Page 164 - Dust Explosions in the Process Industries

Dust Explosions: An Overview 137

Scherrer (1984) and Wehland (1984) discussed prevention of self-ignition in dust deposits
and explosions in dispersed dust in plants for the production and storage of pulverized
coal, by inerting with combustion gases, nitrogen, or carbon dioxide.
The overall dust explosion protection of coal pulverizing plants was discussed by
Birolini and Sammartin (1979), Wibbelhoff (198l), Diliberto (1983, Garini and Hules
(1987), and Dansk Brandvaerns-Komitk (1987). Fire and explosion protection of sys-
tems for the conveyance and storage of pulverized coal was treated by Korner (1984)
and Ghauvin et al. (19871, whereas dust removal from pulverized coal plants was con-
sidered by Parpart (1979). Mullinger (1987) was concerned with fire and explosion grs-
tection of pulverized firing systems, and Egesoe (1978) discussed dust explosion
prevention in systems for preparing and burning coal dust in cement kilns. Patzke (1984)
considered venting of dust explosions in plants for milling and drying coal.
Finally, Ruygrok et al. (1983) were concerned with the prevention and mitigation of
coal diust explosions in surface facilities for the transport, storage, and handling of coal.
The possibility of gas explosions due to release of methane from the coal, in particular
from ,anthracites,was also investigated.

1.5.3.5
Polyester and Epoxy Powders for Electrostatic Powder Coating

Electrostatic powder coating, to an increasing extent, is replacing traditional liquid paint
spraying systems for painting industrial metal products. The basic principle is that the
metal object is first covered with an even layer of electrostatically bound epoxy/poly-
ester powder. By subsequent treatment in an oven, the powder melts and hardens to an
even, strong protective, decorative coating.
In the actual process, the powder is transported pneumatically from a powder hopper
to an electrostatic spraying gun. As the powder particles flow through the spraying gun,
they become electrostatically charged by passing a strong electrostatic field on the order
of tens of kilovolts. The charged particles are then attracted to and deposited on the
grounded workpiece. The powder continues to be deposited on the grounded workpiece
until, at a certain powder layer thickness, the layer acts as an insulator and prevents fur-
ther deposition of powder. Powder not deposited on the workpiece is normally collected
in a powder recovery unit by a dust extraction system.
As technology developed and knowledge increased, the overall concepts of pre-
venting and mitigating dust explosions in electrostatic powder coating systems were
revised periodically. An early summary was given by Eckhoff and Enstad (1975). One
of the preventive measures recommended was to keep the dust concentration in the
spraying boot lower than the minimum explosible concentration. In a later paper, Liere
(1983) omitted this possibility, concentrating instead on inerting, automatic flame
extinction, and isolation. Bartknecht (1986) and Liere (1989) conducted realistic full-
scale explosion experiments in a powder spraying cabin and showed that dust flames
in cloiilds of concentrations just above the minimum explosible concentrations are weak
and s1,ow. Bartknecht and Liere also determined ignitability and explosive properties
of typical polyester and epoxy powders used for electrostatic powder coating. Ec
Pedersen, and Arvidsson (1988) were unable, in a subsequent investigation, to repro-
duce the lowest minimum explosible dust concentrations of 15 g/m3 reported by
artknecht. In view of the fact that the minimum explosible concentration of typical