Page 96 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 69

unable to detecthazardous oxygen levels in regions where they are likely to occur.Wiemann
(1989)recommended that the maximum permissible oxygen content in practice be 2-3 ~01%
lower than the values determined in standard laboratory tests (see Chapter 7 and TableA.2
in the Appendix).
Various types of oxygen detectors are in use. The fuel cell types are accurate and fast.
However, their lifetime is comparatively short, of the order of 6 months-1 year, and they
operate only within a comparatively narrow temperature range. Zirconium dioxide detec-
tors are very sensitive to oxygen and cover a wide concentration range with hig
racy and fast response. They measure the partial pressure of oxygen irrespective of
temperature and water vapor. However, if combustible gases or vapors are present in the
gas, they can react with oxygen in the measurement zone and cause systematically lower
readings than the actual overall oxygen content, which can be dangerous. There are also
oxygen detectors that utilize the paramagnetic or thermomagnetic properties of oxygen.
Even these detectors are sufficiently fast and accurate for monitoring inerting systems
for industrial process plants. However, nitrogen oxides can cause erratic results.
Wiemann emphasized two limitations of the gas inerting method when applied to dust
clouds. First, as already illustrated by Figure 1.67, inerting to prevent dust explosions
does not necessarily inert against self-heating and smoldering combustion. Second, also
mentioned earlier, the use of inert gas in an industrial plant inevitably generates a risk
of accidental suffocation. The limit where significant problems start to arise is I§ ~01%
oxygen. If flue gases are used, there may also be toxic effects.
Fischer (1978) also mentioned several technical details worth considering when design-
ing systems for inerting of process plants to prevent dust explosions. He discussed spe-
cific examples of protection of industrial plants against dust explosions by gas inerting.
Heiner (1986) was specifically concerned with the use of carbon dioxide for inerthg silos
in the food and feed industry.
The actual design of gas inerting systems can take many forms. Combinations with
other means of prevention and mitigation of dust explosions are often used. Figure 1.73
illustrates nitrogen inerting of a grinding plant. More recent works on inerting are
reviewed in Section 9.3.6.1 in Chapter 9.
In Table 1.9, partial inerting, as opposed to the complete inerting discussed SO far, is
included as a possible means of mitigating dust explosions.This concept implies the addi-
tion of a smaller fraction of inert gas to the air than required for complete inerting. In this
way, the ig&on sensitivity, the explosion violence, and the maximum constant-volume
explosionpressure all can be reduced appreciably, which means a correspondingreduction
of the explosion risk. Partial inerting may be worth considering in combination with other
means of prevention or mitigation when complete inertingis financiallyunacceptable. More
recent works on partial inerting are reviewed in Section 9.3.7.4 in Chapter 9. See also
Section 1.3.6.

1.4.3.2
Dust Concentration Outside the Explosible Range

In principle, one could avoid dust explosions by running the process in such a way that
explosible dust concentrations are avoided (see Section 1.3.4). In practice, however,
this is difficult in most cases, because the dust concentration inside process equipment
most often varies in unpredictable and uncontrollable ways.