Pipe Rupture Research Articles

Development of wall-thinning evaluation procedure for nuclear power plant piping - Part 2: Local wall-thinning estimation method

Flow-accelerated corrosion (FAC), liquid droplet impingement erosion (LDIE), cavitation and flashing can cause continuous wall-thinning in nuclear secondary pipes. In order to prevent pipe rupture events resulting from the wall-thinning, most NPPs (nuclear power plants) implement their management programs, which include periodic thickness inspection using UT (ultrasonic test). Meanwhile, it is well known in field experiences that the thickness measurement errors (or deviations) are often comparable with the amount of thickness reduction. Because of these errors, it is difficult to estimate wall-thinning exactly whether the significant thinning has occurred in the inspected components or not. In the previous study, the authors presented an approximate estimation procedure as the first step for thickness measurement deviations at each inspected component and the statistical & quantitative characteristics of the measurement deviations using plant experience data. In this study, statistical significance was quantified for the current methods used for wall-thinning determination. Also, the authors proposed new estimation procedures for determining local wall-thinning to overcome the weakness of the current methods, in which the proposed procedure is based on analysis of variance (ANOVA) method using sub-grouping of measured thinning values at all measurement grids. The new procedures were also quantified for their statistical significance. As the results, it is confirmed that the new methods have better estimation confidence than the methods having used until now.

Preliminary safety analysis of an in-vessel LOCA for the EU-DEMO WCLL blanket concept

Abstract In-vessel Loss Of Coolant Accident (LOCA) is one of the Design Basis Accident to be considered to support the future DEMOnstration power plant safety assessment. The water-cooled lithium-lead (WCLL) Breeding Blanket (BB) concept relies on Lithium-Lead as breeder, neutron multiplier and tritium carrier. The breeding modules are cooled by two independent pressurized water systems: the fist-wall (FW) and the breeding zone (BZ) coolant systems. The postulated initiating event (PIE) considered for this safety analysis is a double ended pipe rupture of the blanket module first wall channels. This event causes the inlet of coolant into the plasma chamber volume triggering an unmitigated plasma disruption and the pressurization of the Vacuum Vessel (VV) volume. The fusion version of MELCOR code (ver. 1.8.6) is used to evaluate accident consequences for two different scenarios, with the presence and absence of the downstream isolation valves, respectively. The chemical reaction between the coolant and the first wall tungsten layer inside the VV has been considered together with the mobilization of the radioactive source term. Pressure and temperature transient behavior in the tokamak volumes demonstrate that safety margins are respected during the accidental sequence.

Response of buried pipeline subjected to reverse faulting

The paper studies the interaction of buried pipelines with a reverse fault rupture, employing 3D finite element (FE) modelling. A thoroughly validated strain softening constitutive model is employed to conduct a parametric study, exploring the effect of sediment thickness, pipeline burial depth and thickness. It is shown that the increase of sediment thickness H leads to a reduction of the normalized fault offset h/H at which buckling is initiated. Two contradicting mechanisms are identified: (i) the increase of H leads to a reduction of the intersection angle α of the fault rupture with the pipe, increasing the compressive component; and (ii) it leads to a reduction of localized deformation close to the surface. From our results, the h/H required for initiation of buckling is insensitive to burial depth Hp/D, but the one required for pipe rupture increases substantially with increase of Hp/D. The increase of the D/t (pipe diameter to wall thickness) ratio leads to a substantial decrease of the h/H at which buckling is initiated, and the same applies to the pipe rupture criteria, which are met at a smaller h/H with the increase of D/t.

