• 대한기계학회:학술대회논문집
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• 대한기계학회 2008년도 추계학술대회B
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• pp.2483-2488
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• 2008

To mitigate the effects of cavitation and flashing, several types of orifices have been installed in the pipeline of new nuclear power plants. To review the effects of wall thinning caused by flow-accelerated corrosion by the types of orifices, which are cone and plate, and the relation between flow behavior and local wall thinning, experiments and numerical analyses for the downstream pipe of two types of orifices were performed. The experimental results in terms of static pressure obtained for the experimental facilities were compared with those of three-dimensional (3D) numerical analyses using the FLUENT code. As the results of review of flow-accelerated corrosion effects based on the experiment and numerical analysis, it was identified that the orifice of cone-type can be comparatively mitigated the effects of cavitation and flashing, but can not be mitigated the effect of flow-accelerated corrosion.

• 대한기계학회논문집A
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• 제31권10호
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• pp.1017-1024
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• 2007

This study performed a series of burst tests at ambient temperature using real-scale elbow specimen containing a local wall-thinning defect at it's intrados or extrados and evaluated failure pressure of locally wall-thinned elbows. In the experiment, a 90-degree 100A, Sch. 80 standard elbow was employed, and various wall-thinning geometries, such as length, depth, and circumferential angle, were considered. From the results of experiment, the dependences of failure pressure of wall-thinned elbows on the defect geometries and locations were investigated. In addition, the reliability of existing models was examined by comparing the tests data with the results predicted from existing failure pressure evaluation models for locally wall-thinned elbow.

• 대한기계학회논문집A
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• 제33권12호
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• pp.1393-1400
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• 2009

The objective of this study is to validate local failure criteria, which were proposed based on the notched-bar specimen tests combining with finite element (FE) simulations, using the results of real-scale pipe failure tests. This study conducted burst test using wall-thinned pipe specimens, which were made of 4 inch Sch.80 ASTM A106 Gr.B carbon steel pipe, under simple internal pressure at ambient temperature and performed associated FE simulations. Failure pressures were estimated by applying the failure criteria to the results of FE simulations and were compared with experimental failure pressures. It showed that the local stress based criterion, given as true ultimate tensile stress of material, accurately estimated the failure pressure of wall-thinned pipe specimens. However, the local strain based criterion, which is fracture strain of material as a function of stress tri-axiality, could not predict the failure pressure. It was confirmed that the local stress based criterion is reliably applicable to estimation of failure pressure of local wall-thinned piping components.

• 대한기계학회논문집A
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• 제37권9호
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• pp.1175-1182
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• 2013

위상잠금모드가 적용된 적외선열화상 시험법은 열전도도가 큰 재료에서 결함의 검출능력을 향상시키기 위해 개발되었으며, 기존의 적외선열화상 기법에 비해 우수한 결함 검출능력을 보이는 것으로 알려져 있다. 따라서, 본 연구에서는 원전 배관 감육결함 검출에서 위상잠금모드 기법의 적용 효과를 살펴보기 위해서, 감육결함이 가공된 배관 시편을 대상으로 위상잠금모드를 적용하여 적외선열화상 시험을 수행하였다. 시험 결과로부터 감육결함에 대한 위상이미지를 얻고, 이것을 기존의 적외선열화상시험법으로 구한 열화상이미지와 비교하였다. 비교 결과, 위상잠금모드의 적용이 감육결함에 대한 형상 결정 능력을 향상시키는 것을 확인하였다. 이러한 개선 효과는 폭과 길이가 작거나 경계가 경사진 감육결함에서 뚜렷하였다. 그러나, 깊이가 얕은 감육결함의 검출능력은 크게 향상되지 않았다.

• Corrosion Science and Technology
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• 제11권4호
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• pp.129-134
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• 2012

UT(Ultrasonic Test), one of the non-destructive tests, is the most common thickness measurement method for evaluating the wear rate in NPPs(Nuclear Power Plants). UT is used widely because it is easy and safe for use. However some amount of error inevitably occurs in attempting to measure the thickness. The error, that could make the thickness data thicker or thinner, may affect estimation of wear rate in pipes. NPPs are composed of a lot of pipes and components. Some of them are tested to check the current status during RFO(Re-Fueling Outage). Reliability analysis of UT is essential for evaluating pipe wear rate and establishing the long-term management plan in NPPs. This paper reviewed the cause of error occurrence and presented the UT data reliability analysis method. Also, this paper shows the application result of reliability analysis to the UT data acquired in NPPs.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2004년도 학술대회지
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• pp.242-246
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• 2004

Effects of circumferentially local wall thinning on the fracture behavior of pipes were investigated by monotonic four-point bending. Local wall thinning was machined on the pipes in order to simulate erosion/corrosion metal loss. The configurations of the eroded area included an eroded ratio of d/t= 0.2, 0.5, 0.6, and 0.8, and an eroded length of I = 10 mm, 25 mm, and 120 mm. Fracture type could be classified into ovalization, local buckling, and crack initiation depending on the eroded length and eroded ratio. Three-dimensional elasto-plastic analyses were also carried out using the finite element method, which is able to accurately simulate fracture behaviors excepting failure due to cracking. It was possible to predict the crack initiation point by estimating true fracture ductility under multi-axial stress conditions at the center of the eroded area.

