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HSEHealth & Safety 

Executive

Recommended practice for the rapid inspectionof small bore connectors using radiography 

Prepared by AEA Technology plc for the

Health and Safety Executive 2005

RESEARCH REPORT 294

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HSEHealth & Safety 

Executive

Recommended practice for the rapid inspectionof small bore connectors using radiography 

S F Burch (BA, Ph D) and N J Collett

 AEA Technology plc

551.11 Becquerel Avenue

Harwell International

Business centre

Harwell

Oxfordshire

OX11 0QJ

This document covers recommended practice for the radiographic inspection of corrosion in small bore

piping and in the small bore connections to larger diameter main pipes. Inspection of this type of

component is not covered by any internationally recognised standards or procedures, and there is

therefore a need for guidance on recommended practice.

Recommendations are given concerning techniques for detection of corrosion within different regions

of small bore connectors and their junction with main pipes. A distinction is drawn between corrosion in

the form of relatively uniform loss of wall and isolated corrosion pits. Recommendations are given

regarding the key technical aspects of in-situ radiographic inspection techniques including radiation

sources, film types, computed radiography systems, positioning of the source and film (or filmless

plate) and methods for reduction in exposure time.

This report and the work it describes were funded in part by the Health and Safety Executive (HSE). Itscontents, including any opinions and/or conclusions expressed, are those of the authors alone and do

not necessarily reflect HSE policy.

HSE BOOKS

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ii

 © Crown copyright 2005 

First published 2005 

ISBN 0 7176 2939 2

All rights reserved. No part of this publication may bereproduced, stored in a retrieval system, or transmitted inany form or by any means (electronic, mechanical,photocopying, recording or otherwise) without the priorwritten permission of the copyright owner.

Applications for reproduction should be made in writing to:Licensing Division, Her Majesty's Stationery Office,St Clements House, 2-16 Colegate, Norwich NR3 1BQor by e-mail to hmsolicensing@cabinet-office.x.gsi.gov.uk

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CONTENTS

Page

1 Introduction ...........................................................................................................................1

 2 Radiographic Methods For Corrosion Detection In Pipes .................................................... 2

2.1 Tangential method.............................................................................................................2 

2.2 Straight-through method ................................................................................................... 3 

2.3 Both methods combined.................................................................................................... 5 

2.4 Inspection of junction with main pipe............................................................................... 5 

3 Radiography Equipment........................................................................................................ 9 

3.1 Radiation Sources.............................................................................................................. 9 

3.1.1 Type of source........................................................................................................... 9 

3.1.2 Size and strength of sources .................................................................................... 10 

3.1.3 Source containers and collimation .......................................................................... 10 

3.2 In-situ inspection of plant................................................................................................ 11 

3.3 Film and screens.............................................................................................................. 11 3.3.1 Conventional film.................................................................................................... 11 

3.3.2 Fluorometallic screens............................................................................................. 12 

3.4 Computed (filmless) radiography.................................................................................... 12 

3.4.1 Image Processing on filmless systems .................................................................... 13 

3.5 Dimensional Indicators ................................................................................................... 14 

4 Equipment setup.................................................................................................................. 15 

4.1 Inspection Techniques..................................................................................................... 15 

4.1.1 Technique 1: Flat film, beam axis perpendicular to main pipe ............................... 15 

4.1.2 Technique 2: Film flat against connector, beam axis perpendicular to main pipe .. 15 

4.1.3 Technique 3: Film flat against connector, beam axis parallel to main pipe ............ 16 

4.1.4 Technique 4: Angled Film flat, beam axis perpendicular to main pipe .................. 17

4.2 Source to film distance .................................................................................................... 17 4.3 Exposure times and film density ..................................................................................... 21 

4.3.1 Fine grain film with lead screens ............................................................................ 21 

4.3.2 Other film/detectors for reduced exposure times .................................................... 22 

4.4 Image Quality Indicators................................................................................................. 23 

5 Overall performance............................................................................................................ 24 

5.1 Image quality................................................................................................................... 24 

5.2 Examples ......................................................................................................................... 25 

5.2.1 Comparison between Ir 192 and Se 75 using fine grain film with lead screens...... 25

5.2.2 Example of application to larger diameter (10 inch) main pipe specimens ............ 27 

5.2.3 Example of use of fine grain film with fluorometallic screens ............................... 29 

5.2.4 Examples obtained with computed radiography systems........................................ 30

5.3 Inspection of approximately uniform corrosion/erosion ................................................. 32 

5.3.1 Wall loss external to the main pipe ......................................................................... 32

5.3.2 Wall loss within the main pipe wall ........................................................................ 32 

5.4 Detectability of isolated corrosion pits............................................................................ 33 

6 Summary of relevant safety standards and legislation ........................................................ 35 

7 Main recommendations ....................................................................................................... 35 

8 Recommendations for further work .................................................................................... 37 

9 Acknowledgements ............................................................................................................. 37 

10 References ...........................................................................................................................37 

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EXECUTIVE SUMMARY

This document covers recommended practice for the radiographic inspection of corrosion in

small bore piping and in the small bore connections to larger diameter main pipes. Inspection of

this type of component is not covered by any internationally recognised standards or procedures, and there is therefore a need for guidance on recommended practice.

The information obtained from the HOIS2000 radiographic evaluation trials is used as the basis

for this recommended practice document. Appropriate standards are also referenced, including

ISO 2919: 1999 and ISO 3999-1:2000, which cover radiation protection aspects of gamma ray

sources and containers. BS EN 1435 which is a standard for radiographic examination of welds

is also referenced.

Recommendations are given concerning techniques for detection of corrosion within different

regions of small bore connectors and their junction with main pipes. A distinction is drawn

 between corrosion in the form of relatively uniform loss of wall and isolated corrosion pits.

Recommendations are given regarding the key technical aspects of in-situ radiographicinspection techniques including radiation sources, film types, computed radiography systems,

 positioning of the source and film (or filmless plate) and methods for reduction in exposure

time.

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1 INTRODUCTION

The inspection of complex geometries was identified as a key area for work within the

HOIS2000 core programme, at the start of the new joint industry programme (HOIS2000) in

April 1999. Standard NDT methods are routinely used to inspect vessels, tanks and medium andlarge bore pipework. However, the standard methods have difficulties in application to more

complex components and fittings such as connectors, especially those of small bore, tight bends,

T-pieces, injectors, etc. Speed of inspection was also identified as a key parameter.

In the year 2001/02, an evaluation of radiographic techniques for the inspection of corrosion in

small bore connectors and weldolets was carried out (Burch & Collett, 2002). For these trials,

specimens containing simulated corrosion in the small bore connectors attached to larger

diameter main pipes were first designed and then manufactured. These specimens contained

controlled and accurately known levels of corrosion. The trials on these small connector

specimens involved evaluation of current radiographic techniques followed by an investigation

of more novel radiographic techniques which would be capable of faster application to plant.

These included the new computed (filmless) radiographic systems which are beginning to findapplication to industrial NDT.

Inspection of this type of component is not covered by any internationally recognised standards

or procedures, and there is therefore a need for guidance on recommended practice.

The information obtained during the radiographic trials is used as the basis for this

recommended practice document, covering the inspection of small bore connectors for corrosion

using radiographic methods. Appropriate standards are referenced, including ISO 2919 and ISO

3999-1, which cover radiation protection aspects of gamma ray sources and containers, as well

as BS EN 1435 which is a standard for radiographic examination of welds.

Recommendations are given concerning techniques for detection of corrosion within different

regions of small bore connectors and their junction with main pipes. A distinction is drawn

 between corrosion in the form of relatively uniform loss of wall and isolated corrosion pits.

Recommendations are given regarding the key technical aspects of in-situ radiographic

inspection techniques including radiation sources, film types, computed radiography systems,

 positioning of the source and film (or filmless plate) and methods for reduction in exposure

time.

It is important to note that the trials were completed under laboratory conditions inside an

exposure compound or similar controlled and uncluttered environment. This made it

straightforward to set-up the radiography equipment in such a way as to achieve safe radiation

dose rates in a small controlled area, whilst using highly penetrating radioisotopes and thesource to film distances (SFD) recommended in this document. The practical application of

these recommendations under typical site conditions may need modification for particular

circumstances. In-situ, local access to small bore connectors is often restricted, limiting the

choices for the source position and shielding. However, the appropriate legislation requires the

radiography contractor to conduct a prior risk assessment of the work and it is at this stage that a

 practical assessment should highlight potential difficulties so that solutions can be planned in

advance.