Comparison between FAC Analysis Result Using ToSPACE & CHECWORKS Programs and Experimental Result

A number of piping components in the secondary system of nuclear power plants (NPPs) have been exposed to aging mechanisms such as FAC (Flow-Accelerated Corrosion), cavitation, flashing, LDIE (Liquid Droplet Impingement Erosion), and SPE (Solid Particle Erosion). Those mechanisms may lead to thinning, leaking, or the rupture of components. Due to the pipe ruptures caused by wall thinning of Surry Unit 2 in 1986 and Mihama Unit 3 in 2004, pipe wall thinning management has emerged as one of the most important issues in the nuclear industry. To manage the wall thinning of pipes caused by FAC and erosion, KEPCO-E & C has developed ToSPACE program. It can predict both FAC & erosion phenomena, and also be utilized in the pipe wall thinning management works such as susceptibility analysis, UT (Ultrasonic Test) data evaluation as well as establishment of long-term inspection plan. Even though the ToSPACE can predict the five aging mechanisms mentioned above, only the FAC prediction result using ToSPACE was compared herein with the experimental result using FACTS (Flow Accelerated Corrosion Test System) to verify the ToSPACE’s capability. In addition, the FAC prediction result using ToSPACE was also compared with that of CHECWORKS that is widely used all over the world.

Pipe thinning model development for direct current potential drop data with machine learning approach

Abstract The accelerated corrosion by Flow Accelerated Corrosion (FAC) has caused unexpected rupture of piping, hindering the safety of nuclear power plants (NPPs) and sometimes causing personal injury. For the safety, it may be necessary to select some pipes in terms of condition monitoring and to measure the change in thickness of pipes in real time. Direct current potential drop (DCPD) method has advantages in on-line monitoring of pipe wall thinning. However, it has a disadvantage in that it is difficult to quantify thinning due to various thinning shapes and thus there is a limitation in application. The machine learning approach has advantages in that it can be easily applied because the machine can learn the signals of various thinning shapes and can identify the thinning using these. In this paper, finite element analysis (FEA) was performed by applying direct current to a carbon steel pipe and measuring the potential drop. The fundamental machine learning was carried out and the piping thinning model was developed. In this process, the features of DCPD to thinning were proposed.

Underground pipelines and railway infrastructure – failure consequences and restrictions

Underground pipelines are an essential part of the transportation infrastructure. The structural deterioration of pipelines crossing railways and their subsequent failures can entail critical consequences for society and industry, resulting in direct and indirect costs for all the stakeholders involved. Therefore, continuous and accurate condition assessment is critical for the effective management and maintenance of pipeline networks within the transportation infrastructure. The aim of this study has been to identify failure modes and consequences related to pipelines crossing railway corridors. Expert opinions have been collected through interviews and two sets of questionnaires have been distributed to the 291 municipalities in Sweden, with 137 responses in total. The failure analysis has revealed that pipe deformation has the highest impact, followed by pipe rupture at locations where pipelines cross railway infrastructure. For underground pipelines under railway infrastructure, ageing and the external load were awarded a higher ranking than other potential causes of pipeline failure.

Leak-Before-Break methodology applied to different piping materials: a performance evaluation

This paper presents a study of the application of Leak-Before-Break (LBB) to nuclear piping using three different materials. Based on the fracture mechanics, the LBB concept considers that a leakage from a crack can be detected before it reaches a critical size that implies the pipe failure, that is, the LBB analysis demonstrates through a technical justification that the probability of pipe rupture is extremely low. Among the aspects that involve the application of LBB, the main ones are: the definition of the material properties, which are extracted through tensile and fracture tests; the leakage analysis, which determines the rate of leakage due to the presence of a through-wall crack; and the analysis that verifies if the crack is stable considering the failure modes by ductile tear and plastic collapse. The materials SA-508 Cl. 3, SA-106 Gr. B and SA-376-TP304 were evaluated in relation to their performances for LBB. Data extracted from literature cases were used for the materials properties, and for the geometry and loadings of the pipe, all corresponding to the primary circuit of a PWR reactor. After application of the LBB, it was verified that all three materials met the limits established in the methodology. SA-508 Cl. 3 and SA-376-TP304 steels showed the best performance for ductile tear failure and plastic collapse failure, respectively, and SA-106 Gr. B steel had the lowest performance in both. All three materials presented plastic collapse as the most likely failure mode. In general, SA-376-TP304 steel obtained the best performance for the LBB among the three materials evaluated in this work.

