Effect of Size of Ignition Energy on the Explosion Behaviour of Selected Flammable Gas Mixtures Effect of Size of Ignition Energy on the Explosion Behaviour of Selected Flammable Gas Mixtures

The determination of explosion indices of flammable gases is an important part of explo - sion prevention. Explosion indices could be influenced by initial temperature, initial pressure, humidity, ignition energy and others. This contribution deals with the effect of ignition energy size on explosion indices of flammable gases. For experimental mea -surements, three flammable gases were chosen—methane, propane and hydrogen. The chapter introduces the measurement results of the gas explosion parameters at various sizes of the ignition energy using explosion autoclave VA-20. Conclusions include the evaluation of influence of the ignition energy size on particular explosion indices.


Introduction
The explosion could happen wherever fuel, oxygen and sufficient ignition source appear together. The ignition energy is really significant. An explosive mixture is not ignited unless energy of the ignition source is sufficient. The ignition energy with a value of 10 J is used by default for the determination of explosion parameters of gases and vapours of flammable liquids. If the mixture is not ignited under given experimental conditions, it does not mean necessarily that the examined mixture is not explosive. When a higher ignition energy is used, the mixture could be ignited and high explosion indices could be reached (Mynarz et al., 2012).
Besides ignition source-the standard (EN 1127-1, 2011) defines 13 groups of ignition sources-duration time also matters. The explosion range is more wide with increasing size of the ignition energy-the lower explosive limit (LEL) is decreasing and the upper explosive limit (UEL) is growing. Table 1 introduces the effect of the ignition energy on the methane explosive limits.
Increasing ignition energy affects the explosive limits but also increases maximum explosion pressure and the maximum rate of explosion pressure. The effect of the ignition energy is significant especially at the rate of explosion pressure rise.
The most common ignition sources suitable for measurement of explosive limits and explosion indices are inductive spark, chemical (pyrotechnic) igniter and fuse wire. Efficiency of each mechanism is different; various results could be reached with the above-mentioned ignition sources. This is manifested by results of measurements of the effect of initial pressure on hydrogen explosive limits using nickel fuse wire and electric spark, see

Tested samples
For experimental measurements of the effect of the ignition energy size on explosion indices, three samples of gases were chosen-methane, propane and hydrogen. Parameters of particular gases are shown in Table 2 (Conrad and Kaulbars, 1995).

Experimental setup
The explosion autoclave VA-20 was used for experimental measurement of the effect of the ignition energy size on gas explosion indices. The setup is made for determination of the explosion indices of dust, gases and hybrid mixtures. The volume of the experimental double-coat chamber is 20 L (Kuhner Safety). Figure 2 presents the scheme of the explosion autoclave VA-20.

Measurement results
Following chapters introduce the experimental results of measurement of explosion indices of methane, propane and hydrogen with air using the apparatus VA-20. The chemical igniter with ignition energies of 80, 160 and 240 J was used. The values of maximum explosion indices and lower explosive limit were determined in a range of minimum 0.5 vol.%.

Methane
Experimental results of the effect of the ignition energy on the explosion indices of methane are presented in Table 3 and Figures 3 and 4.       Table 5 and Figures 5 and 6 show experimental results of the effect of the ignition energy on the explosion indices of propane. Table 6 compares the maximum explosion pressure, maximum rate of explosion pressure rise and the lower explosive limit of propane for particular energies of ignition sources. Percentage changes related to the measurement with the lowest energy are also listed.    Table 8 compares the maximum explosion pressure, maximum rate of explosion pressure rise and the lower explosive limit of hydrogen for particular energies of ignition sources. Percentage changes related to the measurement with the lowest energy are also listed. (dp/dt) m (bar s −1 ) 2 3 9 ---261

Conclusion
While methane was measured, the rate of explosion pressure rise increased by 32.2% using double energy (160 J) and it increased by 35.5% using triple energy (240 J). Maximum explosion pressure increased by 5.5% using double energy (160 J) and it increased by 4.1% using triple energy (240 J). The lower explosive limit did not change at double energy (160 J). LEL decreased by 22.2% using triple energy (240 J).
While propane was measured, the rate of explosion pressure rise increased by 20.0% using double energy (160 J) and it increased by 23.6% using triple energy (240 J). Maximum explosion pressure increased by 1.2% using double energy (160 J) and it had not changed using triple energy (240 J). The lower explosive limit did not change at double energy (160 J). LEL decreased by 25% using triple energy (240 J).
While hydrogen was measured, the rate of explosion pressure rise decreased by 5.1% using double energy (160 J) and it increased by 2.1% using triple energy (240 J). Maximum explosion pressure decreased by 1.2% using double energy (160 J) and it had not changed using triple energy (240 J). The lower explosive limit increased by 14.3% at double energy (160 J). LEL did not change using triple energy (240 J).
According to experimental data, the inference was made that the size of ignition energy affects especially the rate of explosion pressure rise and the lower explosive limit. Its effect on explosion pressure is only minimal.