Review Article

J. Pressure Vessel Technol. 2019;141(5):050801-050801-14. doi:10.1115/1.4043843.

Structural discontinuities (e.g., nozzle, hole, and groove) widely occur in many high temperature components of nuclear and fossil power plants. In general, the notched component is used for simplified tests and analyses due to the complexity of the introduction of a practical component. In the previous work, the effects of the notch on failure life of the components have been reported experimentally, including the strengthening and weakening effects; however, the internal mechanisms have not been clearly demonstrated. This work reviews the notch effects on the structural strength of the notched components at elevated temperatures under creep, fatigue, and creep-fatigue loading conditions. Experimental phenomena (i.e., strengthening or weakening effects) for typical notched specimens subjected to the above three loading conditions are summarized, and the related factors for notch effects on creep rupture life or cycle to failure of the components are discussed. The mechanisms for the strengthening or weakening effects induced by a notch are described. Evaluation procedures for notch effect analysis under complex loading conditions are also included, and the primary challenges concerning the notch effect are provided for further investigations.

Commentary by Dr. Valentin Fuster

Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2019;141(5):051201-051201-11. doi:10.1115/1.4043591.

In recent years, a few new methods of achieving autofrettage in thick-walled hollow cylinders have been developed. Rotational autofrettage is one of the new methods proposed recently for prestressing thick-walled cylinders. The principle of rotational autofrettage is based on inducing plastic deformation in the cylinder at the inner side and at its neighborhood by rotating the cylinder about its own axis at a certain angular velocity and subsequently bringing down it to zero angular velocity. However, the analysis of the process is still in its nascent stage. In order to establish the rotational autofrettage as a potential design procedure for prestressing thick-walled cylinders, accurate modeling of the process is necessary. In this paper, the rotational autofrettage for thick-walled cylinders is analyzed theoretically based on the generalized plane strain assumption. The closed form analytical solutions of the elasto-plastic stresses and strains and the residual stresses after unloading during the rotational autofrettage of a thick-walled cylinder are obtained. In Part II of the paper, the numerical evaluation of the theoretical model will be presented in order to assess its feasibility.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051202-051202-7. doi:10.1115/1.4044173.

The theoretical modeling of the rotational autofrettage of a thick-walled cylinder based on the generalized plane strain assumption has been presented in part I of the paper. In order to access the potentiality of the proposed theoretical model, the numerical evaluation of the analytical solutions is important. This part of the paper presents numerical evaluation of the generalized plane strain model for typical thick-walled cylinders. The residual hoop stress generated in the rotational autofrettage of a typical gun barrel is compared with the residual hoop stresses in the conventional hydraulic and swage autofrettage processes. Comparison shows that the rotationally autofrettaged gun barrel is capable of producing the same level of compressive residual hoop stress at the inner surface as that of the hydraulic autofrettage. In order to corroborate the analytical solution, a three-dimensional finite element method (3D FEM) analysis of the rotational process is carried out in ANSYS finite element package and the results are compared with the theoretical results. The comparison shows a good matching of the results between the theoretical evaluation and the 3D FEM analysis. Finally, a short feasibility analysis of the rotational autofrettage process of typical cylinders is carried out for the practical realization of the process.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051203-051203-11. doi:10.1115/1.4043915.

Finite element analysis (FEA) of bolted flange connections is the common methodology for the analysis of bolted flange connections. However, it requires high computational power for model preparation and nonlinear analysis due to contact definitions used between the mating parts. Design of an optimum bolted flange connection requires many costly finite element analyses to be performed to decide on the optimum bolt configuration and minimum flange and casing thicknesses. In this study, very fast responding and accurate artificial neural network-based bolted flange design tool is developed. Artificial neural network is established using the database which is generated by the results of more than 10,000 nonlinear finite element analyses of the bolted flange connection of a typical aircraft engine. The FEA database is created by taking permutations of the parametric geometric design variables of the bolted flange connection and input load parameters. In order to decrease the number of FEA points, the significance of each design variable is evaluated by performing a parameter correlation study beforehand, and the number of design points between the lower and upper and bounds of the design variables is decided accordingly. The prediction of the artificial neural network based design tool is then compared with the FEA results. The results show excellent agreement between the artificial neural network-based design tool and the nonlinear FEA results within the training limits of the artificial neural network.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051204-051204-9. doi:10.1115/1.4044115.

