Remigijus Janulionis

Integrated assessment of failure probability of the district heating network

Abstract The aim of the research presented in this paper is the assessment of failure probability of the district heating network piping. The applied methodology for assessment of failure probability of the piping network energy systems includes three types of analyses: probabilistic mathematical, deterministic thermal-hydraulic and integrated deterministic–probabilistic structural integrity analyses. The analysis of Kaunas (Lithuania) district heating (DH) network was performed. First of all, the statistical analysis was performed and the piping with the highest failure rate was determined. The thermal-hydraulic analysis was performed and loads for deterministic–probabilistic structural analysis were calculated for the selected part of DH network. The integrated deterministic–probabilistic structural integrity analysis was performed in two steps—general structural integrity evaluation and probabilistic analysis of chosen piping part. Finally, the probabilistic mathematical method was applied for the integrated assessment of failure probability of the DH network piping. This method takes into consideration statistical information about Kaunas DH piping failure data, system structure, and pipe failure probability received by integrated deterministic–probabilistic structural integrity analysis.

Integrated failure probability estimation based on structural integrity analysis and failure data: Natural gas pipeline case

In this paper, the authors present an approach as an overall framework for the estimation of the failure probability of pipelines based on: the results of the deterministic-probabilistic structural integrity analysis (taking into account loads, material properties, geometry, boundary conditions, crack size, and defected zone thickness), the corrosion rate, the number of defects and failure data (involved into the model via application of Bayesian method). The proposed approach is applied to estimate the failure probability of a selected part of the Lithuanian natural gas transmission network. The presented approach for the estimation of integrated failure probability is a combination of several different analyses allowing us to obtain: the critical cracks length and depth, the failure probability of the defected zone thickness, dependency of the failure probability on the age of the natural gas transmission pipeline. A models uncertainty analysis and uncertainty propagation analysis are performed, as well.

Evaluation of the Inter Granular Stress Corrosion Cracking Defects in Austenitic Stainless Steel Piping

The Inter Granular Stress Corrosion Cracking (IGSCC) is a dominant damage mechanism of the austenitic stainless steel. The primary circuit piping of RBMK type reactors is produced from austenitic stainless steel 08×18H10T. Defects in welded joints of pipes with nominal diameter of 300 mm were detected during In-service inspections [1]. Metallographic investigations defined that crack growth mechanism is IGSCC. The appearance of defects increases the probability of RCS piping failures of these pipes. A leak or break in RCS piping is not acceptable from safety and political (society risk) points of view. According this the evaluation of these cracks is very important for safe operation of this type reactor. The procedures for IGSCC crack evaluation consist of two parts. The first part is determination of the acceptable crack size for the component with crack, and the second part is the crack growth calculation. The acceptable flaw size provides information about the largest flaw size which component can tolerate without failure with accepted safety factors. The crack growth calculation determines how long does it take for the existing crack to reach the maximal acceptable size. The results of these calculations (acceptable crack size and crack growth) determine the further inspection schedule of the components with crack. The objective of this paper is the evaluation of the IGSCC defects detected during In-Service inspection in the primary circuit piping which outside diameter of piping is 325 mm, the wall thickness – 16 mm. Detected cracks were evaluated using method R6 [2]. The IGSCC crack growing analysis was performed using methodology presented in document [3]. The prognosis results were compared with crack data detected during In-service inspection. According analysis results were determined that the IGSCC defects detected during In-service inspection can be left without repairing for 1.5 years operation.Copyright

Evaluation of the Influence of Hydrides to the Stress Intensity Factor of Zirconium–2.5% Niobium Alloy

The NPP containing RBMK-1500 type reactor is in operation in Lithuania. The RBMK-1500 is graphite moderated, boiling water, multi-channel reactor. Zr-2.5% Nb alloys are used as a constructional material for manufacturing claddings of both fuel assemblies and fuel channels (FC). Fuel channels of RBMK-1500 reactors are the major structural elements of the reactor core that have to meet strict requirements in terms of operational reliability. Therefore it is necessary to evaluate the influence of ageing mechanism on mechanical properties of zirconium alloys during operation of the reactor. Hydrogen absorption by zirconium alloy during corrosion process is the ageing mechanism of Zr-2.5% Nb fuel channel. When hydrogen concentration in fuel channel exceeds solubility limit, formation of hydrides under certain conditions can reduce resistance to brittle fracture and cause initiation and development of hydride cracks. Therefore the evaluation of the resistance to brittle fracture of zirconium alloy is important. The objective of this paper is modelling and assessment of the influence of hydrogen to the stress intensity factor of Zr - 2.5% Nb alloy using finite element method. The stress intensity factor is a parameter, which is used to estimate a material resistance to brittle fracture. Modelling of the stress intensity factor was performed using finite element method. The influence of the hydrogen concentration to stress intensity factor was evaluated. The prognosis results and experimental data are in close agreement. It was demonstrated that applied methodology for modelling of the influence of hydrogen on the stress intensity factor can be used.Copyright