Occurrence of cone discharges in production Silos

On-line Monitoring of Electrostatic Charge in Powder Pneumatic Transportation Process [Translated]^†

“…The Gauss's law calculates an electrical charge q, which is induced by the electrostatic field periodical change, q =ES£o (4) where E shows an electrostatic field. A current Is running through an electric resistance Rs which connects the electrode and ground is calculated as,…”

“…For example, in the process of powder pneumatic transportation, powders are tremendously charged due to the collision between powders and inside wall of the transportation pipe. In the case such charged powders are fed into a silo, a lightning discharge (atmospheric discharge) in the upper space of the silo or cone discharge along with the accumulated powder surface are frequently observed, which sometimes induce explosion and fire [4][5][6]. In order to prevent these hazards beforehand, continuous motoring of powder electrical charge is required.…”

“…So far, a Faraday cage [4][5][6] has been well used to statically measure the electrical charge of powders. However it is not available for the continuous measure- [7] has also been developed and applied for detecting the surface potential of a flat sheet of paper and nylon film.…”

Electric charge limits on settled powders

“…The type of discharge that occurs depends on the depth of the packed powder. For depths of the order of a meter, the total charge accumulated can trigger a type of electrical discharges known as cone discharges 11 in which the electrical breakdown of the surrounding air occurs through the formation of a conductive channel that carries the charge from the powder to its surroundings: typically, the walls of the silo holding the powder. 12 However, in the initial stages of the formation of heap of powder, there is no enough charge accumulated to form a conductive channel and it is expected that a corona discharge from the surface of the powder to the surrounding air takes place, as it happens in some situations in electrostatic powder coating.…”

Suppression of cone discharges in a powder silo using a grounded metal rod

“…The cone discharge, which can be incendiary to a flammable dust cloud, has been extensively studied experimentally and theoretically (Glor, Lüttgens, Maurer, & Post, 1989;Glor & Schwenzfeuer, 1997;Ohsawa, 1999). Preventive methods for a cone discharge, however, are still being developed, although some static chargeneutralizing devices have been introduced (Kodama, Suzuki, Nishimura, Yagi, & Watano, 2002;Kodama, Yamaguma, Suzuki, Mogami, & Watano, 2005;Mogami, Suzuki, Choi, & Yamaguma, 2008;Taillet, 1993).…”