In addition it should be noted that the laboratory samples on which the trials were carried out

did not contain liquid or scale. However, due to the relatively small diameter of the small bore

 piping and weldolets (the maximum internal diameter was 26 mm), it is not anticipated that the

 presence of a low density liquid such as oil or water, or relatively low density scale would

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significantly affect the performance of the radiographic techniques evaluated. Nevertheless

further trials could be carried out to check this, if considered necessary.

 Note that specific recommendations are highlighted in the document using bold text.

2 RADIOGRAPHIC METHODS FOR CORROSION DETECTION

IN PIPES

Using radiography, corrosion can be detected using two different exposure set-ups or “modes”,

 both of which can often be used on the same radiograph of a small diameter component such as

a small bore connector.

2.1 TANGENTIAL METHOD

Firstly, when used in tangential mode, a radiograph shows a direct image of the pipe wall.

Corrosion producing loss of wall can then be detected and measured by the reduction in theimaged wall thickness, as illustrated in Figure 2.1.

X or gamma-ray source

Pipe

Film

Extended area of corrosion

Image of pipe wall on film

 Figure 2.1 Principle of tangential radiography

The extent of the loss of wall due to extended areas of either internal or external corrosion can

 be measured directly from the radiographic images, provided appropriate calibration techniques

are used to allow for the enlargement of the radiographic image caused when the distance

 between the object and film is non zero - see Section 3.4 for further details.

The tangential method inspects only a small extent of the circumference of the pipe for a single

source/detector position. Thus an isolated pit is likely to fall outside the inspection region,

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unless a several radiographs are taken around the circumference of the pipe (by moving the

source and detector together circumferentially around the pipe).

For small bore connectors attached to larger diameter pipes, geometric constraints generally

 preclude use of many different circumferential angles for the radiation beam (see Section 4.2 for

further details).

Also, even if an isolated pit is located within the inspection region, its through wall extent will

 be underestimated unless it is positioned very close to the exact position at which the radiation

 pipe forms a tangent to the pipe wall, as illustrated in Figure 2.2.

The tangential radiography method is recommended for the detection and through-wall

sizing of extended areas of corrosion, and should not generally be used for isolated

corrosion pits, unless their circumferential location has already been established using for

example the density difference or straight-through method (see below).

Figure 2.2 Illustration of coverage region for tangential radiography and the location of anisolated pit for which the through wall extent would be underestimated

2.2 STRAIGHT-THROUGH METHOD

Radiography can also be used in a “straight-through” mode, whereby corrosion can be detected

 by the increased film density produced by the loss of material (see Figure 2.3).

With this method, the size of the corrosion pits in the circumferential and axial directions can be

measured from the radiograph, provided methods are used for calibration of distances - see

Section 3.4. However, the through wall extent of the corrosion cannot be measured directly,

although some information on this can be gained from the magnitude of the change in filmdensity.

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This method is recommended for the detection of isolated corrosion pits, and can also be

used for detection of extended areas of corrosion.

X or gamma-ray source

Pipe

Film

Corrosion

Image of corrosion pit on

film 

Figure 2.3  Detection of corrosion by increased film density in ‘straight-through’ mode

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2.3 BOTH METHODS COMBINED

For relatively thin-walled, small bore pipes and weldolets, a single radiograph has sufficient

dynamic range and size to show the presence of corrosion by both of the tangential and density

difference methods. Note that the source position should then be central to the small bore pipe,as shown in Figure 2.4.

X or gamma-ray source

Small bore pipe

Film 

Figure 2.4  Radiography of a small bore pipe, combining both the tangential and straight-

through modes in a single radiograph

For this combined type of radiography, as shown in Figure 2.4, the tangential method can be

applied to both sides of the pipe on the same radiograph, and the region in between will show

any loss of wall by increased film density.

2.4 INSPECTION OF JUNCTION WITH MAIN PIPE

For small bore connectors, the junction with the main pipe leads to there being a distinction

 between the sections of the connector which are external to the larger main pipe wall, and theregion within the main pipe wall, as illustrated in Figure 2.5.

For detection of corrosion within the wall of the main pipe, substantially larger metal paths need

to be penetrated, compared with the sections external to the main pipe wall.

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Source

Film/detector 

Corrosionexternal to

main pipe wall

Corrosion withinwall of main pipe

Tangentialpath

  T

 

Figure 2.5  Small bore connector inspection showing different locations of potential corrosion

sites, and maximum path through wall of main pipe, T

For a pipe with wall thickness WT and outside diameter OD, the maximum metal path, T,

through the pipe wall occurs for a line forming a tangent with the inner diameter (as shown onFigure 2.5). This maximum path is given by

( )WT-ODWT2T =   (2.1)

 Note that this applies to any line drawn through the pipe, forming a tangent to the inner surface

of the pipe. Thus T is independent of the source position.

Values for the maximum path, T, through schedule 40, 80 and 160 pipes of various diameters

are given in Table 2.1, for ease of reference. Note that these paths are generally much larger

than twice the wall thickness of the main pipe. For example, T is 100 mm or larger for 8 inch

schedule 80 pipes and above.

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Table 2.1  Maximum paths through schedule 40 & 80 pipes of various diameters

 Nominal Bore

(inches)

Outside

diameter, OD

(mm)

Schedule Wall

thickness, WT

(mm)

Max

Tangential path

(mm)

2 60.3 40 3.9 29.7

80 5.5 34.7

160 8.7 42.4

3 88.9 40 5.5 42.8

80 7.6 49.7

160 11.1 58.8

4 114.3 40 6.0 51.0

80 8.6 60.3

160 13.5 73.85 141.3 40 6.6 59.6

80 9.5 70.8

160 15.9 89.3

6 168.3 40 7.1 67.7

80 11.0 83.2

160 18.3 104.8

8 219.1 40 8.2 83.2

80 12.7 102.4

160 23.0 134.3

10 273.0 40 9.3 99.0

80 15.1 124.8160 28.6 167.2

12 323.8 40 10.3 113.6

80 17.5 146.4

160 33.3 196.7

The same information is presented graphically in Figure 2.6, which shows the maximum path

lengths as a function of pipe diameter.

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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 40Schedule 80

Schedule 160

Se 75

Ir 192

Co 60

 Figure 2.6  Maximum (tangential) path lengths through the walls of pipe of different diameter.

The maximum penetrated thicknesses for different isotope sources, as recommended in BS EN

1435 are also shown.

Also shown on Figure 2.6 are the maximum penetrated thickness for Se 75, Ir 192 and Co 60, as

recommended by BS EN 1435 for class A radiography. It can be seen that even the smaller

diameter main pipes have maximum path lengths which exceed the recommended limit of

40 mm for Se 75. For Ir 192, the limit of 100 mm is reached for a 6 inch 160 schedule pipe, a8 inch schedule 80 pipe and a 10 inch schedule 40 pipe. Note that in some cases it is possible to

exceed the thickness limits shown in Figure 2.6, using for example computed radiography.

For larger path lengths, Co 60 has more penetrating power but is much less widely used in the

oil and gas industry due to greater radiation safety hazards.

For further information on wall thickness and source selection, see Section 3.

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3 RADIOGRAPHY EQUIPMENT

3.1 RADIATION SOURCES

3.1.1 Type of source

The majority of in-situ on site radiography is carried out using gamma-ray emitting isotope

sources, although portable, light-weight X-ray sources can also be used in cases where the need

for electrical power and high voltages do not cause significant safety issues.

For the inspection of the sections of the small bore connectors external to the main pipe,

the two recommended isotope sources are Iridium 192 and Selenium 75. 

Of these, Iridium 192 is a conventional isotope source used for inspection of medium steel

thicknesses (c. 10-100 mm, Halmshaw, 1995). The energy of the majority of the gamma ray

 photons emitted is around 300 keV and the half-life is 74 days.

The isotope source Selenium 75 has been developed for industrial radiography more recently

than Ir 192. It has a lower energy gamma ray spectrum than Ir192 with main peaks at 137keV

and 265keV and a longer half life (120 days). Source strengths are available between about 2

and 80 Curies with physical sizes ranging from 1x1 mm to 3x3 mm.

