharpness in the plane of the object (Ug') is given by:

SDD = (d . b) / Ug' (8.3)

Where d is the effective source size for geometric unsharpness calculations and b is thedistance between the detector and the source side of the external diameter of the pipe (OD).

For small bore connector inspection, [4] recommended a value of Ug' of 0.3mm, and it isrecommended that this value is used for the higher quality of wall loss inspection as definedin the present document.

Thus, for higher quality wall loss inspection, the following equation should be used for SDD:

SDDmin = (d . b)/0.3 (8.4)

For the standard 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 sourcesizes, as given by equations (8.4) and (8.5).



HOIS(09)RP1 Issue 1

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 20 40 60 80 100 120 140 160 180 200 220

Distance from Detector to Source side of pipe OD, b (mm)

S D D ( m m )

Standard Quality

Higher Quality

Approximate pipe nominal bore for detector in contact with pipe

1 2 43 5 6 8

Figure 8.3 Recommended Source to Detector distances (SDD) for DWDI, for anassumed effective source size of 2.3mm.

Table 8.1 Recommended source to detector distances (SDD) for DWDI, for an

assumed effective source size of 2.3mm and for the detector in contact with the pipewall.

SDDmmb

mm

Pipe Nominal Bore(if detector in contact with

pipe wall)in

StandardQuality

HigherQuality

33.4 1 128 256

60.3 2 231 462

88.9 3 341 682

114.3 4 438 876

141.3 5 542 1083

168.3 6 645 1290

219.1 8 840 1680

It should be noted that the SDD values given in equations (8.4) and (8.5) are constant factorsof the pipe diameter, for detectors in contact with the pipe wall. For a 2.3 mm source size, theSDD for the standard quality is 3.8b, or 3.8 times the pipe diameter, and for the higher qualitythe SDD is 7.7b, or 7.7 times the pipe diameter.

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HOIS(09)RP1 Issue 1

8.3 Tangential Inspection

8.3.1 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

b'

Source, sized

Film/Detector

SPD

Figure 8.4 Dimensions and distances for tangential radiography (source on pipecentre line)

For tangential radiography, there are two main factors which affect the accuracy of wallthickness measurements and hence the recommended SDD’s.

Dimensions measured in the CR image are progressively distorted away from thesource axis due to the finite source to detector distance. This has differing effects onthe accuracy of the various methods used for calibration of dimensions in the CRimages.

Geometric unsharpness.

Consideration of the accuracy of dimensional measurements is given below. It is likely thatthis will lead to a greater value for SDD than those based on geometric unsharpnessconsiderations (see equations (8.7) and (8.8) below. If this is the case, the larger of the twovalues for SDD should always be used, where practicable.

Figure 8.4 shows calculations of the errors on the measured wall thickness as a function ofthe ratio (source to pipe centre, SPD)/OD, for four different dimensional calibrationtechniques, as described further in Section 14. Note that these calculations apply toconditions in which the pipe may not be in contact with the detector, due for example to thepresence of pipe insulation, as well as cases in which the pipe OD is in contact with thedetector.

The centre line magnification calculations are based on the approximate equation (14.1). Theball bearing comparator calculations are based on the equations (14.3) and (14.4). The “fan-

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HOIS(09)RP1 Issue 1

beam” geometry calculations assume that the radiation beam diverges from a point source ina thin, two-dimensional fan beam, as shown in Figure 14.3, and that the actual wall thicknessWT is related to the measured value WT’ by equation (14.2).

In reality, for all usual forms of radiography, the radiation beam diverges as a three-dimensional cone from the source. However, if the line joining the source to the centre of thedetector is perpendicular to the detector plane, then the fan-beam geometry applies to anyline 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 complication isignored, and it is assumed that the “fan-beam” geometry calculation method gives no errorsin the derived wall thicknesses (provided the distances are measured accurately). For theother calibration methods, the wall thickness measurement accuracy decreases withdecreasing SDD.

0

1

2

3

4

5

6

7

8

9

10

0 1 2 3 4 5 6 7 8 9

SPD / OD

E r r o r

i n W T m e a s u r e m e n

t ( % )

10

Centre line Magnification

Pipe Diameter measurement

Ball bearing comparator

Fan-beam geometry calculations

Figure 8.5 Calculations of effects of different dimensional calibration techniques on

accuracy of wall thickness measurements using the tangentialtechnique.

From Figure 8.5, it can be seen that the accuracy of all the calibration techniques decreasesrapidly for SPD /OD less than about 3.5 (with the exception of the exact fan beam geometrycalculation method).

It is therefore recommended that for tangential radiography with the source on thepipe centre line, the minimum source to pipe centre distance, SPD should be at least3.5 times the pipe OD.

Using the distances given in Figure 8.4, this corresponds to:

SDD = 3.5 OD + b’ (8.6)

Where b’ is the distance from the pipe centre to the film/detector.

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HOIS(09)RP1 Issue 1

33

It should also be verified that the SDD is not smaller than the values given below. These arebased on geometric unsharpness considerations similar to those used for DWDI (see Section8.2), but adapted for the different plane of interest on the pipe (tangent position instead of thesource 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 centreline.

The SDD’s for this combined technique should be those for DWDI, i.e. as given in equations(8.4) and (8.5), unless the value given by equation (8.6) is larger, in which case that shouldbe used where practicable.

8.3.3 Source offset from pipe centre line

For tangential radiography with the source offset from the pipe centre line, the effects of thedifferent dimensional calibration techniques on measured wall thickness accuracy are muchless significant than with the source on the pipe centre line.

It is therefore appropriate to use recommended SDD’s based solely on an unsharpnesscriterion. In this case, the plane of the object of interest is the pipe tangent position, asillustrated in Figure 8.6 below.



HOIS(09)RP1 Issue 1

SDD

b’

Source, sized Detector

SPD

Figure 8.6 Distances for offset source tangential radiography

The recommended SDD is then given by equations (8.7) and (8.8) for the higher andstandard qualities of inspection.

These recommended SDD values for offset tangential radiography are plotted in Figure 8.7.

