Page 609 - Dust Explosions in the Process Industries

576 Dust Explosions in the Process Industries

As a rule, enclosures designed according to the IP 6X or 5X requirements are fully
satisfactory for preventing hazardous ingress of dust.

8.6.3.3
Molding

A special dust standard for enclosures sealed by molding seems unnecessary. The stan-
dard proposed by the IEC (2002f) is, to a large extent, an edited copy of the correspon-
ding gas standard, IEC (2002g). The basic European enclosure standard for dusts
(CENELEC, 1998a) contains a paragraph on “materials used for cementing,” which
may be expanded to cover any additional aspects of encapsulationby molding that may
be relevant for dusts.

8.6.3.4
Flameproof Enclosures

This concept, as understood in the context of explosive gases, is not applicable to dusts.

8.6.4
INTRINSIC SAFETY

In some very special cases, there is a genuine need for intrinsically safe apparatus even
in environments containing combustible dusts. Electric spark ignition sensitivities of dust
clouds, expressedin terms of their minimum ignition energies,vary substantially,across
at least 8 orders of magnitude. Therefore, substantial differentiationof current and volt-
age limitation requirements for intrinsically safe apparatuses is essential in an intrinsic
safety standard for dusts. Such differentiation is missing in the current IEC (2002h)
draft standard. However, experimentalconservativeMIEs for dusts, obtained by current
standard test methods, constitute an obvious basis for adequate differentiation.
Conservative “ignition curves” can be estimated from the MIE using only simple
theory or empirical correlation. Hence, for capacitive circuits, conservative safe combi-
nations of circuit parameters are obtained by satisfyingthe criterion 0.5CU2< ME. For
inductive circuits, correspondingconservative safe combinationsare given by the crite-
rion 0.5L1’ < MIE, as long as L > 1mH. For resistive circuits, the simple empirical rela-
tion I < 15,000MIE0.451/U2was shown to apply to the available data.
There is a need for a standard method for experimental determination of the MIE for
dust clouds in the important energy range below 1 mJ, which is not covered by typical
standardtest apparatuses.Aresearch program aimed at developingsuch a method is cur-
rently being conducted in Norway.

REFERENCES

“Atex 100a” Directive. (1994)Directive 94/9/EC of the European Parliament and the Council, March
23, 1994, on the approximation of the laws of the member states concerning equipment and pro-
tective systems intended for use in potentially explosive atmospheres.