Identification of hydrogen flammability in steam generator compartment of OPR1000 using MELCOR and CFX codes

Abstract The MELCOR code useful for a plant-specific hydrogen risk analysis has inevitable limitations in prediction of a turbulent flow of a hydrogen mixture. To investigate the accuracy of the hydrogen risk analysis by the MELCOR code, results for the turbulent gas behavior at pipe rupture accident were compared with CFX results which were verified by the American National Standard Institute (ANSI) model. The postulated accident scenario was selected to be surge line failure induced by station blackout of an Optimized Power Reactor 1000 MWe (OPR1000). When the surge line failure occurred, the flow out of the surgeline was strongly turbulent, from which the MELCOR code predicted that a substantial amount of hydrogen could be released. Nevertheless, the results indicated nonflammable mixtures owing to the high steam concentration released before the failure. On the other hand, the CFX code solving the three-dimensional fluid dynamics by incorporating the turbulence closure model predicted that the flammable area continuously existed at the jet interface even in the rising hydrogen mixtures. In conclusion, this study confirmed that the MELCOR code, which has limitations in turbulence analysis, could underestimate the existence of local combustible gas at pipe rupture accident. This clear comparison between two codes can contribute to establishing a guideline for computational hydrogen risk analysis.

Control of the condition of heat exchange surfaces by free vibration method

In power system the main heat carrier is the fresh water received from natural sources, and containing a large amount of various impurity – from the dissolved mineral salts to organic compounds. In housing and communal services and the industry cases of operation of heat-exchange devices and package boilers without special preparation of water aren’t rare. During the operation of the heat-exchange equipment of impurity are allocated in a firm phase both in the form of deposits and in the form of slime. Sediments cause deterioration in a heat transfer that leads to decrease in overall performance of the equipment (to an excessive consumption of fuel, metal overheating, etc.). The radiation surfaces of modern steam generators heating are intensively warmed with a furnace torch. Density of a thermal stream in them reaches 600-700 kW/sq.m, and local thermal streams can be even higher. Therefore even short-term deterioration in coefficient of a thermolysis from a wall to the boiling water leads to significant increase in temperature of a wall of a pipe (500-600 °C and above), durability of metal can it is insufficient to sustain tension which has arisen in him. The metal damages which are characterized by emergence of blisters, fistulas are a consequence of it, and it is frequent also a rupture of pipes. The scum reduces the useful section of pipes, increasing the hydraulic resistance of pipes of the heat exchanger. Everything together it involves an excessive consumption of fuel, and according to expert data the layer of scale up to 0.2 mm thick increases fuel consumption by 3%, and a layer in 1 mm for 7%.

Hydrogen assisted cracking of an AISI 321 stainless steel seamless pipe exposed to hydrogen-containing hot gas at high pressure

A seamless pipe made of AISI 321 stainless steel represented a part of a transportation pipeline system for hydrogen-containing hot gas in a hydrocarbon cracking unit. After a service period of approximately 21 months, a segment of such pipe demonstrated the cracks, causing leakage and respective fire. For clarification of a failure root cause, various metallurgical investigations combined with numerical simulations have been applied. The results revealed that the rupture of seamless pipe was evidently influenced by hydrogen assisted cracking (HAC). An increased susceptibility of the alloy to HAC had to be attributed to its sensitive microstructure which was related to the occurrence of slip bands with a high quantity in austenite grains, particularly in the specific region underneath the outer wall surface. In addition, the intensity of restraint resulting from the T-joint weld configuration caused respectively higher triaxial stresses in the confined area on the outer wall surface where the crack started. The numerical simulations of hydrogen diffusion revealed that a uniform hydrogen concentration profile over the pipe wall thickness was reached when the service period was more than 20 months. This duration agreed well to the time-to-failure of the actual component. Considering additionally that the final stage of rupture by overload was preceded by severe HAC, as confirmed by the respective intergranular fracture topography.

