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HSE

Health & Safety Executive

Evaluation of CEN ultrasonic testing standards for in-service inspection

Prepared by Mitsui Babcock for the Health and Safety Executive 2005

RESEARCH REPORT 299

HSE

Health & Safety Executive

Evaluation of CEN ultrasonic testing standards for in-service inspection

NS Goujon and BWO Shepherd Mitsui Babcock Porterfield Road Renfrew Renfrewshire PA4 8DJ

This report discusses the capability of European based UT procedures (which supersede the equivalent British Standard, BS 3923) to detect service induced defects and looks at the limitations which could have significant safety implications across a broad range of industrial sectors. The aim of this project was to evaluate the capability of procedures based on the EN standards for inservice inspection. The project has concentrated on inherent capability with no blind trials, and therefore complements projects like the PANI programme which has concentrated on human reliability rather than procedure capability. The work involved ultrasonic inspection of a number of testpieces containing both synthetic and real service-induced defects (mainly cracks in welds). The project concentrated on detection, although some sizing trials were also performed. A steering committee was established with representation drawn from a variety of organisations to help direct and review the work. Recommendations based both on the results of this project and the views of the Steering Committee members are provided. This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

HSE BOOKS

© Crown copyright 2005 First published 2005 ISBN 0 7176 2945 7 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means (electronic, mechanical, photocopying, recording or otherwise) without the prior written 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 1BQ or by e-mail to [email protected]

ii

Evaluation of CEN UT Standards for In-Service Inspection

Final Report

CONTENTS

SUMMARY

V

1.

INTRODUCTION

1

2.

STEERING COMMITTEE

2

2.1

MEMBERS

2

2.2

ISSUES RAISED BY THE COMMITTEE

2

3. REVIEW OF MAIN DIFFERENCES BETWEEN BS 3923 AND THE EN STANDARDS 3 3.1

REVIEW OF PREVIOUS ASSESSEMENT TRIALS

3

3.2

COMPARAISON OF STANDARDS

3

4.

EXPERIMENTAL INVESTIGATION

9

4.1

ULTRASONIC MODELLING

9

4.2

TESTPIECES

10

4.3

INSPECTION TECHNIQUE & PROCEDURE

10

4.4

RESULTS

11

4.5

DISCUSSION

22

5.

CONCLUSIONS

27

6.

RECOMMENDATIONS

30

7.

REFERENCES

31

APPENDIX A: EXAMINATION REQUIREMENTS

33

APPENDIX B: ULTRASONIC MODELLING

40

APPENDIX C: TESTPIECES

45

APPENDIX D: TRIALS RESULTS

48

iii

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Final Report

SUMMARY This report describes the work carried out on the HSE funded project “Evaluation of CEN UT Standards for In-Service Inspection”. European standards covering pulse echo ultrasonic inspection of welds have recently been issued, and supersede the equivalent British Standard BS 3923. These standards (BS EN 1712; 1713; 1714), referred to below as “EN” standards, are now specified by various manufacturing standards and are intended to result in a more harmonised approach to in-manufacture inspections within the EC. There are no national or international UT standards specifically intended for inservice inspections. It is therefore possible that in-service inspection procedures will tend to be based “by default” on the new EN standards, just as previously they were based (in the UK) on BS 3923 even though it had been written largely with in-manufacture inspections in mind. It is therefore important that the capability of EN based UT procedures to detect and characterise service induced defects is properly understood, since limitations (particularly reduced capability compared to BS 3923) could have significant implications for plant safety and availability. It should also be noted that more influence can be exerted over in-service inspections since these are generally “non-codified” than over in-manufacture inspections which are performed in accordance with internationally agreed codes and standards. The aim of this project was therefore to evaluate the capability of procedures based on the EN standards for in-service inspection. The project has concentrated on inherent capability with no blind trials, and therefore complements projects like the PANI programme which has concentrated on human reliability rather than procedure capability. The work involved ultrasonic inspection of a number of testpieces containing both synthetic and real service-induced defects (mainly cracks in welds). The project concentrated on detection, although some sizing trials were also performed. A steering committee was established with representation drawn from a variety of organisations to help direct and review the work. The recommendations based on the results of this project and the views of the Steering Committee members are as follows: BS EN 1714 should not be used as a “stand alone” procedure for in-service inspection. In-service inspection should be performed in accordance with a written procedure and work instruction / technique sheet which is more prescriptive than this standard, and includes detailed specification of the scans required, probe characteristics and reporting criteria. These should be based on a knowledge of the defect and degradation types, locations, morphologies and sizes of interest. v


Final Report

In defining these requirements, consideration should be given to the following: 1. Although the BS EN amplitude report threshold of 32% DAC (10dB below the DAC curve) may be appropriate for harmonising in-manufacture inspections, a more sensitive threshold may be appropriate for in-service inspections (even though all the defects studied exceeded this threshold with at least one beam). Previous experience has demonstrated that a report threshold of 10% DAC (20dB below the DAC curve) is sufficiently low for most signals of interest to be reported (including tip diffracted signals) but is still high enough to avoid false calls from noise. However there may be cases where it is appropriate to report all indications detected above the noise level. 2. Choice of beam angle is at least as important as choice of reporting threshold. Both capability and reliability for defect detection will tend to increase, the greater the number of different beam angles employed (different probe angles and / or scan surfaces). The results from this project suggest that for weld thicknesses below 20mm, a 70º scan should be included for detection of opposite surface cracks oriented in the through wall direction. An additional 45º scan is recommended for critical applications. For greater thicknesses a 45º scan may provide increasingly better capability than a 70º scan for the detection of such defects. 3. In this study, the 2MHz probes generally provided better capability for detection than the 4MHz probes. However there may be circumstances where 4MHz probes provide the better detection capability, for example it can be easier to distinguish between defects at the weld root, and the root bead reflection, when using higher resolution (higher frequency) probes. 4. A fixed root scan is highly recommended for inspection of welds with an undressed root, where there is a requirement to detect defects near the root, such as inner surface cracks. 5. The fixed amplitude method of measuring length described in BS EN 1712 (acceptance criteria) is not recommended for in-service inspection applications. These should be based instead on the types of techniques described in BS EN 583-5, e.g. 6dB drop sizing for defects longer than the beam width. 6. Acceptance criteria should be agreed between the contracting parties.

vi


1.

Final Report

INTRODUCTION This final report describes the work carried out on the HSE funded project “Evaluation of CEN UT Standards for In-Service Inspection”. European standards covering pulse echo ultrasonic inspection of welds have recently been issued and supersede the equivalent British Standard, BS 3923 [1] . These standards BS EN 1712 [2], BS EN 1713 [3] and BS EN 1714 [4], are now specified by various manufacturing standards and are intended to result in a more harmonised approach to in-manufacture inspections within the EC. The implications for manufacturers and end-users of these European standards (previously referred to as “CEN” standards, from now on referred to as “EN” standards throughout this report) have previously been evaluated through a TWI group sponsored project GSP 5657 [5], which involved Mitsui Babcock performing part of the work. The project involved performing EN based inspections on a selection of welded testpieces which contained predominantly manufacturing defects such as lack of side-wall fusion, lack of inter-run fusion, weld cracking, slag and porosity. The main conclusion was that procedures based on EN standards were likely to result in a lower detection rate than procedures based on BS 3923. There are no national or international UT standards specifically intended for inservice inspections. It is therefore likely that in-service inspection procedures will tend to be based “by default” on the new EN standards, just as previously they were based (in the UK) on BS 3923 even though it had been written largely with in-manufacture inspections in mind. Inspection capability can be very sensitive to defect type so that the global capability for defects produced during manufacture is unlikely to provide a true picture of the capability for service induced cracks. It is therefore important that the capability of EN based UT procedures to detect and characterise service induced defects is properly understood, since limitations (particularly reduced capability compared to BS 3923) could have significant implications for plant safety and availability. It should also be noted that more influence can be exerted over in-service inspections since these are generally “non-codified” (EN is likely to be used as an “unofficial default”) than over in-manufacture inspections which are performed in accordance with internationally agreed codes and standards (EN is mandatory). The aim of this project was therefore to evaluate the capability of procedures based on the EN standards for in-service inspection. The work has involved ultrasonic inspection of testpieces containing both synthetic and real service induced defects (mainly cracks). The project has concentrated on inherent capability with no blind trials, and therefore complements projects like the PANI programme which has concentrated on human reliability rather than procedure capability.

