Showing papers on "Overpressure published in 1996"

Strombolian explosions: 1. A large bubble breaking at the surface of a lava column as a source of sound

Abstract: Strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column Acoustic pressure has been measured for 36 explosions at Stromboli We propose that sound is generated by the vibration of the bubble before it bursts Oscillations are driven by an initial overpressure inside the bubble, assumed to be initially at rest, just below the magma-air interface Inertia effects cause the bubble to overshoot its equilibrium radius Then the bubble becomes underpressurized and contracts because of gas compressibility These oscillations are only slightly damped by viscous effects in the magma layer above the bubble The bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking, near the minimum radius Assuming a simple geometry, we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure The fit between synthetic and observed waveforms is very good, both for frequency, ≈60 rad s−1, and amplitude The initial bubble radius is ≈1 m, and the length varies between several and a few tens of meters From the initial overpressure, approximately 105 Pa, we calculate the maximum radial velocity of ejecta, ≈30 m s−1 The generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics

Strombolian explosions 1. A large bubble breaking at the surface of a lava

Abstract: Strombolian activity consists of a series of explosions caused by the breaking of large overpressurized bubbles at the surface of the magma column. Acoustic pressure has been measured for 36 explosions at Stromboli. We propose that sound is generated by the vibration of the bubble before it bursts. Oscillations are driven by an initial overpressure inside the bubble, assumed to be initially at rest, just below the magma-air interface. Inertia effects cause the bubble to overshoot its equilibrium radius. Then the bubble becomes underpressurized and contracts because of gas compressibility. These oscillations are only slightly damped by viscous effects in the magma layer above the bubble. The bubble cannot complete more than one cycle of vibration because of instabilities developing on the magma layer that lead to its breaking, near the minimum radius. Assuming a simple geometry, we model this vibration and constrain the radius and length of the bubble and the initial overpressure by fitting a synthetic waveform to the measured acoustic pressure. The fit between synthetic and observed waveforms is very good, both for frequency, m 60 rad s -1, and amplitude. The initial bubble radius is m 1 m, and the length varies between several and a few tens of meters. From the initial overpressure, approximately 10  Pa, we calculate the maximum radial velocity of ejecta, m 30 m s -1. The generally good agreement between data and predictions of our model leads us to suggest that acoustic measurements are a powerful tool for the understanding of eruption dynamics.

Side-vented gas explosions in a long vessel: the effect of vent position

Abstract: Currently, the experimental data on the influence of vent position on the overpressure development during gas explosions in large length to diameter ratio (L/D) vessels are very sparse. Design guides such as NFPA 68 recommend the use of side-venting, but this is based on few experimental data and it was the objective of the present work to provide further information on vented explosions in tube configurations with side vents. Methane-air explosions (10 gas by volume) were undertaken in large L/D vessels and the influence of the side-venting position relative to the spark was investigated and compared with end-venting. The pressure development during side-venting was investigated and five stages of the explosion development were identified. The five pressure peaks were identified as follows: P1 due to the initial elongated flame acceleration, P2 due to the turbulence flame acceleration in the downstream gases ahead of the flame set in motion by the initial explosion gas expansion, P3 due to the flame acceleration around the 90° bend of the side vent, P4 due to the external gas cloud explosion downstream of the vent, and P5 due to the oscillatory combustion of trapped unburnt mixture in the vessel between the side vent and the far end of the tube. All these pressure peaks were not present for each vent position and the effect depended on the distance of the side vent from the ignition source. Very high flame speeds and overpressures were measured as the distance between the side vent position and the spark was increased. In the end-venting explosions the maximum overpressure was higher than the side-vented tests, except for the case where the side vent was placed at the middle of the vessel, which had a similar overpressure to end-venting. Induced gas velocities were also measured and associated turbulent parameters were calculated and were found to increase with an increase in the distance of the vent position from the spark.

Abnormally High Fluid Pressures and Associated Porosities and Stress Regimes in Sedimentary Basins

Abstract: The relationships between overpressures and porosity and overpressures and stress regime are found to differ significantly according to the geological setting and overpressure generating mechanism. These factors must be taken into account in pore pressure prediction techniques and in correlations between pore pressure and minimum horizontal stress. Two examples from overpressured Canadian basins are presented to illustrate the above.

