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Dust Flammability – Part 2 of 3 in a Series on Explosions

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Dust Flammability

In this second part in our Series on Explosions and Flammability we will discuss Dust Flammability.

The dust pentagon below shows that there are five (5) elements necessary for a dust deflagration  to occur.

Dust Flammability

Dust Flammability Pentagon

  1. Combustible dust
  2. Oxidant
  3. Ignition source
  4. Dispersion of dust
  5. Confinement of dust

Although confinement is considered to be a fifth requirement; this is not an essential condition for a dust deflagration to occur.

Testing for Dust Flammability

The lower explosion limit, or Cmin, for dusts is given in grams per m3 of powdered material in a volume of air.  About 30 grams per cubic meter is the lower explosive limit for most flammable dusts. Typically, upper explosion limits are not defined or determined for dusts.

The “Explosion Severity Test” is a standard dust test used to quantify the maximum pressure of a dust cloud explosion (Pmax) and the speed of the pressure rise (KSt).

Explosion pressures are measured for a range of dust concentrations using piezo-electric pressure transducers. Explosibility testing in these chambers are performed per ASTM E1226, ASTM E1515 and ASTM E2931, as well as the EN 14034. The tests are carried out over three series to ensure a thorough investigation of the explosion properties. From the tests, the arithmetic mean of the maximum values (both maximum pressure and maximum rate of pressure rise) is obtained. The Kst value is calculated as the equivalent pressure in a 1 m3 sphere from the cube law (Kst value = cube root of volume x explosion pressure rise). The ST class is based on the Kst value as follows:
ST class 0 – Kst value = 0
ST class 1 – Kst value less than 200 bar m/sec
ST class 2 – Kst value between 200 and 300 bar m/sec
ST class 3 – Kst value greater than 300 bar m/sec

One of the best ways to prevent dust explosion is to minimize dust accumulations on equipment surfaces, floors and high beams.

References:

Dust Explosions in the Process Industries – Third Edition – Rolf K Eckhoff

Industrial Explosion Prevention & Protection – Frank T. Bodurtha

NFPA 68 – Standard on Explosion Protection by Deflagration Venting – Current Edition: 2018

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