1


2.

STEERING COMMITTEE

2.1

MEMBERS

Final Report

A steering committee was established with representation drawn from a variety of organisations. The membership of the steering committee was as follows: Member

Organisation

Mr H Bainbridge

HSE

Mr R Lyon (Chairman)

RWE npower

Dr G Georgiou

Jacobi Consulting Ltd (representing BINDT)

Mr R Gregory

ExxonMobil

Mr S Hewerdine

Oceaneering OIS (latterly with Aker Kvaerner)

Dr B McGrath

Serco

Mr D Ritchie

Conoco Phillips Petroleum

Mr S Smalley replaced by Mr C Latter Royal & Sun Alliance / / Mr M Miller Oceaneering (representing SAFed)

2.2

Mr A Kirkham, Dr L Morgan (MIC Ltd)

National Grid Transco

Dr B Shepherd Mr O Shafi replaced by Ms NS Goujon

Mitsui Babcock

ISSUES RAISED BY THE COMMITTEE The main areas of interest agreed at the beginning of the project were: •

Testpiece thickness range: 10mm to 40mm (main interest up to 20mm)

•

Concentrate the work (around 80%) on surface breaking defects

•

Mainly cracks, but some interest in selective root corrosion

•

Butt, nozzle and attachment welds

2


Final Report

3.

REVIEW OF MAIN DIFFERENCES BETWEEN BS 3923 AND THE EN STANDARDS

3.1

REVIEW OF PREVIOUS ASSESSEMENT TRIALS Mitsui Babcock previously performed work as part of TWI group sponsored project 5657 [5] to compare procedures based on BS 3923 and the draft EN standards for manufacturing inspections. This involved applying both types of procedures to production and test welds and also retrospectively analysing previous inspection reports. As a part of the current workscope this previous Mitsui Babcock work was reviewed. Inspections with the BS EN 1714 standard reported and rejected considerably fewer defects than with BS 3923. Most of the defects reported by the BS 3923 inspections but not by the EN inspections tended to be threadlike defects. However there were cases where lack of side wall fusion defects were not recordable by the EN standard, and were characterised as non-planar using the EN cascade method for defect characterisation (which is the basis of BS EN 1713). Further information on GSP 5657 and related projects is provided in references [6] to [8].

3.2

COMPARAISON OF STANDARDS

3.2.1

Introduction This section summarises the main differences between the European standard BS EN 1714 and the superseded British standard BS 3923. Since BS EN 1712 defines evaluation levels and includes the description of the fixed amplitude level technique for length measurement, it is also compared to BS 3923 where appropriate. These European standards are: •

BS EN 1714: 1998 – Non-destructive testing of welded joints – Ultrasonic testing of welded joints. This standard specifies methods for manual ultrasonic testing of fusion welded joints in metallic materials equal to and above 8mm thick which exhibit low ultrasonic attenuation. The standard specifies four testing levels, each corresponding to a different probability of detection of imperfections.

•

BS EN 1712: 1997 – Non-destructive testing of welds – Ultrasonic testing of welded joints – Acceptance levels. This standard specifies ultrasonic acceptance levels, 2 and 3, for full penetration welded joints in ferritic steels (with thicknesses from 8mm up to 100mm) which correspond to the quality levels B and C of BS EN 25817 [9], in which flaws are accepted or rejected principally on the basis of their ultrasonic echo amplitude and their length. It also describes the fixed amplitude level technique for length measurement.

3


Final Report

The European standard BS EN 1713 was not considered. This standard defines the classification of internal indications as planar or non-planar. It was studied as a part of the previously referenced TWI GSP and some of the results are summarised in reference [6]. The more general guidance on UT provided in BS EN 583 parts 1-5, references [10] to [14], has not been studied in any detail.

The most stringent examination requirements for longitudinal indications in various types of weld, that is examination level 1 under BS 3923 and examination level C under BS EN 1714 are provided in Table A1 of Appendix A1 (note that examination level D is more stringent than level C but is for special applications, see section 3.2.3). The requirements for the testpieces used for this study are provided in separate tables in Appendix A2. 3.2.2

Scope The thickness ranges for which the standards apply are: •

BS 3923: t ≥ 6mm and ≤ 150mm

•

BS EN 1714: t ≥ 8mm; no upper limit.

BS EN 1714 states that it can be applied directly as an inspection procedure (although how this is practical given the range of options is not clear). BS 3923 states specifically that it does not in itself constitute a test specification. 3.2.3

Choice of examination / testing levels Both standards have 3 examination levels providing different degrees of rigour (+ 4th level for special applications). However: •

BS 3923 examination levels differ in terms of both sensitivity and number of techniques.

•

BS EN 1714 testing (the term ‘examination’ has been dropped) levels differ only in terms of number of techniques. Four testing levels are specified: A, B, C and D. From testing level A to C, an increasing probability of detection will be achieved (through additional scanning and/or surface dressing). The quality level corresponding to this test level is defined in BS EN 25817 such that: test level A corresponds to a quality level C (intermediate), test level B corresponds to a quality level B (stringent), test level C is by agreement and test level D is for special applications.

•

To achieve an examination level 1 under BS 3923, the component surfaces must be dressed. BS EN 1714 is not prescriptive regarding surface condition.

•

The highest level under which a butt joint (plates and pipes) inspection can be carried out on an as-welded component, under BS 3923, is level 2B.

•

BS EN 1714 standard makes no distinction between as-welded and dressed welds.

4


3.2.4

Final Report

Coverage / beam angles

3.2.4.1 Parent material inspection As a part of the general requirement under BS 3923, the examination of the parent material using a single or twin 0º probe should be carried out to achieve a complete weld examination. Whether or not the parent material has been ultrasonically tested previously, an examination shall be made after welding. Under the BS EN standard, the parent material , in the scanning zone area, shall be tested with 0º probes prior to or after welding, unless it can be demonstrated that the angle probe testing of the weld will not be influenced by the presence of the imperfections or high attenuation. 3.2.4.2 Weld inspection The standards are broadly similar with the exception of: •

The BS EN 1714 standard states that if individual sections of the testing volume (which includes weld and parent material for at least 10mm on each side of the weld) cannot be covered in at least one scanning direction or if the angles of incidence with the opposite surface is less than 35º and greater than 70º alternative UT or other NDT shall be agreed upon. Removal of the weld cap may have to be considered.

•

There is no requirement in BS EN 1714 for a separate root scan.

•

Little guidance is provided in the BS EN 1714 standard on choice of beam angle, other than that one of the beams should inspect the weld fusion faces at as near as possible to normal incidence. (Clearly more appropriate for inmanufacture inspection than in-service inspection).

•

When there is a requirement for scanning both upper and lower surfaces under the BS EN 1714 standard, there is no alternative proposed when scanning access is restricted to the upper surface.

•

Under BS 3923, the most stringent inspection level (level 1) can only be achieved for set-on connections if the bore of the nozzle/branch is accessible and the root bead has been removed.

•

Unlike BS 3923, BS EN 1714 does not differentiate between single sided and double-sided welds for plates and pipes and set-through nozzles.

5


3.2.5

Final Report

Sensitivities The sensitivities provided in the tables below are for angled beam examinations. Note: SDH = side-drilled hole and in the case of BS 3923 scanning sensitivity, ‘+’ means more sensitive.

3.2.5.1 BS 3923 Weld examination Longitudinal defects

Transverse defects

Examination Level 1

Grass or at least DAC + 14dB

Grass or at least DAC + 20dB

Examination Level 2

3mm SDH DAC + 14dB

3mm SDH DAC + 14dB

Examination Level 3

3mm SDH DAC + 8dB

3mm SDH DAC + 8dB

Examination Level 4

Minimum scanning sensitivity Minimum scanning sensitivity by agreement. by agreement.

Table 3.1:

Minimum scanning sensitivity level under BS 3923.

