Page 139 - Dust Explosions in the Process Industries

7 12 Dust Explosions in the Process Industries

1.4.9
PREVENTION AND REMOVAL OF DUST ACCUMULATIONS
OUTSIDE PROCESS EQUIPMENT: GOOD HOUSEKEEPING

1.4.9.1
General Outline

The main prerequisite for disastrous secondary explosions in factories is that sufficient
quantitiesof combustible dust have accumulated outside the process equipmentto permit
development of large secondary dust clouds (see Section 1.1.3). In other words, the pos-
sibility of extensive secondary explosions can be eliminated if the outside of process
equipment and shelves, beams, walls, and floors of workrooms are kept free of dust.
Significantquantities of dust may accumulate accidentallyoutside process equipment
due to discrete events, such as bursting sacks or bags or erratic discharge from silos or
filters. In such cases, it is important that the spilled dust be removed immediately. In case
of large dust quantities, the main bulk may be sacked by hand using spades or shovels,
but industrial, explosion-proof vacuum cleaners should be used for the final cleaning. In
the case of moderate spills, dust removal may be accomplishedby vacuum cleaning only.
Effective dust extraction should be provided in areas where dust occurs as part of
normal operation, such as bagging machines.
Considerable quantities of dust can accumulate outside process equipment over time
due to minor but steady leaks from process equipment. The risk of such leaks is com-
paratively large if the working pressure inside the process equipment is higher than the
ambient pressure, whereas running the process at slightly lower than ambient pressure
reduces the leaks.
It is important that process equipmentbe inspectedregularly to discover and seal off obvi-
ous accidental leak points as early as possible. However, often one has to accept a certain
unavoidable level of dust leaks from process equipment.It is then important to enforcegood
housekeeping routines by which accumulationsof explosible dust outside process equip-
ment are removed at regular intervals, preferably by explosion-proof vacuum cleaning.
Use of compressedair to blow away spilled dust should be prohibited. By this method,
dust is not removed, only transferred to another location in the same room. In addition,
dust explosions can result if the dust concentration in the cloud so generated is in the
explosible range and an ignition source exists in the same location.

1.4.9.2
Industrial Explosion-ProofVacuum Cleaners

The subject of industrial explosion-proof vacuum cleaners was discussed by Kuhnen
(1978b), Wibbelhoff (1984), and Beck and Jeske (1989). Beck and Jeske listed the
requirements for mobile type 1vacuum cleaners recommended in the Federal Republic
of Germany for removal of combustible dusts:

The fan must be on the clean side and protected against impact by foreign bodies.
The electric motor and other electric components must satisfy the generalrequirements
for such components to be used in areas containing explosible dusts. Motors must be
protected against short circuit and overheating.