This paper reports the results of experiments, analytical models, and finite element simulations on the structural response of piping systems to internal detonation loading. Of particular interest are the interaction of detonations with 90° bends and the creation of forces that lead to axial and bending structural response of the piping system. The piping systems were fabricated using 304 stainless steel, 2-in. (50 mm) diameter schedule 40 commercial pipe with a nominal wall thickness of 0.154-in. (3.8 mm) and welded construction to ASME B31.3 standards. The piping was supported using custom brackets or cantilever beams fastened to steel plates that were bolted to the laboratory walls. Nearly-ideal detonations were used in a 30/70 H2-N2O mixture at 1 atm initial pressure and 300 K. The detonation speeds were close (within 1%) to the Chapman-Jouguet velocity and detonation cell sizes much smaller than the tube diameter. Pressure, displacement, acceleration and hoop, longitudinal, and support strains were measured using a high-speed (1 MHz) digital data acquisition system and calibrated signal conditioners. Detonation propagation through a bend generates a longitudinal stress wave in the piping that can be observed on the strain gauges and is predicted by both analytical models and finite element simulations. The peak magnitude of the bend force is approximately twice that due to the pressure alone since the peak momentum flux of the flow behind the detonation front is comparable to the pressure in the front. With relatively simple models, quantitative predictions of the bend forces can be made for the purposes of design or safety analysis of piping systems with internal detonations.
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.
The focus of this paper is on gaseous deflagration in piping systems and the corresponding implications on piping analysis and design. Unlike stable detonations that propagate at a constant speed and whose pressure-time histories can in some cases be predicted analytically, deflagration flame speeds and pressure-time histories are transient and depend on both the gas mixture and geometry of the pipe. This paper presents pressure and pipe strain data from gaseous deflagration experiments in long and short test apparatuses fabricated from either 2-inch or 4-inch diameter pipes. These data are used to demonstrate a spectrum of measured pressure-time histories and corresponding pipe response. It is concluded that deflagrations can be categorized as either “high” or “slow” speed with respect to pipe response. Slow deflagrations can be treated as quasi-static pressurizations, but high speed deflagrations can generate shock waves that dynamically excite the pipe. The existence of a transition from quasi-static to dynamic response has ramifications in regards to piping structural analysis and design, and a method for predicting the expected deflagration structural response using a semi-empirical flame acceleration model is proposed.
This paper describes hydrogen and nitrous oxide detonation experiments that were performed using an approximately 200-ft long 2-inch schedule 40 piping system. The objective of these experiments was to develop an understanding of the loads and forces imposed by internal detonation on piping combinations representative of a typical industrial piping system. The apparatus contained numerous straight pipe lengths with 90° and 45° bends, 90° elbows, and a tee along with rigid foundation supports that were connected to the pipe using typical u-bolt fasteners. As a detonation wave propagates through a gas-filled piping system, the pipe begins to respond globally once a detonation encounters a change in flow direction, such as a bend, causing a pressure imbalance due to both the internal detonation pressure and change in momentum of the reaction products. The resultant force imparts both axial forces and moments on the pipe exciting both extensional and bending modes. The test data was used to validate two finite element (FE) models developed using the ANSYS finite-element program: a hybrid model that made use of both shell and beam elements, to determine the interaction between shell and beam modes, and an all beam element model. An additional beam element model was developed using the Bechtel National Inc. software ME101 that was also found to be in agreement with the measured and ANSYS calculated frequencies and support loads. In addition to the detonation testing, the finite-element models were validated against experimental modal analysis data of the piping system that identified the primary modal frequencies and vectors. These data were compared to the modes extracted from finite-element models of the piping system.
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