Evaluation and Recording Level For level 1, all indications detected above grass shall be evaluated. For other levels, only indications which exceed the DAC curve at the minimum scanning sensitivity defined above shall be evaluated. Thus, for example, for a scanning sensitivity of DAC +14dB, indications which exceed DAC –14dB (20% DAC) will break the marked DAC curve and should be evaluated. Isolated point reflectors are ignored unless their amplitude is above a specified level in which case they are recorded. Indications which are above the evaluation threshold and interpreted as being planar shall be recorded. 3.2.5.2 BS EN 1714 / BS EN 1712 Weld examination - BS EN 1714 Scanning sensitivity not specified. A choice of reference and evaluation levels is given, see below, these are for both longitudinal and transverse defects. Method 1: 3mm SDH

Reference level: 3mm SDH DAC Evaluation level: Reference level - 10dB

Method 2: distance gain Reference level: based on a Disc Shaped Reflector (DSR) size (DGS) dimension varies with the parent material thickness and the (not covered by BS 3923) probe frequency used. Evaluation level: Reference level - 4dB Method 3: 1mm deep Reference level: DAC curve for a 1mm deep rectangular rectangular notch notch. Evaluation level: Reference level - 10dB Tandem testing

DDSR = 6mm (for all thicknesses) Table 3.2:

Weld examination level under BS EN 1714.

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Final Report

Recording Level - BS EN 1712 •

•

•

Recording level for methods 1 & 3: o

Acceptance level 2: reference level – 6dB (50% DAC)

o

Acceptance level 3: reference level – 2dB (80% DAC).

Recording level for method 2: o

Acceptance level 2: reference level

o

Acceptance level 3: reference level + 4dB

Recording level for tandem testing: o

DDSR = 6mm (for all thicknesses)

Note that BS EN 1714 states that the amplitude of indications exceeding the DAC -10dB (32% DAC) evaluation level shall be recorded and the length determined (and also height if required). The BS EN 1712 recording level of 50% DAC (for acceptance level 2) appears to conflict with this. The BS EN standards also equate DAC -10dB to 33% DAC, whereas it is in fact 31.6% DAC. In this project we have taken the BS EN recording level to be 32% DAC. 3.2.6

Length measurement

3.2.6.1 BS 3923 Under BS 3923 the length measurement of an indication is typically performed using the 6dB drop tip location technique (for defects larger than the beam width). Sizing of imperfections should be carried out at ranges not exceeding the halfskip range (except when agreed with the contracting parties). 3.2.6.2 BS EN 1714 / BS EN 1712 Under BS EN 1714:1998: the length of the indication, in either the longitudinal or transverse direction shall, where possible, be determined using the technique specified in the acceptance levels standard or 6dB drop tip location technique, unless otherwise agreed. However, the BS EN 1712 acceptance standard provides no choice since the 6dB drop method is not referenced. The BS EN 1712:1997 (Acceptance levels) specifies that: the length of the indication shall be determined by measuring the distance along the length over which the echo amplitude is above the evaluation level, using the fixed amplitude level technique described in Annex B of the standard. The technique measures the lateral dimensions of an indication over which the echo is equal to or greater than the evaluation level. To make a measurement the beam is scanned over the indications, and the probe position and beam path range, at which the echo has fallen to the evaluation level provide the extremities of the measured defect length. A narrow beam width probe is suggested in order to achieve a more accurate measurement or corrections for the influence of the beam width may be performed. 7


3.2.7

Final Report

Other •

BS EN 1714 (Section 12.3) recognises the difficulty of detecting subsurface planar defects perpendicular to the surface. It recommends consideration of specific testing techniques but does not provide further guidance. BS 3923 does not recommend other techniques for such defects, except that for double sided welds if the weld root before welding exceeds 3mm, then either the tandem technique or a beam within 10º of normal to the root face must be applied.

•

BS EN 1714 requires personnel to be “…familiar with testing problems specifically associated with the type of weld joints to be tested.” Under BS 3923, “the successful application of manual ultrasonic testing depends on the knowledge and experience of the personnel responsible for producing the test procedures, and the technical competence and ability of the ultrasonic practitioner to carry out the procedural requirements and interpret results.”

•

It should be noted that the EN standard recommends as low a frequency for initial testing (detection) as possible within the range 2MHz to 5MHz. Under BS 3923 probes in the range 4MHz to 6MHz are preferable when the parent examination is carried out in order to note possible variations in attenuation in the material. There are no specific requirement when it comes to the initial testing of the weld. The standard provides guidance in the choice of probe frequency to be used for the weld inspection in relation to the resolution, attenuation and coupling which can be achieved: better resolution is achieved with higher frequency probes; at long ranges or in more highly attenuative materials, shear probe frequency should be in the range 2MHz to 3MHz. Lower frequency probes are noted as being more tolerant of surface roughness.

8


4.

Final Report

EXPERIMENTAL INVESTIGATION This section describes the following:

4.1

•

Ultrasonic modelling to help guide testpiece design

•

Sourcing of testpieces for trials

•

Techniques and procedures adopted for trials

•

Trials results

•

Discussion of results

ULTRASONIC MODELLING In order to guide the design and selection of testpieces, some modelling of the 45° corner reflection response from vertical and tilted internal surface cracks has been performed. The mathematical model of ultrasonic response is based on elastic Kirchhoff theory applied to the case of the pulse-echo corner-effect technique, where the intention is to detect defects close to the inner surface using probes on the outer surface. The results are presented in Appendix B. With reference to Figures B1 to B4 inclusive, the shapes of the corresponding families of curves are fairly similar but with slightly higher amplitude for the 4MHz probe compared with the 2MHz probe for untilted cracks, and slightly lower amplitude for tilted cracks (as expected). For the most part there are similar trends observed in both the 2MHz and 4MHz output data sets. Within the approximate limits of ±5° reflector tilt there is a clear correspondence of increase in signal amplitude with increase in reflector throughwall extent - this response does not appear to demonstrate any strong bias with regard to sign of tilt. For both the 2MHz and 4MHz probes the maximum response is obtained when the throughwall extent of the reflector is largest (i.e. 5mm) and tilt is small or zero (i.e. vertical). The mathematical model is based on specular reflection and therefore does not take any account of diffraction by the top edge of the slot. It is expected that this mechanism would become the dominant detection mechanism at increasing angles of tilt. The results indicate that for the situation modelled, defects exceeding 1mm in height and tilted up to 20° should be above the evaluation threshold for both the BS and EN standards. Note that these results assume that the weld cap does not impose a restriction to scanning, and that there is no complication due to an undressed root or counterbore.

9


4.2

Final Report

TESTPIECES The work involved inspection of real or synthetic service induced defects within a variety of plates, pipes, T-butts and nozzles. The testpieces were ferritic steel testpieces of thickness range 10mm to 65mm. The testpiece list is provided in Tables C1 and C2 in the Appendix C. The defect dimensions refer to intended dimensions unless otherwise stated since sectioning was only performed on a limited number of testpieces. 26 testpieces were used providing a total of 76 defects (4 of which were erosion/corrosion simulated defects). Most of the defects were manufactured the exceptions being the ten selected stress oriented hydrogen induced cracking defects in the SOHIC pipe, the smooth thermal fatigue root crack in pipe P257 and the two toe cracks of the furnace testpiece. Several of the testpieces were provided by members of the Steering Committee (some of which were lent from the PANI programme). Note that the testpieces used for the study contain both in-service and manufacturing type defects. Only defects representing service induced defects are of main interest for this project.

4.3

INSPECTION TECHNIQUE & PROCEDURE The inspections were generally carried out using 45º, 60º and 70º, 10mm diameter crystal probes at both 2MHz and 4MHz frequencies, and scanning from all available surfaces. Signal amplitude was recorded without applying any threshold. It has therefore been possible to analyse the results to investigate which defects exceed the EN evaluation threshold of 32% DAC as a function of frequency and beam direction, and the extent to which lowering the evaluation threshold increases the number of reportable defects. Since the object of this project was to investigate procedure capability rather than human reliability these trials were not blind, i.e. the operators were informed of the location and types of defects in each testpiece. The inspection was carried out using the following probes: •

45°, 2MHz, Ø10mm single crystal probe

•


•

45°, 4MHz, 6x6mm single crystal sub-miniature probe

•


•


•

60°, 4MHz, 6x6mm single crystal sub-miniature probe

•


•


•

70°, 4MHz, 4x9mm twin crystal probe

10


4.4

Final Report

RESULTS The results of the trials are presented in Appendix D. Note that most of the defect dimensions refer to intended dimensions since only a limited number of testpieces were sectioned. The sectioned testpieces include: three plates (2557-003, 2557-004 and P2401), one pipe (P2301), one T-joint (P2201) and two set-on nozzles (P2101 and P2102). This providing sectioning sizing measurements for a total of 26 defects out of the 76 defects used for the study. A summary of the results is provided in the following sub-sections. Note that the comments relating to the BS 3923 standard refer to an examination level 2B. Refer to the schematic diagrams of Appendix A1 (Figure A1) when references are made to a scanning side. The BS and EN examination requirements specific to each testpiece used for this study are provided in Appendix A2.

