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profile of a dust explosion
|3. Maximum rate of pressure rise
|4. Secondary explosion risks
A dust explosion, when a dust cloud is ignited within a confined space, generate a pressure increase that develops quickly to reach a maximum pressure, then the pressure slowly releases.
The pressure profile will depend on the material generating the explosion.
It is necessary to know the characteristics of explosion of a particular material in order to protect the process, especially through pressure release systems or suppression systems.
The maximum pressure recorded during a powder explosion is called Pmax.
Measuring how fast the pressure increases during a dust explosion is critical to correctly design protection measures like explosion panels or suppression system, which must act before the pressure reaches unsustainable levels.
The pressure increase rate is generally represented by a constant Kst,, different for every material, and which can be measured experimentally by making powder explosed in instrumented pressure resistant vessels. The maximum rate of pressure rise can be measured on the pressure vs time graph (see above) and then Kst calculated knowing the volume V of the test vessel :
(dP/dt)max = Kst.V-0.33 [Laurent]
Kst is expressed in bar.m/s
Depending on the value of Kst, powders can be classified in classes
St1 : 0 < Kst < 200
St2 : 200 < Kst < 300
St3 : 300 < Kst < 600
It must be kept in mind that a 1st explosion may actually triggers other. Indeed, the pressure shock and burnt particles can travel through pipes to another process equipement like a silo and trigger an explosion there. It is through this mechanisms that some grain silos were downed altogether.
As well, the pressure shock can put in suspension dust layers that was covering floor or equipment in a production area that was not well house kept and then trigger another explosion that could damage the building.
Please find below some Pmax and Kst data given in the literature. WARNING : these are general values given without guarantee, a risk assessment and design must ALWAYS refer to the MSDS of the ACTUAL product used for tests carried out specifically on the ACTUAL material by a reputable institute.
The determination of this value is difficult thus sometimes a range
is given when different research results have been reported. As
results reported may have been obtained following different
protocols, not always precised, it adds to the care needed when
using those values. Always consult the references given in
brackets for more precisions.
Table 1 : Pmax and Kst of common materials
|Material||Pmax bar||Kst bar.m/s|
||8 [Rhodes]||97 [Rhodes]|
||3.4 [Mills]||17 [Mills]|
||9.6 [Rhodes]||202 [Rhodes]|
||10.3 [Rhodes]||202 [Rhodes]|
||8.8 [Rhodes]||106 [Rhodes]|
||6.6 [Mills]||190 [Mills]|
||6.5 [Mills]||270 [Mills]|
||5.4 [Mills]||510 [Mills]|
||6.2 [Mills]||480 [Mills]|
||6.1 [Mills]||340 [Mills]|
||6.8 [Rhodes]||151 [Rhodes]|
||6.4 [Mills]||250 [Mills]|
||3.4 [Mills]||120 [Mills]|