Dust explosions in totally enclosed interconnected vessel systems

Abstract: Some industrial dust-handling plant is designed so that, should an explosion occur, the overpressures can be totally contained. Such plant needs to be designed to withstand at least 10 bar overpressure. This may suffice for single compact vessels but experience indicates that in interconnected plant, explosion overpressures can be substantially increased. This effect is due to a process known as pressure-piling. There is no adequate guidance on how to take the effects of this process into account. This paper describes a project in which the behaviour of dust explosions in systems of linked enclosed vessels has been studied. Explosions of coal dust and toner dust have been produced in a number of linked systems using vessels ranging in size from 2 to 20 m3 and connected by 5 m lengths of pipe with diameters of 0.15 m, 0.25 m and 0.5 m. Pressure changes that occur when a dust explosion is ignited in one of the vessels and the flame transmits into the other have been measured. Pressure enhancement due to pressurepiling effects occurs in some systems, and pressures approaching 20 bar have been measured. The pipe diameter, vessel volumes and the volume ratio are important variables. These results are discussed and a simple method for the calculation of explosion pressure enhancement effects is described. Comparisons are made between the model predictions of the maximum explosion pressure and experimental measurements. The experiments have shown that, in some circumstances, there is a low probability of an explosion propagating along the pipe to produce an explosion in the second vessel. It appears that although the flame travels through the pipe and a jet flame enters the second vessel, it does not act as an effective ignition source. The wider the pipe diameter, the higher the frequency of an explosion in the second vessel.

PCBoom3 Sonic Boom Prediction Model - Version 1.0c

Abstract: : PCBoom3 is a PC-based program that computes single-event sonic boom footprints from any supersonic vehicle exercising any maneuver in a real atmosphere, including winds. The user specifies the aircraft, the maneuver, and the atmosphere. The primary output is the sonic boom footprint in terms of contours of equal overpressure (or other amplitude metric) on the ground, relative to the aircraft's position. PCBoom3 also generates sonic boom signatures, the pressure-time history of the boom at a particular location on the ground. Spectra of these signatures are also computed. The program is operated through a menu interface, which simplifies its use and the presentation of results. The program is designed for use by personnel who are planning specific supersonic missions where sonic boom will be an issue or who are investigating sonic boom incidents. While the program is relatively simple to operate, it provides access to analysis which requires an understanding of sonic boom phenomena. A copy of this program can be obtained by contacting AFRL/HECB at (937) 255-3605 x423 or downloading the file directly from the public website at www.afcee.brooks.af.mil/EC/noise/noisemodels.htm.

Tank ventilation for direct fuel injection IC engine with external ignition

Abstract: The method involves sucking fuel from an adsorption container (32) with an overpressure pump electrically powered or driven via a mechanically coupling from the internal combustion engine which can be connected and disconnected. The overpressure pump is a diagnosis pump used for diagnosing the tank ventilation system using the overpressure method.

Overpressure and hydrocarbon trapping in the Chalk of the Norwegian Central Graben

Abstract: The hydrodynamic system within the Chalk of the Greater Ekofisk area and its effect upon pore pressure distribution and hydrocarbon trapping has been analysed. The Chalk of the area is a relatively closed system for fluid movement and has large pressure cells characterized by their own pressure regime, fluid type and trapping potential. The aquifer is near static and there is no significant regional hydrodynamic flow. Overpressure is higher at Valhall than at Eldfisk, Ekofisk or Albuskjell, but Valhall is structurally shallower than Albuskjell. This trend corresponds to a maturity trend for the reservoir fluids, and is related to burial depth, temperature and hydrocarbon charging. As total pore pressure at the top of the structures reaches fracture pressure, leakage occurs. The gas cloud observed on seismic data above Valhall may be due to present-day leakage, whilst the gas clouds above other fields could be related to leakage which occurred when the fracture pressure was lower than today. Overpressuring is a dynamic process dependent on burial, hydrocarbon generation and migration, diagenesis and seal integrity and it controls both chalk porosity and hydrocarbon trapping.

Shock waves from an open-ended shock tube with different shapes

Abstract: A new method for decreasing the attenuation of a shock wave emerging from an open-ended shock tube exit into a large free space has been developed to improve the shock wave technique for cleaning deposits on the surfaces in industrial equipments by changing the tube exit geometry. Three tube exits (the simple tube exit, a tube exit with ring and a coaxial tube exit) were used to study the propagation processes of the shock waves. The detailed flow features were experimentally investigated by use of a two-dimensional color schlieren method and by pressure measurements. By comparing the results for different tube exits, it is shown that the expansion of the shock waves near the mouth can be restricted by using the tube exit with ring or the coaxial tube exit. Thus, the attenuation of the shock waves is reduced. The time histories of overpressure have illustrated that the best results are obtained for the coaxial tube exit. But the pressure signals for the tube exit with ring showed comparable results with the advantage of a relatively simple geometry. The flow structures of diffracting shock waves have also been simulated by using an upwind finite volume scheme based on a high order extension of Godunov's method as well as an adaptive unstructured triangular mesh refinement/unrefinement algorithm. The numberical results agree remarkably with the experimental ones.