4.4.1

Detection

4.4.1.1 Plates The results collected from the plates are summarised in the following tables. TESTPIECES

COMMENTS

2557-003 (10mm thick) (4MHz probes only)

• • • •

• • 2557-004 (12mm thick)

• • • • •

•

Investigation of 4 embedded cracks. Best responses from the twin 70° 4MHz probe. The presence of the weld cap restricted the coverage of the 45° (1/2 skip and full skip) and the 60° probes (1/2 skip). Defect No3 (smooth crack) could have been missed using 60° 4MHz probe as this probe provided responses of very low amplitude. The full skip responses were just reaching the recording threshold for the BS EN 1714 standard. 2MHz probes could not be applied as the testpiece was no longer available (the testpiece has been sectioned). The results show that the defects should have been detected and reported using either the EN or the BS standard. Investigation of 2 embedded cracks. Best responses from the twin 70° 4MHz probe. The 45° probes provided poor results due to restricted coverage created by the weld cap. The 2MHz lower beam angle probes provided higher responses than the 4MHz equivalents. The results show that the two defects should have been detected and reported using either BS EN 1714 or BS 3923. (Note that under BS EN 1714, if restriction to one side of the weld, an additional beam angle has to be used and if scanning cannot cover the whole inspection volume alternative or supplementary UT should be used or the weld reinforcement be removed). The results from the dressed testpiece allowed for detection of the defect by all the beam angles and provide responses above the BS EN threshold of 32% DAC. 11


TESTPIECES

COMMENTS

TPF7 (14mm thick)

• • • •

•

• RSA/PLW/15/ ET001 (15mm thick)

• • • • •

• •

•

Final Report

Investigation of 2 cracks: 1 toe crack and 1 at the root. Best responses from the single 70° 2MHz probe. In general, the 2MHz probes provided higher responses than the 4MHz probes. Defect No1 (Toe crack) was only detected from one side and at full skip from the upper surface and at ½ skip from the bottom surface. A full skip examination is critical. The signal amplitudes from the defect are ≥45% DAC. Care must be taken to ensure total coverage of the weld volume. The results show that due to the weld cap restriction, the information on the defects is mainly provided from the full skip scanning. The results show that the defects should have been detected and reported using either the EN or the BS standard. Investigation of 7 upper surface breaking cracks: 3 transverse cracks and 4 longitudinal cracks (testpiece as welded). All the defects in this plate were upper surface breaking cracks, three of which were transverse. The inspection from the top side was carried out using full skip and the inspection from the bottom side was carried out using half skip. A transfer correction of 15 dB was applied when inspecting from the top side of the test plate to compensate for the zinc coating. The transverse defects were still detectable from the upper surface even though the weld cap was present. The signal amplitude from defects 4 and 5 are all ≤ 28% DAC which is below the reporting threshold of BS EN 1714. These defects should have been reported under BS 3923 (even if most of the responses were ≤20% DAC). Defect 6 would have been reported under both standards if the 45° 4 MHz probe was used. Best responses from the 45° probes when scanning from the upper surface. Best responses from the single 70° 2MHz probe when scanning from the bottom surface (with the exception of defect 7, which is not a toe crack, for which the 45° 4MHz probe provided the best response). The 2MHz probes provide in general higher responses than the 4MHz probes.

12


TESTPIECES

COMMENTS

TPF1 (20mm thick)

• • •

•

• • PL8155 (20mm thick)

• • • • •

PL8153 (25mm thick)

• • • • •

PL8156 (25mm thick)

• • • •

PL8157 (25mm thick)

• • • • •

Final Report

Investigation of 5 root cracks. Defects of height ≥5mm were detected with a response ≥ 36% DAC regardless of the probes angle and frequency. The signal amplitude from the Defect 1 (height = 2mm) was below the reporting threshold of BS EN 1714 for most of the scans. Only the 60° and the 70° 2MHz probes provided signal amplitude above the BS EN recording threshold. The signal amplitude from the Defect 2 (height = 3.5mm) was below the reporting threshold of BS EN 1714 for the 45° 2MHz and 4MHz and for the 70° 4MHz probe. The defect would have been reported using all the probes under the BS standard (signal at the limit of reportability using the 45° probes). Best responses from the 70° 2MHz probe. The 2MHz probes provide in general higher responses than the 4MHz probes. Investigation of 1 toe crack. Best responses from the single 70° 2MHz probe. Testpiece not inspected in full skip with the 70° probes. The 4MHz probes provide in general higher responses than the 2MHz probes. The results show that the defect should have been detected and reported using either the EN or the BS standard. Investigation of 2 cracks: 1 toe crack and 1 root crack. Best responses from the 60° and 70° 2MHz probes. Testpiece not inspected in full skip with the 70° probes. The 2MHz probes provide in general higher responses than the 4MHz probes. The results show that the defects should have been detected and reported using either the EN or the BS standard. Investigation of 1 embedded crack. Best responses from the single 70° 2MHz probe. The 2MHz probes provide in general higher responses than the 4MHz probes. The results show that the defects should have been detected and reported using either the EN or the BS standard. Investigation of 1 toe crack. Best responses from the single 60° and 70° 2MHz probes. Testpiece not inspected in full skip with the 70° probes. The 2MHz probes provide in general higher responses than the 4MHz probes. The results show that the defects should have been detected and reported using either the EN or the BS standard.

13


TESTPIECES

COMMENTS

PL8158 (25mm thick)

• • • • •

• PL8159 (30mm thick)

• • • •

PL8160 (30mm thick)

• • • •

P2401 (40mm thick)

• • • • •

P24T (40mm thick)

• • • • •

Final Report

Investigation of 1 embedded crack. Best responses from the single 60° 2MHz. The 2MHz probes provide in general higher responses than the 4MHz probes. Testpiece not inspected in full skip with the 70° probes. The signal amplitude of some of the indications was below the reporting threshold of BS EN 1714 for some scans (but above BS 3923 threshold). However, this should not have prevented the defects being reported. The results show that the defects should have been detected and reported using either the EN or the BS standard. Investigation of 1 embedded crack. Best responses from the single 45° 2MHz probe. Testpiece not inspected in full skip with the 70° probes. The signal amplitude of some of the indications was below the reporting threshold of BS EN 1714 for some scans (but above BS 3923 threshold). However, this should not have prevented the defects being reported. Investigation of 1 transverse crack (testpiece dressed). Best responses from the single 45° 2MHz probe. Testpiece not inspected in full skip with the 70° probes. This transverse crack should have been reported regardless of the standard used. Investigation of 2 cracks: 1 root crack and 1 embedded crack. Best responses from the 60° and 70° 2MHz probes. Testpiece not inspected in full skip with the 70° probes. The 2MHz probes provide in general higher responses than the 4MHz probes. The results show that the defects should have been detected and reported using either the BS EN 1714 or the BS 3923 standard. Investigation of 1 upper surface breaking crack. Best responses from the 45° 2MHz probe. Testpiece could not be inspected in full skip with the 70° probes due to restricted scanning surfaces. The 2MHz probes provide in general higher responses than the 4MHz probes. The results show that the defects should have been detected and reported using either the BS EN 1714 or the BS 3923 standard.

