Mixture Reactivity Effects on Explosion Venting

Free vented explosions were investigated for 10% methane, 4.2 and 4.5% propane, 6.5 and 7.5% ethylene, 30% and 40% hydrogen in a 10 litre cylindrical explosion vessel for vent coefficients of 4.3 and 21.7. The cylindrical vessel volume was 10L and had a diameter of 162mm and an L/D of 2.8. End ignition was used on the wall opposite the vent. The results are presented against KG and the laminar burning velocity as measures of the mixture reactivity. It is shown that the correlation of the KG effect by Bartknecht does not agree with other experimental data, although the hydrogen results are closer to the present results than the other gases. In contrast the laminar flame venting theory, as used in NFPA68 (2013), does correlate the data well, even though it is not supposed to apply to hydrogen explosions. There was evidence of very fast flames at the vent for hydrogen explosions. Acceleration of the flames towards the vent was demonstrated, due to the expansion of the burnt gases in the direction of the vent. The laminar flame venting theory that is used in NFPA68 (2013) over predicts the measured Pred due to the assumption of the vessel surface area as the area of the flame at Pred. It was shown that the flame arrives at the wall after the flame has vented the vessel and well after the time that Pred occurs. At Kv 4.3 the external overpressure was responsible for Pred, although the difference from Pfv was small for methane, propane and ethylene but for hydrogen the flow through the vent Pfv was the highest overpressure. At Kv = 21.7 the pressure loss due to the unburnt gas flow through the vent was the largest overpressure. For hydrogen sonic flow at the vent occurs and at high Kv sonic flow is predicted to occur using the laminar flame venting equation modified for sonic flow at the vent. Sonic flow at the vent is not taken into account in current venting guidance

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

Gas Explosion Venting: External Explosion Turbulent Flame Speeds that Control the Overpressure

In most vented explosions the peak overpressure is controlled by turbulent flame propagation external to the vent. This has been known for many years, but a method to predict the overpressure from the external flame speed has not been developed. Current vent modelling is based on the assumption that the unburned gas flow through the vent controls the overpressure and does not address the issue of the external explosion. This work shows that the external flame speeds in a small vented explosion test facility can be predicted from Taylors’s acoustic theory (1946). Vented explosion data is presented for vent coefficients from 3 – 22 for the most reactive mixtures of methane, propane and ethylene in terms of the overpressure and the external flame speed. The overpressure from Taylors’s acoustic theory give a good prediction of the measured overpressure