Flame Acceleration in Tube Explosions with up to Three Flat-bar Obstacles with Variable Obstacle Separation Distance

The effect of obstacle separation distance on the severity of gas explosions has received little methodical study. It was the aim of this work to investigate the influence of obstacle spacing of up to three flat-bar obstacles. The tests were performed using methane-air (10% by vol.), in an elongated vented cylindrical vessel 162 mm internal diameter with an overall length-todiameter, L/D, of 27.7. The obstacles had either 2 or 4 flat-bars and presenting 20% blockage ratio to the flow path. The different number of flat-bars for the same blockage achieved a change of the obstacle scale which was also part of this investigation. The first two obstacles were kept at the established optimum spacing and only the spacing between the second and third obstacles was varied. The profiles of maximum flame speed and overpressure with separation distance were shown to agree with the cold flow turbulence profile determined in cold flows by other researchers. However, the present results showed that the maximum effect in explosions is experienced at 80 to 100 obstacle scales about 3 times further downstream than the position of maximum turbulence determined in the cold flow studies. Similar trends were observed for the flames speeds. In both cases the optimum spacing between the second and third obstacles corresponded to the same optimum spacing found for the first two obstacles demonstrating that the optimum separation distance does not change with number of obstacles. In planning the layout of new installations, the worst case separation distance needs to be avoided but incorporated when assessing the risk to existing set-ups. The results clearly demonstrate that high congestion in a given layout does not necessarily imply higher explosion severity as traditionally assumed. Less congested but optimally separated obstructions can lead to higher overpressures

Vent static burst pressure influences on explosion venting

The overpressure generated in a 10L cylindrical vented vessel with an L/D of 2.8 was investigated, with end ignition opposite the vent, as a function of the vent static burst pressure, Pstat, from 35 to 450mb. Three different Kv (V2/3/Av) of 3.6, 7.2 and 21.7 were investigated for 10% methane-air and 7.5% ethylene-air. It was shown that the dynamic burst pressure, Pburst, was higher than Pstat with a proportionality constant of 1.37. For 10% methaneair Pburst was the controlling peak pressure for K Pburst in the literature and in EU and US standards. For higher Kv the overpressure due to flow through the vent, Pfv, was the dominant overpressure and the static burst pressure was not additive to the external overpressure. Literature measurements of the influence of Pstat at low Kv was shown to support the present finding and it is recommended that the influence of Pstat in gas venting standards is revised

Comparison of central and end spark position for gas explosion vessels with L/D of 2.8 and 2.0

Current explosion vent design correlations and guidance are based on an experimental data base of centrally ignited vented tests. However, there is evidence in the literature that ignition positions other than central produce higher overpressures. The objective of this work was to compare central and end ignition of vented explosions in a 10L and a 200L cylindrical vessels of L/D of 2.8 and 2 respectively, with vent area coefficients of 10.9, 5.4 and 3.1 for free venting. Methane-air (10% v/v) and ethylene-air ( 7.5%) explosion tests were carried out using a 16J spark ignition at the far end wall opposite the vent and half way along the length of the vessel. The results showed that for both vessels and for both gas/air mixtures end ignition produced the highest overpressures. This was attributed to the higher axial flame speed towards the vent with far end ignition, inducing higher vent mass flows and higher external flame speeds and associated overpressures. The present results and other data from the literature show that the vent design guides may not be based on sufficiently conservative data and need to be reviewed

Turbulent Combustion Parameters in Gas Explosions with Two Obstacles with Variable Separation Distance

Most of the congested gas explosions studies have focused on quantifying global flame acceleration and maximum overpressure through obstacle groupings rather than detailed analysis of the flame propagation through the individual elements of the congested region. Fundamental data of the turbulent flow and combustion parameters would aid better understanding of gas explosion phenomena and mechanisms in the presence of obstacles in addition to the traditional flame speeds and overpressures that are usually reported. In this work we report near stoichiometric methane/air explosion tests in an elongated vented cylindrical vessel 162 mm internal diameter with an overall length-to-diameter, L/D of 27.7. Single and double obstacles (both hole and flat-bar types) of 20-40% blockage ratios, BR with variable obstacle scale were used. The spacing between the obstacles was systematically varied from 0.5 m to 2.75 m. Turbulence parameters were estimated from pressure differential measurements and geometrical obstacle dimensions. This enabled the calculation of the explosions induced gas velocities, rms turbulent velocity, turbulent Reynolds number and Karlovitz number. This allowed the current data to be plotted on a premixed turbulent combustion regimes diagram. The bulk of the data fell in the thickened-wrinkled flames regime. The influence of the calculated Karlovitz number on the measured overpressures was analysed and was related to obstacle separation distance and obstacle scale characteristics

