Effects of significant damage of flame gap surfaces in flameproof electrical apparatus on flame gap efficiency

Electrical apparatus for use in the presence of explosive gas atmospheres has to be specially designed to prevent the apparatus from igniting the gas. Flameproof design is one of several options, and one requirement is then that any holes and slits in the enclosure wall be designed to prevent a possible gas explosion inside the enclosure from being transmitted to an explosive gas cloud outside it. Current standards (IEC) require that joint surfaces have a surface roughness of <6.3 μm. Any damaged joint surface has be restored to this quality. The present investigation has demonstrated that flame gap surfaces in flameproof electrical apparatuses can suffer considerable mechanical and corrosive damage before the flame gaps no longer function satisfactorily. In some cases very significant mechanical surface damage in fact improves the gap performance. This indicates that current high costs of repairing and replacing flameproof electrical apparatus in process plants offshore and onshore can be reduced considerably without any increase of the explosion risk.

► Sandblasting of gap surfaces gave no significant reduction of flame gap performance.
► Severe rusting of gap surfaces improved flame gap performance.
► Multiple milled parallel crosswise grooves in one gap surface improved flame gap performance.
► Multiple milled lengthwise grooves of 1 mm width and depth 4 mm improved flame gap performance slightly.
► One single lengthwise groove of width 1 mm and depth 4 mm had no influence flame gap performance.