The use of a material in a design where failure carries environmental or health and safety risk requires computational design using a conservative strength model. The materials design engineer must determine and be able to justify a lower-bound stress-strain curve. This paper uses the AISI 304 / 304L system to demonstrate how such a model might be derived. It addresses the question of variability in materials properties. Also discussed is the contribution of strain-rate dependence that is relevant to applications involving impact or explosive loading conditions. Designers and those reviewing and approving designs are more comfortable with the use of temperature dependent than strain-rate dependent material properties. Ignoring strain-rate dependence can lead to overly conservative designs, even when the objective is a conservative design. The Mechanical Threshold Stress model is used to demonstrate, in the 304 / 304L system, a lower-bound temperature and strain-rate dependent constitutive model. This model implements a state-variable approach and easily accounts for temperature and strain-rate dependent material properties as well as strain hardening. A wealth of literature data is used to benchmark and validate the model. Comparison of model predictions with applicable ASME and ASTM code standards is described.
The new rules for seismic piping design in Section III that were developed and included in the requirements in 1994 Addenda of the ASME Boiler and Pressure Vessel Code (B&PV Code) generated considerable discussion within the industry and from the United States Nuclear Regulatory Commission, (USNRC). The USNRC initiated a review of the results of the previous EPRI/NRC experimental program and the Japanese industry started its own experimental program. To accommodate and address developments resulting from these efforts, the ASME, B&PV Code established a Special Working Group (SWG) to continue the review and study of the questions and information generated. This paper reports on the efforts of this SWG which resulted in refinements of the revised rules. These refinements have been accepted for inclusion in Section III of the ASME, B&PV Code.
Stresses in piping systems subject to hydrogen detonation loading are complex. There are high-frequency localized shell-type stresses, such as hoop membrane, longitudinal through-wall bending and hoop through-wall bending due to asymmetric modes. There are low-frequency gross beam-type stresses, similar to those from a waterhammer, as the unbalanced forces excite the beam bending and bar wave modes in the piping system. From a code compliance standpoint, all the stresses need to be considered and categorized in terms of the type of failure that they can cause. Part 1 developed a method to estimate the local shell stresses due to the detonation wave. This paper, Part 2, discusses an investigation into the global beam bending effects. It proposes a methodology for combining the beam and local shell effects, and evaluating the results in terms of complying with a typical piping code. The gross stresses due to the propagating detonation wave can be evaluated using beam-finite element models and time-history analysis, similar to analyses for waterhammer. As with waterhammer, these stresses are typically considered “occasional” loads. However, the beam stresses can coincide with very high hoop or radial shell stresses, due to the high peak pressures involved, so that the simple comparison of using longitudinal stresses may not be an adequate design check. This paper recommends a combination of shell-equivalent stresses and beam-stress intensities that result in a conservative comparison, when compared to a full time-history analysis, but one which is not overly conservative. With the exception of ASME Section III, Class 1, most U.S. piping codes do not provide rules for fatigue evaluation for loads other than displacement controlled loads. ASME B31.3 Appendix P provides guidance for pressure fluctuations, but the focus is primarily gross stress effects. The local effects from a detonation wave include both a “skin” stress effect on the inside surface and a through-wall bending effect due to the dynamic nature of the effects of the propagating wave. Both of these must be considered if the number of occurrences is significant. This paper proposes a method to consider these localized stresses that is patterned after the guidance in ASME Section III, NB-3600.
This paper addresses the local effects of hydrogen detonations inside piping. It is the first in a two-part series of papers which assess the effects of detonations in piping systems relative to ASME Code allowables. The effects of internal detonations in piping systems are typically separated into two regimes: local effects and system effects. Local effects are often simplistically represented as pure hoop stresses resulting from the pressure acting radially on the inside circumference of the pipe. In reality, the interaction of the pipe wall and the propagating detonation wave is relatively complex, resulting in “waves” or “ripples” in the pipe wall. These areas of local, through-wall curvature lead to substantial axial stresses which may even exceed the hoop stresses. Furthermore, in the elastic regime, there is very little damping present in the pipe wall, leading to numerous stress cycles as the local bending waves move axially along the pipe wall. Fatigue effects of the combined hoop and axial cycling were evaluated using ASME Code Section VIII, Division 2 fatigue evaluation methodology. Analysis of strain gage data from a number of hydrogen detonation experiments in 2-inch and 4-inch Schedule 40 piping showed that the fatigue damage is generally driven by fewer than 10 large-magnitude fatigue cycles, which account for an average of 75% of the total fatigue damage. However, the results also demonstrate that for two detonation events with similar measured peak hoop or axial strain, the number of fatigue allowable events may vary dramatically depending on the shape of the strain response.
This paper proposes Piping Code rules to address the effects of hydrogen deflagrations inside piping. Previous work proposed a set of criteria for piping subject to detonation loading [PVP2012-78519, PVP2012-78525]. This paper provides criteria to evaluate the effect of deflagrations, which typically have a slower rise time and lower energy, inside the piping. These deflagration criteria, coupled with the previously cited detonation criteria, are being used at the Hanford Tank Waste Treatment and Immobilization Plant to evaluate piping systems subject to hydrogen accumulation. The previous papers did not investigate or propose criteria for deflagrations, as these were known to have lower pressures and slower pressure rise times, but are still of some significance for piping design. Recent work has shown that there exists a scenario in which the deflagration loading may be very significant: deflagrations in small gas pockets surrounded by large waste slugs. Depending on the assumptions used to develop the loading, the unbalanced forces on piping segments in a long piping system can become high during a deflagration event. Thus, for the set of criteria chosen for deflagration, the deflagration event may become the limiting event, especially if it is the more frequent event. The criteria proposed need to recognize this scenario and guide the user to possible solutions. This paper presents the original methodology for evaluating these “slug” events, briefly discusses the recent testing and theory being pursued to reduce the effect of the loading [PVP2015-45970, PVP2016-63260, PVP2016-63262], and then proposes criteria for evaluating deflagration induced stresses and loads.
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