(13h) Limited Applicability of Gas Explosion Venting Due to Turbulence Generation

There is a general lack of knowledge on the effects of turbulence on gas explosion venting of vessels at elevated initial pressure. Moreover, with increasing technical complexity - e.g. biogas or hydrogen plants - requirements for safety increase the effort of gas explosion protection substantially. In particular if turbulence is generated, e.g. by flow over obstacles, there is little data on gas explosion development.

An important step during the design of venting devices for gas explosions in enclosures is the calculation of the vent area [1,2]. Widely accepted rules and most sizing methodologies are based on the maximum pressure rise (dp/dt)_max and/or according to the cubic law the K_G-value (1), which are proportional to the vent area and depend significantly on whether turbulent combustion exists [3]. However, standards or technical recommendations, like NFPA 68 or EN 14994, are still limited to ordinary apparatus and boundary conditions. Therefore, designing pressure relief devices is still a scientific challenge.

K_G = (dp/dt)_max • V^1/3 (1)

Hence, the purpose of the present investigation was to gain a deeper knowledge of the influence of full-bore obstacles or initial pressures on explosion venting. Here, a systematic study was performed to investigate the influence of turbulence on the overpressure development during gas explosions using a 6 and/or 86-litre-autoclave and different methane-air mixtures at 1, 2 and 5 bar. The latest research finding includes results of hydrogen-air-mixtures at elevated initial pressure, where a turbulent gas explosion venting supports the deflagration to detonation transition (DDT).

Turbulence inducing obstacles and over sized vent areas lead to enhanced pressure development [4]. A comparison of K_G-values for explosions with initial pressures between 1, 2 and 5 bar with and without obstacles, shows that in all cases the turbulence caused by the obstacle increases the K_G-value by a minimum of 10 times compared to the laminar case. Preliminary results showed that current venting guidance is unsuitable for predicting overpressures in these systems, due to them exceeding the condition of K_G < 550 bar m/s. This research will enable the derivation of design criteria for emergency relief systems for gas explosions under various boundary conditions in complex geometries.

[1] Bartknecht W., Explosionsschutz : Grundlagen und Anwendung. Berlin, Heidelberg: Springer-Verlag, 1993.

[2] Razus D. M., Krause U., "Comparison of empirical and semi-empirical calculation methods for venting of gas explosions," Fire Safety Journal, vol. 36, pp. 1-23, 2001.

[3] Steen H., Handbuch des Explosionsschutzes: Wiley-VCH, 2000.

[4] Poli M., Grätz R., Blanchard R., "Venting of turbulent gas explosions," in 13th International Symposium on Loss Prevention 2010. Brügge, Belgien, 2010.