The analysis has shown that high pressure and high temperature piping in fossil and nuclear power plants suffer from unexpected and rarely predictable failures. To guarantee operational safety and to prevent failures authors have performed the complex investigations and have created Quantitative Acoustic Emission NDI technology for revealing, identifying and assessing flaws in equipment operated under strong background noise condition. These enabled:
Overall inspection of the piping operated under stress, temperature, pressure, steam flow and loading, variation.
Locating suspected zones and zones of flaw development with low J-integral value and the great variation of the dynamic range of flaws danger level.
Identification of flaw types and their danger level.
Detection of defective components in service prior to shut down.
The continuous and the burst Acoustic Emission (AE) were used in combination as an information tool. As result, the significant number of flaws such as creep at stage 3a-3b, closed-edge micro-cracks, systems of randomly dispersed pores and inclusions, plastic deformation development around them, or/and individual micro-cracking were revealed, identified and assessed in 50 operating high energy piping. The findings and assessing flaw danger level obtained by QAE NDI were confirmed by independent NDI methods as TOFD, X-ray, replication, metallurgical investigations, etc. The findings and assessing flaw danger level obtained by QAE NDI were confirmed by independent NDI methods such as TOFD, X-ray, replication, metallurgical investigations, etc
Quantitative Acoustic Emission (QAE) technology, physical and mathematical models were created for the reliable and precise identification and evaluation of the danger level (the J-integral value) of a developing main crack in a system of interacting micro- cracks, and the reliable assessment of the remaining lifetime of low density polyethylene (LDPE) reactor tubes that contain cracks. These innovations made it possible to carry out pioneer investigations and established previously unknown dependences, phenomena and criteria, such as:
Interdependence between the J-integral value of the flaw and the remaining lifetime of tubes from steel in design condition that contained system of inclusions, micro- cracks or had undergone stress corrosion cracking (SCC) attack and/or hydrogen embrittlement.
Criteria for tube rejection.
The optimal interval between repeated inspections (monitoring) of the reactor together with the time of analysis and decision.
Criteria for acceptable flaw danger level that would allow continued use of tubes in operation for two years.
It was established that a main crack in a system of micro-cracks under dynamic pulse loading could start to propagate earlier and faster, and reach greater lengths and take longer time to brake than an individual main crack. At the same time it was shown that the remaining lifetime could decrease significantly when a main crack interacts with a field of micro-cracks. The danger level (the J-integral value) of combined flaw increases significantly and may provoke dramatic failure within a few weeks. Therefore, only continued monitoring can eliminate the risk of tube fracture in this case.
Tension tests, optical and electron fractography, micro-sclerometric and AE image recognition investigations, spectral and chemical analysis, TOFD, X-ray, all established a good correlation between the results obtained from steel specimens and full-size tube specimens tests, LDPE reactor tubes examinations and theoretical calculations.