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In our factory, we have some large (8 meters tall and 3 meters in diameters) iron silos for storing the plastic compounding material. Every time the silos are getting unloaded from the granules, the plastic powders stick to the inner metal layer due to the static electricity, so that the operator needs to knock on the wall severely in order to make them fall. I want to know:

1- How can I instantly make this static load discharged and get rid of this problem (Right now, the resistance value between the earth system and Null is almost 0.03 Ohm) I would appreciate if you mention the standards and the detailed answers.

2- How can I measure the quality of the grounding system and what would be the standard range of this value?

Ben Aveling
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Farzin Karimi
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    How do you know that the static charge isn't being held in the center of the plastic granules, where it can't be discharged at all? – Ignacio Vazquez-Abrams Oct 03 '16 at 09:57
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    With assuming that the static charge is still held in the center of the granules, this mass finally reached the iron wall in one point. So if the charge of this point is discharged (**this point** gets earthed) the entire of this mass drops. – Farzin Karimi Oct 03 '16 at 10:20
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    The static charge is caused by friction and will be on the surfaces of the granules. – Leon Heller Oct 03 '16 at 10:24
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    The plastic granules are charged *with respect to the silo*, the earth resistance is irrelevant. Can you raise the humidity in the silo to make the air 'conductive' with respect to the small amount of charge involved? – Neil_UK Oct 03 '16 at 10:24
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    I think Neil's onto something with humidity ... blow warm humid air through the silo. But you need to carefully control the humidity or the particles will clump due to water's viscosity instead of electrostatic charge... –  Oct 03 '16 at 10:34
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    Any humidity will cause additional problems. This air cannot reach the depth of this huge material (8 Tons). I addition, since the granules are the first material to enter the production line, any impurity is a trouble. – Farzin Karimi Oct 03 '16 at 10:51
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    I wonder which polarity the charge is and whether an air ioniser would help? – pjc50 Oct 03 '16 at 10:58
  • If humid air causes other problems, can you blow ionized air though it? – winny Oct 03 '16 at 11:32
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    I googled "powder storage static charge". I found a bunch of answers, including a 1992 paper entitled "Guidelines for the control of static electricity in industry". If this is your job, you need to research it. Downvoted your question because you didn't even JFGI. – Graham Oct 03 '16 at 12:53
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    My reflection is.. why bother? Don't you fill the silos with identical granules next time anyway? – pipe Oct 03 '16 at 13:20
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    This isn't a good place to be inventing 'novel' industrial processes. Find someone competent with the relevant experience - with professional insurance too. – Sean Houlihane Oct 03 '16 at 13:39
  • If your present solution is to manually knock the side of the silo then install vibrators to do it automatically – You'll Delete This As Always Oct 03 '16 at 21:08
  • @IgnacioVazquez-Abrams How we know that the charge isn't held in the centers of the plastic granules is that we have certain assurances from physics that the charges are actually on the surfaces of the granules. :) – Kaz Oct 04 '16 at 05:30
  • @Graham I have already studied some articles including the one you have mentioned.They have led me no-where . Although I don't care about your vote, I gotta mention that one of our Chemical engineers have developed a kind of special material in nano-scale which has reduced the static charge to a great extent. However, I have intended to solve it entirely and electrically. – Farzin Karimi Oct 04 '16 at 19:05
  • @You'llDeleteThisAsAlways This is my ultimate priority. Thank you by the way. – Farzin Karimi Oct 04 '16 at 19:06
  • @Kaz I guess you have misunderstood him ( IgnacioVazquez-Abrams ). He means this charge is being held inside a mass (= too many) of the granules not inside each of them. So that there is no touch with the tank wall. – Farzin Karimi Oct 04 '16 at 19:11

6 Answers6

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If your plastic pellets are getting charged, you can flood the inside of the silo with ionized air using a commercial generator. These are not uncommon in the plastics industry.

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Spehro Pefhany
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    Is the mouse pointer a subliminal hint to make us upvote this answer? :) – pipe Oct 03 '16 at 14:13
  • @pipe My screen grab software captures the mouse pointer from time to time, so it's entirely coincidental, definitely. ;-) – Spehro Pefhany Oct 03 '16 at 14:21
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    Can't.. resist.. following.. mouse.. pointer :P – slebetman Oct 03 '16 at 16:08
  • I was wondering why my mouse wasn't moving haha. – Kevin Oct 03 '16 at 16:38
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    I think the mouse pointer is pointing to a down arrow on my screen... – Alex Oct 04 '16 at 03:14
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    Huh? The mouse pointer is usually a hardware sprite these days, so it's not in the frame buffer at all and so shouldn't be captured by a frame buffer grab! Maybe you have a "mouse trails" feature enabled that causes cursors to be drawn into the frame buffer. – Kaz Oct 04 '16 at 05:34
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    @Kaz its blatantly off-topic, you are right but screen capture APIs and programs often have an option to still capture the cursor, and do so by drawing the cursor sprite on the captured picture, example [here](http://stackoverflow.com/questions/6750056/how-to-capture-the-screen-and-mouse-pointer-using-windows-apis) – dlatikay Oct 04 '16 at 08:49
  • @SpehroPefhany Thank you for this suggestion. Some body else has also brought it up. This would be a good idea. However, I gotta do more researches on it. Thank you. – Farzin Karimi Oct 04 '16 at 19:20
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You can't easily do what you ask since the problem is charge gets built up on the plastic particles. Since they are all enclosed in one uniform conductor, it won't matter what the voltage of this conductor (the tank wall) is. There will still be charges on the plastic particles relative to the tank wall. Changing the voltage of the tank wall will only effect the electric fields between the tank and the ground and other things outside the tank. It won't do anything to the electric field inside the tank.