0

200

400

600

800

1000

1200

0 20 40 60 80 100 120 140

Distance from detector to centre of pipe, b (mm)

S D D ( m m )

Standard Quality

Higher Quality

Approximate pipe nominal bore for detector in contact with pipe (in)

1 2 43 5 6 108

Figure 8.7 Recommended Source to Detector distances (SDD) for offset tangentialradiography, for an assumed effective source size of 2.3mm.

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HOIS(09)RP1 Issue 1

9 Circumferential and axial coverage and overlap

9.1 Circumferential coverage

When using the DWDI and DWSI techniques, then full circumferential coverage of a pipe isachieved 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 circumferentialexposures to be calculated on the basis of a maximum permissible increase in penetratedthickness due to inclined penetration at the edges of the diagnostic area. This increase inpenetrated thickness is a function of:

SDD Wall thickness, WT

Pipe OD

Following EN1435 class A for DWSI, with the standard and higher quality classes of CR wallloss inspection, it is recommended that for wall loss CR, the maximum permissible increasein penetrated thickness should be 20%. The minimum number of exposures is then given byFigure A.4 of EN 1435 :1997 (page 17).

For the source positioned outside the pipe, Figure 9.1 shows the number of exposuresneeded, as a function of two dimensionless variables – WT/OD and OD/SDD.

Minimum number of exposures DWSI

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

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

O D / S D D

> 3

> 4

> 5

> 6

> 7

> 84

5

7

6

8

9

3

Figure 9.1 Minimum number of DWSI exposures circumferentially around a pipe as

a function of the ratios WT/OD and OD/SDD. This graph shows the sameinformation as Figure A.4 from EN 1435: 1997, for source positionsoutside the pipe.

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HOIS(09)RP1 Issue 1

If the detector is offset from the pipe wall due for example to the presence of insulation, thenFigure 9.1 is not applicable, and the values given in Figure 9.2 should be used instead. InFigure 9.2 note that the vertical axis is the pipe OD divided by the distance from the sourceto the pipe axis (SPD).

To obtain the circumferential angular difference (in degrees) between exposures, thefollowing formula should be used for DWSI:

Angular difference = 360 / (Number of exposures)

Minimum number of exposures DWSI

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

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

O D / S P D

> 3

> 4

> 5

> 6

> 7

> 84

5

7

6

8

9

3

Figure 9.2 Minimum number of DWSI exposures circumferentially around a pipe, asa function of the ratios WT/OD and OD/SPD, where SPD is the distancefrom the source to the pipe axis (centre). This figure should be usedinstead of Figure 9.1 if the detector is not in contact with the pipe wall(e.g. due to the presence of insulation).

For DWDI (see Section 9.1.2), recent trials [5] have shown that the maximum permissibleincrease in penetrated thickness can be greater than 20%, without significant loss of

sensitivity for pitting type flaws in pipes.

Thus for DWSI, it may also be possible to reduce the number of exposures below the valuesshown in Figure 9.2. However further experimental trials would be needed to investigate this.For DWSI large variations in source to detector distance occur across typical images, whichresult in significant changes in background image brightness. This effect should also beconsidered if new criteria are to be established for DWSI circumferential coverage.

9.1.2 DWDI

For DWDI, the detectability of flaws at the tangential position represents the most demandingposition for circumferential coverage. Recent trial results [5] showed that the detectability of

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HOIS(09)RP1 Issue 1

37

pitting flaws at or very close to the tangent position was generally reduced compared withthat obtained when the flaws were on the pipe centre line, especially with pipes having largermaximum penetrated thickness values, wmax. However rotation of the source/detector so thatthe flaws were 15-20º from the tangent position then gave detectability similar to thatachieved with the flaw on the pipe centre line.

In general, at least two exposures are recommended for DWDI, separated in circumferentialangle by at least 30-40º (for best results the angle would be 90º).

However, for those pipes having maximum tangential paths, wmax, less than about 50% of themaximum 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 estimatepenetrated thickness (Section 12), it is important to ensure that the feature being measuredon the CR image is as close as possible to the centre of CR image. This is likely to requireadditional circumferential exposures to align the flaw of interest more closely with the sourceaxis.

9.2 Axial coverage

The maximum axial coverage for a single CR image can be determined in a similar manner

to that for DWSI circumferential coverage (see Section 9.1.1). For axial distances, thegeometry is then simpler, as illustrated in Figure 9.4. For a 20% increase in penetratedthickness at the edge of the area to be inspected, then it can be shown the total extent, Ld, ofthis area on the detector is

Ld= 1.32 SDD

The corresponding axial coverage on the source side of the pipe, Lp, is then:

Lp = 1.32 f

where f is the distance between the source and the source side of the pipe. Lp should beused when determining the interval along the pipe of exposures.

Note that for DWDI, typical CR image plates are likely to be significantly less than Lp, so thisis unlikely to be a constraint found in practice, and the axial interval between exposures willbe determined by the dimension of the CR image plate.

However, for DWSI, especially on smaller diameter pipes, the value for Lp needs to becalculated and used when determining the axial interval between exposures.

As for circumferential coverage, if quantitative analysis of the image grey level information isbeing used to measure penetrated thickness (Section 13), it is important to ensure that the

feature being measured on the CR image is as close as possible to the centre of CR image.



HOIS(09)RP1 Issue 1

SDD

Source, size

d

Detector

fLd

Lp

Figure 9.4 Axial cross section for DWDI inspection (also applicable to DWSI),showing the maximum permissible axial length, Ld, of the evaluated areafor a single source position, on the detector and along the pipe, Lp.

9.3 Overlap of images

The separate CR images shall overlap sufficiently to ensure that no portion of the componentremains un-examined. Unless otherwise specified, the minimum overlap shall be 25 mmaxially either side of the diagnostic area, measured on the source side.

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HOIS(09)RP1 Issue 1

39

10 CR image quality indicators

10.1 Background

Experience has shown that some form of objective check on image quality is important for in-service computed radiography. Operators should be aware that very short exposure timeswith computed radiography can give CR images which may appear to the eye, subjectively atleast, 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, andhence poor sensitivity for wall loss detection by image grey level variations.

It is strongly recommended that some form of image quality indicator, or objectivecheck on image quality is used on all CR exposures, as described below.

