Flow-accelerated corrosion is a common type of damage to heat engineering equipment and pipelines. This process is subject to almost all elements of the condensate-feed and steam pipelines of turbine sets of nuclear and thermal power plants. Other types of metal thinning, in most cases, occur in conjunction with this process. In accordance with the requirements of the Federal Norms and Rules in Nuclear Energy, it is necessary to conduct regular non-destructive testing of thermal mechanical equipment using the method of ultrasonic thickness measurement to measure the thickness of walls, followed by assessment of compliance of the product quality and determination of the predictive value for the next in-service inspection. To estimate the rate of thinning and predict the value of thinning, it is necessary to know the values of actual wall thicknesses of pipeline systems at the initial time, which is usually not known. The actual wall thickness at the initial time is usually assumed to be equal to the nominal value without taking into account possible deviations, which is not always a reasonable solution in terms of assessing the rate of flow-accelerated corrosion. Functions are obtained that describe the profile of thinning with high accuracy. The coefficients included in the equation have a physical meaning: the actual wall thickness, the value of thinning and the area of local corrosion on the inner surface of the reducer. The developed approach is based on approximation of results of non-destructive testing, which allows to distinguish between areas exposed and not exposed to the mechanism of flow-accelerated corrosion in conical reducers. This makes possible to determine the thickness of the wall of a conical reducer of pipeline at the time of its being put in operation and to assess the rate of thinning based on a single control procedure, which reduces the possible rejection of metal and increases the accuracy of predictive calculations. This approach increases the safety and reliability of operation of conical reducers.
The destruction of equipment metal by a brittle fracture mechanism is a probabilistic event at nuclear power plants (NPP). The calculation for resistance to brittle destruction is performed for NPP equipment exposed to neutron irradiation; for example, for a reactor plant such as a water-water energetic reactor (WWER), this is a reactor pressure vessel. The destruction of the reactor pressure vessel leads to a beyond design-basis accident, therefore, the determination of the probability of brittle destruction is an important task. The research method is probabilistic analysis of brittle destruction, which takes into account statistical data on residual defectiveness of equipment, experimental results of equipment fracture toughness and load for the main operating modes of NPP equipment. Residual defectiveness (a set of remaining defects in the equipment material that were not detected by non-destructive testing methods after manufacturing (operation), control and repair of the detected defects) is the most important characteristic of the equipment material that affects its strength and service life. A missed defect of a considerable size admitted into operation can reduce the bearing capacity and reduce the time of safe operation from the nominal design value down to zero; therefore, any forecast of the structure reliability without taking into account residual defectiveness will be incorrect. The application of the developed method is demonstrated on the example of an NPP reactor pressure vessel with a WWER-1000 reactor unit when using the maximum allowable operating loads, in the absence of load dispersion in different operating modes, and taking into account the actual values of the distributions of fracture toughness and residual defectiveness. The practical significance of the developed method lies in the possibility of obtaining values of the actual probability of destruction of NPP equipment in order to determine the reliability of equipment operation, as well as possible reliability margins for their subsequent optimization.
Reliability theory makes it possible to determine the probabilities of destruction, reaching limit states, equipment and pipeline failures (E&P). Normative documents, as a rule, define requirements for integral indicators of reliability or safety of objects, but do not establish requirements for admissible values of probabilities for individual systems and data elements of objects. This article proposes the approach for determining the permissible values of the probabilities of reaching the yield stress or ultimate strength by operating stresses on the basis of design data on loads and data from certificates on the mechanical properties of metal. During operation, the achievement of the working stresses in metal of E&P value of the permissible stress values is a probabilistic event, in this fact, the probabilistic approach has been developed to obtain the safety factor of probability reaching limiting states. The developed approach is based on the methods of strength science, statistical analysis and probability theory. The object of application of the developed approach is thermal mechanical equipment used in different branches of technology. Using the example of the main circulation pipeline of the NPP with WWER-440, the presence of a reserve in the probability of reaching the limit state is shown and a relationship is established between the stress variation coefficient and the calculated probability for normal operation and abnormal operation. This approach determines the reserve for the probability of destruction, which will justify the extension of the resource or optimize the operating parameters of the objects under consideration.
The reliability of nuclear power plants (NPPs) has an influence on power generation safety and stability. The reliability of NPP equipment and pipelines (E&P), and the frequency of in-service inspections are directly linked with damage mechanisms and their development rates. Flow accelerated corrosion (FAC) is one of significant factors causing damages to E&P because these components experience the influence of high pressure, temperature, and high flow velocity of the inner medium. The majority of feed and steam path components made of pearlitic steels are prone to this kind of wear. The tube elements used in the coils of high pressure heaters (HPH) operating in the secondary coolant circuit of nuclear power plants equipped with a VVER-1000 reactor plant were taken as the subject of the study. The time dependences of changes in the wall thickness in HPH tube elements are studied proceeding from an analysis of statistical data of in-service nondestructive tests. A method for determining the initial state of the E&P metal wall thickness before the commencement of operation is proposed. The article presents a procedure for predicting the distribution of examined objects' wall thicknesses at different times of operation with determining the occurrence probability of damages caused by flow accelerated corrosion to calculate the time of safe operation until reaching a critical state. A function that determines the boundary of permissible values of the HPH wall thickness distributions is obtained, and it is shown that the intervals of in-service inspections can be increased from 6 years (the actual frequency of inspections) to 9 years, and the next in-service inspection is recommended to be carried out after 7.5 years of operation. A method for determining the existence of FAC-induced local thinning in the examined object has been developed. The developed approaches and obtained study results can be adapted for any pipelines prone to wall thinning to determine the frequency of in-service inspections (including an express analysis based on the results of a single nondestructive in-service test), the safe operation time, and quantitative assessment of the critical value reaching probability.
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