14


Final Report

4.4.1.2 Pipes The results collected from the pipes are summarised in the following tables: TESTPIECES

COMMENTS

P2301 (11mm thick)

• •

• •

Investigation of 6 defects: 4 cracks (3 root cracks and 1 toe crack) and 2 erosion/corrosion defects. With the exception of the toe crack, all the defects were located at the inner surface. The presence of the weld cap did not allow coverage of the weld using the 45° at half skip. The investigation range was therefore increased to 1.5 skip. The results from all the defects show that the detection and the reporting of the defects should have been the same regardless of which standard was used for the inspection. The 2MHz probes provide in general higher responses than the 4MHz probes for the defects other than the erosion/corrosion defects for which the 4MHz probes tended to provide higher responses.

Cracks: • Best responses generally from the 70° 2MHz probe. • The 2MHz probes provide in general higher responses than the 4MHz probes. • The results show that the defects should have been detected and reported using either BS EN 1714 or BS 3923 standard.

P23T (11mm thick)

Erosion/corrosion defects: • Best responses from the 60° and 70° 4MHz probes. • The 4MHz probes provide in general higher responses than the 2MHz probes. (May have been due to morphology of simulated corrosion). • The weld geometry restricted the weld inspection from the cone resulting in poor results. The indications were detected with responses below 20% DAC which is below the reporting threshold of both BS EN 1714 and BS 3923. This would have prevented the defects being reported. • Investigation of one incomplete root penetration which simulates a root corrosion defect. • The presence of the weld cap did not allow coverage of the weld using the 45° at half skip. The investigation range was therefore increased to 1.5 skip. • The indication was detected with responses above 40% DAC which is above the reporting threshold of both BS EN 1714 and BS 3923. • The results show that the defects should have been detected and reported using either the BS EN 1714 or the BS 3923 standard.

15


TESTPIECES

COMMENTS

TPF6 (12.5mm thick)

• • •

• • SOHIC (12.7mm thick)

• • • • •

•

•

•

TPF8 (35mm thick)

• • • •

•

Final Report

Investigation of 4 cracks: 2 embedded cracks, 1 toe crack and 1 root crack. Best responses from the 70° 2MHz probe. The signal amplitude of some of the indications was below the reporting threshold of BS EN 1714 for some scans (but above BS 3923 threshold). However, this should not have prevented the defects being reported. The 2MHz probes provide in general higher responses than the 4MHz probes. Defect 6 (Toe crack) was only reported at full skip due to the presence of the weld cap. Investigation of 10 cracks - the cracks are in the parent material. Best responses from the 0° probe. The amplitude responses from the 0° probe (all above 40% DAC) would have allowed all the detected defects to be reported with either standard. The maximum recorded level with the shear probes was 56% DAC. Most of the recorded levels were below 32% DAC. The highest responses from the shear probes were generally provided by the 45° 4MHz probe. Only four out of the ten defects examined provided indication level ≥32% DAC. Nine out of ten defects provided indication level ≥20% DAC using this probe. All the detected defects (10 defects) would have exceeded the reporting threshold of the BS 3923 standard using the 45° 2MHz probe and only one would have exceeded the reporting threshold of the BS EN 1714 standard. Seven out of ten defects would have exceeded the reporting threshold of BS 3923 using the 60° 2MHz probe and only one would have exceeded the recording threshold of BS EN 1714. Both the 60° and the 70°, 4MHz probes provided indication level below 14% DAC which is below the recording threshold of both standards. Investigation of 7 cracks: 1 HAZ crack, 3 root cracks and 3 embedded cracks. Best responses from the 70° probes. Testpiece not inspected in full skip with the 70° probes. All the defects should have been reported using the 70° probes. The detection was achieved from either side of the weld cap with the exception of defect 4 (toe crack) which was only detected from one side. The signal amplitudes from the indications were all above 32% DAC with the 70° probes. The 2MHz probes provide in general higher responses than the 4MHz probes.

16


TESTPIECES

COMMENTS

P257 (65mm thick)

• • • • •

Final Report

Investigation of 1 smooth crack at the root. Best responses from the 45° probes. Testpiece not inspected in full skip with the 70° probes. The signal amplitudes from the indications were all very large (above 140% DAC). The defect would have been reported under BS EN 1714.

4.4.1.3 T-Joints The results collected from the T-joints are summarised in the following tables: TESTPIECES

COMMENTS

P2201 (20x10mm thick)

• • • •

Investigation of 7 defects: 4 cracks (3 root cracks and 1 toe crack), 1 laminar tear and 2 erosion/corrosion defects. The toe crack (defect 6) and the two erosion/corrosion defects (defects 5 and 7) were not detected from the attachment plate. Additional inspection was carried out from side F for defects 5, 6 and 7 (not required by either the EN or the BS standard). The probe beam angle and frequency providing the best responses is highly dependent on the defect being investigated and the scan direction.

Cracks & laminar tear: • Best responses from the 45° 2MHz and the 70° probes. • The toe crack was only detected from sides D, E and F, as expected (see schematic diagram in Appendix D.3.1). The defect would therefore have only been reportable under the BS EN 1714 standard if examination level C or better was required, as the examination only needs to be carried out from sides A and B under examination level B. The defect would have been reported under BS 3923. • With the exception of the toe crack, the reporting of the cracks would have been the same regardless of which standard was used for the inspection. Erosion/corrosion defects: • Best responses from the 45° 2MHz probe. • Scanning restriction due to the weld cap prevented detection of these small defects (≤3mm height) at 0.5 skip from side F. • To detect both defects the inspection has to be carried out from the component plate.

17


TESTPIECES

COMMENTS

P22T

• • •

(20x10mm thick)

•

Furnace testpiece • (24x21.5mm thick)

•

• •

•

•

Final Report

Investigation of 1 toe crack in the component plate. Best responses from the 70° 2MHz probe. The 2MHz probes provide higher responses than the 4MHz probes. The defect was detected only from the component plate surfaces and would therefore not have been reported under BS EN 1714 examination level B. Investigation of 2 toe cracks one in the furnace plate and one in the tubeplate. The inspection scans for this component would normally be limited to surfaces B & D and C (0° inspection) - see schematic of the testpiece Appendix D.3.3. The limitation is mainly due to access restriction. For this study, all the scanning surfaces were investigated to maximise the amount of useful data available. To allow better coverage, sub-miniature probes were used from surface A. Defect No1 (Tubeplate crack): Best response from surface B. Best responses achieved using the 70° 2MHz probe. The 2MHz probes provide higher responses than the 4MHz probes. Reportable by both BS EN 1714 and BS 3923 standards. Defect No2 (Furnace crack): Best response from surface F. Note that inspection from surface F is not required by either BS EN 1714 or the BS 3923 standard and is not included for the in-service inspection. This result is unexpected since half skip from surface D or E would normally be expected to be better. The results may be due to the orientation of the defect or quality of the scan surface in this particular testpiece. The defect was only detected from sides D, E and F. It would therefore have only be reportable under the BS EN 1714 standard if examination level C or better was required, as the examination only needs to be carried out from sides A and B under examination level B. The defect would have been reported under BS 3923. Best responses achieved using the 45° and 70° 2MHz probes. Therefore, both defects should have been reported under BS EN 1714 with an examination level C. If the requirement was to examination level B, the defect in the furnace plate would not have been reported. Both defects would have been reported under BS 3923 (the 70° probes alone would have allowed the detection of both defects.

18


Final Report

4.4.1.4 Nozzles The results collected from the nozzles are summarised in the following tables: TESTPIECES

COMMENTS

P2101 Set-on nozzle

• •

(12mm thick)

• • • •

• •

P2102 Set-on nozzle

• •

(12mm thick)

• • • • • •

Investigation of 3 root cracks. The restriction related to the weld cap limited the volume coverage around the root for the 45° probes and (in some instances) the 60° probes when scanning from the branch. Inspection carried out from the outer surfaces of the branch (side A) and the plate (side E – not required by BS EN 1714 or BS 3923). Difficult to resolve signals from the defect and those from the root. Inspection from the plate allows detection of the three defects with amplitudes above 40% DAC. Inspection from the branch allows detection of the three defects with amplitudes at least 40% DAC using the 70° probes and amplitudes at least 20% DAC using the 60° probes (with signals not always maximised). Best responses achieved using the 45° 4MHz probe (from the plate) and 70° 4MHz probe (from the branch). The results show that the defects would have been detected and reported using either the BS EN 1714 or the BS 3923 (examination level 2) standard. Investigation of 1 root crack and one toe crack (15° tilt). The restriction related to the weld cap limited the volume coverage around the root for the 45° probes. Inspection carried out from the outer surfaces of the branch (side A) and the plate (side E – not required by BS EN 1714 or BS 3923). Difficult to resolve signals from the defect and those from the root. Best responses achieved using the 70° probes for the root crack and the 45° 4MHz probe for the toe crack. Inspection from the branch and the plate allows detection of the root crack with amplitudes above 40% DAC. There are no responses from the plate scans which are between 20% DAC and 32% DAC. The toe crack would have been reported if the inspection included scanning from the plate. As no scanning from the component is required, the toe crack would not have been reported under either standard.