10.2 Image Quality Measures for Double Wall Techniques (DWDI &DWSI)

10.2.1 Target Grey level range

For the standard quality class of wall loss inspection CR, a minimum check on CR imagequality is to ensure that a specified (target) grey level range is achieved in the area of interestof the CR image. The grey level range achieved for a particular radiation exposure alsodepends on the IP type and the scanner gain (or sensitivity) used, so it is also necessary tospecify the IP type and all user selectable scanner parameters when setting a target greylevel range (pixel size, 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) shouldfirst be made for a range of exposures. The results should be plotted as a function of greylevel, as illustrated in Figure 10.1. This allows the grey level corresponding to a normalisedsignal to noise ratio of 50 to be determined, for a specified scanner/IP combination and thespecified scanner user settings including gain/sensitivity and pixel size.

In the example shown in Figure 10.1, for low gain, SNR_N > 50 can be achieved for greylevels >3700, whereas for medium gain grey levels must be >24000. These target grey levelranges are specific to the scanner, the IP and all the scanner settings and the CR analysissoftware. If changes are made to any of these variables, a repeat calibration is required to

establish the target grey level for which SNR_N = 50.



HOIS(09)RP1 Issue 1

0

10

20

30

40

50

60

70

80

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000

Grey Level

S N R_

N

CR50P

Low gain

CR50P

Medium gain

Figure 10.1 Measurements of normalised signal to noise ratio as a function of local

image grey level, for two different gain/sensitivity settings with Ir 192and a specific CR scanner/IP and pixel size.

For the higher quality class of wall loss inspection CR, it is required that one of the morerigorous measures of image quality described below is also applied to ensure the requiredimage quality has been achieved.

10.2.2 Signal to Noise Ratio (SNR)

For the standard and higher quality classes, it is recommended that CR image quality isassessed by measuring the normalised signal to noise ratio (SNR_N) in the area of interest,using appropriate software if available.

Note that it is important when making SNR measurements for the images to have grey levelsdirectly proportional to radiation intensity (i.e. linear response). For non-linear response CRscanners it is necessary to select an appropriate look-up table (LUT), to achieve a linearrelation between radiation intensity and CR image grey level. Different CR scanners canhave different characteristics (e.g. logarithmic or square-root, as well as linear), and the

correct LUT must be selected for the scanner used. If the CR images are not linearisedcorrectly, the values measured for SNR_N can be misleadingly high. For typical CR images,

SNR_N values will typically be in the range 50 – 200. Values in excess of this may be dueto use of an incorrect LUT, and should be checked carefully.

When making this measurement, it is important to ensure the analysis area does not includeany significant variations in grey level due to changes in penetrated thickness. Thus the SNRarea should not include, for example, component edges, areas of wall loss, or any otherimage areas where there are significant changes in penetrated thickness.

The size of the SNR area should be 20 pixels horizontally x at least 55 pixels vertically toprovide a dataset of at least 1100 values, as required by EN 14784-1.

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HOIS(09)RP1 Issue 1

41

In all cases, SNR_N measurements should be made at several locations (minimum of four)within the area of interest, and a mean value derived.

Note that for some components it may not be possible to find a homogeneous area of theimage suitable for SNR_N measurements. An example would be a pipe containinggeneralised irregular corrosion over the whole area of the image. In such cases, the targetgrey 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 depends onthe 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 to noise ratio is:

SNR_N = SNRmeas (88.6/BSR) (10.1)

Where

SNRnorm is the normalised signal to noise ratioSNRmeas is the signal to noise ratio measured on the CR imageBSR is the basic spatial resolution of the CR system in microns (depends on the

scanner pixel size and the model of CR plate)88.6 is the length of the side of a square having the same area as a circle with

diameter 100 microns (see EN14784-1, p11 for explanation)

For double wall techniques (DWSI and DWDI), and the standard quality class, thenormalised signal-to-noise ratio (SNR_N) as calculated from equation (10.1) in the pipecentre 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 pipecentre should be at least 80.

10.2.3 BSR Measurement

The basic spatial resolution (BSR) can be measured according to EN 14784-1, whichinvolves use of orthogonal duplex wire IQIs. An improved BSR measurement technique forisotope sources, to minimise the effects of noise, is described in [5]. This method involvesthe analysis of the responses obtained with a Duplex wire IQI, as shown in Figure 10.2.



HOIS(09)RP1 Issue 1

Figure 10.2 CR image of the Duplex wire IQI, rotated so that the wires are asaccurately vertical as possible.

The modulation of the different wire pairs can then be plotted as a function of wire diameter.The variation of modulation with wire diameter is linear on a log – log plot, which allowsready derivation of the wire diameter for which the modulation is 0.2 (or 20%), as illustratedin Figure 10.3.

Derivation of BSR based on measured wire pair modulations

0.1

1

0.1 1

Duplex wire diameter (mm)

M o

d u

l a t i o n

BSR

Figure 10.3 Measured Duplex wire pair modulation plotted as function of the wire

diameter. The BSR is derived from the fitted line for a modulation of 0.2.

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HOIS(09)RP1 Issue 1

43

The advantages of this more quantitative method for BSR determination include improvedrepeatability, better noise tolerance, less subjectivity, and determination of values to aprecision 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 case, itis important to ensure consistency between the directions of the SNR_N and BSRmeasurements. For the default SNR measurement direction of horizontal within a CR image(along the fast scan direction), the wires themselves within the duplex wire IQI should bealigned orthogonal to this direction (i.e. vertical in the displayed image). This is achievedwhen the long axis of the duplex wire IQI assembly is horizontal in the image, i.e. alignedwith the SNR measurement direction.

Measured values for BSR for some current CR systems are given in Table 10.1, taken from[5]. Note that the values depend on the radiation source as well as the imaging plate, pixelsize and scanner model.

If a value for the BSR for the screen/scanner combination in use is not available, aconservative estimate of 200 microns can be used, provided the screen resolution is knownto be similar to those screens given in Table 10.1, and the scanner pixel size does notexceed 100 microns.