Sources and their containers should comply with the requirements of ISO 2919: 1999 and ISO

3999-1:2000, which include integrity testing to 800° C.

Due to the lower gamma ray energies emitted by Selenium 75 compared with Iridium 192,

Se 75 gives radiographs with higher contrast on components with moderate steel thicknesses,

including the majority of small bore piping.

Source selection should be made according to the following criteria:

(i) Corrosion in small bore piping external to main pipe wall

• For detection of small isolated corrosion pits and sizing in the circumferential and axialdirections, by the density difference (or “straight-through”) method described in Section 2,

use of Se 75 would be preferred to that of Ir 192 due to the higher contrast obtained with the

former source.

• For detection and through wall sizing of generalised loss of wall by the tangential method

described in Section 2, both Ir 192 and Se75 have similar fit-for-purpose capabilities.

(ii) Corrosion in main pipe wall at junction with small bore connectors

• For main pipes which have maximum penetrated paths less than 50 mm (see Table 2.1 andFigure 2.6), both Ir 192 and Se75 have similar fit-for-purpose capabilities.

• Ir 192 is recommended for main pipes for which the main path T exceeds c. 50 mm (seeTable 2.1 and Figure 2.6), but is less than c. 100 mm.

• For maximum path lengths of order 100mm and larger even Ir 192 will not provideadequate penetration and use of a gamma ray source generating a higher photon energy (e.g.

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the 1 MeV photons generated by Cobalt 60) should be considered, although it is recognised

that there are additional safety issues involved in the use of this type of source.

• Se 75 with computed radiography (see Section 3.4) may be applicable to paths greater than

50 mm (up to 70 mm), but this was could not be verified for small bore piping inspectionduring the HOIS2000 radiographic evaluation trials due to the limited range of specimens

available.

3.1.2 Size and strength of sources

In the UK offshore environment, the limit for Ir 192 is normally 25 Ci, but depending on any

other sources are already in store, this may limit the maximum size of a single source. Thus

sometimes single sources can be limited to c. 10 Ci, but higher strength sources in the range 15

to 40 Ci have been used with special risk assessments.

For Se 75, higher activity sources can be used, due to the lower penetrating power of the

radiation produced. Source strengths of up to 30 curies can be used but there is no generally

accepted upper limit offshore.

 Note that radiography contractors should state the maximum strength isotopes within their Local

Rules as required by IRR 1999.

The source size for inspection of small bore piping should be chosen taking account of the

limits on geometric unsharpness given in Section 4.2. Typically source dimensions for this

application will be about 2mm.

On occasion, use of smaller sized sources (e.g. 1 x 1 mm) may be considered preferable, as

these will lead to smaller geometric unsharpnesses for the same source to film distances. ForIr 192, newly manufactured high activity sources of size 1.2 x 1.2 mm can have a strength of c.

8 Ci, and a 2 x 2 mm source can have an activity of c. 44 Ci.

The effective source size for geometric sharpness calculations should be used to calculate the

required source to film distance, as given in Section 4.1. Larger physical sized sources will

require greater source to film distances than smaller sized sources to obtain the same geometric

unsharpness.

3.1.3 Source containers and collimation

The source containers should conform with 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 for the radiography of small bore piping,

 provided the requirements of the current radiation safety regulations are complied with. For

these systems, a large radiation controlled area (radius ≈100 m) is normally needed, which oftenrequires out of hours working, or even shutdown of plant.

Systems which keep the source within a single container or single container/collimator assembly

are now available (e.g. SCAR, SafeRad) which allow a much smaller size of controlled area (of

order 1m - 5 m). These systems reduce radiation doses to operators, and the small size of the

controlled area generally means that plant operation does not need to be interrupted when site

radiography is underway.

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The selection of an appropriate source container and deployment system depends on the balance

of the above factors for each individual site and inspection application, together with economic

considerations.

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

3.2 IN-SITU INSPECTION OF PLANT

Use of gamma ray radiography equipment for in-situ inspection of plant involves significant

safety issues associated with the use of ionising radiation. The appropriate mandatory safety

regulations appropriate to the plant must be adhered to (IRR 1999 in the UK). These include the

construction and maintenance of radiation controlled areas, by means of appropriate barriers.

Pre-planning of the inspection work to be carried out on a plant is required, to include both a

risk assessment and a practical assessment of how the source container and shielding will be

 placed (IRR 1999).

3.3 FILM AND SCREENS

3.3.1 Conventional film

Radiographs with the highest quality in terms of spatial resolution, contrast and granularity will

 be achieved with a fine grain high contrast film.

All other detection techniques described in this document give some reduction in image quality,

 but with the main benefit of reduced exposure time.

Use of different types of film for weld radiography is covered by BS EN 1435. In this standard,

two classes of radiography are defined, class A (basic) and class B (improved). The class B isgenerally used for more demanding applications such as detection of fine cracks.

The detection of corrosion, which is a form of volumetric defect, is considered less demanding

for radiography than fine crack detection, and hence class A radiography, as defined by BS EN

1435 should generally be sufficient.

For Class A radiography and Se 75/Ir 192 sources, BS EN 1435 recommends a C 5 film class

(as defined is BS EN 584-1), common examples of which are Agfa D7, Fuji 100, Kodak AX

and DuPont 70. Use of front and back lead screens is also required; a thickness of 0.1mm for

 both screens is consistent with the requirements of BS EN 1435.

Thus the recommended standard film class is C 5 (Agfa D7, Fuji 100, Kodak AX and DuPont70), although it is recognised that for particular applications such as detection of very slight

corrosion in very small wall thickness pipes, a finer grain film may be required.

The advantages of fine-grained film used with lead screens are:

• Provides the highest spatial resolution and overall image quality of all currently availabledetection systems.

• Low-cost, unless used in very large quantities.• Well established, well understood characteristics and universally accepted.• Forms a direct hardcopy which can be readily archived

Disadvantages of film include:

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• Relatively long exposure times•  Need for film development which involves use of a dark room, and safe use and disposal of

the necessary chemicals.

• Accurate dimensional measurements not straightforward

3.3.2 Fluorometallic screens

Fluorometallic screens provide an alternative to lead intensifying screens and, when used with

fine grain film (Agfa D7), give significant reductions in exposure times, albeit at the expense of

some loss of image sharpness.

The results of the HOIS2000 evaluation trials (Burch and Collett, 2002) showed that when used

with Ir 192, NDT 1200 fluorometallic screens with fine grain film gave approximately a factor

five reduction in exposure time compared with fine grain film used with lead screens (see

Section 4.3).

Use of fluorometallic screens with medium grain film (Agfa F8) was also investigated in the

HOIS2000 evaluation trials. A further reduction in exposure time was obtained, but with a

significant reduction in image quality. This combination is not therefore generally

recommended for applications requiring high sensitivity. However if the tolerable defect

sizes are large and hence the required sensitivity is low, then this technique could provide a fit

for purpose method for reducing exposure time.

3.4 COMPUTED (FILMLESS) RADIOGRAPHY

Technical developments have led to systems that do not rely on film as the detection medium.

These fall into two categories, those which use a storage phosphor technology and those

employing an array of semiconductor detectors. Only the phosphor technology has been tested

within this programme of work. Originally developed for medical applications the storage

 phosphor technology is now finding use in industrial radiography.

Storage phosphor systems use a flexible plate coated with a type of phosphor that stores the

incident radiation as a latent image within the phosphor (composition used depends on the

manufacturer). The latent image can be read off the plate into a computer as a digital picture

using a special scanner. This system is sometimes referred to as computed radiography (CR) or

digital radiography.

Imaging plates have good linearity and high dynamic range, exceeding conventional film in

 both cases. They also have a good sensitivity to gamma and X-rays. The same plate can be used

many times. The limiting factor is mechanical wear and damage to the plate rather thandegradation in the luminescent properties.

In practical implementations the phosphor is coated onto a flexible substrate and the plate can

 be used like a conventional film. Readout is achieved by scanning over the plate with a fine spot

of laser light at a suitable wavelength to stimulate the photoluminescence of the phosphor

material. Typically the scan lines are 100 microns apart (10 lines per mm ) although some

scanners can achieve 50 microns ( 20 lines per mm ). After readout some residual image may

remain on the plate so to erase it fully a very bright light is used. Ambient light does not cause

an image on the plate unlike conventional film, although very bright light can cause erasure of a

stored image as noted. However overall the plates require less precautions against ambient light

exposure than conventional film.