If you really wanted to counter the attractive force between the plastic particles and the metal wall, you'd have to introduce a different conductor into the tank and drive it with a voltage relative to the tank wall. One polarity will drive the particles more towards the wall, and the other will drive them away from the wall. Experimentation is the easiest way to find the right polarity.

However, even if you put a narrow cylinder in the middle of the tank and drove it with the appropriate voltage so that the particles fly off the wall, you'll have the same problem as now they'll be stuck to the center cylinder.

A different strategy would be to reduce the charges built up on the plastic particles in the first place. Plastic can shed or grab electrons as it is rubbed against other material. This is exactly what is happening as the tank is drained. Since the plastic is a good insulator, those charges stay on the particles for a long time.

The first possibility that comes to mind is to increase the humidity in the tank during draining operations. The plastic still sheds or grabs electrons, but the humidity makes surfaces a little more conductive so that these charges bleed off faster. Whether adding humidity is feasible, and whether it decreases the charge bleed time sufficiently is something you have do decide.

There are also various chemicals that essentially coat surfaces with something at least a little conductive. Again, whether you can tolerate those is something you have to decide.

Olin Lathrop
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  • Thank you for the precious comment, What I get from your reply is that **the static charge on a thick enclosed metal object cannot be discharged through the grounding system**, am I right? So the question is, how basically an enclosed iron container is grounded in the electrical standards? is the grounding system nonfunctional in our case? – Farzin Karimi Oct 03 '16 at 11:25
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    @Farz: As I said, the voltage of the container relative to things around it, and therefore the grounding, have nothing to do with this issue. – Olin Lathrop Oct 03 '16 at 11:52
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    @Farz: Like Olin indicates the potential of the metal enclosure has nothing to do with this situation. The movement of electrons from the plastic to the wall or from the wall to the plastic ( do not know the polarity) is causing the static field between the container and the plastic. – Decapod Oct 03 '16 at 12:32
  • You are right. If we put a vertical cylinder inside the metal silo, then what kind of voltage (AC or DC) and to what magnitude do you think it should be charged? Would it be an appropriate solution? – Farzin Karimi Oct 03 '16 at 12:40
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Ionized air is frequently used for static control in industrial processes. If you simply google for "ionized air static control", you'll find many commercial products.

Dave Tweed
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Plastics have the tendency of giving up or collecting electrons (depends on the kind of plastic) and become positively or negatively charged. If this charge is disturbing a proces then you need to treat the plastic by adding surface conductivity thereby preventing this kind of behaviour. Such an additive could be moisture or anti-static spray. Such an anti-static spray is often made up from a soap based material dilluted in a solvent (mild alcohol). A fire retardant is then added to combat flammability. The plastic has now become conductive and as long as the coating is not disturbed it will be difficult to generate static electricity in this material.

If additives in whatever form are not possible then it might be a solution to make the plastics adhere to a place where the unloading of the silo takes place. In that case you are not changeing the plastics and even make use of the fact that they are charged. I would have to see more for further assistance.

Decapod
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  • Thank you dear Decapd. The fact is adding any additional solvent or material to this compound is strongly forbidden since it results in impurity in the produced material after the molding and all the process. Despite, we cannot prepare this huge amount of solvent (each silo contains 8 Tons of plastic granules!). The humidity also brings additional troubles (in packaging and the quality of the final product). – Farzin Karimi Oct 03 '16 at 10:46
  • See my edited answer – Decapod Oct 03 '16 at 11:22
  • I did, I am looking forward to your ideas, – Farzin Karimi Oct 03 '16 at 11:28
  • Your comment is not clear. – Decapod Oct 03 '16 at 12:33
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What about if a metal rod was added to the centre of the tank? If that rod was charged with an AC potential, that went both below and above the outer wall's ground potential, then you may get what you are looking for. Not very much - I'm not trying to electrocute anyone - but if the attractive force changed between the tank and the rod then the plastic would in turn be attracted to the tank wall and the rod. Given the dimensions involved, it woudn't take long for gravity to win overall, and have the plastic fall to the output feed.

John Burger
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Ionized air seems to be the best solution to your problem. Moisture is verboten for plastic moulding granules and chemical additives, ammonium acetate in alcohol, as used in many anti static sprays would not be tolerated also, fire risk, etc. Delivery of ionized air into silo during discharge may have some issues, pellet dispersal, ozone production etc. Further thought. Recirculating portion of pellet discharge through deionizer and back into hopper would reduce problems with ecessive ozone and reduce amount of ionization required during discharge process.

graham foulkes
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