HOIS(09)RP1 Issue 1

44

Table 10.1 Basic spatial resolution (BSR) of some CR scanners and imaging plates,measured using Duplex wire IQI

CR System Basic spatial resolution,BSR*

(microns)

Scanner IP Pixelsize

(m)

Radiationsource

Accordingto

EN14784-1

Morequantitative

method[see ref 5]

HD CR 35NDT

White, pre April2008

100 Iridium 192 160

HD CR 35NDT

White, pre April2008

100 Selenium 75 160

HD CR 35NDT

White, post April 2008

100 Iridium 192 145 163

HD CR 35NDT


50 Iridium 192 115 133

HD CR 35NDT


100 Selenium 192 145

HD CR 35NDT


50 Selenium 192 100

HD CR 35NDT

Blue 50 Iridium 192 100

HD CR 43NDT


100 Iridium 192 145

HD CR 43NDT

Blue 50 Iridium 192 90

Fuji systemDynamiX

ST-VI 100 Iridium 192 160 176

Fuji systemDynamiX

ST-VI 100 Selenium 75 160

GE CR50P IPC2 100 Iridium 192 200 236

GE CR50P IPS 50 Iridium 192 130

GE CR50XP IPC2 100 Iridium 192 200

GE CR50XP IPS 50 Iridium 192 145 165

GE CR50XP IPS 50 Selenium 75 130

GE CR100 IPC2 100 Iridium 192 200 230 GE CR100 IPC2 100 Selenium 75 200

GE CR100 IPS 100 Iridium 192 160

GE CRXTower IPC2 100 Iridium 192 160

GE CRXTower IPC2 100 Selenium 75 160

GE CRXTower IPS 50 Iridium 192 130

GE CRxFlex IPC2 100 Iridium 192 160 222

GE CRxFlex IPS 100 Iridium 192 145

GE CRxFlex IPS 50 Iridium 192 100 100

* The effective spatial resolution or unsharpness of the CR plate/scanner isapproximately twice the BSR [see equation (4) of EN14784-1].



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 toprovide a quantifiable measure of image quality. In general, the minimum IQI wire diametervisible on a CR image will be limited by the total unsharpness of the image, which is made upfrom contributions from the screen unsharpness and geometric unsharpness. The CR imagesignal to noise ratio and radiographic contrast will also affect the minimum visible IQI wirediameter.

For DWDI, the IQIs should be placed on the source side of the pipe, where possible. If thepipe is insulated, the IQI will need to be placed on the detector side. For DWSI, the IQIs needto be placed on the detector side (between pipe wall and detector).

For both DWDI and DWSI, the IQIs should be positioned close to the centre of the resultingCR image. If the IQIs are close to the edges of the images, a smaller number of wires maybe detected than for centrally placed IQIs.

For DWDI inspection using Ir 192, source-side measured and target values for IQI wirenumbers (as defined in EN 462-1) are shown in Figure 10.4. These measurements coverpipes with wall thicknesses in the range c. 3 mm to 22 mm. Separate values are shown forthe standard and higher image quality classes (SNR_N ≥ 50 and ≥ 80 on the pipe centre line,respectively). For product filled pipes, the total equivalent steel penetrated thickness shouldbe 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 factor

representing the lower attenuation of the product compared with steel. Estimates for f are 9

for water and 11 for oil (no measured value is available for oil).

DWDI - Ir 192

5

6

7

8

9

10

11

12

13

14

15

0 10 20 30 40 50 6

Equivalent steel total penetrated thickness (mm)

I Q I v a l u e W

0

DWDI Bergen Oct 08 IPC2/CR100DWDI IPC2/CR50P NDT ServicesDWDI Bergen Feb 08 IPC2/CRX Tower DWDI Bergen May 09 ST6/FujiDWDI Water filled Bergen Feb 08 IPC2/CRX Tower DWDI Water filled Bergen Oct 08 IPC2/CR100DWDI Water Filled Bergen May 09 ST6/Fuji & HD CR 35/WhiteDWDI Ir 192 Target valuesDWDI HIGHER Bergen Feb 08 IPS/CRX Tower DWDI HIGH Bergen Oct 08 IPS/CR50XPDWDI HIGHER Bergen May 09 HD CR 35 White IPDWDI Water filled HIGHER Bergen Feb 08 IPS/CRX Tower DWDI Water filled HIGH Bergen Oct 08 IPS/CR50XPDWDI Ir 192 High Target valuesEN1435 Class A (DWDI)

Figure 10.4 Measurements of smallest IQI wires visible on DWDI Ir192 CR images, asa function of equivalent steel total penetrated thickness (on pipe centreline). The values from EN1435 for film radiography weld inspection areshown for comparison purposes. The IQI wires were on the source side

of the pipe.

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Corresponding measured IQI values and target values for DWSI inspection using Iridium 192are given in Figure 10.5.

DWSI - Ir 192

5

6

7

8

9

10

11

12

13

14

15

16

0 10 20 30 40 50 6


I Q I v a l u e W

0

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

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 Tower

DWSI Water filled HIGHER Bergen Oct 08 IPS/CR50XP

DWSI HIGHER Water Filled Bergen May 09 HD CR 35/White IP

DWSI Ir 192 High Target values

EN1435 Class A (DWSI)

Figure 10.5 Measurements of smallest IQI wires visible on DWSI Ir192 CR images, asa function of equivalent steel total penetrated thickness (on pipe centreline). The values from EN1435 for film radiography weld inspection areshown for comparison purposes.

With Selenium 75, increased radiographic contrast is generally obtainable compared withIridium 192, within the range of applicability of the source. This results in higher IQI valueswith Selenium 75 for images having the same SNR_N values. The available measurements

for Selenium 75 DWDI and DWSI inspections are given in Figures 10.6 and 10.7respectively.

DWDI : Se 75

6

7

8

9

10

11

12

13

14

15

0 5 10 15 20 25 30 35 40


I Q I v a l u e W

DWDI SELENIUM Bergen Feb 08 IPC2/CRX Tower DWDI Bergen Oct 08 IPC2/CR100DWDI Bergen May 09 HD CR 35/WhiteDWDI Water Filled Bergen Oct 08 IPC2/CR100DWDI Water Filled Bergen May 09 HD CR 35/WhiteDWDI Ir 192 Target valuesDWDI HIGHER Bergen Oct 08 IPS/CR50XPDWDI HIGHER Bergen May 09 HD CR 35/WhiteDWDI Ir 192 High Target valuesEN1435 Class A (DWDI)

Figure 10.6 Measurements of smallest IQI wires visible on DWDI Se 75 CR images,as a function of equivalent steel total penetrated thickness (on pipecentre line). The IQI wires were on the source side of the pipe.

