Page 485 - Dust Explosions in the Process Industries

452 Dust Explosions in the Process Industries

geometrical similarity even of vent areas, was not fully supported by the experiments.
However, as long as the dust clouds were generated in similar ways in all three vessel
sizes and the ignition points were at the vessel centers, the experiments were in agree-
ment with the law A, =Al(V21V,)o~52.
Pineau, Giltaire,and Dangreaux (1978) presented a seriesof experimentallybased cor-
relations for various dusts between vent area and vessel volume for open and covered
vents, with and without vent ducts. Both bursting membranes and spring-loaded and
hinged vent covers were used in the experiments.
Zeeuwen and van Laar (1985) and Wingerden and Pasman (1988) studied the influ-
ence of the initial size of the exploding dust cloud in a given vented enclosure on the
maximum pressure developed during the vented explosion.
The investigationshowed that the pressure rise caused by the explosionof a dust cloud
filling only part of a vented enclosureis higher than would be intuitively expected. Even
if the dust cloud is considerably smaller than the enclosure volume, it is usually neces-
sary to size the vent as if the entire volume of the enclosure were filled with explosible
cloud.
Gerhold and Hattwig (1989) studiedthe pressure developmentduring dust explosions
in a vented steel silo of rectangular cross section. The length-to-equivalent-diameter
ratio could be varied between 2 and 6. The explosion pressure and flame front propaga-
tion histories were measured using a measurement system similar to that illustrated in
Figure 6.6. The influence of the key parameters of industrial pneumatic dust injection
systems on the explosion development was investigated, in particular injection pipe
diameter, airflow, and dust-to-air ratio. The general conclusion was that the maximum
pressures generated with realistic pneumatic injection were substantially lower than
those predicted by the VDI 3673 (1979 edition) guideline.

6.3
VENT SIZING PROCEDURES FORTHE PRESENT

AND NEAR FUTURE

6.3.1
BASIC APPROACH AND LIMITATIONS

As shown in Section 6.2, realistic vented dust explosion experiments,conducted mostly
during the 1980s,demonstrated that none of the vent sizing codes in use up to 1990 are
fully adequate. It is proposed, therefore, that for the present and near future, sizing of
dust explosionvents be primarily based on the total evidence from realistic experiments
that is available at any time.
The following suggestions presuppose that the initial pressure in the enclosure to be
vented is atmospheric. Furthermore,the vent covers must open completely within times
comparable to the opening times of standard calibrated rupture diaphragms. In the case
of heavier, and reversible, vent covers, such as hinged doors with counterweights or
spring-loaded covers, additionalconsiderations are required. The same applies to the use
of vent ducts and the new, promising vent closure concept that relieves the pressure but