Static Electricity + Powder Feed

[Rich]

What effect would a build up of a static electric charge in a powder feed line and hopper have on a fine ceramic powder?

I have found an old Metco 4MP hopper lying around and had a play with it out of curiosity. It has a varying powder feed rate (nominally 50g/min). When doing manual powder feed rate checks* a static charge builds up in the powder feed line and is transmitted to the person holding the hose.

What effect would this have on the powder, and could it be causing the variation in feed rates? A nominal feed rate of 50g/min might vary +/- 10g/min, when tested over a 1 min period.

I would be interested if anyone has experienced any variations due to static charges before, and how they were dealt with. Thanks!


* Hopper on, torch off, powder fed into a pot of known mass for 1 minute and change in mass measured, i.e. g/min.

[Gordon]

Hi Rich

First, the 4MP system relies on metering the powder via differential pressure between hopper and carrier gas and not by mechanical means. This makes powder feed checking using the old methods of collecting powder in pots or bags unreliable. Any changes in back pressure from the powder powder port will effect the pressure differential between hopper and carrier gas and lead to changes in powder feed rate.

The most accurate and reliable way is to measure powder feed rate under actual spraying conditions. Measure weight of hopper + powder before spraying and then after spraying. Time period may need to be longer than 1 minute though to even out effects of start up and shut down. You can also use this method to accurately check feed rate during actual job processing.

Static charges I think could cause problems, other than operator discomfort possibly with powder feed pulsing. Difficult to confirm this though, as pulsing can also occur without noticeable static build-up. Tried winding wire along length of hose and connecting to earth, but don't remember any real positive effect on feeding characteristics. If the hopper is building charge, an electrical contact to earth should be cure.

[Rich]

Hi Gordon, thanks for your reply.

Interesting comment about collecting powder in a pot and weighing being unreliable. I'm not sure how this would cause back pressure in the feed line which would cause the feed rate to vary. Can you elaborate please? The powder pot we use has a hole to allow powder in and also a filter to allow gases to escape without releasing powder from the pot.

Still, weighing the powder pot would be useful if I could have the whole setup on a balance and could keep a visual tab on rate of change of mass. It might allow me to pinpoint exactly when the feed rate is varying and make it easier to identify the reason/s for it...interesting idea!

As for the static charge, earthing the equipment (again!) would be the simplest solution. It will either stop the powder pulsing or not! At the least it should improve operator comfort. Thanks for the ideas.

[Gordon]

Hi Rich

Interesting comment about collecting powder in a pot and weighing being unreliable. I'm not sure how this would cause back pressure in the feed line which would cause the feed rate to vary. Can you elaborate please? The powder pot we use has a hole to allow powder in and also a filter to allow gases to escape without releasing powder from the pot.

The 4MP relies on differential pressure between hopper and carrier gas to control feed rate. Anything that effects this differential pressure, possible like different back pressures imposed from the outlet powder port in spraying process or powder feed checking in pot or bag may effect feed rate. Also, very important consideration, do you check feed rate with powder port attached? I think you will find feed rate will be higher when port is not attached.

I know many use the collect in pot method and not really experience any problems, but I think its good to know the method has it flaws. Anyway, I like to know how much powder is actual going through the spraying process as a posed to how quick I can fill a pot

[LEN WOOD]

Hello Rich,

I believe that you are using an F4 gun? Since the F4 was designed for use with the Twin 10 feeder you may be unable to achieve everything perfectly. With the 4MP I believe that the powder hose/line is orange for this type of powder? Maybe you are using the Plasma Technik supposedly 'static free' powder hose/line or?

As Gordon mentioned the injector/powder port should be attached for accurate feed rate assessing. But you will find it more difficult to catch all the powder in the measuring pot with this arrangement. As Gordon also stated this method is precarious and a reliable/accurate setting is not easily or fully attained. Due to supply, conditions & other day-to-day variations regular checking is required to maintain approximate settings.

Why not invest in a second hand unit that has a scale or a Twin 10?
Time is money!

[Arina]

When an excess of either positive or negative charge is confined into a relatively small volume (far from any excess charge of the opposite polarity) mutual repulsion occurs between the charges within that volume. This repulsion causes the charges to try to leave the confining volume and fly apart, releasing energy. This energy is available to do damage by sparking. If two neutral but dissimilar materials are rubbed together, causing one to become + and the other -, there will be very little repulsion energy available on either object while they are close together.