19


TESTPIECES

COMMENTS

P21T Set-on nozzle

• •

(11mm thick)

• •

• • • • •

4.4.2

Final Report

Investigation of 1 root crack. Inspection carried out from the outer surface (side A) and the inner surface (side B1) of the branch and from the outer surface of the plate (side E). Restriction was created by the weld cap limiting the volume coverage when scanning from the branch and the plate. Due to the restriction, the defect was only detected in full skip from the branch side A and B1. The 60° probes detected the defect from side A but the signal could not be maximised. A 0º 5MHz Ø10mm twin probe was used from the internal surface of the plate. The defect was detected providing a signal equivalent to 141% DAC. Sub-miniature probes did not provide better results. Difficult to resolve signals from the defect and those from the root. Best responses achieved using the 70° 4MHz probe. The inspection from the outer surface of the branch (side A) allows the detection of the defect with amplitudes of 100% DAC with the 70° probes. The defect should have been reported under both BS EN 1714 and BS 3923 standards.

Length measurement In addition to the study on the amplitude response, length measurement was carried out on selected testpieces: plates TPF7, PL8155, PL8156 and P24T; pipes TPF6 and TPF8; T-joints P22T and the furnace testpiece, and the P21T nozzle. This provided length measurement on 20 defects (that is on: 6 toe cracks, 6 root cracks, 5 embedded cracks, 2 centreline cracks at surface and 1 HAZ crack). The defect lengths were measured using both standards (that is, using the 6dB drop technique and the fixed amplitude level technique) and compared to the intended defects length. The length of the indications was determined under BS 3923 using a 6dB drop tip location technique (6dB drop from the last peak) and under BS EN 1712/ BS EN 1714 by measuring the distance along the length over which the echo amplitude is above the evaluation level. The results from individual testpieces are provided in Appendix D. A summary of the length measurement is provided in the table below (these results are illustrated by the figures in Appendix D5 - note that none of the defects used for this sizing exercise was sectioned).

20


Beam angle (º) Probe frequency (MHz) Defect ref.

Final Report

45 2MHz

Intended BS

60 4MHz

2MHz

70 4MHz

2MHz

4MHz

EN

BS

EN

BS

EN

BS

EN

BS

EN

BS

EN

TPF6-3

14

14

15

15

11

13

15

14

15

15

17

14

14

TPF8-10

14

17

17

16

17

18

11

16

10

17

10

17

11

TPF8-9

15

17

15

17

14

19

17

19

16

19

17

18

16

P22T-1

15

14

14

15

15

14

14

15

14

14

14

14

13

TPF8-4

16

16

16

17

16

18

17

18

-

16

17

16

12

TPF6-6

17

20

20

20

21

18

20

18

18

17

17

16

21

TPF7-1

18

17

18

17

17

19

20

19

10

17

18

18

18

TPF6-1

18

-

-

-

-

16

17

18

18

18

22

19

19

TPF6-8

18

-

-

-

-

20

22

19

23

20

25

17

20

TPF8-5

18

20

19

18

20

17

20

19

22

20

18

18

21

TPF8-8

18

19

17

20

16

20

21

20

20

19

21

17

20

TPF8-11

18

17

17

17

19

17

21

17

19

22

23

18

23

TPF7-4

20

19

11

20

11

22

10

19

10

20

24

19

12

PL8155-1

20

19

22

20

21

22

22

22

20

21

23

22

22

PL8156-1

20

19

30

19

27

22

27

22

27

23

30

21

27

TPF8-7

20

21

24

20

23

21

28

19

30

20

31

22

30

P21T-2

20

-

-

-

-

18

8

18

15

17

15

20

20

P24T-1

47

50

50

49

54

48

49

48

53

51

56

50

25

Furnace-1

220

40+ 40+ 228 214 225 210 217 40+ 218 35+ 215 215 216 210 70 75

Furnace-2

220

220

20+ 30+ 30+ 25+ 20+ 20+ 214 214 212 215 10+ 210 20 20 18 20 10 10+7

Note: the reference in the table to BS refers to BS 3923 and EN refers to BS EN 1712. Table 4.1: Defect Length – including intended values and measurement values using the 6dB drop technique and the fixed amplitude level technique.

Note: the signal amplitudes of the defects in the furnace testpieces were close to threshold for some defects so reported length was segmented along full length of defect. 4.4.3

Echodynamic pattern The operators were asked to record the echodynamic pattern observed for the individual beams used for each of the defects examined for length measurement (see above section 4.4.2). The results for the relevant testpieces are provided in Appendix D. 21


4.5

DISCUSSION

4.5.1

Detectability

Final Report

4.5.1.1 Plates and Pipes There are little differences between the two standards as far as coverage is concerned as they both specify that full coverage of the inspection volume needs to be achieved in both cases. If there are access restrictions additional scanning (or other NDT) is required to allow coverage (removal of the weld cap may have to be considered to achieve better coverage). The EN standard is more general, with very little specific requirements on scan directions or probe type. The fact that most of the defects were synthetic defects may have influenced the detectability of the defects. Detection of defects at the root was generally very hard. The geometric echo related to the root bead made the differentiation between defect and root difficult. The operators reported that some of the defects might not have been detected under normal circumstances i.e., if the position of the defects were not known. Therefore, a dedicated scan parallel to the root bead is strongly recommended for root inspection. All of the weld defects provided signal amplitudes which were above both EN and BS report thresholds for at least one probe and scanning direction. However since the EN standard provides little guidance on the selection of probes or beam direction, it is reliant on the operator using an appropriate technique, or a procedure being available which specifies appropriate techniques. One pipe, the SOHIC testpiece, contained stress oriented hydrogen induced cracking (SOHIC) located in the parent material. Due to the thickness of the testpiece, the scanning requirements for weld defects are very similar for both the EN and the BS standard with the exception of the scanning sensitivity. The amplitude responses from the defects were mostly below 32% DAC. Therefore, these defects may not have been reported under the EN standard. Moreover, the best responses from the defects were obtained with a 0° probe. The amplitude responses from the 0° probe were all above 40% DAC which would have allowed all the defects to be reported with both standards. However, under the EN standard, the inspection of the parent material with a 0º probe does not need to be carried out if the inspection was carried out prior to welding. According to both standards, if there is a restriction to one side of the weld a second probe must be used and total coverage of the weld volume must be ensured. Therefore, all the weld defects from the plate and pipe testpieces used for the study should have been reported regardless of the standard used (provided an appropriate beam angle was used). The 2MHz probes provided in general higher responses than the 4MHz probes for testpieces 14mm thick and above. However, the results showed that the responses from the erosion/corrosion defects were slightly higher with the 4MHz probes. Also, there may be circumstances where 4MHz probes provide 22