Turbulent premixed flame propagation in a cylindrical vessel

Abstract: An experimental and theoretical study of premixed flame propagation in a cylindrical vessel, containing turbulence inducing rings is described. The study was undertaken to provide further understanding of flame propagation and the generation of overpressure in explosions and to assess a mathematical model of explosions through comparison with detailed experimental data. Data gathered on flame location and shape, flow velocities, and overpressure during the course of an explosion are used to elucidate the dynamics of the various combustion processes occurring in the different chambers of the vessel. In particular, flame propagation through the vessel, until the flame front vents from the tube, is found to be substantially laminar, with significant overpressure only being generated in the later stages of the explosion due to rapid turbulent combustion in the shear layers and recirculation zones induced by the obstacles. Comparisons between measurements and numerical predictions obtained using an explicit, second-order accurate, finite-volume integration scheme, coupled to an adaptive grid algorithm, demonstrate that the mathematical model used provides a reasonable simulation of combustion within the vessel In particular, good qualitative agreement with observations is obtained for flame speed and shape, with rapid turbulent combustion being predicted with sufficient accuracy to yield reliable results for flow velocities and the overpressures generated.

Process for transporting a thermally unstable viscous mass

Abstract: A process is disclosed for transporting a thermally unstable viscous mass through a pipe with a predetermined breaking point for letting out an overpressure. When an overpressure is reached, said predetermined breaking point breaks and lets out the overpressure. The predetermined breaking point is located inside the pipe but is not part of the inner wall of the pipe.

Starbursts Triggered by Central Overpressure. II. A Parameter Study

Abstract: A parameter study is made of the radiative shock compression of a disk molecular cloud due to the high pressure of the central molecular intercloud medium, after the cloud has fallen into the central region of a galaxy following a galaxy interaction. The dependence of the compression on the disk cloud and central gas parameters has been studied. We show that fshell, the fraction of cloud mass compressed in the outer shell that becomes unstable, is a function of only the external pressure, the cloud radius, and the cloud density. We find that for a wide range of values for the input parameters, fshell is high and lies between 0.75 to 0.90. The fraction fshell is not sensitively dependent on the value of the central gas pressure because the initial inward shock velocity is proportional to the square root of the central pressure, and at later stages it is determined mainly by the self-gravity of the shocked shell. Thus, star formation triggered in disk clouds by compression by the central overpressure, as proposed by Jog & Das, is a general triggering mechanism and is valid as long as the central gas has an overpressure of even a factor of a few higher than that of the incoming disk molecular clouds.We apply the mechanism to the galaxies for which the central gas parameters are known; for example, IC 342 and NGC 1808. We find that in both cases a large fraction of the cloud mass will be compressed. Hence, these galaxies should show a central starburst, provided that there is a substantial gas infall rate from the disk to the central region. This agrees with observations, in that NGC 1808 with gas infall due to galaxy interaction has a central starburst, while IC 342 does not.

The effect of vent size on pressure generation in explosions in large L/D vessels

Abstract: There is a need for more explosion relief data in large L/D vessels. In this paper we report an experimental investigation of the effect of vent size on the overpressure development in a large L/D, end-vented vessel. Three pressure peaks were identified: P 1 due to an initial elongated flame (characteristic of large L/D explosions); P 2 due to turbulent combustion in the middle and end sections of the vessel and P 3 due to the external explosion. P 1 was found to be the dominant overpressure for small vent areas and to increase with decreasing vent size, whereas the flame speed was reduced (as well as P 2 and P 3) due to reduction of unburnt gas flow through and out of the vessel.

Overpressuring in the Kutei Basin: Distribution, Origins and Implications for the Petroleum System

