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For hazard assessment purposes, the dispersion of gases in complex urban areas is often a scenario to be considered. However, predicting the dispersion of heavy gases is still a challenge. In Germany, the VDI Guideline 3783, Part 1 and 2 is widely used for gas dispersion modelling. Whilst Part 1 uses a gauss model for calculating the dispersion of light or neutrally buoyant gases, Part 2 uses wind tunnel experiments to evaluate the heavier-than-air gas dispersion in generic built up areas. In practice, with this guideline, it is often not possible to adequately represent the existing obstacle configuration. To overcome this limitation, computational fluid dynamics (CFD) methods could be used. Whilst CFD models can represent obstacles in the dispersion area correctly, actual publications show that there is still further research needed to simulate the atmospheric flow and the heavy gas dispersion. This paper presents a modified k-ε-turbulence model that was developed in OpenFOAM v5.0 (England, London, The OpenFOAM Foundation Ltd Incorporated) to enhance the simulation of the atmospheric wind field and the heavy gas dispersion in built-up areas. Wind tunnel measurements for the dispersion of neutrally buoyant and heavy gases in built-up environments were used to evaluate the model. As a result, requirements for the simulation of the gas dispersion under atmospheric conditions have been identified and the model showed an overall good performance in predicting the experimental values.
CFD simulations of near-ground gas dispersion depend significantly on the accuracy of the wind field. When simulating wind fields with conventional RANS turbulence models, the velocity and turbulence profiles specified as inlet boundary conditions change rapidly in the approach flow region. As a result, when hazardous materials are released, the extent of hazardous areas is calculated based on an approach flow that differs significantly from the boundary conditions defined. To solve this problem, a turbulence model with consistent boundary conditions was developed to ensure a horizontally homogeneous approach flow. Instead of the logarithmic vertical velocity profile, a power law is used to overcome the problem that with the logarithmic profile, negative velocities would be calculated for heights within the roughness length. With this, the problem that the distance of the wall-adjacent cell midpoint has to be higher than the roughness length is solved, so that a high grid resolution can be ensured even in the near-ground region which is required to simulate
gas dispersion. The evaluation of the developed CFD model using the German guideline VDI 3783/9 and wind tunnel experiments with realistic obstacle configurations showed a good agreement between the calculated and the measured values and the ability to achieve a horizontally homogenous approach flow.
A steel pipeline segment of 2.5 m length was subjected to quasi-static four-point bending load in three steps for studying the initial cracking and damage accumulation based on the Acoustic Emission (AE) technique and by the direct current potential drop (DCPD) technique. For the latter, a new post-test analysis method was established.
AE is found more sensitive to crack initiation than DCPD. Formation of mesoscopic and macroscopic cracks as well as their closure and the resulting friction generate weighted peak frequencies below 400 kHz, whereas microscopic cracking produces AE with broad band spectra identifiable by weighted peak frequencies above 400 kHz. Critical states alike the maximum load level and the leak opening were accompanied by peak amplitudes above 85 dBAE. This rather fundamental study provides a data base for possibly developing advanced strategies of detection and alarm systems based on acoustic monitoring of pipelines, or at least, steel structures.
In the presented work, the potential of fiber-optic distributed acoustic sensing (DAS) for detection of small gas pipeline leaks (<1%) is investigated. Helical wrapping of the sensing fiber directly around the pipeline is used to increase the system sensitivity for detection of weak leak-induced vibrations. DAS measurements are supplemented with reference accelerometer data to facilitate analysis and interpretation of recorded vibration signals. The results reveal that a DAS system using direct fiber application approach is capable of detecting pipeline natural vibrations excited by the broadband noise generated by the leaking medium. In the performed experiment, pipeline vibration modes with acceleration magnitudes down to single ug were detected. Simple leak detection approach based on spectral integration of time-averaged DAS signals in frequency domain was proposed. Potential benefits and limitations of the presented monitoring Approach were discussed with respect to its practical applicability. We demonstrated that the approached is potentially capable of detection and localization of gas pipeline leaks with leak rates down to 0.1% of the pipeline flow volume and might be of interest for monitoring of short- and medium-length gas pipelines.
