Fundamentals of dust explosion protection
The manufacturer of equipment intended for use within areas that contain potentially explosive concentrations of dust and air must indicate the maximum surface temperature of the equipment in °C (prefixed with the letter ‘T’). This temperature is part of the dust Ex-designation.
- Ex II 2D Ex tb IIIC T90°C Db – alternatively Ex II 2D Ex tb IIIC T90°C
(Here the ignition protection type is based on the housing indicated with the letter ‘t’ in the code.)
- or Ex II 1D Ex ia IIIC T96°C Da – alternatively Ex II 1D Ex ia IIIC T96°C
(Here the ignition protection type is based on the intrinsic safety type of protection indicated with the letter ‘i’ in the code.)
Dust explosion protection - temperature
Combustion and explosion parameters for dusts depends on their condition specific properties. Parameters that affect combustion and explosion behavior include particle size, particle shape, water content, purity and where applicable the content of the flammable solvents. The particle size distribution and the mean value (value for average particle size) should also be known.
In accordance with Directive 1999/92/EG, the system operator / employer is obliged obligated to make a hazard assessment and must therefore be aware of the minimum dust cloud and layer ignition temperature of the dust being used.
There are simple calculations to determine the two "temperatures" and they are carried out thus (taken from EN/IEC 60079-14, clause 5.6.3):
a. Limit temperature 1 = 2/3 of minimum ignition temperature (dust cloud)
b. Limit temperature 2 = minimum ignition temperature minus 75 °K (dust layers up to 5mm)
(Note: for dust layers greater than 5mm standard EN/IEC 60079-14 should be consulted)
These two limit temperatures must now be examined to confirm which guarantees the greater safety.
a. Minimum ignition temperature (dust cloud) = +330 °C,
b. Minimum ignition temperature (dust layer up to 5mm) = +300 °C
a. Limit temperature 1 = 2/3 x +330 °C = +220 °C
b. Limit temperature 2 = +300 °C - 75 °K = +225 °C
Greater safety: Limit temperature (1) = +220 °C
Here a device with a max. surface temperature in the event of failure ≤ +220 °C must be used. As stated, the device designation ‘T‘ includes a corresponding value.
a. Minimum ignition temperature (dust cloud) = +186 °C,
b. Minimum ignition temperature (dust layer) = +180 °C
a. Limit temperature 1 = 2/3 x +186 °C = +124 °C
b. Limit temperature 2 = +180 °C - 75 °K = +105 °C
Greater safety: Limit temperature (2) = +105 °C
Here a device with a max. surface temperature in the event of failure ≤ +105 °C must be used.