Effect of Equipment Placement in a Flameproof Enclosure

The main purpose of this research was to simplify the process of pressure testing flameproof enclosures to allow for some degree of flexibility in the layout of internal components without the need for retesting. Because of the cost and time spent in the testing of a flameproof enclosure, manufacturers are constantly seeking to gain flexibility with their product.

Present testing of flameproof enclosures in accordance with AS/NZS 60079.1:2005 determines the pressures to which the enclosure may be subjected in the event of an internal ignition. Simtars is involved in testing and certification of flameproof enclosures and is one of only two testing stations in Australia accredited to undertake flameproof testing. The standard covering this testing leaves it to the discretion of the testing authority to ensure they find the highest explosion pressure possible in an enclosure.

The objective of the project was to investigate the effect of equipment placement within a flameproof enclosure and define limitations on such placement. By providing guidelines in regard to the limitations within which component placement can be changed or additional equipment included, the whole process of testing flameproof enclosures will be simplified.

Testing was carried out using cubic and elongated enclosures and gave results that varied depending on whether the components were in block form or thin plate form (i.e. mounting plate). This meant that the limitations required to avoid high pressures developing in the enclosures were different to cover both situations.

To satisfy testing to Clause 15 of AS/NZS 60079:1:2005 and be able to ensure that testing the enclosure empty represents the most severe conditions for explosion pressure development, the following limitations had to be applied. The apparatus within the enclosure may be placed in any arrangement, provided that an area of at least 10% of each cross-sectional area remains free to permit an unimpeded gas flow and, therefore, unrestricted development of an explosion. Separate relief areas may be combined provided that each area has a minimum dimension in any direction of 20 mm. However if a thin mounting plate is used it must provide an area of at least 30% of each cross-sectional area free to permit unimpeded gas flow, and , therefore the unrestricted development of an explosion.

Some limitations had to be placed on the enclosures covered by this finding due to the limited testing undertaken. Enclosures are limited to Group I application only and they are to be of basically simple geometry of only square or rectangular cross-section and no major dimension shall exceed any other major dimension by more than 3:1 and shall have an internal volume when empty no greater than 0.5 cubic metres.

These limitations do not preclude existing limitations which apply due to other requirements of the standard, AS/NZS 60079.0:2005 and AS/NZS 60079.1:2005.