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DWSI : Se 75

6

7

8

9

10

11

12

13

14

15

0 10 20 30 40 50 60


I Q I v a l u e W

DWSI SELENIUM Bergen Feb 08 IPC2/CRX Tower DWSI Bergen Oct 08 IPC2/CR100DWSI Water Filled Bergen Oct 08 IPC2/CR100DWSI Bergen May 09 HD CR 35/WhiteDWSI Water Filled Bergen May 09 HD CR 35/WhiteDWSI Se 75 Target valuesDWSI SELENIUM HIGHER Bergen Feb 08 IPS/CRX Tower DWSI HIGHER Bergen Oct 08 IPS/CR50XPDWSI HIGHER Bergen May 09 HD CR 35/White IPDWSI Higher Water Filled Bergen Oct 08 IPC2/CR100DWSI HIGHER Water Filled Bergen May 09 HD CR 35/White IPDWSI Se75 High Target valuesEN1435 Class A (DWSI)

Figure 10.7 Measurements of smallest IQI wires visible on DWSI Se 75 CR images, asa function of equivalent steel total penetrated thickness (on pipe centreline).

The tables below summarise the target IQI values derived from the experimental CR imagescollected during the HOIS trials, as given in Figures 10.4 to 10.7 above. Note that thethickness t is the equivalent steel total penetrated thickness on the pipe centre line, wt, asgiven by equation 10.2.

DWDI Iridium 192 (source side IQIs)

Standard Image quality Higher Image quality

Thicknessrange (mm)

IQI value, W Thicknessrange (mm)

IQI value, W

5 wt < 15 9 5 wt < 8 12

15 wt < 25 8 8 wt < 12 11

25 wt < 40 7 12 wt < 15 10

40 wt < 60 6 15 wt < 20 9

DWSI Iridium 192 (detector side IQIs)


Thicknessrange (mm)


IQI value, W

5 wt < 10 11 15 wt < 20 10

10 wt < 15 10 20 wt < 35 9

15 wt < 25 9 35 wt < 60 8

25 wt < 30 8

30 wt < 60 7

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DWDI Selenium 75 (source side IQIs)


Thicknessrange (mm)


IQI value, W

5 wt < 12 11 5 wt < 10 12

12 wt < 18 10 10 wt < 20 11

18 wt < 25 9

25 wt < 35 8

DWSI Selenium 75 (detector side IQIs)


Thicknessrange (mm)


IQI value, W

5 wt < 15 11 5 wt < 25 11

15 wt < 25 10 25 wt < 40 10

25 wt < 30 9 40 wt < 45 9

60 wt < 60 8

10.2.5 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 toassess IQI visibilities in any meaningful way.

However, very noisy CR images will give lower wall thickness measurement accuracies thanless noisy CR images. In addition, the unsharpness of the CR image will influence the WTmeasurement accuracy.

Hence, some form of quality control for tangential CR radiography is considered necessary,as follows.

Ensure that the SDD’s recommended in Section 8.3 are used.

The exposure time should be adjusted so that the un-impeded radiation beam outsidethe pipe wall does not exceed 80-90% of the CR imaging system’s saturation value(see Sections 12 and 14.4 for further information on exposure times and burn-offeffects).

The resulting exposure time depends on the gain setting of the CR system. It isimportant that a sufficiently low system gain is used to allow the SNR_N values givenbelow to be achieved.

If the available software provides this function, the normalised signal to noise ratio(SNR_N) should be measured with appropriate image analysis software. The averageSNR_N values obtained in the free beam outside the pipe should be at least 70 forthe standard quality and 110 for the higher quality classes of wall loss inspection.



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If the pipe centre line is available for measurement of SNR_N, then the DWDI SNR_Nvalues of 50 and 80 for standard and higher qualities respectively can be used as analternative 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 formhaving the image grey levels directly proportional to radiation intensity, otherwise the valuescan 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 has a lower average photon energy than that of the primaryradiation beam. It is therefore important to reduce scattered radiation by means of leadscreens.

The recommended values given in EN 14784-2 are reproduced for convenience inTable 11.1. These screen thickness values are generally greater than those recommendedfor film radiography and are thicker than standard.

Thinner screens with standard thickness can be used provided the specified image qualityvalues are achieved (see Section 10).

Table 11.1 Recommended metal screens for computed radiography taken from

Table 2 of EN 14784-2. Thinner (standard) screens can be used provided the specifiedimage quality values are achieved (see Section 10).

Type and minimum thickness in mm of metalscreens

Radiation Source Penetrated WT, wmm

Front Back

w < 5 Pb 0.1 Pb 0.1Yb 169, Tm 170

w > 5 Pb 0.1 Pb 0.1

w < 50 Pb 0.3 Pb 0.3Ir 192, Se 75

w > 50 Pb 0.4 Pb 0.4

w < 100 Fe 0.5/Pb 1.5 Fe 0.5/Pb 1.0Co 60

w > 100 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 minimumvalues, and thicker screens should be used, where possible, to reduce burn-off effects at theouter edges of the pipes, and lower the overall image contrast.

For pipes containing product, increased levels of scatter may be obtained, depending on thepipe wall thickness, and the penetrated thickness of product. Increased levels of scatter arelikely to be associated with greater penetrated thicknesses of product, and reduced pipe wallthicknesses.

Front and back screens with increased thickness (e.g. up to 1.5 mm thick front screens) maythen be beneficial in some cases. Note however that use of thicker lead screens will thenrequire increased exposure times to achieve acceptable SNR_N values, as defined inSection 10.

For high strength sources, significantly thicker back screens can be needed to compensatefor increased scatter from the plant environment in the vicinity of the detector.



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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 quality,and hence poor sensitivity to wall loss with the DWDI and DWSI techniques. Correspondinglylower wall thickness measurement accuracies can be anticipated with the tangential methodfor short exposure time images.