Final Report

better detection capability, for example it can be easier to distinguish between defects at the weld root, and the root bead reflection, when using higher resolution (higher frequency) probes. For the defects and testpieces studied, the 70º probes provided best overall performance for thicknesses up to 20mm, suggesting that a 70º probe should be included as a minimum for this thickness range. 4.5.1.2 T-joints Under BS EN 1714 examination level B, scanning with angled beams needs only to be carried out from the two faces of the attachment plate (see Table A3 & Figure A1 of Appendix A). No scanning from the component is required. Therefore, defects at the upper and rear surface of the component plate cannot be detected. This is in contradiction with the required volume coverage as stated in the European standard since, by limiting the inspection to the attachment plate surfaces, the coverage of the inspection volume of the attachment welds cannot be achieved. The results from this study highlighted this point. For example, defects such as toe cracks and erosion/corrosion defects would not have been reported under the BS EN 1714 standard. The EN standard specifies a 0° scan from the main component surface (opposite surface from the attachment). This is certainly appropriate for manufacturing inspections, to detect e.g. lack of fusion between the attachment weld and the main component. However, for in-service inspection, angle beam scans to detect cracks initiating at the weld toes and propagating into the main component would be more appropriate (access permitting). In general, it is difficult to identify the best probe to allow the detection of the defects as it varies with the type and position of defect being inspected. 4.5.1.3 Nozzles All the nozzles inspected for this work had thicknesses below 15mm. The scanning requirements for the testpieces are summarised in Table A4 of Appendix A2. BS 3923 level 1 does not cover set-on nozzles less than 15mm thick. The requirements under the examination level 2 are: to scan from the branch to full skip using two angles and an independent root scan with one angle scanning from side A (if the root bead is removed, the separate root scan can be omitted). The requirement for the EN standard is that the inspection has to be carried out from one of the nozzle surfaces toward the shell (scan from side A to 1.25 skip or side B1 to 0.5 skip). There is no requirement for scanning from the shell. Therefore, inspection from the shell is not required under either of the standards which is a major omission for in-service inspection. The geometry and the weld finish often restricted the coverage for the 45° and 60° beams. The 70° beams would have allowed the detection and reporting of

23


Final Report

all the root defects with responses >40% DAC from the branch. They would have therefore been reported under both standards. Difficulty was reported in resolving signals from the root crack and those from the root. However, the toe defects would not have been reported under either of the standards as inspection from the plate is not required. The results show that the responses from the defects with the 70° beams scanning from the plate would have provided responses >45% DAC which is above the reporting threshold of both standards. 4.5.2

Length measurement Length measurement using the fixed amplitude level technique of the EN standard can vary depending on the beam angle and frequency used, since length measurement is related to the defect response at the evaluation level. Defects for which the response varies below the evaluation level along their length can be reported as a succession of smaller defects. For example, defect furnace-2 length was reported as three discrete short lengths when using the 60º probes. The length measurement using the fixed amplitude level technique of the EN standard for the two longest defects (furnace-1 and furnace-2 both having a length of 220mm) varies from 150mm to 215mm for furnace-1 and 30mm to 50mm furnace-2 depending on the probe used (the 70° probes provided the larger values). The variation as a function of probe in measured length for the defects of intended length 14mm to 20mm (this excludes the three longest defects), illustrated by Figures 4.1 and 4.2, are: L being the intended length value, •

with the British Standard: L - 3mm to L + 4mm

•

with the European Standard: L - 22mm to L + 11mm

The standard deviations of the difference between the length measured using the 6dB drop technique and the fixed amplitude level technique were calculated. The calculation was carried out for all the defects and for the defects of intended length between 14mm and 20mm only. The results are provided in Table 4.2 below. Note that the measured length of the defects in the furnace testpiece used for the calculation were the addition of the segmented lengths, when appropriate. Probe characterisation

45° 2MHz

45° 4MHz

60° 2MHz

60° 4MHz

70° 2MHz

70° 4MHz

standard deviation (including all the testpieces)

42.6

37.7

49.1

48.4

37.2

37.1

standard deviation (excluding the three longest defects)

3.6

3.6

5.1

5.0

4.0

4.1

Table 4.2:

Standard deviation of the difference between the length measured using the 6dB drop technique and the fixed amplitude level technique.

24


Final Report

35 Intended length smaller length recorded larger length recorded

30

Length (mm)

25

20

15

10

5

0 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Defect

Figure 4.1: Variation of length measured using the 6dB drop technique (for defects of intended length between 14mm and 20mm).

35 Intended length smaller length recorded larger length recorded

30

Length (mm)

25

20

15

10

5

0 1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

Defect

Figure 4.2: Variation of length measured using the fixed amplitude level technique (for defects of intended length between 14mm and 20mm).

25


Final Report

In general, the experimental results show (regardless of the probe angle and frequency used) that the lengths measured using the 6dB drop technique of the British Standard are very close to the intended values. However, the results collected using the fixed amplitude level technique of the BS EN 1712 show more discrepancies against the intended length values. Moreover, the error of measurement seems to increase with defect length and long defects, where the amplitude fluctuates about the BS EN evaluation level along their length, can be reported as segmented (note that the study only included three long defects, of intended length: one 47mm long and two 220mm long). The accuracy of defect length measurement depends on the shape of the defect and its size. This is particularly the case for the EN fixed amplitude technique, since parts of the defect which provide responses below the evaluation threshold of 32% DAC will not be included in the measured length. This can be put into perspective by considering the results from testpiece TPF1 which contained a number of vertical surface breaking cracks. Defect 1 was of nominal height 2mm and out of six probes (45º, 60º, 70º each at 2MHz and 4MHz), it was only above 32%DAC for one (70º 2MHz) and was borderline for one (60º 2MHz). If the length of this type of defect was measured using the EN fixed amplitude technique, it is therefore possible that parts of the defect below 2mm height would not be included in the measured length. These results are consistent with a more detailed study of length sizing methods performed by TWI [15] on a variety of defect types. Fixed amplitude techniques were reported to be less subject to operator variability than the 6dB drop (from the last peak) method, but more prone to gross deviations from length measured by radiography. Note that undersizing is generally of more concern than oversizing. 4.5.3

Echodynamic pattern The patterns recorded were all as expected with the exception of two cracks which exhibited a pattern 2 instead of pattern 3. These two cracks were two inservice cracks from a T-joint testpiece (furnace testpiece, see Appendix D.3.3). This helps to confirm that the intended defects are indeed planar.

26


5.

Final Report

CONCLUSIONS The data generated from the test samples show that the defects should have been reported in most cases with both BS 3923 and BS EN 1714 standards (referred to below as “BS” and “EN” respectively). However choice of probes played an important part in the detection of the defects. Defects that were missed or below the EN report threshold with one probe were often reportable with another. The orientation and size of the defect both influence the detection capability. The EN standard does not include a requirement for a separate root scan to be performed. Inspection with a root scan made detection easier on samples with an undressed root. An operator with limited experience may miss root defects without a root scan. Care must be taken when choosing the probes to use for the inspection. The choice of probe(s) to be used for the detection of the defects under the EN standard is even more critical than for the BS standard since the report threshold is higher (less stringent). The results from this study show that in most cases 2MHz has provided higher response than 4MHz, although this may not be the case under other circumstances. A 70° probe should be included to inspect components of thickness 20mm or less. Maximum coverage of the inspection volume of interest must be achieved. Note that both the BS and EN standards specify that this should include the whole weld volume. However for in-service inspection there might be no need to inspect the full weld, but only those parts where service induced defects could occur depending on whether or not there is a need to detect or monitor defects produced during manufacture. Dressing of the weld cap and root improves the reliability of the inspection. Inappropriate choice of probes or scan surfaces using the EN standard could result in defects being missed (it provides less guidance than the BS standard). Moreover, the results from the T-joints and the nozzles studied showed that a literal interpretation of the EN minimum requirements (which limit scans to the branch/attachment) prevents the detection of defects extending into the main component. Defects such as toe cracks growing from the attachment welds as well as erosion/corrosion defects created at the inner surface of the component could therefore be missed under the EN standard. The requirement for parent material inspection prior to weld inspection with a 0° probe is not compulsory with the EN standard as long as the inspection was performed before or after welding. This information would not generally be available to those performing in-service inspection. In any event, if 0º inspection is not performed because it was already done at the manufacturing stage, it could lead to service induced laminar cracking in the parent material being missed. The error in length measurement (compared to intended length) using the EN 1712 standard (acceptance criteria) was up to +11/-22 mm compared to an error of +4/-3 mm using the BS standard. As the length measurement technique specified by this EN standard is directly related to the defect response at the inspection level, length measurement is dependent on the probe used. Due to the variation of the signal amplitude along the defect, longer defects can be 27