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The advantages of the computed radiography (filmless) systems include:

• Significantly reduced exposure times compared with fine grain film used with lead screens.• Greater exposure latitude than film (i.e. acceptable images can be obtained over a broader

range of exposures than with film).• Greater exposure latitude also permits reasonable interpretation of a large range of materialthicknesses within the same image reducing the need for multiple exposures required by

film.

•  No chemical processing needed (no disposal of chemicals, no need for dark room).• The filmless plates can be re-used many times.• Images directly read into a computer for subsequent enhancement and straightforward on-

screen measurements of wall-thickness.

• Multiple ‘film-like’ hard copies of images can be produced which are interpretable onstandard film viewers.

The disadvantages of this technology include:

• Relatively new recording medium, characteristics not fully understood or documented.• Resulting radiographs have lower spatial resolution than fine grain film used with lead

screens (comparable with fluorometallic screens used with fine grain film).

• High capital cost of system.• ‘Film-like’ hardcopying facility is non-standard and additional cost.

3.4.1 Image Processing on filmless systems

For computed (filmless) radiography, the raw digital images are generally stored on disc in a

 proprietary 12-bit/pixel format.

On some systems, various image processing and enhancement techniques can then be applied to

obtain the image displayed on the PC monitor. Various enhancement options can be available,

including:

• Contrast amplification• Edge contrast• Latitude reduction•  Noise reduction

Usually the amount of enhancement within each of the above categories can be controlled by an

integer parameter.

This processing can have a significant effect on the on the perceived image characteristics. To

the eye, the processed image can be made to appear substantially sharper than the one without

 processing, but the apparent noise level (granularity) on the processed image is then higher than

that without processing. By varying the processing parameters it is possible to obtain many

different effects, with different trade-offs between noisy/granularity level and apparent

image/edge sharpness.

It is clear that the values selected as optimum for the various image processing parameters are

somewhat subjective, and different operators may well prefer to use different combinations of

values. In addition, the values can be varied depending on the requirements of the inspection

application.

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 Nevertheless, this type of image processing can be an effective additional option, when using

computed (filmless) radiography. It should be noted that this processing is not necessarily

available on all computed radiography systems.

The above spatial image processing operations can be followed by interactive adjustment of

contrast and brightness (sometimes referred to as level control) to obtain the “optimum” display

of the processed images on the PC monitor. Images in standard file formats (e.g. jpeg) of the

current enhanced image can be created and stored on disc. Note that once in this format, only

very limited further processing or enhancement is possible.

3.5 DIMENSIONAL INDICATORS

For applications requiring accurate dimensional measurements, such as wall thickness

measurement by the tangential radiography technique, it is necessary to calibrate the dimensions

measured from a radiograph. For a film radiograph, this is because a non zero component to

film distance causes the image of the component to be magnified by the ratio (source to film

distance)/(source to component distance). For a digital radiograph, the pixel size needs to be

calibrated in terms of real dimensions (e.g. mm on the component) in order to make accurate on-

screen dimensional measurements.

Techniques for calibrating dimensions include use of known pipe or small bore ODs or IDs if

available, but inaccuracies may arise due to possible presence of corrosion. An alternative is the

inclusion of an appropriate calibration object(s) in the radiograph of precisely known

dimension, such as a ball bearing of known diameter, at the same distance from the film/detector

as the object of interest, as illustrated in Figure 3.1.

Figure 3.1  Comparators for calibration of dimensional measurements taken from radiographs

Radiography contractors should produce a written procedure detailing how wall thickness

measurements will be made in a consistent and accurate manner, particularly if this

technique is used for condition monitoring of specific inspection points

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4 EQUIPMENT SETUP

This section gives recommendations concerning the setup of the radiographic equipment

detailed in Section 3, for the inspection of small bore pipes, and their connection with the main

 pipes.

4.1 INSPECTION TECHNIQUES

The junction between small bore piping and a larger diameter pipe can cause physical

restrictions on the placement of the source and film/detector plate. In particular, the presence of

the larger diameter pipe can make it difficult to place the film/detector plate in close proximity

to the small bore connector. Various different techniques can therefore be used to inspect these

components, as follows.

4.1.1 Technique 1: Flat film, beam axis perpendicular to main pipe

In technique 1, the radiation beam axis is arranged perpendicular to the axis of the main pipe,

and the film/detector is kept flat and perpendicular to the radiation beam, as shown inFigure 4.1.

The simplest and most convenient location for the film/detector is then tangential to the OD of

the main pipe, which gives an appreciable stand-off between the film and the small bore

connector under inspection. This stand-off clearly increases with the diameter of the main pipe

and requires a corresponding increase in the source to film distance to maintain a small

geometric unsharpness as recommended in Section 4.2.

4.1.2 Technique 2: Film flat against connector, beam axis perpendicular to main pipe

To reduce the large source to film distances needed for the larger diameter main pipes,

technique 2 was investigated in the HOIS2000 evaluation trials (Burch & Collett, 2002). Thistechnique involves the film placed in direct (flat) contact with the small bore connector and

curved around the circumference of the adjacent section of the main pipe, as shown in

Figure 4.1. This allows smaller source to film distances to be used for larger diameter pipes,

while maintaining the recommended maximum geometric unsharpness of 0.3 mm.

Technique 2 is applicable to the detection of corrosion in the section of the small bore connector

external to the main pipe. A source to film distance of c. 300 mm is recommended.

Information can also be obtained on any corrosion within the wall of the main pipe, but as

shown in Figure 4.1, the change of direction of the film as it curves around the circumference of

the main pipe will lead to some distortion of the image.

In the HOIS2000 evaluation trials this technique was applied successfully to specimens

containing 10 inch schedule 40 pipes with a 300 mm SFD. Corrosion in the section of small

 bore connector external to the main pipe wall was clearly seen, with significantly reduced

exposure times compared with Technique 1. Some information on the ID of the hole in the main

 pipe was also obtained, but the large metal path (c. 100 mm) made this feature difficult to

image, even with increased exposure, in this case. With smaller diameter pipes (e.g. 6 inch or

8 inch) this would be a less significant problem.

However, by its nature Technique 2 requires the detector to be flexible, so that if film is used, it

cannot be housed in a rigid cassette. A possible further problem with the use of curved films, is

 pressure marking of the films, particularly on tight bends. Faster films (e.g. Agfa F8) are

understood to be more prone to this than the fine grain films.

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With computed radiography systems it can be difficult to use this technique as the filmless

 plates, although in themselves are flexible, may be deployed within a rigid cassette. Some

computed radiography systems do not however have this restriction.

Technique 1 Technique 2

Technique 3

Source Film

 

Technique 4 

Figure 4.1 Different techniques for the radiographic inspection of the connections between

small bore piping and larger diameter pipes. Technique 1 - beam axis perpendicular to pipe, flat

film offset from small bore pipe; Technique 2 beam axis perpendicular to pipe, film flat against

the small bore connector and curved around the main pipe; Technique 3 beam axis parallel to pipe, film flat against the small bore connector and main pipe, and bent at 90° at the joint.Technique 4 –- beam axis perpendicular to pipe, flat film angled to reduce gap behind the

connector

4.1.3 Technique 3: Film flat against connector, beam axis parallel to main pipe

Technique 3 is used to obtain radiographs with the beam axis parallel with the main pipe axis, as

illustrated in Figure 4.1. In this case, inspection of only the section of the small bore connector

external to the main pipe is possible, but small SFD’s (e.g. 300 mm) can be used.

The comments about the use of curved films given in Section 4.2.2 are equally applicable to this

technique.

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4.1.4 Technique 4: Angled Film flat, beam axis perpendicular to main pipe

Technique 4 is a variant of Technique 1. Here the film/imaging plate was kept flat but is angled

as shown to reduce the gap between the film and the connector. The reduction in the film to

object distance (OFD) with this technique depends on the angle used for the film, relative to the beam direction. As shown in Figure 4.1, an angle of 45° would give an approximate factor 2reduction in OFD, which would lead to a factor 2 reduction in the required SFD, resulting in an

reduction in exposure time by a factor c. 4.

The image distortion caused by the angled direction of the film was not a significant problem in

the result obtained using this technique in the HOIS2000 evaluation trials. However the use of

dimensional indicators (see Section 3.5) in this technique may lead to inaccuracies because of

the varying object to film distance.

