Revealing systems of flaws in high-pressure reactors operating under dynamic pulse loading

Abstract

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. GENERAL INFORMATION AND MOTIVATIONSPreviously, we have created our QAE Image Recognition method for revealing, identifying and assessing flaws in LDPE reactors, operating at high pressure (3000 bar) 3-5 . This enabled increasing the operational safety of reactors and prevents unexpected and rarely predictable failures. Nevertheless there were no:• Reliable methods for assessing the remaining lifetime of LDPE reactor tubes with individual cracks or systems of cracks having known J-integral values. • Relevant design criteria for acceptable J-integral values of flaws for the specific tubes and criteria for rejecting tubes from operation. • Universal tools for analytical or numerical calculations of flaws in tubes that undergoing dynamic loading. These problems have motivated us to clarify complications, and create and develop technology for the reliable assessment of the remaining lifetime of LDPE reactor tubes that contain individual and multiple flaws.

Construction of a Generalized Fatigue Diagram of Metallic Materials

“…The separation is performed according to the parameters of the two-parameter distribution "Energy E -frequency coefficient K f " [14][15][16]. In the diagrams E-K f allocated three characteristic "field distribution of AE signals corresponding to the three different types of AE sources (Figure 1 The areas were determined on the basis of experimental studies of various materials in static and cyclic tests [9,[17][18][19]. The allocated distribution areas overlap partially, and the AE signals located at the boundary of the regions can be assigned to both groups.…”