To avoid these issues, the exposures times for computed radiography should be sufficient togive 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 sourceout of the container into the exposure position and back again should be less than 10% ofthe 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:

S

SDD2

tot0

4-

w wexpE2.4x10E (12.1)

Where

Ew is the exposure time in sec for penetrated thickness w.E0 is the exposure needed to achieve the required SNR_N value for zero penetrated

thickness in units of Ci.min @ 500mm.

S is the source strength in Ci.SDD is the source to detector distance in mm.

is the measured effective material attenuation coefficient (0.04 /mm for Ir 192 and0.08 /mm for Se 75).

wtot is the total steel equivalent penetrated thickness, including any product in the pipe.

Note that if the exposure, E0, is expressed in units of gBq. sec @ 1000mm, then the constantin equation (12.1) is 2.7 x 10-8.

For pipes fully filled by product, the steel penetrated thickness (2 WT) on the pipe centre lineshould be increased by approximately ID/9 for water and c. ID/11 for oil (density of 0.8

gm/cm

3

) to give the total steel equivalent penetrated thickness, wtot.

The exposure times required to obtained the recommended SNR_N values on the pipecentre line depend on the imaging plate and scanner.

For the standard image quality class (SNR_N ≥ 50 on the pipe centre line), exposures, E0 in

the range 3 - 15 Ci.Min @ 500mm have been measured in practical trials [5], depending onthe CR scanner and imaging plate being used.

For the higher image quality class (SNR_N ≥ 80 on the pipe centre line), exposures, E0 in the

range 10 - 30 Ci.Min @ 500mm have been measured in practical trials [5], depending onthe CR scanner and imaging plate being used.

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Significantly longer exposures may be needed if high-resolution imaging plates intended for

weld inspection are used for in-service examination (E0 30 Ci.Min @ 500mm for standard

image quality and E0 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 aguide.

It is also important to ensure that the scanner gain/sensitivity setting is adjusted so that theunimpeded radiation beam outside the pipe wall is not saturated, which will cause “burn-off”and errors in wall thickness measurement.



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13 Penetrated Thickness measurements

13.1 Introduction

For the DWDI and DWSI techniques, computer analysis of the CR image grey levels can, incertain conditions, be used to estimate wall thickness changes. It is important to notehowever 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 thecomponent under inspection using test components with closely similar wall thickness andpipe OD. These test components should contain areas of wall loss of known through-wallextent, to allow validation of the penetrated thickness analysis method, described below.

It should be emphasised that this is a relative method for measurement of wall loss, andunlike the tangential method (Section 14), the penetrated thickness method does not providedirect measurements of remaining wall thickness.

13.2 Principle of method

The software used for this purpose generally assumes that the detected radiation intensity isrelated to penetrated thickness by:

I(w) = I0 exp(- w) (13.1)

Where

I(w) is the intensity for penetrated thickness wI0 is the unimpeded beam intensity

is the effective linear attenuation coefficient of the material

13.3 Effects of scattered radiation

In practice, it is important to appreciate that a number of effects, in particular the presence ofscattered radiation, complicates the analysis. Thus equation 13.1 is often only approximate,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 attenuationvalues unreliable. For example, a theoretical value (with no scatter) for the linear attenuation

coefficient, , for Ir 192 and steel is 0.084 /mm. However, measured values can vary widelybetween at least 0.02 /mm to 0.05 /mm, depending on scatter levels.

13.4 Calibration using step wedge

For this method, it is important to calibrate using a step wedge placed on the componentunder inspection, as close to the area of interest as possible. For the DWDI method, the stepwedge can be positioned on the source side of the pipe. For DWSI, the step wedge needs tobe positioned between the pipe wall and detector, which leads to certain practical issues (seeSection 13.6).



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For both DWSI and DWDI methods, the penetrated thickness increases away from the pipecentre-line, so it is important that the step wedge is positioned so that the penetratedthickness for the step wedge is similar to that for the wall loss being measured.

The effective attenuation coefficient should be measured from at least the first and secondsteps on the step wedge. If the attenuation values derived from the different steps are ingood mutual agreement, an average value can be used, and this provides some confidencein the accuracy of the subsequent measurements of wall loss. However, if the attenuationcoefficients derived from the different steps are not in good mutual agreement, the valuederived from the first step (smallest increase in penetrated thickness) should be used whenestimating wall loss. Such differences can also indicate the presence of significant levels ofscatter 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 grey levels must be linearised using the correct look-up table for the CRscanner, if a non linear amplifier is built into the CR scanner.

A step wedge must be used to measure the effective attenuation coefficient, andlocated such that the local penetrated thickness is close to that for the wall loss beingmeasured.

The reference and measurement areas should be as large as possible given the sizeof features in the image to measure. (Small areas are more affected by noise).

The reference and measurement areas should be as close together as possible,given the limitations of the image content.

With this technique, initial results suggest that accurate (±1mm) results can only obtained forrelatively small wall losses of c. 30 – 50%.

If the actual total penetrated thickness is not known accurately, differences from an assumedvalue can be used to measure small percentage wall losses with reasonable accuracy.

Use of substantially thicker lead screens may preferentially absorb the lower energyscattered radiation and hence increase accuracy in some cases.

13.6 Limitations

If this method is used to estimate wall loss, it is important to appreciate its limitations and tonote that it is a relatively new method which has not been fully validated.

Practical issues can include:

1. For the DWDI method, if wall loss occurs on both pipe walls (those near to and far fromthe source) the corresponding radiation intensity changes can be superimposed on theCR image, leading to an overestimation of the wall loss, if it is assumed to be present in asingle pipe wall only.

2. For DWSI it can be difficult to place a step wedge between the pipe wall and detector,without distorting the detector and increasing the object to detector distance significantly

(which can lead to a need to increase the SDD – see Section 8.1).



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3. For DWSI, and some DWDI images, the CR images can contain significant variations inbackground grey level, due the rapidly changing SDD and penetrated pipe wall thicknessvalues across the image. These background variations introduce uncertainties into themeasurement of an appropriate reference grey level for the area of wall loss, especially ifa single measurement area on only one side of the indication is used.