Thus this technique is not recommended if the application requires accurate dimensional

measurements to be taken from the radiographs.

4.2 SOURCE TO FILM DISTANCE

In general, a large source to film distance will minimise the unsharpness in the radiograph

caused by the size of radiation source (known as geometric unsharpness). However, large source

to film distances can lead to very long exposure times, and increased shielding difficulties. Thus

trade-offs must be made, whilst ensuring acceptable image quality.

However, there is no universally accepted method for the choice of the source to film distance

which will provide a satisfactory radiographic technique (Halmshaw, 1995, p125) and existing

standards from various countries differ widely.

For example, BS EN-1435 uses unexplained formulae to determine the minimum source to film

distance, as follows:

For class A ("basic" techniques):

3/2

mm

 b5.7

d 

f ⎟ ⎠

 ⎞⎜⎝ 

⎛ ≥  

For class B ("improved" techniques):

3/2

mm

 b15

d 

f ⎟ ⎠

 ⎞⎜⎝ 

⎛ ≥  

where

f is the source to object distance

d is the source size

 b is the object to film/detector distance (which must be expressed in mm).

The basis for the above formulae is not clear, and appears to be empirical at best since the 2/3

 power law produces a dimensionally inconsistent equation, and b must be expressed in mm.

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An alternative approach for determining the minimum source to film distance is to ensure that

the source to film distance is large enough to keep the geometric unsharpness of the radiographs

 below a certain value, hence giving radiographs with similar overall spatial resolution.

For inspection of a small bore connector, the distances which should be used in calculating the

geometric unsharpness due to the effective source size, d are shown in Figure 4.2. The

terminology follows that used in BS EN 1435.

The source to film distance (SFD) is the distance from the source position to the film or detector

 plane. The distance between the source and the object, f, should be measured from the source to

the point on the connector nearest to the source, to give the most conservative value for the

geometric unsharpness.

For film/detectors which are not aligned normal to radiation beam, the distance to the film

should be taken as the maximum distance from the source side of the component to the position

on the film furthest from the component.

SFD

b

Source, sized   Film/detector 

f 

 

Figure 4.2  Distances used in calculation of geometric unsharpness in small bore connector

inspection

The corresponding distance from the object to the film b is then simply given by:

 b = SFD – f

The geometric unsharpness on the film/detector is then given by

Ug = (d . b)/ f

However, the magnification, M, of the image is given by:

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M = SFD / f

Calculating the geometric unsharpness in the plane of the object then gives:

Ug' = Ug / M

= (d . b) / SFD

 Note that this measure of unsharpness refers to the value appropriate to the object under

inspection, and differs from the value Ug which refers to the value on the film/detector. For

large object magnifications, it is clear that Ug' is the appropriate measure to use for assessing

image quality in relation to object detail, and by extension, the same is true for the smaller

magnifications normally used in site radiography of small bore piping (and other components

for that matter).

For the HOIS2000 evaluation trials on the small bore connectors (Burch and Collett, 2002), thegeometric unsharpness in the plane of the object was c. 0.3 mm or better for most radiographs.

This is considered to be a reasonable target value for the detection of a volumetric defect such

as corrosion.

For a given object to film distance, b, and source size, d, the source to film distance, SFD,

should therefore be set so that Ug' is not more than 0.3 mm, i.e.

SFDmin  = (d . b)/0.3

It should be noted that fine grain film has a lower unsharpness than 0.3mm. For Ir 192

Halmshaw (1995, p 90), quotes a value of 0.17 mm for the unsharpness of fine grain film. The

film unsharpness for Se 75 is less than this due to the lower photon energies involved. Thuslarger SFD values than those given by the equation above would give some slight improvement

in detail resolution.

Table 4.1 shows calculated source to film distances needed to achieve the recommended

geometric unsharpness of 0.3 mm, assuming an effective source size of 2.3 mm. The same

information is shown graphically in Figure 4.3.

Table 4.1  Source to film distances (SFD) recommended for Technique 1, assuming an effective

source size of 2.3mm and a geometric unsharpness of 0.3 mm

 Nominal Bore

(inches)

Outside

diameter, OD

(mm)

Small bore

 piping OD

(mm)

SFD required for

Technique 1

(mm)

2 60.3 17 296

3 88.9 17 405

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4 114.3 17 503

5 141.3 21 622

6 168.3 21 725

8 219.1 27 943

10 273.0 27 115012 323.8 27 1345

 Note that for the 2 inch – 3 inch OD main pipes, the SFD values are relatively small (300 –

400 mm), but for the 10 inch OD pipes and larger, the required SFD’s exceed 1000 mm,

requiring substantial exposure times (see Section 4.3) when used with fine grain film.

Source to film distances recomm ended for Technique 1

0

200

400

600

800

1000

1200

1400

0 2 4 6 8 10

Main pipe nominal bore (inch)

   S  o  u  r  c  e   t  o   f   i   l  m    d

   i  s   t  a  n  c  e   (   S   F   D   )   (  m  m   )

12

 Figure 4.3 Recommended source to film distances for application of Technique 1 to the small

 bore connections, for an effective source size of 2.3 mm. The pipe nominal bore refers to that of

the main pipe to which the small bore pipe is connected. Smaller sized sources would require

 proportionally smaller source to film distance to maintain a geometric unsharpness of 0.3 mm.

For Techniques 2 and 3, a constant SFD of c. 300 mm is recommended to ensure the

geometric unsharpness is within the limit set by the equation above.

For Technique 4, a larger SFD is required, intermediate between the minimum value of

300 mm, and the values shown in Figure 4.3. The exact value should be calculated

according the equation above and will depend on the angle of the film to the small bore

pipe, and hence the maximum distance between the small bore pipe and the film/detector.

 Note that for a source size of smaller than 2.3 mm, the above recommended source to film

distances can be reduced in direct proportion to the source size relative to 2.3 mm. Thus a

1.2 mm source would allow the above source to film distances to be reduced by a factor of two.

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4.3 EXPOSURE TIMES AND FILM DENSITY

4.3.1 Fine grain film with lead screens

For conventional radiography using fine grain film, exposures times should be chosen to

obtain a density of c. 3 in the centre of the small bore connection, so that the density of the

tangential wall thickness area is not too low.

Table 4.2 gives a guide to the exposure times required for the inspection of small bore

connectors using Technique 1 (i.e. flat film placed tangentially to the main pipe OD, radiation

 beam perpendicular to the main pipe). The times given are for an Ir 192 source with a strength

of 10 Ci, and are based on exposures used for the HOIS2000 evaluation trials. For Se 75

sources, very similar exposure times were needed.

Exposure times for different strength sources can be readily calculated, as exposure time is

inversely proportional to source strength.

For weldolets with larger (10 mm) wall thickness, increased exposure times will be needed byfactors of c. 2 (for Ir 192) and c. 7 (for Se 75 – larger factor due to greater attenuation with this

lower photon energy source).

For Techniques 2 & 3 (see Section 4.2), the SFD is maintained at c. 300 mm irrespective of the

nominal bore of the main pipe, so c. 3 min exposure time is required with a 10 Ci Ir 192 source.

Table 4.2  Approximate exposure times for fine grain film for small bore connector inspection

using Technique 1 with a 10Ci Ir192 source

 Nominal Bore

(inches)

SFD required for

Technique 1

(mm)

Approximate exposure time for a

sch. 40-80 small bore connector

(10 Ci Ir 192 source)

(min)

2 296 3

3 405 6

4 503 8

5 622 13

6 725 18

8 943 30

10 1150 45

12 1345 60

The approximate exposure times are also shown in Figure 4.4 in graphical form.

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0

10

20

30

40

50

60

0 2 4 6 8 10

Main pipe nominal bore (inch)

   E  x  p  o  s  u  r  e   t   i  m  e   (  m   i  n   )

12

 

Figure 4.4  Approximate exposure times for application of Technique 1 to schedule 40/80 small

 bore connections, assuming use of fine grain film with lead screens and a 10 Ci Ir 192 source of

size c. 2 x 2 mm.

4.3.2 Other film/detectors for reduced exposure times

The exposure times given in the section above can be reduced using various alternatives to fine

grain film with lead screens, albeit with some reduction in image quality.

