Page 658 - Dust Explosions in the Process Industries

Research and Development 625

from propagation from one process unit to neighboring units (see Section 1.4.4 in
Chapter 1).The paper also provided some quantitative design guidance. Faber (1999)
discussedthe various types of active and passive explosionisolationtechnologies in use.
These can be grouped in three main categories: complete isolation devices, pressure wave
interrupters, and flame interrupters. For some applications,the different concepts can be
combined. Xu et al. (1996) investigated experimentally constructive measures, that is,
vents in the pipe wall and flame arrestors inside the pipe, for mitigating or interrupting
dust explosions in pipelines for pneumatic transport of coal powders.

9.3.7.4
Partial lnerting by Inert Gas

This relatively new, promising concept for mitigating dust explosions deserves further
attention. The idea is that, as the oxygen content in the atmosphere is decreased, there is
a systematic decrease of both ignition sensitivity and combustion rate of the dust cloud.
Some data illustratingthis are given in Section 1.3.6in Chapter 1.In some cases, the explo-
sion hazard may be reduced substantiallyby only a moderatereduction in the oxygen con-
tent. Section 6.6 in Chapter 6 provides a philosophical basis for reducing explosion vent
areas by partial inerting. More research seemsnecessary to establish correlationsbetween
the oxygen content in the gas phase and ignitability and explosibility parameters of dust
clouds. Some data were produced in thejoint European researchprogram CREDIT (1995),
focusing on the influence of oxygen content of the atmosphere on the minimum spark
energy and hot-surfacetemperature for ignition of dust clouds.
Zeeuwen et al. (1996) confirmed experimentally that even modest reductions in the
oxygen concentration in the atmosphere by partial inerting can increase the minimum
ignition energies of dust clouds substantially. The minimum ignition temperatures of
clouds and layers were found to be less sensitive to reductions in the oxygen content.
The effect of partial inerting on the explosion violence (important for full confinement,
venting, suppression, and isolation) was not investigated in this study. Glor and
Schwenzfeuer(1996,1999) provided further informationon the influence of the oxygen
content in the atmosphere on the minimum ignition energy of dust clouds. Devlikanov,
Kuzmenko, and Poletaev (1995) found that, for clouds of nutrient yeast dust in mixtures
of air and nitrogen, the explosionviolence index k,, was a linear functionof the percentage
of oxygen in the gas phase.
Schacke and Walther (1998) suggested that partial inerting may, in fact, also be a
useful concept in preventing gas and vapor explosions. In some cases, it may be possi-
ble, by using only modest quantitiesof inert gas, to reduce the ignitionsensitivityof the explo-
sive atmosphereto a level at which marginal potentialignitionsources,like electrostaticbrush
discharges and single metal sparks, are no longer able to cause ignition. Conde-Lazaro
and Garcia-Torrent (1998, 2000) carried out a series of partial inerting experiments at
12bar initial pressure, in a demonstrationPFBC pulverized coal power plant. The effects
of adding inert gas and inert dust (see below) were investigated.

9.3.7.5
ExplosionVenting

This widely used protective method is explained and discussedin Section 1.4.6in Chapter 1
and in Chapter 6.