Investigation Into Thermal Stress Characteristics of Pipe-in-Pipe Under High Temperature Condition

In this paper, the thermal stress characteristics of the pipe-in-pipe (PIP) system under high temperature condition are analyzed. The PIP is a type of pipe applied in sodium-cooled faster reactor (SFR) and has a different geometry from a single pipe. In particular, under the high temperature condition of the SFR, the high thermal stress is generated due to the temperature gradient occurring between the inner pipe and outer pipe. To investigate the thermal stress characteristics, three cases are considered according to geometry of the support. The fully constrained support and intermediate support are considered for case 1 and 2, respectively. For case 3, both supports are applied to the actual curved pipe. The finite element (FE) analyses are performed in two steps for each case. Firstly, the heat transfer analysis is carried out considering the thermal conduction, convection and radiation conditions. From the heat transfer analysis, the temperature distribution results in the piping system are obtained. Secondly, the structural analysis is performed considering the temperature distribution results and boundary conditions. Finally, the effects of the geometric characteristics on the thermal stress in the PIP system are analyzed.

Three-Dimensional Numerical Analysis for Bolted Flange Joints Considering Effect of Creep

Bolted flange joints are widely used in petroleum, chemical, nuclear and power industries, etc. With more and more devices are used at high temperature, the performance of flange connections becomes more complex, especially with creep of different components in flange connection. At elevated temperature, with the loss of bolt force and gasket force due to creep, the joints are prone to leak. Based on this, this paper analyzed the relaxation of bolt force at elevated temperature due to creep of bolt, flange and gasket separately and simultaneously. Besides, the influence of different initial installation stress of bolts was also studied. The results showed bolted flange joints relaxed due to gasket creep during early short term service. However, contribution of bolt and flange creep became more and more significant with the extension of time. With considering the creep of bolt, flange and gasket simultaneously, 50% to 60% of the bolt material yield strength at room temperature was recommended as the bolt initial installation stress for the joint case studied in this paper.

Proposal of the Local Failure Evaluation Method With Stress Parameters

The failure mode known as local failure could occur at structure discontinuities with multiaxial stress conditions. Experiments and analyses of notched bars, which generate multiaxial stress, were conducted. The experiments showed that the tensile strength of a notched bar was stronger than that of a smooth bar. The ratio of the maximum and minimum diameter has become the important factor of this notch strengthening. In addition, the initiation of failure was observed at the inner location from the notch root. According to the analysis results, the Mises-stress became the maximum at the notch root. On the other hand, hydrostatic stress became the maximum at the inner location from the notch root, and this location corresponded to the initiation of fracture. The maximum hydrostatic stress has good correlation with the notch strengthening ratio. These facts reveal that hydrostatic stress must be taken into account for strength evaluation as a dominant factor in addition to the Mises-stress. However, only Mises-stress is considered in the present structural design code of nuclear plants. From above results, the new criterion based on fracture surface, where the coordinate plane consists of hydrostatic stress and Mises-stress, was proposed for local failure. Furthermore, this fracture surface was extended to an isochronous fracture surface in a creep region based on isochronous stress-strain curves.

Application of Genetic Algorithm on Optimal Pipeline Route Considering Complex Terrains and Obstacles

Gathering network, which is usually characterized by various and complex structure, takes a large proportion of the overall construction cost of gas field. Optimization of pipeline routes is an effective way to reduce the investment. In this paper, a novel model for optimal route of pipeline considering complex terrains and obstacles is proposed and solved by Genetic Algorithm. Minimizing the total investment is the object of this model. Since the construction costs under different terrains are different, the distance factor Li, slope factor Di and elastic factor Si are introduced into the objective function to represent the length of the pipeline, the gradient of the pipeline, and the fluctuation of terrain. In addition, the performance of the model is verified by taking three typical situations of different terrains and obstacles as examples. The results illustrate that the proposed model can address the optimal design of pipeline routes in complex terrains. Moreover, the effects of different genetic operators on solutions are investigated, including three selection operators and two crossover operators. The study provides a guideline for designing pipeline routes in complex terrains and is also applicable to the analogous problem.