Table 4.3 gives a guide to the exposure times needed with different film/screen combinations

and also the newer computed (filmless) radiography systems. The exposure times are given for a

10 Ci source of either Ir 192 or Se 75 and for a source to film distance of 300mm.

Table 4.3  Approximate exposure times for different detection media and source combinations

using Technique 1 with 300 mm SFD (10Ci source in all cases).

Technique Exposure time for

10 Ci source, SFD

= 300

(min)

Exposure reduction 

factor relative to fine

grain film

Fine grain film with lead screens & Ir 192 3 1

Fine grain film with lead screens & Se 75 3 1

Fine grain film with fluorometallic screens &

Ir 192

0.6 5

Computed (filmless) radiography systems &

Se 75

0.2 – 0.8 4 - 15

Computed (filmless) radiography systems &

Ir 192

0.06 50

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4.4 IMAGE QUALITY INDICATORS

In weld radiography, image quality indicators (IQIs) are used to verify that the sensitivity of the

radiographs obtained during an inspection is within acceptable limits. For small bore connector

radiography, this is most relevant to the detection of corrosion in the straight-through/density

difference method (Section 2.2), but is less critical for the tangential technique which images theloss of wall directly.

Various different IQI types are in use world-wide, including single wire IQIs (EN 462 Part 1),

step hole IQIs (EN 462 Part 2) and duplex wire IQIs (EN 462 Part 5).

The IQI should generally be placed on the source side of the small bore pipe. For wire IQIs, the

wires should run across the pipe. The IQI needs to be positioned in a section of the pipe unlikely

to be affected by corrosion, otherwise it is possible that the IQI will mask the corrosion.

Table 4.4 is adapted from that shown in EN 1435 for class A radiography, and shows IQI

sensitivities which should be achieved as a function of the wall thickness of the small bore pipe.

 Note that the minimum penetrated thickness is twice the values shown in Table 4.4, as the X and

gamma rays penetrate both walls of the piping.

For Ir 192, EN1435 allows lower sensitivities to be accepted for some thickness ranges, as

shown in Table 4.4.

Table 4.4  IQI sensitivities for radiography of small bore piping, adapted from EN 1435

For all radiation sources other than Ir

192

For Ir 192

Small bore pipe

wall thickness(mm)

Wire IQI value Mean %

sensitivity

Wire IQI value Mean %

sensitivity

>2.5 to 3.5 W14 2.7 W14 2.7

>3.5 to 5.0 W13 2.4 W13 2.3

>5.0 to 7.5 W12 2.0 W10 3.2

>7.5 to 12.5 W11 1.6 W9 2.5

>12.5 to 16 W10 1.4 W9 1.8

In the HOIS2000 evaluation trials(Burch and Collett, 2002), the sensitivities of the radiographs

were not assessed using IQIs, as the main aim of the trials was to establish detectability of

different levels of corrosion.

Further work is recommended to quantify the sensitivity of radiographs of small bore

connectors using IQIs, and to compare the values obtained with those given in Table 4.4.

For the computed (filmless) systems, use of Duplex IQIs which measure more directly the

spatial resolution of the image should be investigated.

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5 OVERALL PERFORMANCE

5.1 IMAGE QUALITY

The different sources and detection media described in this document give differing imagequalities. Table 5.1 gives a qualitative comparison between the results obtained, in terms of

resolution/sharpness, contrast and noise/granularity level. These parameters were assessed using

a simple scoring system, involving a number of stars between 1 and 3, with the “best” being 3

stars. It should be noted that the range from 1 to 3 stars was qualitative, and chosen to span the

differences between the available results. The number of stars assigned to a particular technique

is not therefore intended to provide a quantitative or linear measure of the parameter concerned

(e.g. image resolution).

Table 5.1  Qualitative comparison between results obtained with different radiographic sources

and detection media on sections of small bore connectors external to the main pipe wall.

Technique Resolution/

sharpness +Contrast

+ Noise/

granularity +

Fine grain film with lead

screens & Ir 192

*** ** ***

Fine grain film with lead

screens & Se 75

*** *** ***

Fine grain film with

fluorometallic screens & Ir 192

** ** **

Computed (filmless)

radiography (Se 75) but without

spatial image processing

** 1 ***

Computed (filmless)

radiography (Se 75) withspatial image processing

2

**(*) 1 * to ***

depending on processing

 parameters

Computed (filmless)

radiography (Ir 192) with

spatial image processing2

** 1 * to **

depending on

 processing

 parameters

 Notes:+  Qualitative measure only – “more stars the better” (high contrast, high

resolution/sharpness, low noise)

1 Intrinsic contrast is difficult to quantify for computed radiography systems, due to use

of on-screen contrast/brightness control.2 Spatial image processing can improve the apparent sharpness of the image, but often

at the expense of a significant increase in the noise/granularity level.

 Note that the highest quality images were the fine grain film radiographs obtained with Se 75.

This is because, for the small bore connectors and weldolets (WT no more than 10 mm), which

have relatively low maximum metal paths, the Se 75 source gave noticeably higher contrast than

Ir 192 due to the lower mean gamma ray energies from Se75.

When fine grain film is used with fluorometallic screens, a significant reduction in exposure

time is obtained (see Section 4.3.2), but the resulting radiographs have appreciably lower spatial

resolution and somewhat reduced contrast.

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The raw images obtained with two different computed radiography (filmless) systems were

similar in quality, and had lower resolution than fine grain film (but shorter exposure times, as

noted in Section 4.3.2). The spatial resolution of the raw images from the computed radiography

systems was comparable with that obtained with fine grain film and fluorometallic screens.

The spatial image processing package available with one of the computed radiography systems

allowed different trade-offs between noise level and apparent image resolution/edge sharpness

to be obtained, by varying the values of four available processing parameters. Thus, it was

 possible to improve the apparent image sharpness but only at the expense of an increase in noise

level.

5.2 EXAMPLES

This Section gives examples of radiography of small bore connector specimens obtained during

the HOIS2000 evaluation trials (Burch & Collett, 2002). These specimens contained simulated

 but realistic corrosion defects introduced into one end of the small bore pipes, prior to their

welding onto the main pipes. In each case, a length of about 10-15 mm of simulated corrosion

was introduced using a variety of “ad-hoc” techniques, which produced an irregular loss of wall

approximately evenly distributed around the circumference of each small bore pipe.

Superimposed on these relatively smooth variations (length scale several mm), were a number

of small isolated “pits”. At around 10 mm from the end of each pipe, the loss of wall tapered

down to zero.

5.2.1 Comparison between Ir 192 and Se 75 using fine grain film with lead screens

Figure 5.1 shows radiographs obtained using technique 1 (flat film, placed tangentially to main

 pipe OD) on 3 inch main pipe specimens, with the small bore connectors containing different

levels of corrosion. The source used was Ir192, and the detection medium was fine grain film

with lead screens. For further details, see the figure caption. Note that inevitably the techniques

used to digitise and display the film radiographs in this report have caused a loss of imagequality, compared with viewing the original film radiographs on a light box.

 Note that the wall loss on the moderate and severe cases of corrosion can be detected by the

tangential method. Also, the hole through the main pipe wall can also be clearly seen, and

corrosion here would have been detected, if present.

On the original film radiographs the increased density in the centre of the connector image

caused by wall loss in the “straight-through” radiography mode can also just be discerned for

 both the moderate and severe cases of corrosion, although the presence of the screw thread

complicates matters.

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(a) (b)

(c)

Figure 5.1  Film radiographs obtained using Technique 1 with Ir 192 and fine grain film with

lead screens on threaded small bore connectors (OD 17 mm, WT 3.1 mm before introduction of

screw thread) connected to 3 inch pipe (WT 7.6 mm). (a) no corrosion, (b) ‘moderate’ corrosion

(mean wall loss 24%), (c) ‘severe’ corrosion (mean wall loss 43%). Results courtesy ofOceaneering OIS plc.

Figure 5.2 shows results on a similar, but unthreaded set of small bore connectors using

technique 1 applied with a Se 75 source, again with fine grain film and lead screens. The use of

the lower photon energy (“softer”) Se 75 source has resulted in radiographs with significantly

improved contrast, compared with the corresponding Ir 192 results given in Figure 5.1.

Thus with Se 75, the corrosion can be readily detected by both the density difference and

tangential methods, although only the tangential method allows the wall loss to be measured

quantitatively.

