Page 91 - Dust Explosions in the Process Industries

64 Dust Explosions in the Process Industries

extinction by precise injection of a small amount of extinguishing agent at a convenient
distancejust when the smolderingor burning nest or fragmentpasses the nozzles. Water
is the most commonly used extinguishing agent, and it is applied as a fine mist. Such
systems are used mostly in the wood industries but also to some extent in the food and
feed and some other industries. The field of application is not only to smoldering nests
but also glowing or burning fragments from, say, sawing machines and mills. Further
information is given in Sections 9.2.3.3,9.3.5.2, and 9.3.5.3 in Chapter 9.

1.4.2.6
Heat from Accidental Mechanical Impact

Mechanical impacts produce two different kinds of potential ignition sources, small
flying fragments of solid material and a pair of hot spots where the impactingbodies touch.
Sometimes, such as in rotating machinery, impacts may occur repeatedly at the same
points on one or both impacting bodies, and this may give rise to hot spots of apprecia-
ble size and temperature. The hazardous sourceof ignition then is a hot surface, and what
has been said in Section 1.4.2.4 applies.
When it comes to single accidental impacts, there has been considerable confusion.
However, research during the last decade has revealed that, in general, the ignition
hazard associated with single accidental impacts is considerably smaller than often
believed by many in the past. This applies,in particular, to dusts of natural organic mate-
rials, such as grain and feedstuffs, when exposed to accidental sparks from impacts
between steel hand tools like spades or scrapers and other steel objects or concrete. In
such cases, the ignition hazard is probably nonexistent, as indicated by Pedersen and
Eckhoff (1987). The undue significance often assigned to “friction sparks” as initia-
tors of dust explosions in the past, was also stressedby Ritter (1984) and Muller (1989).
However, if more sophisticatedmetals are involved, such as titanium or some aluminum
alloys, energetic spark showers can be generated, and in the presence of rust, luminous,
incendiary thermite flashes can result. Thermite flashes may also result if a rusty steel
surface covered with aluminum paint or a thin smear of aluminum is struck with a
hammer or another hard object. However, the impact of ordinary soft, unalloyed alu-
minum on rust seldom results in thermite flashes but just a smear of aluminum on the
rust. For a given combination of impacting materials, the incendivity of the resulting
sparks or flash depend on the sliding velocity and contact pressure between the collid-
ing bodies (see Chapter 5).
Although the risk of initiation of dust explosionsby accidental single impacts is prob-
ably smaller than believed by many in the past, there are special situations where the igni-
tion hazard is real. It would in any case seem to be good engineering practice to
Remove foreign objects from the process stream as early as possible.
Avoid construction materials that can produce incendiary metal sparks or thermite
flashes.
Inspect work processes and remove the cause of impact immediately in a safe way
whenever unusual noise indicating accidentalimpact(s) in process streamis observed.

Figures 1.70 and 1.71 show two examples of how various categories of foreign objects
can be removed from the process stream before they reach the mills.