Effects of obstacle separation distance on gas explosions

The separation distance (pitch) between obstacles is an area that has not received adequate attention by gas explosion researchers despite general recognition of the important role it plays in determining the explosion severity. Either too large or too small a separation distance between the obstacles would lead to lower explosion severity. Therefore obstacles would need to have “optimal” separation distance to produce the worst case explosions overpressures and flame speeds. Most studies to date with multi-obstacles had the obstacles too closely packed resulting in data that most likely do not represent the worst case scenarios.
The major objective of this project was to investigate the influence of spacing between obstacles in gas explosions by systematically varying the distance in order to determine the worst case separation that will produce the maximum explosion severity. A long vented cylindrical vessel 162 mm internal diameter with an overall length to diameter ratio (L/D) of 27 was used in the experimental study. The vessel was closed at the ignition end and its open end connected to a large cylindrical dump-vessel with a volume of 50 m3. The spacing between the obstacles in the test vessel was systematically varied from 0.25 m to 2.75 m. The influence of obstacle spacing was studied with obstacles of different blockage ratios, shapes, number and scale. Tests were carried out with methane, propane, ethylene and hydrogen mixtures with air.
A correlation was developed and applied in this research to predict the position to maximum intensity of turbulence downstream of an obstacle, xmax dimensionalised with obstacle scale, b as a function of obstacle blockage ratio, BR, using steady state experiments from the limited available data in the literature as,
( ) for t/d < 0.6 (thin/sharp obstacles)
A clearly defined separation distance which gave the most severe explosions in terms of both maximum flame speed and overpressure was found in this research. The profile of effects with separation distance agreed 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 further downstream than the position of maximum turbulence determined in the cold flow studies. It is suggested that this may be due to the convection of the turbulence profile by the propagating flame, after the flame has moved passed the obstacle. The predicted model on position to maximum intensity of turbulence from cold flow data agreed with the worst case obstacle separation distance in the current research if multiplied by a factor of three.
Turbulence parameters were estimated from pressure differential measurements and geometrical obstacle dimensions. This enabled the calculation of the explosions induced gas velocities, r.m.s turbulent velocity, turbulent Reynolds number and Karlovitz number. By expressing these parameters in terms of turbulent combustion regimes, the bulk of the tests in this study was shown to be within the thickened-wrinkled flames regime.
Turbulent burning velocity, ST models with dependence on obstacle scale, higher than the ones in the existing gas explosion scaling techniques were obtained as,
for single hole-obstacles
for single flat-bar obstacles
From the newly obtained ST correlation for single flat-bar obstacles, an overpressure correlation, P for scaling relationship was derived and validated against both small and large scale experimental data and the results were encouraging.
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In planning the layout of new installations, it is appropriate to identify the relevant worst case obstacle separation in order to avoid it. In assessing the risk to existing installations and taking appropriate mitigation measures it is important to evaluate such risk on the basis of a clear understanding of the effects of separation distance and congestion. The present research would suggest that in many previous studies of repeated obstacles the separation distance investigated might not have included the worst case set up, and therefore existing explosion protection guidelines may not correspond to worst case scenarios.