The prediction of leak rate through porous gaskets for different gases based on test conducted on a reference gas can prevent bolted joint leakage failure and save the industry lots of money. This work gives a basic comparison between different gas flow models that can be used to predict leak rates through porous gasket materials. The ability of a model to predict the leak rate at the micro- and nanolevels in tight gaskets relies on its capacity to incorporate different flow regimes that can be present under different working conditions. Four models based on Navier–Stokes equations that incorporate different boundary conditions and characterize specific flow regime are considered. The first- and second-order slip, diffusivity, and molecular flow models are used to predict and correlate leak rates of gases namely helium, nitrogen, SF6, methane, argon, and air passing through three frequently used porous gasket materials which are flexible graphite, polytetrafluoroethylene (PTFE), and compressed fiber. The methodology is based on the determination experimentally of the porosity parameter (N and R) of the microchannels assumed to simulate the leak paths present in the gasket using helium as the reference gas. The predicted leak rates of different gases at different stresses and pressure levels are confronted to the results obtained experimentally by measurements of leak rates using pressure rise and mass spectrometry techniques. The results show that the predictions depend on the type of flow regime that predominates. Nevertheless, the second-order slip model is the one that gives better agreements with the measured leaks in all cases.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051205-051205-13. doi:10.1115/1.4043916.

An integrated software platform of high-temperature design evaluation and defect assessment for a nuclear component and piping system subjected to high-temperature operation in creep regime has been developed. The program, called “HITEP_RCC-MRx,” is based on French nuclear grade high-temperature design code of RCC-MRx and enables a designer to conduct not only elevated temperature design evaluation but also elevated temperature defect assessment. HITEP_RCC-MRx consists of three modules: “HITEP_RCC-DBA,” which is programmed for the design-by-analysis (DBA) evaluation for class 1 pressure boundary components such as the pressure vessel and heat exchangers according to the RB-3200 procedures; “HITEP_RCC-PIPE,” which is programmed for the design-by-rule (DBR) evaluation according to the RB-3600 procedures; and “HITEP_RCC-A16,” which is programmed for high-temperature defect assessment according to the A16 procedures. The program has been verified with a number of related example problems on modules of DBA, Pipe, and A16. It was shown from the verification examples that integrated software platform of HITEP_RCC-MRx is capable of conducting three functions of an elevated temperature design evaluation for pressure boundary components and for piping, and an elevated defect assessment in an efficient and reliable way.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051206-051206-11. doi:10.1115/1.4043844.

In this study, the integrity of a manhole structure made of a 78 in. high density polyethylene (HDPE) stub-end, steel ring, and blind flange, sealed with a compressed nonasbestos fiber (CNAF) gasket is investigated by means of a parametric finite element analysis (FEA). A coupled thermomechanical nonlinear FEA model is built, comprising of a heat transfer and a structural model, which allows modeling the complex thermal and mechanical loads and their interactions present during the operation of the manhole. The temperature-dependent elastic–plastic HDPE material constitutive behavior and the temperature-dependent nonlinear response of the CNAF gasket are accounted for in the model. Factors influencing the performance and integrity of the manhole such as stud-bolt pretorque level (Tb), internal pressure (Pi), and outer temperature (To) are considered. Based on the results, the integrity and performance of the structure are assessed in view of a leakage through the gasket criterion and a yielding of the HDPE stub-end criterion. The FEA results reveal that both Tb, Pi, and To significantly influence the performance (i.e., leakage) of the gasket and the integrity (i.e., yielding) of the HDPE stub-end. At 40 °C, it is possible to find a safe operational window for a range of Tb and Pi values, where no leakage through the gasket or yielding of the stub-end occurs. However, as the temperature is increased this safe operational window decreases considerably, and at 80 °C safe operation cannot be guaranteed where leakage, yielding, or both simultaneously, will lead to loss in performance and integrity of the manhole structure.