Abstract: Overpressuring is ubiquitous in the Kutei Basin and occurs over a wide range of geologic ages and depths. Sequences affected by overpressuring become younger from west to east, from Eocene through Pliocene, and this is consistent with the overall easterly progradation of the sedimentary regime through the Tertiary. Overpressuring has been penetrated from as deep as 15,000 ft to as shallow as the surface. The thickness of overpressured section penetrated ranges up to more than 10,000 ft; however, the base of the overpressure has not been encountered within a sedimentary section. The priamry mechanism for generation of the overpressuring is considered to be Disequilibrium Compaction, as a results of inefficient expulsion of pore fluids during burial. The overpressuring is pervasive in sand-deficient, fine grained clastics deposited in distal and deeper marine environments. However, relatively sand-rich sequences, deposited in more proximal settings, also experience severe overpressuring with deep burial. Sonic log crossplots from wells have been used to define three pressure zones - Hydrostatic, Transition and Hard Overpressure. Hydrocarbons are produced from both the Hydrostatic and Transition Zones; basinwide, a large percentage of reserves occur in the Transition Zone. Implications of overpressuring to the Petroleum System are discussed. Commercially productive hydrocarbon reservoirs have not been encountered within the Hard Overpressure Zone in the Sanga-Sanga PSC. Sands present in this zone typically exhibit characteristics of a limited reservoir and contain gas dissolved in water - so called "Coke-Bottles". Seal capacity of shales within the Transition Zone is considerably enhanced relative to shales in the Hydrostatic Zone and results in the trapping of significantly larger hydrocarbon columns. Hydrocarbon source potential in the Hard Overpressure Zone is generally poor and, due to very low expulsion efficiency, an effective kitchen in this zone is unlikely.

Process and device for transporting a thermally unstable viscous mass

Abstract: A process is disclosed for transporting a thermally unstable viscous mass through a pipe with a predetermined breaking point for letting out an overpressure. When an overpressure is reached, said predetermined breaking point breaks and lets out the overpressure. The predetermined breaking point is located inside the pipe but is not part of the inner wall of the pipe.

Small Scale Tests on Mitigation Effects of Water in a Model of the KLOTZ Club Tunnel in Alvdalen

Abstract: : The effects of water in close contact with detonating high explosives have been studied in several test series and it has been determined that water has a mitigation effect on the blast wave from an explosion. Both maximum overpressure and impulse density will be reduced. The effective charge size can be reduced with more than 50%, possibly as much as 80%. This means that possible equivalence factor is 0.2 (amount of high explosive with/without water giving the same effects). It has been suggested as a very interesting concept to store ammunition or high explosive together with containers of water to reduce the effects of accidental explosions. A full scale test with real ammunition and water used for mitigation is planned to take place in The KLOTZ Club tunnel in Aelvdalen in fall 1996. For planning and preparation purpose of the full scale test a comprehensive test series in a small scale model of the tunnel has been performed. Different locations of water and charge to water ratio have been studied. Pressure time histories have been recorded inside and outside the tunnel. Some simple experiments were also made to compare the effects of debris throw, with and without water mitigation.

Formation and mechanism of the abnormal pressure zone and its relation to oil and gas accumulations in the Eastern Jiuquan Basin, northwest China

Abstract: Three abnormal overpressure zoes with a planar top at different depths occur in the Ying'er Depression in the Eastern Jiuquan Basin. The distance and the temperature difference between them are about 1 000 m and 30℃, respectively. The studies of sedimentary history, nature of formation water, variation of geothermal gradient and examination of thin sections, and the relationship between lithologic section and formation pressure show that there are conditions for formation of abnormal overpressure zones in the Ying'er Depression. Aquathermal pressuring and the overlying sediment load are main factors forming the abnormal overpressure zones. The study indicates that most of oil and gas in the Ying'er Depression accumulated in reservoirs above or under the seals or in the top of the compartments.

An analysis of the response of Sooty Tern eggs to sonic boom overpressures.

Abstract: It has been proposed that sonic booms caused a mass hatching failure of Sooty Terns in the Dry Tortugas in Florida by cracking the eggshells. This paper investigates this possibility analytically, complementing previous empirical studies. The sonic boom is represented as a plane-wave excitation with an N-wave time signature. Two models for the egg are employed. The first model, intended to provide insight, consists of a spherical shell, with the embryo represented as a rigid, concentric sphere and the albumen as an acoustic fluid filling the intervening volume. The substrate is modeled as a doubling of the incident pressure. The second, numerical model includes the egg-shape geometry and air sac. More importantly, the substrate is modeled as a rigid boundary of infinite extent with acoustic diffraction included. The peak shell stress, embryo acceleration, and reactive force are predicted as a function of the peak sonic boom overpressure and compared with damage criteria from the literature. The predicted peak sonic boom overpressure necessary for egg damage is much higher than documented sonic boom overpressures, even for extraordinary operational conditions. Therefore, as with previous empirical studies, it is concluded that it is unlikely that sonic boom overpressures damage avian eggs.

