Generating confidence through inspection qualification - the surest route to effective NDT

TWI Bulletin, March/April 1999

John Wintle is a Consultant Engineer in the Structural Integrity Department. He is responsible for developing reliability engineering at TWI and has acted as Chairman of an Inspection Qualification Body. 

Bryan Kenzie is a Principal Project Leader in the Structural Integrity Department. He has experience of serving on performance demonstration and inspection qualification bodies and in the setting up and supervision of blind trials.

Charles Schneider is a Principal Project Leader in the Structural Integrity Department. He has twelve years' experience of assessing the capability of non-destructive testing, mainly through technical justification and modelling.

TWI has recently been assisting the qualification of inspections of repair welds to pressure vessels to increase confidence in their effectiveness and reliability. John Wintle, Bryan Kenzie and Charles Schneider describe how inspection qualification has developed and the benefits that can be obtained from its wider use.

How confident are you that your inspection can meet its objectives and will detect the flaws you really need to find? Inspection qualification is the formal process for gaining this confidence and convincing your management and regulators that the component or weld is fit for its purpose. The process covers the entire inspection process by examining individually and in combination the elements of the equipment, the procedure and the personnel.

Qualification increases the basic confidence gained from quality assurance, of using inspectors with general competence certification, and approved procedures conforming to codes and standards. It provides the additional evidence that the required inspection performance can actually be achieved in practice.

Evolution through the nuclear industry

The roots of inspection qualification lie in the nuclear industry during the early 1970s with the development of ultrasonic inspection techniques linked with fracture mechanics based engineering critical assessment of flaws. ^[1] The ability, in principle, of ultrasonic techniques to detect and size planar defects in thick welds created the possibility of a safety case based, not only on conformance with codes and practice, but also on a detailed assessment of the structural integrity of the component in question. This placed the reliability of the manufacturing and in-service inspections at the heart of nuclear safety.

The PISC I trials ^[2] in the 1970s were one of the first investigations of the capability of ultrasonic inspection techniques to detect and size defects of concern in nuclear components. A series of test pieces containing implanted flaws were inspected by national nuclear inspection teams who had no previous knowledge of the flaws. Surprisingly, the results showed that certain procedures in use at the time gave levels of performance much lower than that expected, and that, if not properly controlled, ultrasonics could give an unacceptably low level of performance.

This was recognised by Sir Alan Cottrell and Sir Walter Marshall who recommended ^[3] in 1982, 'independent validation to ensure that the Licensing Authority can assess the adequacy of the chosen inspection procedures'. Following this recommendation and support from the UK regulatory body, the Nuclear Installations Inspectorate, the CEGB proposed, as part of the safety case for the UK's first PWR at Sizewell B, setting up an Independent Validation Centre. This was to qualify the inspection of safety related components, including the reactor pressure vessel and steam generators.

The experience gained from the UK of independent validation for Sizewell B components and the results from the later PISC II and PISC III inspection trials led to qualification becoming recognised at a European level as having benefit for nuclear safety. A European Network for Inspection Qualification (ENIQ) was set up to develop a process for inspection qualification that could be generally applied. The result was the publication in 1996 of the European Methodology for Qualification. ^[4] Although developed with the nuclear industry in mind, the document sets a general framework that can be used by any industry or company wishing to gain additional confidence in the effectiveness of its inspection strategy. A detailed account of the basis of inspection qualification and the ENIQ methodology may be found in the literature. ^[5]

Elements of inspection qualification

The process of qualification of inspection consists of compiling theoretical and practical evidence to demonstrate that the inspection can meet its required objectives. This evidence is then presented to a body of experts in inspection qualification, the Qualification Body (QB), to judge and to decide whether the inspection is qualified for its objectives. Indeed, one of the primary benefits of qualification is that it forces the plant operator to define the objectives and level of inspection performance required. Whilst manufacturing inspection may be determined from codes and standards, the level of in-service inspection performance required is usually for the plant operator to decide considering the fitness for purpose of each component.

The level of rigor for inspection qualification is something that will vary from case to case and is to be agreed between the different parties at the outset. As time passes, there will be an increasing understanding of what level of evidence is judged acceptable in particular circumstances as more case histories are generated.

