A study on the actuator efficiency behavior of safety-related motor operated gate and globe valves

Periodically, the operability of the safety-related motor-operated valves (MOVs) in nuclear power plants must be verified. Because the actuator efficiency is one of the most important factors in the determination of the actuator output, it should be considered in ensuring the operability of MOVs during the verification duration. In particular, special consideration should be paid to its potential degradation, but the design efficiency provided by manufacturers is usually used because the actuator efficiency calculation is difficult and requires considerable time and effort. In this paper, a method is introduced to calculate actuator efficiency by using diagnostic signals acquired in field tests. The actuator efficiency was calculated from the estimated motor torque, the stem thrust measured in field tests, and overall gear ratio provided by manufactures. The motor torque was estimated by using an algorithm, which can calculate electric torque from the three phases of currents and voltages, resistances between phases acquired in field tests. The validation of the design efficiencies was evaluated by comparing those efficiencies with the calculated actuator efficiencies. And, the age-related degradation was analyzed through the behavior analysis over time of the calculated actuator efficiencies. Most of the actuator efficiencies were found not to be degraded over time and kept efficiency greater than the design efficiency. However, two actuator efficiencies with lower motor speed, overall gear ratio, and maximum motor torque rating are susceptible to be lower than the design efficiencies. For the two actuators, threshold efficiencies were calculated and provided to replace their design efficiencies.     

A study on a characteristic of stem friction coefficient for motor operated flexible wedge gate valve

Stem friction coefficient is a coefficient that represents friction between thread leads of the stem and stem nut. It is an important factor to determine output thrust delivered from the actuator to the valve stem in assessing performance of motor operated valves. This study analyzes the effects of changes in differential pressure on stem friction coefficient, and determines the bounding value of stem friction coefficient. A dynamic test was conducted on multiple flexible wedge gate valves in various differential pressure conditions, and the test data was statistically analyzed to determine the bounding value. The results show that stem friction coefficient in middle and high differential pressure is influenced by fluid pressure, while stem friction coefficient in low differential pressure is almost not affected by fluid pressure. In addition, it is found that the bounding value of stem friction coefficient is higher in a closing stroke than in an opening stroke.     

A study on the stem friction coefficient behavior of motor-operated valves

This paper describes how to calculate the stem friction coefficient of safety related motor operated valves (MOVs) that reflects potential degradation with time by using diagnostic signals acquired in static field tests that have been conducted more than two times per valve. Based on the calculated stem friction coefficients, their behaviors with time were analyzed considering various parameters that could cause potential degradation. Most friction coefficients change randomly rather than increasing or decreasing continuously over time. From those trends, a threshold coefficient, which represents the highest expected value of the friction coefficient, was calculated and provided.     

大亚湾核电站反应堆厂房仪用压缩空气系统手动隔膜阀失效分析及改进

针对反应堆厂房内的SAR供气常开隔离阀频繁发生失效外漏事件,采用根本原因分析方法对相关系统设备进行了调查和试验,对可能存在的故障模式排查、论证和分析,提出了该类事件的根本原因,并就今后SAR系统的可靠运行提出改进建议。     

Analysis of the effect of lubrication performance on motor operated valve actuator output thrust

In this study, we performed theoretical analysis and test to identify an effect that lubricant performance has on actuator output thrust of MOV. Particularly, we introduced Stem Friction Coefficient (SFC) concept that means friction coefficient between a stem nut and a stem screw. By using SFC concept, an effect of lubricant performance on the actuator output thrust was analyzed quantitatively. The first phase test was performed to compare the lubrication performance between new lubricant and old lubricant and the second phase test was performed to compare the lubrication performance for the three kinds of representative lubricants (EP0, EP1, EP2) being used in Korea nuclear power plants. According to the test results, SFC value of old lubricant appeared higher than that of new lubricant and a old lubricant at TST point generated lower thrust value than a new lubricant at the same torque value. We can verify that the effect of lubrication performance has on power generated in an actuator was very large and SFC was a very useful variable in analyzing lubrication performance quantitatively.     

Seismic qualification of motor operated valves—alternate approach

This paper presents a potential alternate method for determining operating capacity of motor-operated valves subjected to seismic and other applicable loadings. As a result of programs at nuclear facilities to ensure the operational capability of MOVs (under NRC GL89-10), extensive analytical focus to develop the structural capability of valves has ensued. In the past, seismic qualification of valves typically addressed the strength of the topwork structure to resist inertial loading from excitation of the large valve actuator mass. These evaluations paid little or no consideration to the loading resulting from valve closing forces. The focus of the recent efforts is to develop the maximum operational capability of the valve, in terms of thrust, with consideration of seismic and other services loading as applicable. The alternate method outlined in this paper presents a series of thrust capacity curves, with reduction factors for seismic loading which can be applied and developed to determine safe thrust loadings without performing extensive analytical effort. A similar approach was put forward by the SQUG GIP approach to MOVs to ensure the safe operation of valves based on past earthquake experience. However, the GIP approach cannot be used to determine safe operational loads and thus has limited use in the necessary analysis required for GL89-10 programs at nuclear facilities.     

