Article,
Guidelines on the electrostatic hazards during pneumatic conveying and storage of flammable powders
Journal of Loss Prevention in the Process Industries, 4 (4): 211--216 (July 1991)
DOI: http://dx.doi.org/10.1016/0950-4230(91)85002-B
Abstract
A purpose-built industrial-sized metal silo facility was used to assess
the electrostatic hazards that may arise during the pneumatic conveyance
of a range of powdered products. This range embraced the insulating
powders, high density polyethylene (HDPE) and phenol formaldehyde
(resistivity >10$^13$ Ømegam); the moderately conducting products,
starch and sugar (resistivity 10$^9$Ømegam); and relatively conductive
wheat (resistivity 10$^7$ Ømegam). Pneumatic transport tests were
conducted and triboelectric charging was monitored. The incendivity
of electrostatic discharges in the receiving silo was assessed by
using an earthed probe shrouded in a propane---air environment. Numerous
non-incendive discharges were promoted by the probe. By increasing
the product charge levels artificially by using a charge-injection
device to simulate the levels of triboelectric charging that might
arise in some industrial circumstances, it was possible to promote
two propane---air ignitions at the probe, one from bulk HDPE and
one from the starch dust cloud. It was concluded that the risk of
discharges from bulked powder igniting the dust cloud depends on
product resistivity. For relatively conducting products of resistivity
$<$ 10$^9$ Ømegam no ignition risk was identified. For more insulating
products when resistivity> 10$^9$ Ømegam no risk was identified
if minimum ignition energy> 10 mJ. For discharges from the product
dust cloud there is no evidence from the silo project to contradict
the guidance in BS 5958.