Performance of Depressurization Devices to Dynamic Loads Generated by Arcing in Liquid Filled Transformers

This study provides a methodology that can be used to evaluate the dynamic performance of fast depressurization devices used in liquid-filled oil transformers. Liquid-filled transformers are susceptible to explosions due to internal arcing if the dielectric insulation fails. The internal arc vaporizes a portion of the liquid and generates a sudden pressure wave. The first peak of the pressure wave has been measured to be as high as 13 bars, with time durations on the order of milliseconds [1]. Transformer tanks have a typical static withstand limit of approximately 1 bar gauge [2]. It is thus imperative that the tank be depressurized before the static pressure reaches such a threshold. One industry-accepted Fast Depressurization System [3] used to depressurize transformers after an internal arc is based on a patented rupture disk design [4]. This study compares the dynamic performance of this disk to results from a successful test campaign using a rupture disk as the depressurization device. Limiting loading rate values from the test campaign are then used to comment on the effectiveness of the design. The evaluation methodology is based on Pressure-Impulse (P-I) curves. The P-I curve was generated by running a series of Implicit Dynamic analysis using Code_Aster [5]. This iterative process first required establishing a failure mode that is consistent with actual observed failure in the field and observable in the Finite Element Analysis (FEA) model. The criteria were then used in interpreting the response of the Rupture Disk to a series of different half-sine wave pulse loading of varying amplitudes and time-periods. The generated P-I curve was then compared to loading rates observed in the test campaign [1] as well as three other higher loading rates (1.28 times, 2 times, 3.8 times, and 10.25 times the reported experimental rate) to qualitatively assess the effectiveness of the design. Results indicated that disk functions extremely effectively as a Fast Depressurization System as also corroborated by the test campaign. Although this methodology is used for the rupture disk, it is expected that this methodology can be extended to compare the dynamic performance of other depressurization devices.

Experimental Study on Characteristics of Condensation and Flow Resistance Inside Horizontal Corrugated Low Finned Tubes

It is the simplest and most feasible method to enhance heat transfer by replacing the smooth tube with various kinds of special-shaped enhanced tubes. In this paper, the characteristics of condensation and flow resistance inside horizontal corrugated low finned tubes were studied experimentally. The effects of steam inlet conditions and condensation tubes structural parameters were analyzed. The results showed that the heat transfer performance inside corrugated low finned tubes was greater than that inside smooth tubes. Like inside smooth tubes, the heat transfer coefficients increased with the vapor quality and steam mass flux. But the enhancement rate showed the opposite trend. And the heat transfer coefficients inside corrugated low finned tubes increased with the decrease of pitch and increase of protrusion height. Meanwhile, the variation trend of pressure drop gradient changing with inlet conditions and construal parameters was consistent with trend of heat transfer coefficient. The performance evaluation criteria were used to evaluate the comprehensive performance. It was found that the maximum performance evaluation factor was acquired at the minimum vapor quality and mass flux. The maximum value was 2.24 happened in the tube with pitch of 6 mm and height of 0.7mm. Finally, both the correlation for heat transfer coefficient and correlation for pressure drop gradient were developed by fitting experimental data. And this would provide calculation foundations for the design of horizontal condensers with corrugated low finned tubes.