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(a) (b)

(c)

Figure 5.2  Film radiographs obtained using Technique 1 with Se 75 and fine grain film with

lead screens on small bore connectors (OD 17 mm, WT 3.1 mm) connected to 3 inch pipe (WT7.6 mm). (a) no corrosion, (b) ‘moderate’ corrosion (mean wall loss 28%), (c) ‘severe’ corrosion

(mean wall loss 52%). Results courtesy of SafeRad Ltd

5.2.2 Example of application to larger diameter (10 inch) main pipe specimens

For larger diameter main pipes, use of technique 1 (flat film, placed tangentially to main pipe

OD – see Figure 4.2) can lead to long exposure times (e.g. c. 45 min for 10 inch pipes with a

10 Ci source), when using fine grain film and lead screens (Table 4.2). To reduce exposure

times, technique 2 can be used. This needs a flexible detection medium placed in direct contact

with the small bore connector, and curved around the main pipe surface (see Figure 4.2).

Corresponding exposure times are then only c. 3 min for fine grain film.

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Results with Ir 192 for technique 2 applied to larger diameter 10” pipe specimens are illustrated

in Figure 5.3. In this case, the small bore connectors are set-through, and the majority of the

corrosion is within the wall of the main pipe. Nevertheless, on Figure 5.3, evidence of wall loss

in the “tangential mode” can be seen in the form of a general increase in the internal diameter of

the connector, close to the main pipe wall, for both moderate and severe levels of corrosion.

(a) (b)

(c)

Figure 5.3  Film radiographs obtained using Technique 2 with Ir 192 and fine grain film with

lead screens on plain small bore connectors (OD 17 mm, WT 3.9 mm) connected to

10” pipe (WT 9.3 mm). (a) no corrosion, (b) ‘moderate’ corrosion (mean wall loss

23%), (c) ‘severe’ corrosion (mean wall loss 53%). Note most of corrosion is

within wall of main pipe. Results courtesy of Oceaneering OIS plc.

For this larger diameter main pipe, the path length through the main wall of the pipe is large

(maximum 100 mm – see Table 2.1) which leads to under-exposure on the radiograph in this

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region of the image, and there is consequently little information concerning the

 presence/absence of corrosion in this region. However, by increasing the exposure time by a

factor two or more, the hole in the wall of the main pipe can be imaged, allowing some

detection of corrosion in this region of the component, albeit at reduced sensitivity compared

with the sections of the connectors external to the main pipe wall.

5.2.3 Example of use of fine grain film with fluorometallic screens

Figure 5.4 shows a radiograph obtained with the faster detection medium of fluorometallic

screens used with the fine grain film, compared with the results obtained with conventional lead

screens. The specimen was that with ‘severe’ simulated corrosion already shown in Figure

5.1(c).

It can be seen that the use of the fluorometallic screens has resulted in some reduction in image

resolution, but, in this case, this has not affected the detectability of the wall loss using the

tangential method. The exposure time was reduced by a factor 5 using the fluorometallic

screens, as shown in Table 4.3.

(a) (b)

Figure 5.4  Film radiographs obtained using Technique 1 with Ir 192 and fine grain film with

lead and fluorometallic screens on threaded small bore connectors (OD 17 mm, WT 3.1 mm

 before introduction of screw thread) connected to 3” pipe (WT 7.6 mm) with severe’ corrosion(mean wall loss 43%). (a) Fine grain film and lead screens, (b) fine grain film and fluorometallic

screens. Results courtesy of Oceaneering OIS plc

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5.2.4 Examples obtained with computed radiography systems

(a) Computed radiography System 1

Figure 5.5 shows an example of results obtained using Se 75 with a computed (filmless)

radiography system on a 3 inch main pipe specimen with severe corrosion in the attached small bore connector (for comparison with the corresponding radiographs obtained with Ir 192 and

film see Figure 5.4). This computed radiography system has additional facilities for spatial

image processing (see Section 3.3.1). To show the effects of this processing, Figure 5.5 shows

two images of the same component with and without spatial image processing.

The unprocessed image shows an appreciable loss of spatial resolution compared with the result

obtained with fine grained film used with lead screens (see Figure 5.1(c)) but the image

 processing has a substantial effect on the perceived image characteristics. To the eye, the image

 processed image appears substantially sharper than the one without processing, but the noise

level (granularity) on the processed image is higher than that without processing.

Thus, the effect of the image processing has been to improve the clarity of image detail (edges

etc), at the expense of an increase in noise level. However, by varying the image processing

 parameters it is possible to obtain many different effects, with different trade-offs between

noisy/granularity level and apparent image/edge sharpness.

(a)(b)

Figure 5.5  Digital images obtained using computed radiography system 1 (technique 1

with Se 75) on a threaded small bore connector (OD 17 mm, WT 3.1 mm before introduction of

screw thread) connected to 3” pipe (WT 7.6 mm) with ‘severe’ corrosion (mean wall loss 43%).

(a) Computed radiography image without spatial image processing, (b) Computed radiography

image after spatial image processing using one combination of processing parameters selected

to highlight edge detail. Results courtesy of Oceaneering OIS plc.

(b) Computed radiography System 2

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Figure 5.6 shows examples of results obtained with a second computed radiography system,

using Se 75 and technique 1 on a 3 inch main pipe specimen. A direct comparison can be made

with Figure 5.2 which shows the corresponding results obtained with fine grain film with lead

screen.

(a) (b)

(c)

Figure 5.6  Digital images obtained using computed radiography system 2 (technique 1 with Se

75) on small bore connectors (OD 17 mm, WT 3.1 mm) connected to 3” pipe (WT 7.6 mm). (a)

no corrosion, (b) ‘moderate’ corrosion (mean wall loss 28%), (c) ‘severe’ corrosion (mean wall

loss 52%). For a comparison with the corresponding fine grain results, see Figure 5.2. Results

courtesy of SafeRad Ltd

The results from the computed radiography system clearly show the moderate and severe levels

of corrosion by both the reduced wall thickness in tangential mode and the increased image

density in straight-through radiography mode. However, as with computed radiography system

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1, some loss of spatial resolution compared with fine grained film used with lead screens is

apparent.

5.3 INSPECTION OF APPROXIMATELY UNIFORM CORROSION/EROSION

Detectability and sizing of approximately uniform corrosion/erosion is best achieved using thetangential radiography method by direct imaging of the resulting wall loss. Detectability is

affected by the location of the wall loss, as follows.

5.3.1 Wall loss external to the main pipe

If the wall loss is in the section of the small bore connectors external to the main pipe wall, then

the techniques 1-4 described in Section 4.2 are applicable, and detectability will not be affected

 by the diameter and wall thickness of the main pipe to which the small bore piping is attached.

The HOIS2000 trials showed that all the source/detector media combinations described in this

document allowed both the “moderate” (20-30% loss of wall) & “severe” (40-60%) levels of

corrosion to be readily detected.

The minimum detectable wall loss is therefore appreciably less than 20%, especially for the

highest quality technique such as fine grain film used with lead screens.

5.3.2 Wall loss within the main pipe wall

Inspection of corrosion within the wall of the main pipe is affected by the significantly

increased metal paths that need to be penetrated by the gamma-ray radiation. For metal paths of

around 50 mm, both Ir 192 and Se 75 gave adequate penetration and hence detectability of wall

loss would be similar to the values for external corrosion/erosion given above.

Table 2.1 gives the maximum metal paths involved in the inspection of schedule 40 and 80

 pipes of differing diameters. From this, it can be seen that metal paths of 50 mm or less areobtained for pipes of diameter 3 inch or less.

For maximum metal paths of 100 mm, only marginal detection of the presence of

corrosion/erosion within the wall of the main pipe was obtained with Ir 192 used with fine grain

film and lead screens. No detection was demonstrated at such high metal paths using Se 75 and

either fine grained film with lead screens or with the computed radiography systems.

Thus, for maximum metal paths between 50 and 100 mm, detectability of corrosion/erosion is

 predicted to decrease relatively gradually with increasing metal path with Ir 192, but more

rapidly for the less penetrating Se 75. Main pipes with maximum metal paths in this range are

typically between 3 inch and 10 inch in diameter (see Table 2.1).

For larger diameter pipes (>10 inch OD), neither of these isotopes would be capable of adequate

 penetration of the main wall. More penetrating isotopes sources such as Co 60 could then be

used, but there are additional safety issues when using such sources.