The QB is usually a small team of three or more independent experts appointed by and reporting to the plant owner. It should be completely independent of the agency carrying out the inspection. It may be possible for the plant owner to be represented. However, for certain nuclear components, the regulator will require the QB to be independent from any operational interests. The QB is responsible for overseeing the conduct of the entire qualification process and for issuing formal qualification certificates when the owner or regulator requires these. TWI is one organisation with the necessary degree of expertise, experience and independence required to form a QB.

One of the key aspects of the qualification process is that it relates the damage or degradation to the inspection outputs. A dossier of input information is assembled that will define:

• the components and welds to be inspected
• the types of defects/corrosion to be detected and/or sized and any limits to their location and orientation
• the inspection performance to be achieved in terms of, for example, reliability targets, minimum flaw sizes to be detected, minimum wall thicknesses to be guaranteed, the accuracy of characterisation and dimensional sizing
• the full NDT system to be used (procedure, equipment and personnel)

Qualification therefore takes place after the inspection procedure has been defined and approved in accordance with codes and standards; it is not part of the development process.

The evidence put forward for qualification is generally a combination of:

1. Practical assessment (open or blind trials) carried out on simplified or representative test pieces replicating or resembling the component or weld to be inspected and which contain flaws relevant to the purpose of the inspection
   
   
2. Technical justification (TJ), which involves assembling other evidence in support of the effectiveness of the inspection. This evidence can include previous experience of its application, laboratory studies, mathematical modelling, physical reasoning and so on.

The appropriate mix of these sources of evidence will depend on each particular case, but the use of technical justification is strongly recommended: practical trials can only cover a limited range of situations and their results are best extrapolated, using theoretical considerations and/or other experimental evidence.

Technical justification - qualification on paper

The TJ provides a body of evidence that the inspection will perform to the required level. It complements and generalises the results from any practical trials, demonstrating that the inspection would perform equally well for any other defect, and under any other conditions, for which qualification is required.

The TJ can include both experimental and theoretical evidence. The evidence may cite custom and practice, previous inspection results and data obtained during procedure development. Theoretical models of the inspection can be especially useful in comparing practical results collected under different conditions and applying them to the actual inspection conditions. Computer modelling of ultrasonic scanning and radiography is now well developed at TWI as a tool to investigate coverage and flaw detectability. As part of its assessment of the TJ, the QB must satisfy itself that appropriate validation evidence exists for the models themselves.

The TJ should identify all the parameters (called essential parameters) that have a significant influence on the inspection capability. These parameters might relate to:

• the component, eg surface curvature, temperature
• the defect, eg size, orientation
• the NDT equipment, eg ultrasonic probe angle.

The TJ identifies the range of values that the essential parameters can assume and assesses their effect on inspection performance. Clearly, the qualification is only valid within the defined limits, and as a rule, the essential parameters in any practical trials are designed to remain within these limits. Indeed, for defect parameters such as size, the 'worst case' limit is generally focussed on. If, exceptionally, a parameter such as surface curvature lies outside the defined limits during the trials, then the trial results are extrapolated to the actual situation, using evidence contained in the TJ. This approach can save time and money by allowing, for instance, the use of test pieces of a simpler geometry than the actual component.

The TJ also serves to justify the design of any practical trials. Since the trials and the TJ complement one another, the design and scope of the practical trials that are needed to underpin the inspection depend on the outcome of the TJ. In practice, time constraints often mean that the test blocks for the trials have to be procured before the full TJ is available. This difficulty can be resolved by presenting that part of the TJ relating to test block design as a separate document. This allows the QB to assess the design of the test blocks before they are procured.

Open and blind trials - the ultimate practical assessment

The ultimate way of generating confidence in an inspection is to manufacture some representative test pieces containing implanted flaws and proving the whole inspection process in open or blind trials. Test piece design for a fair and informative trial requires some thought so as to cover the range of essential inspection and defect parameters identified in the TJ. The QB normally oversees test piece design.

The manufacture of test pieces is a specialised process that presently exists at TWI and a small number of other companies. Special techniques and control must be used to obtain flaws of the required size, orientation and morphology in welded test pieces of appropriate geometry. TWI supplies test pieces under the WELDTEST trade name and has a library of test pieces covering many common welding geometries.

