Page 655 - Dust Explosions in the Process Industries

622 Dust Explosions in the Process Industries

9.3.7
PROTECTIVEAND MITIGATORY MEASURES

9.3.7.1
Various Measures

Lunn (1999) and Lunn et al. (2001) discussed important additional aspects requiring con-
sideration when implementing various concepts for dust explosion protection in prac-
tice. Examples are the use of dejector plates for guiding blasts and flames expelled
from dust explosion vents in a safe direction, accountingfor vent cover inertia andpres-
sure losses in vent ducts when assessingrequired vent areas, and essential considerations
when complex systems of process equipment connected by pipelines or ducts are to be
protected. Siwek (2001) presented new German guidelines for dust explosion venting
at large and special guidelines for protecting fluidized bed installations.

9.3.7.2
Full Confinement by Using Explosion-Pressure-ResistantProcess Equipment
An outline of this concept is given in Section 1.4.5in Chapter 1.The applicabilityof the
concept is limited due to high equipment costs. However, the method is used in some
special cases, for example, when the powder or dust is highly toxic and reliable con-
finement is absolutelynecessary. Current experimentalmethods allow sufficiently accu-
rate prediction of maximum explosion pressures in simple vessels with point source
ignition. It should be pointed out that closed-bomb explosion test data for elevated ini-
tial pressures may be of limited value for predicting maximum explosion pressures in
the case of complex dynamic pressure development, such as in pressure-pizing situations.
There is considerable room for further improvement in the design of pressure-resistant
process equipment,with respect to minimizing its heaviness.The German concept of pres-
sure-shock-resistantdesign should be developed further. Crowhurst (1993b) has a useful
overview of the state of the arton design of equipment to withstand a given internal over-
pressurecausedby a fully confinedor vented explosion.Bartenev,Gelfand,and Frolov (1993)
developed a mathematical model for the failure of a process vessel subjected to an internal
exothermic,comparatively slow, process creating an overpressure by which the maximum
initial velocities of fragments or vent covers may be estimated. Bartenev et al. (1994)
extendedthis work to the case where the pressurized vessel is filled with dust. Experiments
revealed that the presence of dust can have a significanteffect on the pressure development
inside the bursting vessel and on the kinematic parameters of the ejected fragments.
Design of protective measures to handle dust explosions at elevated initial pressures
requires special consideration. Garcia-Torrent and Menkndez (1993) found that the pro-
portionalitybetween initial and finalpressurefound previouslyin smd laboratory-scaleves-
sels (see Section 1.3.8in Chapter 1)also holds in vessels of 1 m3volume, at least up to 12
bar initial pressure. In later investigationswith various coals, Dennison et al. (1995) and
Conde-Lazaroand Garcia-Torrent(1998,2000) confirmed the proportionalitybetween ini-
tial pressure and maximum explosion pressure in closed vessels up to 20 bar initial pres-
sure. This proportionality was also confirmed for biomass dusts by Rautalin and Wilen
(1996)and Garcia-Torrent et al. (1998).Pilao,Ramalho,and Pinho (2002) studiedthe influ-
ence of the initial pressure of clouds of cork dust in airon the maximum explosion pressure