At the NDT Conference hosted by SAINT on the occasion of its 50th anniversary, Manfred Johannes highlights challenges facing the industry and suggests some new directions for a more modern and successful NDT industry.
NDT is definitely a mature technology, but despite this, there are definitively problems, begins Johannes. “It is increasingly necessary for NDE to become part of the product development cycle so that engineers are less likely to incorporate impossible requirements into their end-products. 90% of the NDT problems we experience in the field are related to poor design. If an inspector can’t get there, or can’t reach the inspection point, then no procedure or modern method will help. And these issues reoccur on a continuous basis,” he begins.
Outlining his talk he says that he will be discussing degradation mechanisms and the need for precursors to help track equipment health; the need for improved probability of detection (POD); structural health monitoring and prognostics; and the technical challenges and how these might be overcome in the future.
“Since the 1970s, all major construction codes have specified NDT requirements and defect acceptance criteria pertinent to the particular construction code. But the defects in the codes are totally different to those that we see in service. So we are performing NDT to construction codes to meet code requirements, but we quite often miss service induced discontinuities as the morphology of these is totally different to construction defects – and designers are not aware of this fact,” Johannes points out.
In addition, although the NDT acceptance criteria are well defined and the testing methodologies are well developed and published, these can often not be applied in practice. “Any mature technology has got to adopt a review and change process,” he suggests. “Following some R&D to develop technologies and equipment – generally with less R and lot more D – calibration and procedure development needs to be done, followed by personnel training and NDT system capability assessment.
“We need to be much more cognisant of human factors, though: safety issues, people near reactors or in hot areas near steam generators. And following its implementation, the technology needs to be subjected to routine surveillance on an ongoing basis, via audits and surveys. And if any one of these links break, then the integrity of the whole technology is compromised,” he warns.
Johannes suggests that surveillance should not be the responsibility of the NDT company. The end user of the equipment, who should know exactly what is required from the NDT process, should be doing ongoing surveillance. “But how many NDT specialists work for our power utility. In general, surveillance by plant and equipment operators in South Africa is very poor,” Johannes believes.
He says that NDT problems mainly concern in-service inspections. “Construction defects are mostly volumetric – porosity, inclusions, lack of fusion – while service-induced defects are planar. So we need to ask the question: can the NDT that we perform find the discontinuities likely to lead to chaotic failure?
“And I think all owners of operating plant should be asking this. It is their duty to ask why they are doing an in-service inspection, what they expect to find and how they will respond to an indication,” he says.
Yet in spite of this need, there is only one in-service inspection (ISI) code – ASME XI for Nuclear plant. “As early as 1992, a general ISI code was being developed, but this has yet to emerge. Why? The developers realised that a code would remove the ultimate responsibility from plant owners and place it onto NDT service providers.”
Johannes believes that plant owners need to make it very clear to NDT specialists what they expect: what the critical failure modes are, exactly where these are most likely to occur, the acceptance criteria and their repair intentions.
Describing a typical plant experience, he says he once received a phone call to come and ‘do some crack testing’. What is the material? What equipment? Where are the cracks, the casing or somewhere else? What kind of cracks? “I was told I was asking too many questions and asked if I knew what I was doing,” he recalls.
“We need to educate operators in what is possible and what is not. What are the failure mechanisms? Where are the previous inspection results and failure reports? We need to know the answers to these issues if we are to do anything meaningful,” he argues.
And meaningful NDT is essential if catastrophic failure is to be avoided, he continues, pointing towards a 3.5 m by 6.0 m fracture surface of a turbine rotor that ‘exploded’ at a power station in Germany the 1988.
What went wrong? In spite of five days of code-compliant testing using good technology, a large planar off-centre defect was completely missed. The crack grew steadily until, only three seconds before failure, a heavy vibration was detected followed by the complete destruction of the turbine. One fracture piece was found 3.0 km away.
Needless to say, the inspection procedure for all rotors was immediately modified from one scan to many from all angles over the entire body of the component. “Accidents and chaotic failures do tend to cause changes to codes and invoke more scans, but shouldn’t we be more proactive in our overall approach?” he asks.
NDT system capability assessments
“The NDT system has got three legs, equipment; procedures/work instructions; and personnel,” says Johannes, and each of these legs must be assessed to determine system capability.
At the core of such assessments is the manufacture of samples containing defect reflectors, which are used to perform open trials to assess equipment and procedure performance. The same samples can be used in blind trials to assess the ability of NDT personnel to detect flaws using the equipment and inspection procedures that have been developed.
“At the EPRI NDT Centre in Charlotte, USA… read more.