Fundamentals of dust explosion protection

Fundamental principles:The manufacturer of operating devices for areas rendered potentially explosive through dust must indicate the maximum surface temperature of all devices that dust can penetrate (usually expressed in °C - indication of the temperature class should be avoided here). This temperature is part of the dust Ex-designation.

Designation examples:

II 2 D T90 °C IP64, II 2 D Ex td A21 T90 °C IP64

 

(If the ignition protection type is based on the housing, thehousing protection 

rating should also be stated as an IP Code). or II 2 D Ex iaD 21T96 °C(This device has already been approved according to the new IEC de-jurestandard "Intrinsic Dust Safety - iaD". This de-jure standardspecifies that the designation also contains the corresponding zone - in thiscase 21)

Dust explosion protection - temperature:

Combustion and explosion parameters for dusts depend on the their condition.Parameters that affect combustion and explosion behaviour include particlesize, particle shape, water content, purity and where applicable the content ofthe flammable solvents.The particle size distribution and the mean value (value for average particlesize) should also be known.In accordance with 1999/92/EG Directive (ATEX 137, replacing: ATEX 118a),thesystem operator /employer is obliged to make a hazard assessment and musttherefore be aware of the minimum glow temperature of the dust.There are simple calculations to determine the two "temperatures" andthey are carried out thus:a) Limit temperature 1 = 2/3 of minimum ignition temperatureb) Limit temperature 2 = minimum ignition temperature* minus75 °KThese two limit temperatures must now be examined to confirm which guaranteesthe greater safety.

Example 1:

Minimum ignition temperature = +330 °C, minimum glow temperature = +300 °C:a) Limit temperature 1    =     2/3    x   +330°C     =     +220 °Cb) Limit temperature 2    =     +300 °C   - 75°K      =     +225 °CGreater safety: Limit temperature (1) = +220 °CHere a device with a max. surface temperature in the eventof failure <= +220 °C must be used. As stated, the device designation includes acorresponding value.

Example 2:

Minimum ignition temperature = +186 °C, minimum glow temperature = +180 °C:a) Limit temperature 1    =     2/3    x   +186°C     =     +124 °Cb) Limit temperature 2    =     +180 °C   - 75°K      =     +105 °CGreater safety: Limit temperature (2) = +105 °CHere a device with a max. surface temperature in the eventof failure <= +105 °C must be used.*The value for the glow temperature applies with a dust layer thickness of 5mm.The temperature safety distance must be increased for larger layer thicknesses.