Page 599 - Dust Explosions in the Process Industries

566 Dust Explosions in the Process Industries

As discussed in Section 8.3, dust particles that enter an enclosure through narrow
unintentional holes and gaps do not accumulate as an explosive cloud of particles sus-
pended in the air but settle out as dust layers on the internal surfaces of the enclosure.
Furthermore, it does not normally represent great difficulties to design dusttight enclo-
sures to satisfy the requirements of the enclosure standard IP 6X, as defined in Section
8.4.1. In comparatively large apparatus enclosures, such as instrument cabinet, with
doors and windows fitted with rubber gaskets and locking arrangements,it may be dif-
ficult to completelyprevent ingress of dust. However, the transmissionof explosive dust
clouds from the outside the enclosure to its interior, through possible narrow gaps and
holes, can be excluded.
A further considerationis that any such larger enclosure that is embracedby an explo-
sive dust cloud becomes covered by dust very soon after the appearance of the cloud,
and regular dust removal from the exterior is required. Therefore, one hesitates to locate
such enclosures in zones 20 and 21 in the first place,just from the point of view of keep-
ing the workplace clean and tidy.
A related concern is that with dusts, as opposed to gases, it is impossible for humans
to perform any installation,maintenance, or repair work inside the explosive atmosphere,
that is, inside a dust cloud having dust concentrations in the explosive range.
The IEC (2001b) Ex“pD’ standard for dusts seems, to a large extent,to be an edited copy
of the IEC (2001d)Ex‘p” pressurization standard for gases and vapors.This is evidentfrom
the scope of the dust standard, which defines the purpose of pressurization as “to prevent
the entry of dust which might otherwise lead to the formation of a ‘combustiblemixture’
within the enclosure.” In the context of this standard, the term combustible mixture means
an explosive dust cloud. Furthermore, in Section 6 of the standard, dealing with temper-
ature limits of hot surfaces inside the enclosure, the overall task is said to be “to ensure
that, if pressurization ceases, any explosive dust atmosphere which may exist [inside the
enclosure], is prevented from making contact with that surface before . . .”A final exam-
ple is found in the introductionto Section7.5in the standard,which is concernedwith guard-
ing “against the possibility that electrical apparatusprotected by pressurizationmay cause
an explosion (inside the enclosure) in the case of failure of the protective gas supply.”

8.4.5.4
The Self-contradiction of the Dust Pressurization Standard

IEC Ex“pD” standardis self-contradictorybecause it rests on the erratic assumptionthat
dust particlesin a dust cloud embracingan enclosure,by enteringit though unintentional
narrow holes and gaps, can accumulate as an explosive dust cloud inside the enclosure.
As discussed in Section 8.3, if significant quantities of dust particles enter an enclosure
at all, they accumulate as a layer not as an explosive cloud. According to the philoso-
phy of the current international area classificationstandards (see Section 8.2), the inte-
rior of a reasonably dusttight nonpressurized enclosure is therefore to be regarded as a
nonhazardous (safe) area.
If the self-contradictioninherent in the new standard is to be resolved, focus must be
shifted from the hazard of dust explosionsto the hazard of smolderingor open dust fires,
which can occur in layers and are also clearly undesirable.But, within the context of the
“Atex lOOa” Directive and the current area classification standards (see Sections 8.1.3