Vented gas explosions

Explosion venting technology is widely accepted as the effective constructional protection measures against gas and dust explosions.The key problem in venting is the appropriate design of the vent area necessary for an effective release of the material i.e. the pressure developed during explosion did not cause any damage to the plant protected.Current gas explosion vent design standards in the USA (NFPA68, 2002) and European (2007) rely on the vent correlation first published by Bartknecht in 1993 (Siwek, 1996).N FPA 68 also recommends the correlation of Swift (Swift,1983)at low overpressures. For a vent to give no increase in overpressure other than that due to the pressure difference created by the mass flow of unburnt gases through the vent, the vent mass flow rate is assumed to be equal to the maximum mass burning rate of the flame and this consideration should be used as the design mass flow through the vent. Two different methods ( Method I and Method 2) have been proposed based on the Sμ and Sμ (E-1) to describe the maximum mass burning rate given as, mb = ASμpμ=CdeA(2pPμred)o.5 mb =ASgPm =AgSμ(E-I)P μ=Cde4,(2pu Pred)0,5 (2) The equation given in (2) is slightly different from (1) as is about 6.5 times the mass flow of the first method as it takes the effect of (E-1) where E is the expansion ratio. A critical review were carried out for the applicability, validity and limitation on the venting correlations adopted in NFPA 68 and European Standard with 470 literature experimental data, covering a wide range of values for vessel volume and geometries, bursting vent pressure, Pv L/D ratio, maximum reduced pressure, Pred and ignition location. The fuels involved are methane, propane, hydrogen, town gas, ethylene, acetone/air mixtures with the most hazardous near-stoichiornetric fuel-air concentration. Besides, Molkov's equation (Molkov, 2001) which is regarded as alternative venting design offered in NFPA 68 and Bradley and Mitcheson's equation for safe venting design were also analysed on the experimental data for their validity and limitation as well as the proposed methods. From the results, it is clear that Bartknecht's equation gave a satisfactory result with experimental data for K <-5 and Swift's equation (Swift, 1983) can be extended to wider range for Pred> 200 mbar, providing the parameter PV is added into the equation. Method 2 gave a good agreement to most of the experimental data as it followed assumptions applied for correlations given by Bradley and Mitcheson for safe venting design (Bradley and Mitcheson, 1978a,B radley and Mitcheson, 1978b). It is also proven that the vent coefficient, K is confident to be used in quantifying the vessel's geometry for cubic vessel and the use of As/Av term is more favourable for non-cubic vessels. To justify the validity and applicability of the proposed methods, series of simply vented experiments were carried out, involving two different cylindrical volumes i.e. 0.2 and 0.0065 M3. It is found that self acceleration plays important role in bigger vessel in determining the final Pmax inside the vessel. Method 2 gave closer prediction on Pmax in respect with other studied correlations. The investigation of vented gas explosion is explored further with the relief pipe been connected to the vessel at different fuel/air equivalence ratios, ignition position and Pv. The results demonstrate that the magnitude of Pmax was increased corresponding to the increase of Pv- From the experiments,it is found that peak pressure with strong acoustic behaviour is observed related to increase in Pv and in some cases,significant detonation spike was also observed particularly in high burning velocity mixtures. It is found that substantial amount of unburnt gases left inside the vessel after the vent burst is the leading factor in increase of Pmax for high burning velocity mixtures at centrally ignited. The associate gas velocities ahead of the flame create high unburnt gas flows conditions at entry to the vent and this give rise to high back pressures which lead to the severity in final Pmax inside the vessel. It was observed that end ignition leads to a higher explosion severity than central ignition in most cases, implying that central ignition is not a worst-case scenario in gas vented explosions as reported previously.