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5.4 DETECTABILITY OF ISOLATED CORROSION PITS

Detection, sizing and positioning of isolated corrosion pits as opposed to generalised wall loss

raises additional issues for radiographic inspection.

Isolated corrosion pits around the circumference of the connector are readily detectable withradiography but cannot be correctly positioned and sized in the through-wall direction unless a

large number of radiographs are taken at different circumferential angles using the tangential

method.

However, the geometry of the small bore connections makes it difficult with flexible film, and

impossible with rigid cassettes, to position the detection medium in such a way as to make the

tangential method possible at all positions around the circumference, apart from the detector

 positions described in this document, i.e. techniques 1-4 described in Section 4.2.

Thus, detection of an isolated corrosion pit at an arbitrary circumferential location would

generally be best achieved by the density difference produced on the radiograph in straight-

though mode (see Figure 3.2).

To find the shallowest pits, use of Se75 instead of Ir 192, would then be preferred to obtain

higher contrast radiographs, provided the pitting is in sections of the small bore connector

external to the main pipe. To measure the through-wall depth of a corrosion pit, a tangential

radiographic image of the pit would still be needed.

Detectability of small corrosion pits using radiography was not assessed in the HOIS2000 small

 bore connector trials, nor is there any published probability of detection (POD) information

relevant to small bore connector inspection.

However, some information on the smallest size of detectable corrosion pit can be estimatedfrom published IQI information. The step/hole type of IQI indicator consists of a cylindrical

hole having a height equal to its diameter, which is a reasonable approximation for the shape of

a small corrosion pit. However, the shape of the pit will affect its detectability. Very narrow

"worm-hole" type pits will be more difficult to detect than wider pits having the same depth.

For Ir 192 and the lower energy Yb-169, Halmshaw (1995, p 185) gives values for attainable

IQI sensitivity values in flat plate steel using fine grain film, which are shown here in Table 5.2.

Table 5.2 Attainable IQI sensitivity values in flat plate steel for Ir 192 and Yb 169 gamma ray

sources and fine grain film with thin lead screens, taken from Halmshaw (1995).

Steel thickness Source Step/Hole IQI(%) Hole diameter (mm)

6 Yb169 5.0 0.32

6 Ir 192 6.6 0.4

12 Yb169 3.3 0.4

12 Ir 192 4.1 0.5

25 Ir 192 2.6 0.63

40 Ir 192 2.0 0.8

50 Ir 192 2.0 1.0

For detection of corrosion pits in piping, the minimum steel thickness in the “straight-through”

technique is twice the wall thickness, so to convert the figures for percentage sensitivity into an

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equivalent loss of (single) wall due to a corrosion pit would require multiplication by a factor of

at least two.

Thus, for example for a small bore pipe with a 3 mm wall thickness using Ir 192, the Step/Hole

IQI sensitivity is 6.6%, from which it can be inferred that a corrosion pit producing a 13.2% loss

of wall may just be detectable using fine grain film with thin lead screens. This assumes that the

corrosion pit is ideally placed in the centre of the pipe image, on the radiation beam axis. For

corrosions pits off-centre the metal path will be higher, and sensitivity will be reduced.

The sensitivity achievable with Se 75 should be somewhat higher than that for Ir 192, as Se 75

generates radiation with a mean gamma-ray energy approximately mid-way between Ir 192 and

Yb 169. Thus the corresponding IQI sensitivity for 6 mm of steel is likely to be c. 5.5%, which

converts to a 11% minimum detectable loss of wall for an idealised corrosion pit in the centre of

the pipe image.

For pipes with larger wall thicknesses, note that the sensitivity when expressed in percentage

terms improves. Thus for a pipe with 6 mm wall thickness, the corresponding minimumdetectable wall loss for an isolated corrosion pit would be 8.8%, with the same assumptions as

detailed above.

In practice, however, detection of corrosion pits in pipes will be more difficult than detection of

a step/hole IQI in a known location within a uniform plate, for the following main reasons:

(a) With an IQI device, the inspector knows exactly where to look for the object of interest.

Isolated corrosion pits on the other hand could occur in any location within a large region of

 pipe material.

(b) With an IQI device on a flat plate, the density of the radiograph is almost constant over the

region of interest. With pipe inspection, the density of the radiograph changes appreciablywith distance from the centre of the image of the pipe.

(c) Corrosion pits can be irregular and less well defined than a machined cylindrical hole

In view of the above factors, more realistic conservative estimates for the minimum detectable

depth of a corrosion pit would be 15 – 20% loss of wall for pipes with wall thicknesses in the

range 3 - 6 mm, provided the pit is approximately centrally placed in the pipe image (not too

close to the tangential view).

Further investigation of this issue is recommended.

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6 SUMMARY OF RELEVANT SAFETY STANDARDS AND

LEGISLATION

• The requirements of all relevant national and international safety standards relating to the

use of ionising radiation must be complied with. In the UK, the Ionising RadiationsRegulations 1999 (IRR 1999) are enforced by the Health & Safety Executive (HSE). These

are legal requirements and include the construction and maintenance of radiation controlled

areas, by means of appropriate barriers. Pre-planning of the inspection work to be carried

out on a plant, to include both a risk assessment and a practical assessment of how the

source container and shielding will be placed is also covered by this legislation.

• The safe transport of radioactive material is covered by IAEA regulations, the latest versionof which is TS-R-1 (ST-1, Revised). Individual countries have their own legislation

covering this area.

•Gamma ray sources and their containers are covered by the standards ISO 2919: 1999 andISO 3999-1:2000, respectively.

• The standard BS 5650:1978, ISO 3999-1977 concerning specification for apparatus forgamma radiography is also still current.

7 MAIN RECOMMENDATIONS

The Sections presents a summary of the main recommendations contained in this document.

 Radiography methods

• The tangential radiography method is recommended for the detection and through-wallsizing of extended areas of corrosion, and should not generally be used for isolated

corrosion pits, unless their circumferential location has already been established using for

example the density difference method.

• The straight-through or density difference radiographic method is recommended for thedetection (but not through-wall sizing) of isolated corrosion pits, and can also be used for

detection of extended areas of corrosion.

• Both the tangential and straight-through methods can be combined on a single radiograph ofa small bore pipe.

 Isotope sources

• For the inspection of the sections of the small bore connectors external to the main pipe, thetwo recommended isotope sources are Iridium 192 and Selenium 75.

• Source selection should be made according to the criteria given in Section 3.1.1.

• The source size for inspection of small bore piping should be chosen taking account of thelimits on geometric unsharpness given in Section 4.2. Typically source sizes for this

application will be about 2 x 2mm (effective size for calculation of geometric unsharpness

2.3 mm).

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• Careful collimation of the sources is recommended to minimise unwanted radiation, andreduce the effects of scatter on the radiographs.

Wall thickness measurements

• Radiography contractors should produce a written procedure detailing how wall thicknessmeasurements will be made in a consistent and accurate manner, particularly if this

technique is used for condition monitoring of specific inspection points

Source to film distances

• The source to film distance, SFD, should be set so that the geometric unsharpness in the plane of the object is not more than 0.3 mm. A minimum SFD of 300 mm should be used.

 Exposure time reduction

• For conventional radiography using fine grain film, exposures times should be chosen toobtain a density of c. 3 in the centre of the small bore connection, so that the density of the

tangential wall thickness area is not too low.

• Recommended measures to reduce exposure time include reduction of the object to filmdistance, and hence SFD, by placing the detection medium in direct contact with the small

 bore connector (a flexible detection medium is needed if the junction with main pipe is to be

inspected).

•The highest quality radiographic images are obtained with fine grain film used with thinlead screens. Alternative detection media can be used to reduce exposure time, including (a)

fine grain film with fluorometallic screens and (b) the reusable phosphor plates in computed

(filmless) radiography systems, provided the reduction in image quality is acceptable for the

application.

It is important to note that the above recommendations are based on trials which were completed

under laboratory conditions inside an exposure compound or similar controlled and uncluttered

environment. This made it straightforward to set-up the radiography equipment in such a way

as to achieve safe radiation dose rates in a small controlled area, whilst using highly penetrating

radioisotopes and the source to film distances (SFD) recommended in this document. The

 practical application of these recommendations under typical site conditions may