Large-scale experiments have been performed to investigate the possible consequences of realistic amounts of liquified hydrogen (LH2) encountering water. The experiments aimed at simulating an accidental release of LH2 onto water, for instance during the fuelling of a ship. For liquified natural gas (LNG), it has been demonstrated that physical explosions may occur when it is spilled onto water. These phenomena are referred as rapid phase transitions (RPTs). It cannot be excluded that RPTs are also possible in the case of LH2. The tests were performed at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Horstwalde, Germany. The tests were performed in a 10 m x 10 x 1.5 m basin filled with water. LH2 releases of up to about 1 kg/s were established releasing directly from a trailer carrying LH2. The releases occurred from a height of 50 cm above the water surface pointing downwards, 30 cm under the water surface pointing downwards and 30 cm under the water surface pointed along the water surface. All release configurations resulted in a very chaotic LH2-water mixing zone, causing considerable evaporation and resulting in minor over pressures. No RPTs were observed. The main phenomenon to be observed is, however, an ignition of the released gas cloud resulting in significant blast wave overpressures and heat radiation to the surroundings. The ignition occurred in all under-water releases and in about 90 % of the releases above the water surface.
Large-scale experiments have been performed to investigate the possible consequences of realistic amounts of liquified hydrogen (LH2) encountering water. The experiments aimed at simulating an accidental release of LH2 during especially the fuelling of a ship. For liquified natural gas (LNG) it has been demonstrated that when spilled onto water the evaporation rate can be that high that physical explosions occur which are referred as rapid phase transitions (RPTs). It cannot be excluded that RPTs are also possible in the case of LH2. The tests were performed in a 10 m x 10 m x 1.5 m basin filled with water, at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Horstwalde, Germany within a research cooperation between BAM and Gexcon as part of
the SH2IFT program. LH2 releases were established releasing directly from a trailer carrying LH2 through a long flexible double vacuum insulated transfer line. The releases occurred from a height of 50 cm above and 30 cm under the water surface both pointing downwards and 30 cm under the water surface pointing along the water surface too. The release rate was determined based on the weight loss rate of the road tanker which was placed onto load cells. Special blast pressure sensors were used to measure the shock waves generated by the release processes both in the water and in the air. At several locations the gas concentration was measured. Heat radiation was measured at 3 distances from the point of release. High speed,
Infrared (IR) and normal cameras were used to record events and to follow the gas cloud behaviour in time. This includes cameras mounted on a UAV and underwater cameras. Two weather stations were used to measure wind speed, wind direction, temperature and humidity during all tests performed. All investigated release configurations resulted in a very chaotic LH2-water mixing zone, causing considerable evaporation but only minor over pressures.
The main phenomenon observed was an ignition of the released gas cloud, resulting in significant blast wave overpressures and heat radiation to the surroundings. The location of the ignition occurred in free air at some distance from the instrumentation and release location.
In case of a vehicle fire, an installed LPG (liquefied petroleum gas) tank with a malfunctioning safety device poses severe hazards. To investigate the consequences in case of tank failure, we conducted 16 tests with toroidal shaped LPG vehicle tanks. Three tanks were used for a Hydraulic Burst Test under standard conditions. Another three tanks were equipped with a statutory safety device and were subjected to a gasoline pool fire. The safety device prevented tank failure, as intended. To generate a statistically valid dataset on tank failure, ten tanks without safety devices were exposed to a gasoline pool fire. Five tanks were filled to a level of 20 %; the re-maining five were filled to a level of 100 %. In order to gain information on the heating process, three tem-perature readings at the tank surface, and three nearby flame temperatures were recorded. At distances of l = (7; 9; 11) m to the tank, the overpressure of the shock wave induced by the tank failure and the unsteady tem-peratures were measured. All ten tanks failed within a time of t < 5 min in a BLEVE (boiling liquid expanding vapor explosion). Seven of these resulted directly in a catastrophic failure. The other three resulted in partial failure followed by catastrophic failure. A near field overpressure at a distance of l = 7 m of up to p = 0.27 bar was measured. All ten tests showed massive fragmentation of the tank mantle. In total, 50 fragments were found. These 50 fragments make-up 88.6 % of the original tank mass. Each fragment was georeferenced and weighed. Fragment throwing distances of l > 250 m occurred. For the tanks with a fill level of 20 %, the average number of fragments was twice as high as it was for the tanks that were filled completely.
In continuation of a preceding test series involving sole LPG vehicle tanks, three passenger cars equipped with identical toroidal steel LPG tanks were set on fire. The tanks were installed in the space normally reserved for the spare tyre, in the car boot. No safety device was installed on the tank, in order to force critical failure of the cylinder. Two of the cars were equipped with a tank filled with liquefied propane to a level of 20 % (5.3 kg), the third one was filled completely (25.5 kg). The partially filled tanks failed critically within a time period of more than 20 min after the initiation of the fire. The fully-filled tank did not rupture; the propane was released continuously through a small leak that appeared during the fire. Comprehensive equipment was used to procure measurement data, enabling an analysis of potential consequences and hazards to humans and infrastructure within the vehicle surroundings. The inner status of the tank (pressure, temperature of the liquid phase and the steel casing), the development of the fire (temperature inside and around the vehicle) and the pressure induced in the near-field in case of tank rupture were recorded. The results were analysed in detail and compared against the data gained in tests involving sole, but identical LPG tanks.
