Re: Static Discharges from PE etc... (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Sat, 17 Jan 2004 13:51:25 -0800
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Static Discharges from PE etc... (fwd)

Similar circumstances occur when moving pellets of material through tubing,
especially if it's done by blowing them. This is where the spark that blows
up the grain or flour silo comes from. A friend who used to work in a bakery
said that they used to get flour explosions when the static dissipation
apparatus failed in the storage silos. Loud and muffled boom, overpressure
vent opens, etc.

I've had first hand experiences with incidental static charging along these
lines, one with a special effect rig that was blowing several cubic meters
of popcorn into a set, another with a machine that pulled and trimmed
lengths of plastic film off a bulk roller. The first generated very
impressive sparks from the blower to ground (or the operator (ouch!)), the
second glowed most impressively in the dark.

Vollrath, and others, have built electrostatic generators using such
techniques as blowing dust.

A friend once wondered if you could do it with steam.  Superheated steam is
a gas (insulating) and you could vent it through a condensing nozzle to form
small particles, which could carry charge, much like a Kelvin water dropper.
He was theorizing that it might be a way to power something floating at a
high voltage, among other things.

----- Original Message -----
From: "High Voltage list" <hvlist@xxxxxxxxxx>
To: <hvlist@xxxxxxxxxx>
Sent: Saturday, January 17, 2004 1:20 PM
Subject: Re: Static Discharges from PE etc... (fwd)
>
> ---------- Forwarded message ----------
> Date: Sat, 17 Jan 2004 14:45:51 -0500
> From: David Sharpe <sccr4us@xxxxxxxxx>
> To: High Voltage list <hvlist@xxxxxxxxxx>
> Subject: Re: Static Discharges from PE etc... (fwd)
>
> Hi Dan!
>
> High Voltage list wrote:
>
> > Original poster: Steven Roys <sroys@xxxxxxxx>
> >
> > ---------- Forwarded message ----------
> > Date: Fri, 16 Jan 2004 11:04:29 -0500
> > From: "Mccauley, Daniel H" <daniel.h.mccauley@xxxxxxxx>
> > To: High Voltage list <hvlist@xxxxxxxxxx>
> > Subject: RE: Static Discharges from PE etc...
> >
> > Dave,
> >
> > My first job out of school was with Anheuser-Busch working on their
> > bottling and packaging conveyor systems.
> > Typical practice is using static-dissipative polyethylene or similar
> > plastic which quickly dissipates any
> > charge built up on the material due to passing packages etc...  Although
> > the static-dissipative PE is much more
> > expensive than standard PE, it works extremely well.
> >
> > Dan
> > ----------------------------
>
> The PE material used was virgin grade, and used as a "wear surface".
Since
> PE has similar self lubrication characteristics as PTFE, improved conveyor
> material wear life was assumed.  Another reason why this situation
escalated
>
> into a major issue, the conveyor belt had metallic fibers in it to
> "dissappate"
> (read equalize) the charge.  Lets see, metal over metal with a PE
insulator
> the width of the converyor, 15 feet long (180") conveyor, with belt
> (electrode)
> width of 6" .  Assuming wear surface was 20 mil thick PE (phi ~2.0) and
> actual active capacitor area is 1/3 total would approach
>
> Cest(pf) = 0.224 * k * A / d
> C          =  0.224 * 2.0 * (6 * 180/3) / 0.02
> C           ~ 8000pF
>
> If you assume 15kVDC / inch (high humidity) environment, and 8" discharges
> (maximum);
>
> V = 8* 15  = 120kV.
>
> E = 0.5 * C * V^2;  with these values stored E is >50J...  < OUCH >
>
> Even if the actual capacitance is 1/4 what is represented here; clearly a
> dangerous and potentially (NO PUN) lethal condition exists.
>
> With the attractive force being presented to cartons and actual jamming
> (and damage) to upstream machines, I have no doubt that these values are
> close to what was seen in field...
>
> Regards
> Dave Sharpe, TCBOR/HEAS
> Chesterfield, VA. USA
>
> >
> > > HV List
> > >
> > > I ran into a very interesting "Van de Graff" type problem while
> > > working on package conveying systems about 12 years ago.
>
> <<SNIPP>>
>
>