An Experimental Study of the Influence of Major Damage of Flame Gap Surfaces in Flameproof Apparatus on the Ability of the Gaps to Prevent Gas Explosion Transmission
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Electrical apparatuses for use in the presence of explosive gas atmospheres have to be specially designed to prevent them from igniting the explosive gas. Flameproof design implies that electrical components producing incendiary electrical sparks, e.g. relays and switches, be contained in enclosures that not only withstand the maximum pressure of an internal gas explosion. In addition any holes or slits in the enclosure wall have to be designed in such a way that they will not transmit a gas explosion inside the enclosure to an explosive gas atmosphere outside it. Designs of a variety of flameproof enclosure joints, including plane flanged joints, are specified in detail in international standards (IEC) requiring that the maximum permissible average roughness of any flame gap surface has to be < 6.3 μm. The standards also require that any damaged joint surface has to be restored to the original quality prescribed in standards (IEC). However, the standards do not provide any guidance as to what level of damage is considered significant. As a result even minor mechanical or corrosive damage of flame path surfaces gives rise to expensive overhaul and repair of flame proof apparatuses. In fact, this is mandatory in spite of the fact that a generous safety factor is included in the requirements to maximum permissible gap widths. For example, for the plane-flange configuration and explosive gas (propane) used in the present investigation, the maximum permissible width in a practical apparatus is only 0.4 mm, whereas the real limiting value is 0.92 mm. The purpose of the present investigation has been to obtain some experimental guidance as to what level of damage of flame gap surfaces is required to significantly reduce the flame-proofing effect of flame gaps in flameproof electrical apparatuses. The maximum experimental safe gap (MESG) of an explosive gas mixture is the largest gap width between the two parts of a circular plane joint of 25 mm breadth in a standardized test, which prevents transmission of a gas explosion on the inside of the gap to an outside explosive gas mixtures. Normally the purpose of MESG experiments is to compare MESGs of different gases and vapours, using the same smooth flame gap surface in all experiments. However, in the present investigation MESG has been used as a parameter for judging whether various kinds of significant damage of the gap surface had any noticeable effect on the ability of the flame gap to prevent flame transmission. A significant reduction of MESG compared with that obtained with a standard undamaged surface (standard roughness of < 6.3 μm) would mean that the particular type of damage under test had destroyed the gap efficiency significantly. On the other hand a significant increase of MESG compared with that for the undamaged surface would mean the damage had in fact significantly increased the gap efficiency. In the experiments performed in the present work premixed 4.2 vol. % propane in air was used as the test gas mixture in all the experiments. Two different apparatuses were used, viz. a plane circular-flange apparatus (PCFA) and a plane rectangular-slit apparatus (PRSA). For both apparatuses the optimal distance between the ignition point and the gap entrance for flame transmission was 14 mm. Consequently this distance was used in all the experiments. The flame gap surfaces were damaged mechanically by...