A study on the evolution of crack networks under thermal fatigue loading

The crack network is a typical cracking morphology caused by thermal fatigue loading. It was pointed out that the crack network appeared under relatively small temperature fluctuations and did not grow deeply. In this study, the mechanism of evolution of crack network and its influence on crack growth was examined by numerical calculation. First, the stress field near two interacting cracks was investigated. It was shown that there are stress-concentration and stress-shielding zones around interacting cracks, and that cracks can form a network under the bi-axial stress condition. Secondly, a Monte Carlo simulation was developed in order to simulate the initiation and growth of cracks under thermal fatigue loading and the evolution of the crack network. The local stress field formed by pre-existing cracks was evaluated by the body force method and its role in the initiation and growth of cracks was considered. The simulation could simulate the evolution of the crack network and change in number of cracks observed in the experiments. It was revealed that reduction in the stress intensity factor due to stress feature in the depth direction under high cycle thermal fatigue loading plays an important role in the evolution of the crack network and that mechanical interaction between cracks in the network affects initiation rather than growth of cracks. The crack network appears only when the crack growth in the depth direction is interrupted. It was concluded that the emergence of the crack network is preferable for the structural integrity of cracked components.     

Influence of loading rate on the crack resistance curve and the initiation temperature

This investigation is concerned with the influence of temperature and loading rate on elastic-plastic fracture in the range of ductile cleavage transition. Side-grooved three point bend specimens of a width of 200 mm and a thickness of 100 mm of two steels of the type 22 NiCrMo 37 of different toughness have been loaded with two deflection rates differing by a factor of 10⁵ in the range between initiation and upper shelf temperature. Due to dynamical loading the transition temperature of these steels has been increased by about 30 K. The slope of the crack resistance curve of the steel of lower toughness was increased considerable due to dynamical loading whereas the slope of the crack resistance curve of the steel of high toughness was increased only negligible. The results suggest that a conservative assessment of dynamically loaded components may be based on quasi-static crack resistance curves. However, in the case of dynamical loading stable crack growth can be supposed only at temperatures being considerable higher than for quasi-static loading.     

The influence of loading rate on fracture resistance of notched and precracked ISO-V-specimens

Higher loading rates are expected to have a disadvantageous influence on fracture parameters. This opinion derives probably from experience with ductile-cleavage transition whose temperature increases with increasing loading rate. As far as results on the influence of loading rate on crack resistance are published in the literature they show mostly increasing initiation values as well as increasing crack resistance with increasing loading rate.The paper deals with the influence of loading rate on fracture resistance of notched and precracked ISO-V-specimens measured on different materials. The crack resistance of notched specimens has been determined in analogy to that one of the precracked specimens. The influence of loading rate was separately investigated with regard to initiation as well as to tearing modulus.The results demonstrate that the ratio of dynamic to quasi-static values decreases with increasing toughness, the initiation values for dynamic loading may drop at high toughness below those for quasi-static loading, the slope of the crack resistance curve for dynamic loading is mostly larger than for quasi-static loading and the ISO-V-specimens fall well into line with the results of valid tests.     

A study of the characteristics of the acoustic emission signals for condition monitoring of check valves in nuclear power plants

The objective of this study is to demonstrate that a condition-monitoring system based on acoustic emission (AE) detection can provide timely detection of check valve degradation and service aging so that maintenance or replacement can be preformed prior to the loss of safety function. This research is focused on the investigation and understanding of the capability of the acoustic emission technique to provide diagnostic information on check valve failures.AE testing for a check valve under controlled flow loop conditions was performed to detect and valve degradation such as wear and leakage due to foreign object interference. It is clearly demonstrated that the distinction of different types of failure were successful by systematically analyzing the characteristics of various AE parameters.     