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reported as segmented. Much of the guidance on defect characterisation and sizing which was in the BS standard is incorporated in BS EN 583-5 (Reference 14). These methods are considered preferable to those specified in BS EN standards 1712 and 1713. Results from the testpieces demonstrated that for a given probe angle, defects were either not reportable by either standard, or above the reporting threshold for both. There were very few instances where signal amplitude from a weld defect was between the BS and EN report thresholds. Note that it is important to distinguish between “reportable” and “detectable”. A defect might not be reportable because its signal response does not exceed the reporting criteria. However it may still be detectable because its signal amplitude is sufficiently above the noise level for it to be visible to the operator. An operator who detects an indication will normally manipulate the probe so as to maximise the signal and determine whether it exceeds the evaluation / report threshold. If an amplitude report threshold is required for ISI to avoid too many nonrelevant indications being reported, previous experience has shown that a threshold of 10% DAC is sufficiently low for most signals of interest (including tip diffracted signals) to be reported. (BS 3923 specifies a threshold of 10% DAC for high integrity inspections, and 20% for medium integrity inspections for quality control purposes. EN specifies a level of 32%) This project deliberately concentrated on the inherent capability of the EN standard rather than human reliability issues (such as those investigated in the PANI programmes). It is not straightforward to determine the influence of reliability on whether a given defect would still have been reported during blind trials or site inspections, since this is likely to depend primarily on whether it would have been detected in the first place, rather than how successful the operator was at maximising its signal and comparing it to the reporting threshold. The EN standard regularly refers to the “required specification” for guidance on which techniques to use, i.e. it assumes the customer or some other party has already specified some requirements. The EN standard states that it will normally satisfy the need for a written procedure. However, it is not very prescriptive, and can be difficult to interpret. One particular area where confusion can rise is over quality levels and examination levels. It should also be noted that the analysis performed in this project has been based on a “literal” interpretation of the EN standard. It is important that those responsible for specifying and performing ultrasonic in-service inspection do not feel constrained to blindly following the EN standard (or, unless with good reason, any other generic standard). Although the scope of the EN standard does not discriminate between application for in-manufacture and in-service inspection purposes, it is much more appropriate as a generic standard for harmonising in-manufacture inspections, than for in-service inspection. For in-service inspection, it is important that those responsible for specifying and performing the inspection take due account of the types, locations, morphologies and sizes of defects of concern for each specific application. 28


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This is needed in order to determine the most appropriate scan surfaces, probe types and reporting criteria. In fact it may be appropriate for all defects or degradation to be reported during ISI regardless of signal amplitude (i.e. no amplitude threshold). This is because even a small defect may be of interest if it provides early warning of degradation, and recording such defects allows growth to be monitored during subsequent inspections.

29


6.

Final Report

RECOMMENDATIONS The following are based on the results of this project and the views of the Steering Committee members. BS EN 1714 should not be used as a “stand alone” procedure for in-service inspection. In-service inspection should be performed in accordance with a written procedure and work instruction / technique sheet which is more prescriptive than this standard, and includes detailed specification of the scans required, probe characteristics and reporting criteria. These should be based on a knowledge of the defect and degradation types, locations, morphologies and sizes of interest. In defining these requirements, consideration should be given to the following: 1. Although the BS EN amplitude report threshold of 32% DAC (10dB below the DAC curve) may be appropriate for harmonising inmanufacture inspections, a more sensitive threshold may be appropriate for in-service inspections (even though all the defects studied exceeded this threshold with at least one beam). Previous experience has demonstrated that a report threshold of 10% DAC (20dB below the DAC curve) is sufficiently low for most signals of interest to be reported (including tip diffracted signals) but is still high enough to avoid false calls from noise. However there may be cases where it is appropriate to report all indications detected above the noise level. 2. Choice of beam angle is at least as important as choice of reporting threshold. Both capability and reliability for defect detection will tend to increase, the greater the number of different beam angles employed (different probe angles and / or scan surfaces). The results from this project suggest that for weld thicknesses below 20mm, a 70º scan should be included for detection of opposite surface cracks oriented in the through wall direction. An additional 45º scan is recommended for critical applications. For greater thicknesses a 45º scan may provide increasingly better capability than a 70º scan for the detection of such defects. 3. In this study, the 2MHz probes generally provided better capability for detection than the 4MHz probes. However there may be circumstances where 4MHz probes provide the better detection capability, for example it can be easier to distinguish between defects at the weld root, and the root bead reflection, when using higher resolution (higher frequency) probes. 4. A fixed root scan is highly recommended for inspection of welds with an undressed root, where there is a requirement to detect defects near the root, such as inner surface cracks. 5. The fixed amplitude method of measuring length described in BS EN 1712 (acceptance criteria) is not recommended for in-service inspection applications. These should be based instead on the types of techniques described in BS EN 583-5, e.g. 6dB drop sizing for defects longer than the beam width. 6. Acceptance criteria should be agreed between the contracting parties. 30


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REFERENCES [1]

BS 3923 Part 1 1986 - Ultrasonic examination of welds – Methods for manual examination of fusion welds in ferritic steels.

[2]

BS EN 1712:1997 – Non-destructive testing of welds – Ultrasonic testing of welded joints – Acceptance levels.

[3]

BS EN 1713:1998 – Non-destructive testing of welds – Ultrasonic testing – Characterisation of indications in welds.

[4]

BS EN 1714:1998 – Non-destructive testing of welded joints – Ultrasonic testing of welded joints.

[5]

Georgiou, G A : ‘Adopting European Ultrasonic Standards for High Quality Fabrications: Implications for Manufacturers and End Users’ TWI Final Report 5657/10/95 for GSP 5657, December 1995.

[6]

Georgiou, G A and Mudge, P J: 'Proposed European Ultrasonic Procedures and Acceptance Criteria for High Quality Fabricators: Their Evaluation and their Implementation'. INSIGHT, European Issue, October 1994, Vol. 36 No 10, pp 792-793.

[7]

Georgiou, G A and Mudge, P J: 'An Evaluation of Proposed CEN Ultrasonic Methods for the Inspection and Sectioning of Fusion Welds in Ferritic Steels'. Proceedings of the 6th European Conference on Non-Destructive Testing, Nice, France, 24-28 October 1994, Vol. 1, pp 117-121.

[8]

Georgiou G A: 'A Comparison of European Ultrasonic NDT National Standards for Pressure Vessels with Proposed European Standards. (Proceedings of ICPVT '95, Paris, October 1995, No 32).

[9]

BS EN 25817:1992 – Arc-welded joints in steel – Guidance on quality levels for imperfections.

[10]

BS EN 583-1:1999 – examination; Part 1: General principles.

Non-destructive

testing

–

Ultrasonic

[11]

BS EN 583-2:2001 – Non-destructive examination; Part 2: Sensitivity and range setting.

testing

–

Ultrasonic

[12]

BS EN 583-3:1997 – Non-destructive examination; Part 3: Transmission technique.

testing

–

Ultrasonic

[13]

BS EN 583-4:2002 – Non-destructive testing – Ultrasonic examination; Part 4: Examination for discontinuities perpendicular to the surface. 31


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[14]

BS EN 583-5:2001 – Non-destructive testing examination; Part 5: Characterisation and sizing of discontinuities.

[15]

Georgiou, G A, Hayward, K S: 'Ultrasonic Techniques for Sizing Flaw Lengths'. British Journal of Non-Destructive Testing, November (1991) Vol. 33 No 11 pp 545-548.

32

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APPENDIX A: EXAMINATION REQUIREMENTS

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Final Report

A1. MOST STRINGENT REQUIREMENTS FOR LONGITUDINAL INDICATIONS - EXAMINATION UNDER BS 3923 (LEVEL 1) AND BS EN 1714 (LEVEL C) The following table summarises the most stringent requirements for longitudinal indications only, in various types of weld: for examination level 1 for BS 3923 and examination level C for BS EN 1714.

• Note 1: Although the probability of detection achieved with a testing level C is greater than the one achieved with testing level B, according to BS EN 25817 [9], quality level B is still categorised as stringent. • Note 2: The examination of butt joints - plates and pipes - under BS 3923 level 2B is as per BS 3923 Level 1 without the separate near surface scans from sides A, B C and D (see Figure A1). If the weld of the component is a single sided weld the scans are only performed from the upper surface (sides A & B) with two angles at 0.5 skip and one angle at 1 skip.

34


Parent material Scanning (1) thickness (mm) side Butt joint – plates and pipes NA 6≤t