Contribution to Safety Enhancement for BDBE in Structure and Material Fields

This paper proposes research issues on contribution to safety enhancement for BDBE in structure and material fields. There are large difference between DBE and BDBE. Objective of DBE is prevention of accident and conservative approach is adopted such as prevention of all assumed failure modes. As for BDBE, objectives are prevention of safety function loss and mitigation of accident consequences, risk approach is expected. Since DEC is a part of BDBE, approach against DEC is different from DBE. DEC requires best estimate. Probabilistic Risk Assessment consists of best estimate plus uncertainty. For stress test, identification of the weakest portions and cliff edges become important. To realize above approach, prediction of realistic failure modes is essential. Furthermore, relative strength evaluation becomes important to predict order of failure location and their mode, even though absolute strength is not clear. After Fukushima daiichi nuclear accident, there is a tendency to apply DBE design criteria to BDBE, however, conservative criteria for design are inappropriate for best estimate. Therefore, authors proposes failure mode maps to identify realistic failure mode and its application to mitigation to accident sequences such as fracture control.

Update on Allowable Limits for Corroded Pressure Boundary Bolted Joints

In a previous paper, guidelines for assessment of bolt and nut corrosion on pressure boundary bolted joints was developed and verified using tests on joints with spiral wound gaskets. The work has since been extended to include a wider range of gasket types, corrosion of the flange itself (in addition to the bolts and nuts) and higher pressure class joints. This paper provides a summary of the additional work performed and updated guidance on general guidance for acceptable levels of corrosion for each component in the pressure boundary bolted joint.

CFD Prediction of Safety Valve Disc Forces Under Two Phase Flow Conditions

At present there are very few published works on prediction based methods to establish the forces that act on safety valves during two-phase operation. This means that the valve dynamics and resulting opening and closure are uncertain for a wide range of complex flow applications. This paper describes a study whereby a safety valve, primarily developed for the industrial refrigeration sector is investigated for a range of steady state high gas mass fraction inlet conditions, (gas mass quality 1-0.2) and the disc force characteristics measured for valve choked conditions. The highly compressible two phase flow processes are modelled using an Euler–Euler two fluid CFD approach and the results compared with the experiments. Results indicate that CFD approaches can reasonably capture the key processes but deficiencies exist due to the prediction of two phase built up backpressure in the valve. The methods and data trends are discussed to show the effectiveness of current modelling approaches.

A Novel Approach for Assessment of Pressurised Equipment for Slow Depressurisation During Fire

Majority of modern design codes and regulations for pressurised equipment mandate that pressurised equipment are equipped with depressurising facilities so that in the event of an over-pressurising scenario or during emergency shut-downs the equipment can be safely depressurised. In the process industry, depressurisation calculations are usually done in accordance with the requirements of API 521 standard. For equipment with a wall thickness greater than 25mm, this standard recommends that depressurising facilities are designed to reduce the pressure of a vapour containing system exposed to external pool fire from the initial internal pressure to the final safe pressure within 15 minutes. There are cases where the depressurising time is even further shortened from 15 minutes by design engineers, e.g. for LPG applications or jet fire scenario. Once depressurisation facilities are sized for the fire-case, depressurising calculations are carried out in order to determine the minimum metal temperatures at coincident pressures reached in the equipment in a non-fire depressurising scenario (called cold-case). This will enable design engineers to analyse equipment for potential brittle fracture of equipment during cold-case depressurisation. Whilst the above mentioned methodology is usually adequate for majority of applications, there may be occasions that achieving API 521 recommended fire-case depressurisation time would require a large depressurising valve. This can potentially cause: • Significantly fast depressurising (and subsequent auto-refrigeration) in the cold-case leading to very low metal temperatures and the need for costly materials, particularly in cold climate environments; • Damage to equipment internals due to high depressurising rate; • Overloading the existing flare network in a brown field project. Increasing the depressurising time can alleviate the operational and/or economic issues arising from rapid depressurisations. However, slower-than-usual depressurisation increases the risk of rupture during fire-case, as equipment will be subject to heat for a longer period of time whilst still pressurised. This paper describes a methodology and identifies the necessary steps for assessment of pressurised equipment for slow depressurisations. The method is based on the provisions in the latest editions of API 521, API 579-1/ASME FFS-1 and Finite Element Analysis (FEA). A sample high pressure vessel is analysed in this paper for both cold and fire depressurisation.