In the presence of significant variations in background grey level, analysis methodsbased on extrapolation of grey-scale profiles extracted from the CR image arerecommended to estimate the background image grey level “underneath” the area of wallloss.



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14 Tangential radiography

This Section contains important recommendations regarding tangential radiography.

14.1 Recommended SDD

The recommended source to detector distances (SDD) for tangential radiography havealready 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 is generally positioned on the pipe central axis, as illustrated in Figure 14.1. Thisallows both walls, on either side of the pipe, to be inspected on a single CR image.

CR plate

Figure 14.1 Tangential radiography with the radiation source on the pipe centre line.

For larger diameter pipes (both insulated and non insulated), the radiation source should beoffset from the pipe centre line, to be instead in-line with the tangential position on the pipe,as shown in Figure 14.2.

In all cases, it is important to check that the tangential path for the pipe is within the sourcepenetration limits given in Table 6.2.

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CR plate

Figure 14.2 Tangential radiography with the radiation source offset from the pipecentre line by half the pipe diameter, so it is in-line with the tangentialposition on the pipe.

There are no specific recommendations concerning the maximum pipe diameter whichshould be inspected using the radiation source on the pipe axis. This will in practice dependon the size of the CR plate available, as well as the method used for dimensional calibration(see Section 14.3 below).

14.3 Dimensional calibration

For tangential radiography, when making dimensional on-screen measurements of wallthickness, it is important to calibrate the distances involved in the radiography, to allow forthe image enlargement or “blow-up”. The geometric magnification effect for tangentialradiography is shown in Figure 14.3.

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Detector

PDD

r

WT’

SPD

x

SDD

Figure 14.3 Geometric magnification for tangential radiography showing themeasured wall thickness WT’.

Two methods can be used for dimensional calibration, to derive the actual wall thickness WTfrom the measured value WT’ as follows.

14.3.1 Measurement of distances

This method involves direct physical measurement of the key distances involved in theradiography.

Note that reliable and accurate physical measurements of distances may be difficult toachieve in a plant environment, and the measurements may not be available at the time ofthe analysis. If this is considered to be the case, the alternative comparator method (Section14.3.2) is recommended.

For calibration by the distances method, two of the following distances need to be measuredaccurately (to within a few percent):

1. Source to detector distance SDD2. Distance from source to pipe centre line, SPD.3. Distance from detector to pipe centre line, PDD.

In addition, the following distance must be recorded:

4. Lateral offset (if any) of source from pipe centre line, x.

This method also assumes that the CR system software already contains correct information

on the scanner resolution (pixel size) used when acquiring the CR image.

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For offset tangential radiography (with x ~ r) , the true wall thickness WT at the tangentialpipe position as shown in Figure 14.2 can be calculated from the projected (measured) wallthickness WT’ on the CR plate using the approximate equation (14.1):

'WT)1(WT SDD

PDD (14.1)

For tangential radiography on the pipe centre line (x =0), equation (14.1) is an approximation

which becomes less accurate as the angle, shown in Figure 14.3 increases. The exactcalculation then requires application of the more involved equation (14.2) [6].

SDD

WT'

r

r 1

SDD

WT'

r

r

r WT

22

22

SPD

SPD

SPD

(14.2)

Where r is half the pipe outside diameter (=OD/2).

Provided all the distances involved can be measured sufficiently accurately, and that the CRimage pixel size is known reliably, then software which applies equation (14.2) to derive theactual wall thickness WT from the measured value WT’ is capable of high wall thicknessmeasurement accuracy, even for values of SDD/OD < 3.

Note however, that equation (14.2) is only valid for “fan-beam” geometries, and will be onlyapproximate for 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 shown

in Figure 14.4 is non zero (see also Section 8.3.1).

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Detector

R

f

Pipe walls

SDD

Figure 14.4 Section along the pipe axis showing the three-dimensional cone beam

effect (for non zero values of the angle ).

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14.3.2 Dimensional comparator

An alternative method for dimensional calibration is the use of ball bearing or similardimensional 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 thetangent position on the pipe wall, as illustrated in Figure 14.5.

Detector

Comparator,diameter c

c

Figure 14.5 Tangential radiography showing use of comparator for dimensionalcalibration. The comparator should be placed as close to the pipe wall aspossible, without overlapping it.

On-screen measurements of the imaged size of the comparator, using the CR systemsoftware, then allow the pipe wall thickness measurement to be calibrated in mm, henceallowing for the radiographic magnification or “blow-up”. The actual wall thickness WT is thengiven by:

c'

c WT'WT (14.3)

Note that if the comparator cannot be placed adjacent to the pipe tangent position, due forexample to the presence of external insulation, it is recommended that the source is offsetfrom the pipe centre-line to be aligned with the pipe wall as shown in Figure 14.6.

As an alternative to the use of a dimensional comparator, if the outside diameter of the pipeis known reliably, then the measured pipe diameter on the CR image can be used fordimensional calibration. This method does not then require an additional calibration object,and can have advantages in terms of accuracy provided the pipe outside diameter is knownaccurately.

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Detector

Comparator

Figure 14.6 Tangential radiography showing use of offset source position withcomparator for dimensional calibration, for insulated pipes, where thecomparator should be placed as close to the outside of the insulation aspossible.

14.4 Use of lead strips to avoid burn-off

Some CR procedures specify the use of lead strips close to, or coincident with edge of thepipe to avoid image burn-off effects at the pipe OD.

However, the use of these strips is not recommended for computed radiography, as theywould adversely affect the accuracy of the preferred wall-thickness measurement methods,which involve analysis of the image values (profiles) extracted along lines orthogonal to thepipe wall, see Section 14.5 below.

Burn-off should instead be minimised by ensuring the exposure time is adjusted so that theunimpeded beam beyond the pipe wall is not greater than 90% of the dynamic range of theCR plate (not too close to saturation).

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.

14.5 Wall thickness measurement

14.5.1 Interactive on-screen measurements

Many CR systems contain software options which allow on-screen interactive dimensionalmeasurements using a cursor overlaid on the CR images. The user then judges by eye thelocations in the image of the inner and outer edges of the pipe wall.