Impact of non-central vents on vented explosion overpressures

It is normal practice to use centrally positioned vents or single vents in most experimental work and in the application of explosion venting in industry. This work seeks to investigate the influence of non-central and multiple distributed vents on the explosion overpressure. A 10L cylindrical vessel of 460mm length and 162mm diameter (L/D=2.8) was used for vented explosion with free venting (without a vent cover). Three different vent coefficient (Kv) were investigated, Kv, 3.6, 5.4 and 10.9 for both non-central and 4 hole vents. 10% methane-air and 7.5% ethylene-air mixtures were investigated to determine the influence of the mixture reactivity. The position of the spark ignition was in the centre of the end flange opposite the vent. It was shown for the non-central vent that the flame speed upstream of the vent was lower than for a central vent and this reduced the mass flow through the vent, which reduced the overpressure and reducing the external explosion due to the lower exit velocity of the unburnt gas and hence lower external turbulence. The external flame jets downstream of the vent was influenced by the increase in characteristic length scale of the vent, which was changed by increasing the number of vents

Gas explosion venting: comparison of experiments with design standards and laminar flame venting theory

European and USA design standards for gas explosion venting are quite different in their predictions, with the European standards always giving a higher predicted explosion Pred for the same vent coefficient, Kv. The format of the two predictions are different with the US standards following the approach of Swift expressing the vent area as a ratio to the surface area of the vessel, As/Av and the European standard using the vent coefficient approach. Kv= V2/3/Av. It is shown that these two approaches are directly related as As is proportional to V2/3. The reactivity parameter in the US standards is the laminar burning velocity, UL, and in the European venting standards it is the deflagration parameter, KG = dp/dtmax/V1/3. It is shown that these two reactivity parameters are linearly related. The USA standard is shown to be compatible with spherical flame venting theory and with experimental data other than that of Bartknecht. There is also good agreement with the present results for a 10L vented vessel for which the spherical laminar flame venting theory gives reasonable agreement but predicts the Pred to be higher than measured. This is because of the assumption that at the maximum value of Pred the bulk flame area is equal to As which is not valid. The US standard also has corrections for flame self acceleration, which is a vessel size effect, and for the influence of vessel size on the external explosion, which are not factors addressed in the European standards. The European standard is the equation for the results of Bartknecht for a 10 m3 vessel and the results of higher and lower volumes in Bartknecht’s results are all lower that for 10 m3. The experimental results reviewed, for methane and propane maximum reactivity vented explosions, include data for vessels larger than that on which the European standards are based and they all give significantly lower values of Pred than those of Bartknecht.

Effect of steam exploded treatment on the reactivity of pine wood

A commercial thermally treated biomass process known as ‘steam exploded biomass’ provided the treated biomass samples for this project together with the original yellow pine wood. The aim was to investigate the change in pulverised biomass reactivity. The steam exploded biomass is processed into pellets in the normal way and are known as black pellets (BP). The material was investigated using the Hartmann dust explosibility equipment. This enables the minimum explosion concentration (MEC) to be determined together with the initial rate of pressure rise and the flame speed and these latter parameters are measures of the mixture reactivity. BP was found to have a higher reactivity than the raw biomass with a much leaner MEC. A good correlation was found between the initial rate of pressure rise and the flame speed for the raw wood sample. Surface morphology was performed to investigate the effects of the steam exploded treatment. This showed the enhancement of the proportion of fines. The particle size distribution was determined and this confirmed the enhancement of the fineness of the treated sample. The enhanced reactivity of BP was found to be due to the greater proportion of fine particles which had a higher heating rate and a greater release of volatiles. The steam explosion treatment was found to be an effective pre-treatment in facilitating the combustion of renewable fuel and the main effect was that it was more easily milled, changes in the biomass chemistry was of secondary importance

Influence of a Wall Close to a Vent Outlet

It is well known that the US NFPA 68 (2013) and the EU standard (EN14994:2007), based on the work of Bartknecht (1993), for gas venting do not agree and the EU standard will require a much larger vent area for the same Pred. The present work offers a possible explanation of the difference in these guidelines: the experimental results of. Bartknecht (1993) were carried out with the bottom of the vented vessel on the ground, so that the vent exit was relatively close to the ground and the interaction increased Pred. In the present work a 0.2 m3 cylinder of 0.5m diameter with end wall ignition was free vented into a large dump vessel with a 0.5m diameter connecting pipe. The wall of the 0.5m connecting pipe was close to the vent and the results showed that there was a wall interaction that gave Pred close to those of Bartknecht (1993) at low Kv. In the vented explosion work of Fakandu (2016b) using a 10L vessel, the discharge area was connected to a dump vessel with a 0.5m diameter pipe, which was much bigger than the 162mm diameter of the vented vessel and this gave overpressures close to those predicted in NFPA 68 (2013) with the turbulence parameter λ set to unity. The critical ratio of the centerline distance of the vented vessel to the external surface (ground in most cases) as a ratio of the distance from the edge of the vent to the external surface (DR) was shown to be 1.8 in this work, with lower values indicating no interaction. The present results show that Bartknecht’s experimental results had high Pred probably due to the presence of the ground as a nearby surface