Experimental investigation of the consequences of release of liquified hydrogen onto and under water
(2022)
Large-scale experiments have been performed to investigate the possible consequences of realistic amounts of liquified hydrogen (LH2) encountering water. The tests were performed at the Test Site Technical Safety of the Bundesanstalt für Materialforschung und –prüfung (BAM) in Horstwalde, Germany.
The experiments aimed at investigating that upon contact with water, the evaporation rate of LH2 can be that high that physical explosions or even rapid phase transitions (RPTs), could occur. RPTs can generate air and underwater blast pressures which could damage adjacent plant or structures (which has been demonstrated when releasing LNG).
The tests were performed in a 10 m x 10 x 1.5 m basin filled with water. LH2 releases with mass flows ranging from approximately 0.25 kg/s to approximately 0.8 kg/s were established releasing directly from a trailer carrying LH2. The LH2 was released through a 46 m long flexible double vacuum insulated transfer line (inner diameter 39 mm). The releases occurred from a height of 50 cm above the water surface pointing downwards, 30 cm under the water surface pointing downwards and 30 cm under the water surface pointing along the water surface. The release system allowed for an initial phase to release flashed LH2 into the air before redirecting the flow towards the water surface for liquid release. A thermocouple inserted near the nozzle is used to indicate the presence of LH2 at the nozzle. The nozzle could be moved up and downwards relative to the water surface and also moved into the water. The release system was purged with helium before starting a release. A mouthpiece at the end of the release line as well as a manually operated valve at the trailer were used to vary the release rate.
The temperature of the water and the air was measured at multiple locations. The temperature of the air was also measured at H2 concentration probe locations. In addition, the temperature in the filling line was measured (1 at the outlet and 1 further down into the filling line). The pressure in the filling line was also measured at 3 locations, one located directly at the outlet. The release rate was determined based on the weight loss rate of the road tanker. To this end the trailer was placed onto load cells. Special blast pressure sensors were used to measure the shock waves generated by the rapid evaporation or other phenomena both in the water and in the air. At several locations the gas concentration development in time was measured. Heat radiation was measured at 3 distances from the point of release. High speed, Infra-Red (IR) cameras and normal cameras were used to record events and to follow the gas cloud behaviour in time. This includes cameras mounted on a drone and an underwater camera. Two weather stations were used to measure wind speed, wind direction, temperature and humidity during all tests performed.
All investigated release configurations resulted in a very chaotic LH2-water mixing zone, causing considerable evaporation and resulting in minor over pressures. The main phenomenon observed was an ignition of the released gas cloud, resulting in significant blast wave overpressures and heat radiation to the surroundings. The ignition occurred in all under-water releases and in about 90 % of the releases above the water surface. The location of the ignition occurred in free air at some distance from the instrumentation and release location.
The feasibility study „AGIFAMOR. Ageing infrastructures – distributed acoustic monitoring of pipes” is an interdisciplinary research project at BAM internally financed from 2015 to 2018. Therefore, the quite young fibre optic sensing technology of distributed acoustic sensing (DAS) was investigated to possibly be extended towards a global condition monitoring system for pipelines operating in real time.
DAS is a highly dynamic fibre optic sensing technology based on the method of coherent optical time domain reflectometry (C-OTDR). DAS allows capturing strain changes in the range of kHz. For the experimental work, the most suitable application yielding an optimum sensitivity was proven by wrapping a standard single-mode silica fibre around the pipe.
The DAS sensitivity was investigated regarding the detection of 1) incidents that initiate propagation of acoustic waves in the pipe wall, 2) changes inside the pipeline causing altered flow and 3) damage development in the pipe wall. Therefore, several testing setups in laboratory as well as in real scale were realized. For comparison purposes, experiments were accompanied by acoustic emission analyses and by measurements with accelerometers.
DAS was found to be very sensitive to gas ignition and its propagation across the pipe. Furthermore, the ability of DAS to detect and localize acoustic signals associated with pipeline leakage was demonstrated. The detection of crack formation and propagation within the pipe wall by means of DAS was studied during bending tests on several pipe segments, but was not proven so far with certainty. As expected, these studies turned out as the most difficult challenge due to the random occurrence and transient nature of microscopic damage phenomena.