A review of CCFL phenomenon

Counter current flow limitation CCFL is an important phenomenon for numerous engineering applications and safety of light water reactors. In particular, the possible occurrence of CCFL in the hot-leg of a PWR during SBLOCA or LOCA accidents is of special interest for nuclear safety research. A review of the related literature has made in order to present the most important studies about the phenomenon and to reach common general understanding of the different factors that govern CCFL. Eventually this will allow explaining contradictions among different explanations provided by different authors. Most important factors were geometrical characteristics, liquid superficial velocity, and physical properties. The review shows that despite numerous experimental works, many scaling and geometrical effects are still not fully understood. For Instance there exist no consistent explanation of the channel diameter and inclined riser length effect upon results. The same can be stated-though to a minimum extent – for the inclination angle while channel length (or channel to diameter ratio) effect was clear and consistent. Since most experimental work was done in down-scaled hot-leg simulators, it becomes interesting to build a coherent knowledge about these effects and to explain arising contradictions in order to safely extrapolate results to full-scale hot-leg. The review has shown that many differences were simply due to geometrical effects, this leads to the need to “standardize” experimental data according to geometrical parameters. This should results in a better understanding of the phenomenon and corresponding scaling effects. Additionally, important variables such as: pressure drop, void fraction and shear stress were also investigated and discussed. A compilation of CCFL data was built and analyzed. Since the new simulation trends tend to use CFD codes where geometrical and spatial deviations are excluded by using 3D modeling, emphasis was placed upon introducing correlations for onset of CCFL out of collected data. Existing correlations for interfacial shear stress friction factor and the void fraction as a function of gas superficial velocity were also gathered and briefly discussed. The effect of condensation, physical parameters, and hysteresis upon CCFL was also introduced.     

A study on the effect of gas flow rate on the wave characteristics in two-phase gas–liquid annular flow

Interfacial waves play a very important role in the mass, momentum and energy transport phenomena in annular flow. In this paper, film thickness time–trace measurements for air–water annular flow were collected in a small vertical tube using a parallel wire probe. Using the data, a typical disturbance wave shape was obtained and wave properties (e.g., width, height, speed and roughness) were presented. The liquid mass flux ranged from 100 to 200 kg/m² s and the gas mass flux ranged from 18 to 47 kg/m² s. Disturbance wave characteristics were defined and the effects of changing the gas flow rate on the wave spacing, wave width, wave peak height and wave base height were studied. An average velocity model for the wave and base regions has been developed to determine the wave velocity. The investigation method could be further extended to annular-mist flow which frequently occurs in boiling water reactors.     

核电阀门国产化研究

介绍了我国核电站阀门的基本情况、工况和技术要求以及我国核电阀门采用的技术标准.说明了当前核电站阀门的发展趋势和我国核电阀门国产化现状.分析了当前我国核电阀门国产化率偏低的原因,指明了当前我国核电阀门国产化存在的不足之处,编者就实现核电阀门国产化的基本目标提出了的自己的建议和措施.     

Modelling of the effect of dislocation channel on intergranular microcrack nucleation in pre-irradiated austenitic stainless steels during low strain rate tensile loading

In the present article, the effect of dislocation channel on intergranular microcrack nucleation during the tensile deformation of pre-irradiated austenitic stainless steels is studied. Because several slip planes are activated within the dislocation channel, the simple dislocation pile-up model seems not well suited to predict grain boundary stress field. Finite element computations, using crystal plasticity laws and meshes including a channel of finite thickness, are also performed in order to study the effect of some microstructural characteristics on grain boundary stress field. Numerical results show that: the thickness and the length of the dislocation channel influence strongly the grain boundary normal stress field. The grain boundary orientation with respect the stress axis does not affect so much the grain boundary normal stresses close to the dislocation channel. On the contrary far away the dislocation channel, the grain boundary stress field depends on the grain boundary orientation. Based on these numerical results, an analytical model is proposed to predict grain boundary stress fields. It is valuable for large ranges of dislocation channel thickness, length as well as applied stress. Then, a macroscopic microcrack nucleation criterion is deduced based on the elastic-brittle Griffith model. The proposed criterion predicts correctly the influence of grain boundary characteristics (low-angle boundaries (LABs), non-coincident site lattice (non-CSL) high-angle boundaries (HABs), special grain boundaries (GBs)) on intergranular microcrack nucleation and the macroscopic tensile stress required for grain boundary microcrack nucleation for pre-irradiated austenitic stainless steels deformed in argon environment. The criterion based on a dislocation pile-up model (Smith and Barnby) underestimates strongly the nucleation stress. These results confirm that pile-up models are not well suited to predict microcrack nucleation stress in the case of dislocation channels impacting grain boundary. The proposed criterion is applied to the prediction of the IASCC macroscopic nucleation stress for pre-irradiated material tested in PWR environment and the predictions are discussed with respect to experimental data. Finally, the limitations of the continuum modelling are discussed.     

