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Research Papers: Design and Analysis

J. Pressure Vessel Technol. 2019;141(6):061201-061201-11. doi:10.1115/1.4043996.

To ensure a reliable power supply with a minimum environmental impact in the future, further increases in efficiency and flexibility of fossil-fired power plants are major challenges to address in recent R&D activities. In order to counterbalance substantial fluctuations in the electricity grid due to the rising share of renewable resources, frequent start-ups and shut-downs of turbomachinery will be claimed by the market. Hence, along with the development of advanced materials for elevated steam parameters, contemplations with respect to altered boundary conditions of established materials, e.g., for bolted pipe flange connections, are required. In this paper, a selection of results from a recently finished research work on stress relaxation will be presented. Together with European power plant component manufacturers, a newly developed test rig comprising a scaled intermediate pressure (IP) steam turbine pipe flange now allows investigations under near-service conditions. Before, throughout, and after performance of the experiments, an extensive measurement setup enables the examination of effects that cannot be studied in standardized relaxation tests. Within this work, a loss in bolt pretension of up to almost 50% over a comparatively short period of experimental time was observed. By means of numerical calculations, creep deformation in the transition areas between pipe and flange plate sections was identified to be a major originator of these pretension drops. Particularly, good correlations between experiments and finite element method (FEM) simulations could be achieved through the implementation of enhancements concerning the fitting strategy of the Graham–Walles-type creep model which was used.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(6):061202-061202-10. doi:10.1115/1.4044116.

Consider the swinging effect of spherical tank, the theory of velocity potential is adopted, and a reasonable potential function is derived according to the boundary conditions. Further, the dynamic fluid pressure, the wave height of the liquid, the shear force and the overturning moment at the bottom of the spherical tank is calculated, and a simplified dynamic model of spherical tank considering liquid sloshing and swinging effect was constructed. The seismic response was studied and compared with the results without considering the swing effect. The results show that: for Ι, II site conditions, base shear force and overturning moment of considering the swing effect is slightly smaller than when nonconsidering and the difference rate between the two is very small. III–IV site conditions, each condition value of considering the swing effect is larger than when nonconsidering and the difference rate between the two is relatively large. Aseismic design of spherical tank and the influence of swing effect should be considered if the site condition is III and IV, and if site I and II, they can be ignored.

Commentary by Dr. Valentin Fuster

Research Papers: Materials and Fabrication

J. Pressure Vessel Technol. 2019;141(6):061401-061401-9. doi:10.1115/1.4043998.

Creep failure of the pressure equipment operated at high temperatures in power and petrochemical industry is a frequent reason of its collapse. Ultrasonic waves' velocity is sensitive to change of material properties due to thermally induced degradation processes, especially plastic strain. The article demonstrates that measurement of ultrasonic waves' velocity ratio is able to localize plastic strain induced by various means including creep. Results are confirmed by comparison with hardness and metallographic analysis. Furthermore, statistical significance of the results is tested by t-statistics. The proposed application may help with selection of potentially critical areas during routine inspections on pressure equipment in operation.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(6):061402-061402-11. doi:10.1115/1.4044211.

This paper proposes a framework to quantify the measurement error associated with lengths of corrosion defects on oil and gas pipelines reported by inline inspection (ILI) tools based on a relatively large set of ILI-reported and field-measured defect data collected from different in-service pipelines in Canada. A log-logistic model is proposed to quantify the likelihood of a given ILI-reported defect being a type I defect (without clustering error) or a type II defect (with clustering error). The measurement error associated with the ILI-reported length of the defect is quantified as the average of those associated with the types I and II defects, weighted by the corresponding probabilities obtained from the log-logistic model. The implications of the proposed framework for the reliability analysis of corroded pipelines given the ILI information are investigated using a realistic pipeline example.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(6):061403-061403-8. doi:10.1115/1.4044263.

The standard master curve (MC) approach has a major limitation in that it is only applicable to homogeneous datasets. In nature, steels are macroscopically inhomogeneous. Reactor pressure vessel (RPV) steel has different fracture toughness with varying distance from the inner surface of the wall due to the higher cooling rate at the surface (deterministic material inhomogeneity). On the other hand, the T0 value itself behaves like a random parameter when the datasets have large scatter because the datasets are for several different materials (random inhomogeneity). In this paper, four regions, the surface, 1/8 T, 1/4 T, and 1/2 T, were considered for fracture toughness specimens of Korean Standard Nuclear Plant (KSNP) SA508 Gr. 3 steel to provide information on deterministic material inhomogeneity and random inhomogeneity effects. Fracture toughness tests were carried out for the four regions at three test temperatures in the transition region and the microstructure of each region was analyzed. The amount of upper bainite increased toward the center, which has a lower cooling rate; therefore, the center has lower fracture toughness than the surface so reference temperature (T0) is higher. The fracture toughness was evaluated using the bimodal master curve (BMC) approach. The results of the BMC analyses were compared with those obtained via a conventional master curve analyses. The results indicate that the bimodal master approach considering inhomogeneous materials provides a better description of scatter in the fracture toughness data than a conventional master curve analysis does.

Commentary by Dr. Valentin Fuster
J. Pressure Vessel Technol. 2019;141(6):061404-061404-12. doi:10.1115/1.4044264.

The influence of oxidation on the estimation of long-term creep rupture strength is investigated for 2.25% chromium (Cr)–1% molybdenum (Mo) steel specified as JIS STBA 24, JIS SCMV 4 NT, and ASTM A542/A542M by the Larson–Miller method using creep rupture data in the National Institute for Materials Science (NIMS) Creep Data Sheets at 450–650 °C for up to 313,000 h. The creep rupture data exhibit a change in slope of the stress versus time to rupture curves due to oxidation in air during 600 °C creep tests at 15,000–40,000 h and 650 °C tests at 2000–3500 h for different size specimens, which indicates degradation in creep life by the oxidation. The estimated 100,000 h creep rupture strength using regression analysis is increased by the elimination of long-term data degraded by the oxidation. Several metallurgical factors, such as the initial strength represented by the 0.2% proof stress at the creep test temperature and the concentration of aluminum (Al) impurity, also affect the creep life of the tested steel.

Commentary by Dr. Valentin Fuster

Technical Brief

J. Pressure Vessel Technol. 2019;141(6):064501-064501-4. doi:10.1115/1.4043812.

This paper highlights about the high temperature creep failures affecting refinery materials, structures, and components for which good operational practices, proper maintenance, and inspection are required to avoid major failures and to maintain safety of personnel and integrity of static equipment and to avoid unplanned shut down.

Commentary by Dr. Valentin Fuster

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