Page 88 - Dust Explosions in the Process Industries

Dust Explosions:An Overview 6 1

e Generally, the processed batches of the powder should be kept as small as feasible.
0 Use of additivesthat suppressthe decompositiontendency may be helpful in some cases.
Section 9.3.5.2in Chapter 9 gives furtherreferences to works on self-heatingand smoldering.

1.4.2.3
Open Flames and Hot Gases

Most potential ignition sources of the open flame type can be avoided by enforcing ade-
quate organizational procedures and routines. This, in particular, applies to the prohibi-
tion of smoking and other use of lighters and matches and to the enforcement of strict
rules for performing hot work. Hot work must not be carried out unless the entire area
that can come in contact with the heat from the work, indirectly as well as directly, is
free of dust, and hazardous connections through which the explosion may transmit to other
areas have been blocked.
Gas cutting torches are particularly hazardous, because they work with excess oxygen.
This gives rise to ignition and primary explosion development where explosions in air
would be unlikely.
In certain situations in the process industry, hot gaseous reaction products may entrain
combustible dust and initiate dust explosions. Each such case has to be investigated
separately and the required set of precautions tailored to the purpose in question.
Factory inspectors in most industrialized countries have issued detailed regulations for
hot work in factories containing combustible powders or dusts.

1.4.2.41
Hot Surfaces

As pointed out by Verein deutscher Ingenieure (1986), hot surfaces may occur in indus-
trial plants both intentionally and unintentionally.The first category includes external sur-
faces of hot process equipment, heaters, dryers, steam pipes, and electrical equipment.The
equipment where hot surfaces may be generated unintentionally include engines, blow-
ers and fans,mechanical conveyors, mills, mixers, bearings, and unprotected lightbulbs.
A further category of hot surfaces arises from hot work. One possibility is illustrated in
Figure 1.10. During grinding and disk cutting, glowing hot surfaces are often generated,
which may be even more effective as initiators of dust explosions than the luminous spark
showers typical of these operations. This aspect has been discussed by Muller (1989).
A hot surface may ignite an explosible dust cloud directly or via ignition of a dust layer
that subsequently ignites the dust cloud. Parts of glowing or burning dust layers may
loosen and be conveyed to other parts of the process, where they may initiate explosions.
It is important to realize that the hot surface temperature in the presence of a dust layer
can, due to thermal insulation by the dust, be significantly higher than it would normally
be without dust. This both increases the ignition hazard and may cause failure of equip-
ment due to the increased working temperature. The measures taken to prevent ignition
by hot surfaces must cover both modes of ignition. The measures include
@ Removal of all combustible dust before performing hot work.
vention or removal of dust accumulations on hot surfaces.
@ Isolation or shielding of hot surfaces.