两种分布加载电磁脉冲天线的FDTD方法模拟

为了研究电磁脉冲(EMP)对各种电子系统的效应规律,需要设计一种合适的EMP辐射天线。采用时域有限差分(FDTD)方法考察了柱形和锥形两种分布加载行波天线的瞬态辐射特性,详细讨论了各种天线参数对辐射特性的影响。给出了FDTD模拟天线瞬态辐射过程的关键算法,并将计算结果与公开文献进行了比较,证明了结论的可靠性。     

Effect of heating and cooling rate on the kinetics of allotropic phase changes in uranium: A differential scanning calorimetry study

The kinetic aspects of allotropic phase changes in uranium are studied as a function of heating/cooling rate in the range 10⁰-10² K min⁻¹ by isochronal differential scanning calorimetry. The transformation arrest temperatures revealed a remarkable degree of sensitivity to variations of heating and cooling rate, and this is especially more so for the transformation finish (T_f) temperatures. The results obtained for the α → β and β → γ transformations during heating confirm to the standard Kolmogorov-Johnson-Mehl-Avrami (KJMA) model for a nucleation and growth mediated process. The apparent activation energy Q_eff for the overall transformation showed a mild increase with increasing heating rate. In fact, the heating rate normalised Arrhenius rate constant, k/β reveals a smooth power law decay with increasing heating rate (β). For the α → β phase change, the observed DSC peak profile for slower heating rates contained a distinct shoulder like feature, which however is absent in the corresponding profiles found for higher heating rates. The kinetics of γ → β phase change on the other hand, is best described by the two-parameter Koistinen-Marburger empirical relation for the martensitic transformation.     

Effect of loading rate upon fracture behavior of ferritic steel under large scaled yielding

Dynamic loading to ferromagnetic materials and large scaled yielding result in peak or valley and non-linear curve, respectively, on the Direct Current Potential Drop (DCPD) versus Crack Opening Displacement (COD) plots, which make it difficult to determine the crack initiation point. In this work high intensity of current up to 100 A was applied to the specimens of SA106Gr.C ferritic steel and the crack growth behavior was directly monitored by a high speed camera to obtain the crack initiation point. The effects of loading rate up to 1200 mm min⁻¹ upon the fracture resistance were explored. As the results, it has been shown that, although no substantial difference was seen in the load–COD plots, the crack initiation and then J_i and J–R curve were quite sensitive to the loading rate. That is, under the loading rate of 300 mm min⁻¹ the material showed the worst fracture resistance than under static loading and even under the higher loading rates of 600 and 1200 mm min⁻¹. Also applying the high speed camera and high current source have been proved to be an effective way to find out the accurate crack initiation point and to compensate the pulse of DCPD due to the ferromagnetic effect.     

Influence of eccentric valves on the vibration of fluid conveying pipes

The dynamic stability of motion of fluid conveying pipes has been the subject of numerous studies. Theoretical interest was stimulated by the non-conservativeness of these systems. Design requirements for pipelines and nuclear related systems provided the additional impetus.Recently, the assessment of the probable influence that large, rigid valves frequently encountered on piping systems might have on the behavior of the overall system was required. In spite of the above mentioned wealth of literature, a great paucity of information was found on the influence of lumped masses. Consideration of rotatory inertia is apparently non-available. The motivation and contents of the present study stem from these facts.A variational formulation is followed by a Galerkin approximation that is applied to simply supported and cantilever pipes. Lumped translational and rotatory inertia are introduced. The results are reported for values of dimensionless parameters selected in a representative manner.Substantial modifications on the magnitudes of the frequencies are shown. The influence of the eccentricity is found of greater importance than that of the mass for constant eccentricity factor. The character of the equilibrium configurations associated with vanishing frequencies is a function highly dependent on the mass and eccentricity of the valve.The need for ensuing research on coupled out-of-plane bending and twisting of the pipe system is also established.     

Application of the local-to-global approach to the study of infilled frame structures under seismic loading

The seismic performance of civil engineering structures may be estimated by using two levels of modelling. At the local level, each constituent has its own constitutive law and geometric finite element support. The main phenomena such as the cracking and the crushing of concrete and masonry could be reproduced by using the continuous damage or plasticity theories. However the cost of the computations does not allow extensive or dynamic studies and thus the global level — where the constitutive laws based on empirical rules reproduce the behaviour of the structural elements — represents the unique strategy for the analysis of complete civil engineering structures under seismic loading. The present paper aims at presenting the application of these two modelling levels in order to assess the seismic performance of masonry infilled R/C frame structures. The one-bay masonry infilled frames tested at Lisbon under cyclic loading and the four-storey building tested at ELSA have been used for the validation of the modelling approach.