Comparative dynamic analyses under HELB-induced jet impingement loads

While ANSI/ANS 58.2 has been widely adopted as a standard to protect nuclear power plants against effects of postulated piping rupture, there are still arguments in determination of dynamic fluid characteristics and application to structural integrity assessment. To solve non-conservativeness of jet impingement loads in the current standard, a numerical approach was proposed that is promising from computational fluid dynamics analyses standpoint but further elaboration seems necessary for FE (Finite Element) analyses due to its implicit feature. This study is to investigate appropriate dynamic FE methods and to quantify differences with the previous one under postulated high energy line break conditions. In detail, modal analyses were performed to assess the vibration characteristic of integrated component and structure as well as individual components. Subsequent vibration analyses were carried out by using the modal analyses results of the integrated full model and specific power spectral density data depend on the jet direction and target area. From the comparative study, the dynamic FE analyses provided approximately 47% higher von Mises stresses and 14% larger radial displacements than the implicit FE analyses at the critical location.

Experimental Investigation of Acoustic Propagation Characteristics in a Fluid-Filled Polyethylene Pipeline

Fluid-filled polyethylene (PE) pipelines have a wide range of applications in, for example, water supply and gas distribution systems, and it is therefore important to understand the characteristics of acoustic propagation in such pipelines in order to detect and prevent pipe ruptures caused by vibration and noise. In this paper, using the appropriate wall parameters, the frequencies of normal waves in a fluid-filled PE pipeline are calculated, and the axial and radial dependences of sound fields are analyzed. An experimental system for investigating acoustic propagation in a fluid-filled PE pipeline is constructed and is used to verify the theoretical results. Both acoustic and mechanical excitation methods are used. According to the numerical calculation, the first-, second-, and third-order cutoff frequencies are 4.6, 10.4, and 16.3 kHz, which are close to the experimentally determined values of 4.7, 10.6, and 16 kHz. Sound above a cutoff frequency is able to propagate in the axial direction, whereas sound below this frequency is attenuated exponentially in the axial direction but can propagate along the wall in the form of vibrations. The results presented here can provide some basis for noise control in fluid-filled PE pipelines.

Development of ToSPACE for Pipe Wall Thinning Management in Nuclear Power Plants

A number of piping components in the secondary system of nuclear power plants are exposed to aging mechanisms such as FAC (Flow-Accelerated Corrosion), cavitation, flashing, SPE (Solid Particle Erosion), LDIE (Liquid Droplet Impingement Erosion), etc. Those mechanisms may lead to thinning, leak, or rupture of the components. Due to the pipe ruptures caused by wall thinning in Surry unit 2 of USA in 1986 and in Mihama unit 3 of Japan in 1994, the pipe wall thinning management has emerged as one of the most important issues in nuclear power plants. To manage the pipe wall thinning in the secondary system, Korea has used a foreign program since 1996. As using the foreign country’s program for long term, it was necessary to improve from the perspective of the users. Accordingly, KEPCO-E & C has started to develop the 3D-based pipe wall thinning management program (ToSPACE, Total Solution for Piping And Component Engineering management) from eight years ago, and the development was successful. This paper describes the major functions included in ToSPACE program, such as 3D-based DB (Database) buildup, development of FAC and erosion evaluation theories, UT (Ultra-sonic Test) data reliability analysis, field connection with 3D, automatic establishment of long-term inspection plan, etc. ToSPACE program was developed to allow site engineers performing the selection of inspection quantity at each refueling outage, UT data reliability analysis, UT evaluation, determination of next inspection timing, identification of the inspecting and replacing components in 3D drawings, etc., to access easily.