Commentary by Dr. Valentin Fuster

Research Papers: Fluid-Structure Interaction

J. Pressure Vessel Technol. 2019;141(5):051301-051301-9. doi:10.1115/1.4044118.

Excitation of acoustic resonance by flow over tube bundles in heat exchangers can cause hazardous levels of acoustic pressure that may pose operational and environmental risks. The previous studies have indicated that inline arrangements of cylinders excite acoustic resonance of a nature different from that of a single cylinder. In this work, the excitation of acoustic resonance by cross-flow around inline arrangements of cylinders is experimentally investigated to identify the role of critical parameters on resonance characteristics. Results show that flow around inline tube bundles can excite acoustic resonance due to periodic flow oscillations over the cavity formed between successive cylinders rather than periodic wake phenomena. Based on precoincidence resonance characteristics, a criterion is introduced to predict the occurrence of acoustic resonance in inline arrangements of cylinders. The proposed parametric criterion does not only identify the potential for resonance excitation for inline arrangements of cylinders experimentally investigated in this work but it also provides a method to separate resonant from nonresonant cases for inline tube bundle data from the literature.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2019;141(5):051401-051401-8. doi:10.1115/1.4043995.

The formation of strain-induced martensite (SIM) is found in metastable austenitic stainless steel (m-ASS) during cold forming, and the presence of SIM may cause reductions in toughness, ductility, and corrosion resistance of m-ASS. These mechanical properties can be restored and improved by proper heat treatment after forming, however, which obviously raises the manufacturing costs. One low-cost way to reduce the SIM amount during m-ASS forming is to maintain the forming temperature at an appropriate level. This paper intends to investigate an approach to determine the optimum forming temperature at which the strain-induced martensitic transformation (SIM-Tr) of m-ASS head during forming can be restrained within a limited intensity. First, static tensile tests were conducted on S30408 conventional cylindrical tensile specimens under different temperatures varying from 20 °C to 180 °C, and then the effect of deformation temperature on SIM was evaluated. Second, according to the stacking fault energy (SFE) calculation method, m-ASS's chemical composition was taken into further consideration to investigate its effect on SIM. Finally, a formula was established based on SIM and chemical composition for optimization of forming temperature. In addition, the results obtained by this formula were compared with those of the experiment by S30408 ASS head stamping tests, and the satisfactory matching is found for the proposed forming temperatures and predicted ferrite number (FN) values (readings of the Ferritescope measurement, as a representation of the amount of martensite in this study). Furthermore, an enhancement in the cryogenic impact properties and a fewer quantity of delta-ferrite in the microstructure of m-ASS heads are observed when warm stamping is performed as compared with the cold stamped head.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051402-051402-10. doi:10.1115/1.4043997.

The mean stress effect on the fatigue life of type 316 stainless steel was investigated in simulated pressurized water reactor (PWR) primary water and air at 325 °C. The tests in air environment have revealed that the fatigue life was increased with application of the positive mean stress for the same stress amplitude because the strain range was decreased by hardening of material caused by increased maximum peak stress. On the other hand, it has been shown that the fatigue life obtained in simulated PWR primary water was decreased compared with that obtained in air environment even without the mean stress. In this study, type 316 stainless steel specimens were subjected to the fatigue test with and without application of the positive mean stress in high-temperature air and PWR water environments. First, the mean stress effect was discussed for high-temperature air environment. Then, the change in fatigue life in the PWR water environment was evaluated. It was revealed that the change in the fatigue life due to application of the mean stress in the PWR water environment could be explained in the same way as for the air environment. No additional factor was induced by applying the mean stress in the PWR water environment.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051403-051403-12. doi:10.1115/1.4044119.