• 대한기계학회논문집A
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• 제34권11호
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• pp.1675-1680
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• 2010

캐나다형 중수로에서 피더관은 가동 중에 유동 가속 부식에 의해 감육이 발생한다. 피더관에 감육이 발생하면 배관 건전성이 떨어진다. 본 논문은 원형 감육이 발생한 피더관에서의 기기신뢰성 평가를 위한 한계 하중을 연구하였다. 유한요소 해석을 통하여 면내 굽힘 하중과 내압을 받는 경우에 대하여 연구하였다. 재료는 대변형 효과를 고려하고 탄성-완전소성 재료로 가정하였다. 원형 감육이 발생한 피더관에 대하여 내압과 닫힘 방향, 열림 방향 굽힘 하중에 대하여 한계하중해를 제시하였다.

• Corrosion Science and Technology
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• 제12권3호
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• pp.125-131
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• 2013

The most common pipe wall thinning degradation mechanisms that can occur in the steam and feedwater systems are FAC (Flow Acceleration Corrosion), cavitation, flashing, and LDIE (Liquid Droplet Impingement Erosion). Among those degradation mechanisms, FAC has been investigated by many laboratories and industries. Cavitation and flashing are also protected on the piping design phase. LDIE has mainly investigated in aviation industry and turbine blade manufactures. On the other hand, LDIE has been little studied in NPP (Nuclear Power Plant) industry. This paper presents the development of prediction system for pipe wall thinning caused by LDIE in terms of erosion rate based on air-water ratio and material. Experiment is conducted in 3 cases of air-water ratio 0.79, 1.00, and 1.72 using the three types of the materials of A106B, SS400, and A6061. The main control parameter is the air-water ratio which is defined as the volumetric ratio of water to air (0.79, 1.00, 1.72). The experiments were performed for 15 days, and the surface morphology and hardness of the materials were examined for every 5 days. Since the spraying velocity (v) of liquid droplets and their contact area ($A_c$) on specimens are changed according to the air-water ratio, we analyzed the behavior of LDIE for the materials. Finally, the prediction equations(i.e. erosion rate) for LDIE of the materials were determined in the range of the air-water ratio from 0 to 2%.

• 한국해양공학회:학술대회논문집
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• 한국해양공학회 2002년도 춘계학술대회 논문집
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• pp.134-139
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• 2002

Fracture behaviors of pipes with local wall thinning are very important for the integrity of nuclear power plant. However, effects of local wall thinning on strength and fracture behaviors of piping system were not well studied. Acoustic emission(AE) has been widely used in various fields because of its extreme sensitivity, dynamic detection ability and location of growing defects. In this study, we investigated failure modes of locally wall thinned pipes and AE signals by bending test. From test results, we could be divided four types of failure modes of ovalization, crack initiation after ovalization, local buckling and crack initiation after local buckling. And fracture behaviors such as elastic region, yielding range, plastic deformation range and crack progress could be evaluated by AE counts, accumulative counts and time-frequency analysis during bending test. It is expected to be basic data that can protect a risk according to local wall thinning of pipes, as a real time test of AE.

• 한국가스학회지
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• 제9권3호
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• pp.1-8
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• 2005

배관은 사용기간 중에 여러 가지 열화에 의해 손상을 받으며, 그 중 감육(두께감소)이 발생하게 되면 심각한 결과를 초래할 수 있다. 따라서, 배관시스템의 보전유지를 위해서는 감육되어진 배관의 강도를 평가하는 것이 중요하다. 어떤 재료가 외부 응력 등에 의해 파괴 혹은 변형된다면 그 특징적인 탄성파가 변형메카니즘에 따라 여러 가지로 다르게 나타날 수 있다. 본 연구에서는 감육을 가지는 배관에서 감육결함의 길이 및 깊이를 다르게 상정하였을 때 나타나는 AE신호들에 대해 Wavelet Transform시스템을 이용하여 평가하였다. 또한,실배관의 굽힘시험을 통하여 기계적인 강도를 평가하였다. 결과로부터, 감육의 깊이비(d/t)의 정도에 따라 주파수 성분을 구별하는 것이 가능하였다.