In open trials, the NDT technicians have knowledge of the flaws in the test pieces and the purpose is to qualify the inspection procedure and the equipment. Any shortcomings in the procedure are usually identified at this stage when there is opportunity for further improvement. As qualification is a confidence generating exercise, the QB is normally present to witness open trials to confirm that the written procedures have been correctly applied and verify the results.

In blind trials knowledge of the implanted flaws is kept from the NDT technicians who are to carry out the inspection. The trial is essentially a test on the personnel's ability to apply proven procedures and equipment under realistic conditions. The aim should be to reproduce as closely as possible the conditions of the actual inspection, and this may include environmental and human factors. The criteria for passing or failing any blind trial must be established in advance since commercial and personal credentials may be at stake. Like any examination strict invigilation is necessary and the QB may have to be present throughout.

Afterwards, the QB will examine the inspection reports against the criteria for pass or fail. A pre-requisite for passing is that the intent of the inspection procedure and reporting has been followed. Ambiguity in the wording of these aspects in the procedure may be revealed at this stage. Successful completion of a properly design blind trial provides compelling and powerful evidence that the whole inspection process can meet its objectives.

Prospects for the wider application of inspection qualification

Worldwide application of inspection qualification to date has been mainly embraced by the nuclear industry. The future application of inspection qualification to non-nuclear industries operating pressure and other equipment subject to statutory inspection will depend on how well the benefits are recognised by operators and regulators within the context of changing industrial practices.

The UK Pressure Systems Regulations place increased importance on in-service inspection results as an indicator of fitness for purpose. In parallel, industry is moving away from codified and prescriptive regimes and towards risk based approaches. These place greater emphasis on the inspection providing accurate data on the condition of component, particularly if there is to be reduced coverage or less frequent inspection. The escalating level of awards made to those that suffer loss through plant failure should serve as a reminder of the value of inspection qualification to provide an independent guarantee of the effectiveness of the whole inspection process.

The role of inspection qualification in the certification of new pressure equipment is harder to justify. NDT during fabrication is primarily a means to provide quality control and assurance of good workmanship for product conformity with codes and standards. The NDT techniques used tend to be well prescribed by the available British Standards and national certification schemes are in place for weld inspection and NDT personnel. Only if novel or non-standard techniques are used or if there are requirements beyond the normal standards of construction codes is there a case for inspection qualification.

Engineering inspection and notified bodies already perform elements of inspection qualification in their role as 'competent persons'. Standardising the process, one of the aims of ENIQ, should mean that qualifications are more portable than is currently the case. Even so, control over site inspections will still need to be exercised to ensure that the intended standards are achieved in the field.

Cost effectiveness is likely to be an issue since the formation of a QB and trials can be expensive. However, economic benefits will be realised if the result is more focused inspection linked to fitness for service. ^[6] Streamlining of the process of qualification may also become possible through the wider use of technical justification and modelling to reduce the need to perform practical trials and as the body of experience increases.

Conclusion

Inspection qualification has four main benefits:

• First, it challenges the operator to define the specific objectives of each inspection with regard to fitness-for-purpose of the component in question.
• Second, the process of peer review leads to improved inspection procedures and clear reporting criteria.
• Third, the functionality and suitability of equipment are verified through the process of practical trials such that fewer problems are experienced on the actual job, thus saving time and money.
• Fourth, there is greater confidence in the inspection personnel, particularly when blind trials have been carried out.

Thus the entire system of inspection procedures, equipment and personnel is validated and a high quality inspection capable of detecting and assessing the defects of concern is assured.

As with the first PISC trials, twenty five years ago, the effectiveness of industrial inspection procedures is now being put to the test. The UK PANI (Programme to Assess the effectiveness of Non-destructive testing in Industry) trials are sponsored by the Health and Safety Executive and involve a wide range of plant operating and inspection companies. A series of test pieces have been manufactured and are being inspected using standard industrial procedures. The results in the next few years could lead, like PISC, to a wider recognition of the benefits of inspection qualification.

Acknowledgement

To Mr Cameron Sinclair of Eagle Star for his helpful contributions to this article.

References