A 3D numerical modeling of polyethylene buried pipes affected by fault movement

The large and sustainable deformations of ground and fault movement are the most serious threats for buried pipelines. Heavy damages resulting from the rupture of buried pipes have caused many researchers analyze this issue in the recent years. In this study, the impact of fault movement on the buried polyethylene pipes was investigated. Initially, numerical results were validated based on the fault test results. The fault movement was simulated using Finite Difference Method (FDM). The numerical results were in good agreement with the experimental results. Next, a sensitivity analysis was performed for various parameters of buried depth, the angle of fault, and the fault vertical displacement. The results showed that axial forces and bending moments in the pipe enhance by increasing each of these parameters. It was also observed that the induced horizontal and vertical displacements exceed the tolerance of connections and they will be disrupted at the location of fault movement.

Numerical simulation on fracture mechanics behavior of high-pressure gas pipeline

The numerical simulation could help to understand the fracture mechanics behavior of high-pressure gas pipeline. The calculation results show that the internal high pressure of gas provides a driving force for crack propagation and tube wall deformation when the pipeline cracks and expands. Therefore, the rapid crack propagation tends to occur when the steel pipe is driven by high-pressure gas. In the process of steel pipe crack extension, immense internal high-pressure gas spewed from rupture and expand rapidly, compressing the air to form a series of compression wave which propagate rapidly and superpose together to form a shock wave eventually. Because the explosive wave of the pipe rupture has a high directivity, the energy distribution of gas is not uniform during the releasing process. What’s more, in pipe blasting, the longer distance, the more the surface vibration velocity decreases. In the vicinity of explosion source, the decreasing trend of the peak vibration velocity is significant. The vertical vibration velocity at the same point is larger than the longitudinal and tangential vibration speed.

Rehabilitation Effect Evaluation of CFRP-Lined Prestressed Concrete Cylinder Pipe under Combined Loads Using Numerical Simulation

Prestressed concrete cylinder pipe (PCCP) has been widely used for water transfer and transit projects. However, prestressing wire breaks may result in the rupture of pipes and cause catastrophes. Carbon fiber reinforced polymer (CFRP) liners adhered to the inner concrete core can provide an effective method of internal repair and strengthening of PCCP. To evaluate the rehabilitation effect of CFRP-lined PCCP under combined loads, two contrasting three-dimensional finite element models that investigated the visual cracking of concrete and the yielding of steel cylinders were developed. A conceptual zone was introduced to analyze the different states of the pipe during the phase of wire break. In particular, the complex CFRP-concrete bonded interface was simulated by a cohesive element layer with a bilinear traction-separation response. The results show that CFRP has a good rehabilitation effect on the inner concrete core and steel cylinder but only a slight effect on the outer concrete core, prestressing wire, or mortar. A one-hoop CFRP layer diminishes the area of a yielding steel cylinder of 4.72 m2. In addition, CFRP works more effectively along with an increase in the number of broken wires. This research can provide a basis for strengthening distressed PCCP pipelines.

Field performance of large section concrete pipes cracking during jacking in Chongqing – A case study

Long-distance large-section pipe jacking inevitably encounters a variety of complex engineering problems in the construction process, one of which is the pipe concrete cracking during jacking. A large-area rupture and leakage occurred at pipe No. 61 during the large-section concrete pipe jacking adopted in the project studied in this paper. After a detailed field investigation and on-site pipe strain monitoring, the possible causes of pipe cracking were analysed. First, a direct compression failure of the pipes was ruled out. Additionally, a large number of pores were found inside the concrete fragments falling from pipe No. 61’s site. Thus, we suspected that the pipe cracking was attributed to the conversion of the pipe’s compressive stress into the concentrated tensile stress by the substantial pores inside the pipes, which resulted in a concrete tension fracture. The core sampling results of the pipes after monitoring confirmed that there were indeed plenty of small pores in multiple pipe concretes, while the strain monitoring results indicated that the compressive stress of the pipes during normal jacking had the potential to produce the tensile stress that could crack the concrete. Combining this with the jacking deviation monitoring curves, it could be deduced that the upper right side of the pipes was most prone to failing initially. This coincided with the rupture area of pipe section No. 61 but was also consistent with the prediction of pipe No. 60 cracking, thus proving the rationality of the pipe rupture cause hypothesis. Finally, no cracking occurred after improving the production quality of the pipes. This not only shows that the pipe quality is indeed the root cause of cracking but also effectively solves the engineering problems and provides an excellent reference for similar projects.