Recently, a heat treatment technique has been proposed to induce compressive residual stresses at the vicinity of the outer wall of a thermally autofrettaged cylinder. In the proposed technique, the outer wall of a thermally autofrettaged vessel is heated above the lower critical temperature while temperature of the inner wall is kept below it. The cylinder is then quenched, which induces compressive residual stresses both at the inner and outer walls. This article presents the construction and working of an experimental setup to carry out the proposed heat treatment coupled thermal autofrettage process. Experiments are carried out on AH36 mild steel cylinders to assess the presence of the compressive residual stresses. Measurement of microhardness and opening angle of cut in a thermally autofrettaged AH36 steel cylinder provided the evidence for compressive residual stresses at the outer wall of the cylinder. A finite element method (FEM) technique was used to predict the opening angle of cut. Predicted opening angle was fairly close to experimental observation.

Commentary by Dr. Valentin Fuster

Research Papers: Operations, Applications and Components

J. Pressure Vessel Technol. 2019;141(5):051601-051601-15. doi:10.1115/1.4043383.

Centrifugal pumps are one of the significant consumers of electricity and are one of the most commonly encountered rotodynamic machines in domestic and industrial applications. Centrifugal pumps operating at off-design conditions are often subject to different periodic flow randomness, which in turn hampers functionality and performance of the pump. These limitations can be overcome by modification in the conventional design of different components of a centrifugal pump, which can assuage flow randomness and instabilities, reconstitute flow pattern and minimize hydraulic flow losses. In this article, flow vulnerabilities like pressure and flow inconsistency, recirculation, boundary layer separation, adverse rotor–stator interaction, and the effects on operation and performance of a centrifugal pump are reviewed. This article also aims to review design modification attempts made by different researchers such as impeller trimming, rounding, geometry modification of different components, providing microgrooves on the impeller and others. Based on the findings of this study, it is concluded that some design modifications of the impeller, diffuser, and casing result in improvement of functionality, efficiency, and reduction in pressure fluctuations, flow recirculation, and vibrations. Design modifications should improve the performance without hampering functionality and useful operational range of the pump. Considerable research is still necessary to continue understanding and correlating flow physics and design modifications for the pump impeller, diffuser, and casing.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051602-051602-10. doi:10.1115/1.4043918.

During compressor operation, vibration of the connecting pipeline often occurs which potentially has an impact on safety. Field vibration measurements are carried out on a pipeline for a centrifugal compressor, and the vibration waveform and spectrum characteristics are obtained. Numerical studies are conducted to investigate the relationship between the natural frequency of the pipeline and the fluid excitation frequencies. These numerical results are verified by comparison with the field measurement values, which shows that the numerical method can be useful for engineering applications. Some preliminary conclusions are drawn from the results of the field measurements and the numerical simulation. Pressure fluctuations spread through the pipeline over an appreciable distance. The sensitive frequencies of the pipeline vibration are determined, and the fundamental causes of the flow induced vibration revealed. Accordingly, the pipeline structure can be modified to reduce the vibration and ensure the safety of the equipment and the pipeline.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(5):051603-051603-14. doi:10.1115/1.4044117.

Reservoir-pipe-valve (RPV) systems are widely used in many industrial processes. The pressure in an RPV system plays an important role in the safe operation of the system, especially during the sudden operations such as rapid valve opening or closing. To investigate the pressure response, with particular interest in the pressure fluctuations in an RPV system, a multidimensional and multiscale model combining the method of characteristics (MOC) and computational fluid dynamics (CFD) method is proposed. In the model, the reservoir is modeled as a zero-dimensional virtual point, the pipe is modeled as a one-dimensional system using the MOC, and the valve is modeled using a three-dimensional CFD model. An interface model is used to connect the multidimensional and multiscale model. Based on the model, a transient simulation of the turbulent flow in an RPV system is conducted in which not only the pressure fluctuation in the pipe but also the detailed pressure distribution in the valve is obtained. The results show that the proposed model is in good agreement when compared with a high fidelity CFD model used to represent both large-scale and small-scale spaces. As expected, the proposed model is significantly more computationally efficient than the CFD model. This demonstrates the feasibility of analyzing complex RPV systems within an affordable computational time.

Commentary by Dr. Valentin Fuster

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