Method for pneumatically checking the operability of a tank-venting system

Abstract: The invention is directed to a method for pneumatically checking the operability of a tank-venting system having a tank made of a material such as plastic. The tank-venting system includes an adsorption filter having a venting line with a closeable shutoff valve mounted in the venting line. The adsorption filter is connected to the tank via a tank connecting line and a tank-venting valve. In the method, the tank-venting system is first charged with a first overpressure or underpressure which exceeds, by a predetermined value, a second overpressure or underpressure corresponding to a diagnostic overpressure or diagnostic underpressure with the tank-venting valve and the shutoff valve being closed. The first overpressure or underpressure is then removed after a pregiven time span has elapsed and an overpressure or underpressure decay gradient measurement is made only after an essentially constant diagnostic overpressure or diagnostic underpressure adjusts in the tank-venting system. A conclusion is then drawn as to the tightness of the tank-venting system based upon the decay gradient measurement.

Exhaust gas feedback control device for IC engine

Abstract: The device feeds back a part of the exhaust gas in a combustion engine (1). An overpressure supply source generates overpressure which is higher than the air pressure. A control valve (50) sets the overpressure to a predetermined pressure in which the air pressure is mixed with the pressure from the overpressure supply. An exhaust gas feedback valve (30) controls a quantity of exhaust gas to be fed back to the combustion engine in accordance with the predetermined pressure. A first switch valve (113) controls the communication between the overpressure source and the control valve. A control device (20) controls the opening or closing of the first switch valve (113). The control device (20) controls the first valve only in the open state when the predetermined pressure fed back to the exhaust gas valve is increased and maintained.

Large-power contactless overpressure alarm pressure meter

Abstract: The utility model relates to a large-power contactless overpressure alarm pressure meter. The utility model is formed by connecting a joint of an entering pressure meter, a spring tube, an indicating pointer, a meter dial, a setting needle with an upper and a lower limits, a regulating column, a draw rod, a machine core, a drive mechanism of a gear, and a fixed bracket. An induction coil is fixedly arranged on the setting needle with an upper and a lower limits. An overpressure setting needle is arranged on the fixed bracket. An overpressure induction coil is fixedly arranged on the overpressure setting needle. A thin metal sheet is fixedly arranged on the indicating pointer. The thin metal sheet moves between the upper and the lower limits and the overpressure induction coil following the indicating pointer. Each induction coil is connected with an on-off control circuit.

A Computational Assessment of Surge-Induced Inlet Overpressure Distribution for Preliminary Design

Abstract: Emphasis on tactical aircraft cost reduction has motivated efforts to reduce air induction system cost, which is mainly driven by structural weight. Historically, inlet structural weight has been set by the peak loads associated with engine surge-induced hammershocks. Therefore, an accurate assessment of peak inlet pressure distribution occurring during an engine surge is critical to successful design of next-generation inlet structure for minimum weight and adequate safety margin. Current methods to quantify these inlet loads during preliminary design stages of an advanced inlet are inadequate. A cost-effective, computational methodology was developed to address this issue. This methodology predicted F-16 Normal Shock Inlet overpressure design loads to within 3% of existing methods. The sensitivity of inlet overpressure distribution to the engine face waveform and inlet geometry was investigated. This technique was also applied to an advanced inlet to study the effects of serpentine ducts and 3-D inlet apertures on hammershock loads.Copyright © 1996 by ASME

Overpressure and Seals in the North Sea - A Model Approach: ABSTRACT

Abstract: Overpressure prediction in the North Sea is complicated by fault seal assumptions. The computer code, BasinMod-2D, is used to simulate overpressure for a small-scale case study, the Puffin structure, located on the western flank of the Central Graben. This area is characterized by tilted fault blocks and syn-rift sedimentation of Upper Jurassic Fulmar sandstones with a post-rift cover of Cretaceous chalk and Tertiary mudstones. The model, a 10 km W-E section based on published seismic data, is tied to detailed well stratigraphy in 29/10 -2 with pressure data from RFT and mudweight. The model fails to replicate observed overpressure data. The simulated overpressure is 20 MPa lower than the measured pressure of 85 MPa in the Puffin structure. The model fails to simulate hydrocarbon accumulation in the Fulmar sandstone reservoir, in contradiction to the observed bitumen indication of paleoaccumulation. The model also raises important questions as to the validity of uncalibrated parameter manipulation as a means of forward modelling fault seal, with specific reference to shale permeability. We contend that these failures are not due to model limitations, but are the result of the size-scale of this case study. The inference is that overpressure and hydrocarbon creation and transmission ismore » on a size scale fault seal at one time can leak hydrocarbons at another time. We consider that the drive for this comes from pressure increase during hydrocarbon generation in the deep kitchen.« less