Process of Air Ingress during a Depressurization Accident of GTHTR300

A depressurization accident is the design-basis accidents of a gas turbine high temperature reactor, GTHTR300, which is JAEA’s design and one of the Very-High-Temperature Reactors (VHTR). When a primary pipe rupture accident occurs, air is expected to enter the reactor core from the breach and oxidize in-core graphite structures. Therefore, it is important to know a mixing process of different kinds of gases in the stable and unstable density stratified fluid layer. In order to predict or analyze the air ingress phenomena during the depressurization accident, we have conducted an experiment to obtain the mixing process of two component gases and the characteristics of natural circulation. The experimental apparatus consists of a storage tank and a reverse U-shaped vertical rectangular passage. One side wall of the high temperature side vertical passage is heated and the other side wall is cooled. The other experimental apparatus consists of a cylindrical double coaxial vessel and a horizontal double coaxial pipe. The outside of the double coaxial vessel is cooled and the inside is heated. The results obtained in this study are as follows. When the primary pipe is connected at the bottom of the reactor pressure vessel, onset time of natural circulation of air is affected by not only molecular diffusion but also localized natural convection. When the wall temperature difference is large, onset time of natural circulation of air is strongly affected by natural convection rather than molecular diffusion. When the primary pipe is connected at the side of the reactor pressure vessel, air will enter the bottom space in the reactor pressure vessel by counter-current flow at the coaxial double pipe break part immediately. Afterward, air will enter the reactor core by localized natural convection and molecular diffusion.

Experiment and Dynamic Simulation of PIG Motion during Pigging Operation in a Slope Pipeline

Abstract Pigging techniques are widely used in the oil and gas industry. The unsteady motion of the PIG in an undulating pipe section during the pigging process after a water pressure test affects the stable operation of the pipeline and also causes a pipe rupture accident in serious cases. First, an experimental study was conducted to investigate the pigging process of air–water two phase pipe flows, and the PIG reverse movement and hydraulic pulse phenomenon were observed. Subsequently, a hydraulic transient model of the pigging process after a water pressure test was established in a dual-grid system. The model combined mass and motion equations of gas and liquid and PIG dynamic equations, considered three types of PIG motion states, namely positive movement, reverse movement and still, and used the method of characteristics to solve the equations. The model exhibits the ability for PIG tracing and hydraulic pulse prediction. It can be used to obtain the position and speed of the PIG. Finally, the field data and simulation results were compared, and the results indicated that they are essentially identical. This verified the accuracy of the model that is established in this study and the reliability of computed results and provided a reliable and effective theoretical basis for the development of field pigging plans.

Structural improvement of a control valve to prevent corrosion in acid gas treating plant pipeline: An experimental and computational analysis

A severe rupture of the carbon steel pipe next to a control butterfly valve caused a terrible explosion in real CO2 removal plant of a petrochemical company. Using data from on-site acid gas treating plant, the valve shortcomings was investigated via experimental analysis and computational fluid dynamics (CFD) methodology. Six various valve degree openings from θ = 5° to 60° was selected as the computational domains to clarify the link between localized corrosion thinning and the working fluid hydrodynamics. The high turbulence and rotational downstream flow can intensify the inhomogeneous deterioration of the surface in butterfly valve, due to its peculiar internal geometry. On this basis, an attempt was numerically made to improve the internal geometry of butterfly valve to avoid the high shear stress. Depending on the opening disk angle, the newly proposed model can reduce the wall shear stress by 20–90 times, that it is in favor of failure mitigation.