This method can however be prone to error, especially if the penetrated thickness of the pipewall at the tangent position (wmax) is approaching the maximum possible (see Section 7.2),

given the radiation source in use. This makes the exact positions of the outer and especially

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inner walls difficult to determine by eye. Furthermore, the apparent locations can be affectedsignificantly 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 andbrightness settings are varied on the same image.

If the on-screen measurement method is used, it should, if at all possible, first be checked foraccuracy using the current contrast and brightness settings of the displayed image, byapplication to a section of the pipe with known wall thickness (e.g. known to be uncorrodedor not eroded).

For pipes having maximum tangential paths which approach the maximum permissible forthe source in use (see Table 6.2), the interactive on-screen measurement method is bestapplied 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 theposition of internal pipe wall (ID).

Logarithmic CR images can be obtained from logarithmic-response CR scanners (e.g. the

CR100) or by application of an appropriate logarithmic look-up table (LUT) to CR imagesresulting from scanners with linear response functions.

In addition, use of high-frequency spatial filtering (sharpening) is recommended since thisimproves the accuracy of this measurement method, by emphasizing the positions of theedges of the pipe wall in the CR images, and reducing any dependence on the contrast andbrightness settings on the image.

It is however recommended that for improved accuracy interactive on-screen measurementsare made in combination with grey-level profile analysis methods as described in thefollowing section.

14.5.2 Grey-level profile analysis methods

Many CR systems have software which allows the user to mark lines on the CR imageorthogonal 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 ofwall thickness can be obtained by either interactive or automated analysis of these grey-levelprofiles.

Automated routines Automated analysis routines can increase the reliability of the measured wall thicknessvalues, unless the maximum tangential penetrated thickness (wmax) is approaching the

maximum possible, given the radiation source in use (see Section 7.2). In addition, otherfactors such as the presence of external scale, corrosion products or irregularinternal/external corrosion may affect the accuracy of these automated routines.

In these cases, the automated routines are subject to uncertainties, and the operator shouldcheck the consistency of the derived values with the density profile.

Interactive methodsIn the absence of automated routines for wall thickness analysis, the operator should use theavailable interactive facilities for analysis of the image profiles. Accuracy is likely to beimproved, especially for pipes having larger wmax values, if the CR images have a logarithmicresponse and are high-pass filtered.



HOIS(09)RP1 Issue 1

Figure 14.7 shows an example of interactive measurement of wall thickness, using cursorson a grey level profile across the pipe wall, after applying a logarithmic look-up table to theCR image, and high-pass filtering to enhance details. The position of the outer diametercorresponds to a clear peak in the profile, and the location of the inner diameter is given bythe 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 wall thickness measurement using cursorssuperimposed on a grey level profile taken across the pipe wall.

It should be noted that the accuracy of all measurement methods decrease as the tangentialpenetrated thickness, wmax, approaches the maximum value recommended for the isotope inuse (see Section 7.2), since the location of the inner wall becomes increasingly difficult todetermine with any reliability due to lack of contrast and increased noise.

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15 Scanner parameters, image recording andprocessing

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 orsensitivity increases the grey levels, but has negligible effect on the image quality, asmeasured by the normalised signal noise ratio (SNR_N). To increase SNR_N, the exposuremust be increased, not the scanner gain.

For scanners with linear responses between radiation dose and grey level, use of lowgain/sensitivity reduces the risk of image saturation. For higher scanner gains, imagesaturation may occur, especially in the free beam areas, for relatively short exposures, whichdo not give sufficiently high image SNR_N values to meet the quality criteria given in Section10.

15.2 Image recording and storage

The CR images from the scanner should be stored in a file format which supports a minimumof 12-bits/pixel.

The images should be stored at full resolution and full dynamic range, as delivered by thedetector system. Only image processing required for detector calibration should be applied tothe images prior to storage of these raw data.

Any data compression techniques used in the storage of these files should be “loss-less”.

The following information should be recorded as header information, attached to each CRimage, to be used for reporting purposes.

(a) Name of company carrying out the inspection(b) Test report number(c) Component under test

(d) Date of test(e) Material tested(f) Nominal pipe diameter and wall thickness(g) Details of any lagging/insulation, including thickness(h) Radiographic technique and class(i) Radiation source: type, size of focal spot & equipment used(j) CR plate used (IP), screens and filters(k) X-ray tube voltage & current or gamma ray source strength(l) Exposure time(m) Source to detector distance (SDD)(n) Component to detector distance and/or source to component distance(o) Type and location of IQIs used(p) Any deviations from this procedure



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(q) Name and certification of responsible person(r) Average pixel values (grey levels) in the area of interest(s) Measured SNR, if software permits derivation

15.3 Image processing

All measures of image quality (mean image grey level in area of interest, signal to noise ratioif available, IQI wire visibility etc) must be made on images which have not been subject toany image processing routines which use spatial filtering.

Spatial filtering is defined as a process in which the value of the current image pixel is alteredby an amount which depends on other (usually adjacent) image pixel values. Examples ofspatial filtering or spatial image processing include noise smoothing/filtering, edgesharpening, edge contrast, un-sharp masking etc. Spatial filtering such as edge sharpeningmay however be beneficial for the interactive measurement of wall thickness using thetangential 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 imagequality, 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, andthat the raw data derived from the CR scanner should not be altered by any subsequentprocessing.

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). Results from HOIS CR Trials in Bergen October 2008. HOIS(08)R8Issue 1. January 2009

2. “Development of protocols for corrosion and deposit evaluation in large diameter pipes byradiography” IAEA report, Vienna, 2008.

3. Halmshaw, R (1995) Industrial Radiology Theory and Practice Second Edition, Chapman& Hall, p125

4. Burch, S F and Collett N J, (2005) Recommended Practice for the rapid inspection ofsmall bore connectors using radiography, HSE Research Report, RR 294.

5. Burch S F (2009). Results from HOIS CR Trials in Bergen May 2009. HOIS ReportHOIS(09)R5 Issue 1. August 2009

6. Willems P, Vaessen B, Hueck W and U Ewert (1999) ‘Applicability of computerradiography for corrosion and wall thickness measurements’. Insight, Vol 41, No 10, 635-637.



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