How to discipate or discharge Static Electricity Safely

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Huntxtrm

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We are installing electrical systems, LV PLC systems, in cement plants. They are having problems with static electricity build up in the tanks systems. They run a bunch of dry product through the piping systems, and it builds up quite a bit of static electricity. They have been having problems with the static electricity blowing the loadcells in the scale system, as well as making some employees rather uncomfortable(they have been getting zapped as well). What would be the best way to discharge or discipate the static electricity. I am bonding around the load cells with straps to connect the lower frame with the upper tank. Should these tanks be solidly grounded to earth as well? Should there be arials installed? What are you fellas thoughts on this? And of course the customer want this to be on the cheap.
 
basically you have to tie all the metal parts together. run a big wire around to all the pieces of equipment and tie all the isolated pieces of metal together.

I would start with the stuff that they are having the worst problems with.

unfortunately, there are often a lot of pieces of isolated metal that can get charged up, especially if there is dry air flowing along or through the equipment.

I was in a plant once where they solved their static problem in one area with sprinklers. they put them on timers and ran them for a few minutes every hour in areas that had static problems. static electricity does not coexist well with humidity. it was a little odd seeing sprinklers hosing down tanks and piping periodically.
 
It has been awhile like over 10 years. So I may be rusty. This was a plant with machines for manufacturing the read/write heads for disk drives. The parts were extremely sensitive to static . There was a separate grounding system all the way back to earth for static dissipation. Every surface had to be conductive and tied to an isolated buss which was tied to the separate building buss. A separate and distinct wire even for covers and doors. This was to save the product. I think for what you need is just really good grounding for all the metal that builds up charge. If you have nonmetallic gaskets separating piping and tanks then all the pieces need to be grounded. If the tanks are not metal ahh big trouble. I also seem to recall pretty big wire , something about static charge dissipating on the surface.
I don't know or remember if for what you need you could tie it to the EGC or not
 
6sunset6 said:
I don't know or remember if for what you need you could tie it to the EGC or not
Click to expand...

the key is to make sure all the metal pieces are tied together, and yes they should be tied to the EGC as well.

plastic or lined pipe is sometimes difficult to deal with static wise. I have seen people install metal mesh or bare wire on the outside of non-conductive pipes. there are also conductive finishes that can be added to the outside of plastic parts if necessary.


there are also static eliminator devices (Exair makes some for instance) that you can sometimes use.

IMO your best first stab at this is to bond the worst offenders together with a big wire to the local EGC and see if it works. maybe run a 2/0 or so sized wire around to all the equipment skids and then bond it to each of the equipment skids and tie all the pieces that are isolated to that point.

this is one of those things that may be pretty simple and cheap or might be really pricey.
 
The wire only has to be big enough that it will not be damaged. A path of 1,000,000 ohms or less will prevent the static build-up.
 
What you fellas are saying, is kind of what I had in mind. I am intending on bonding across the load cells with braided bonding straps. That should direct the static away from the loadcells. I may have to start bonding across the "VIC's" to completely discipate it. Here is a nother notion that puzzles me. We run all of our LV in GRC, which is tied to each and every tank in the system via strut, and strut straps, welded onto stands protruding from the tanks. Why is that not discipating the static? It is a comlplete conduit system point to point. Maybe if I effectively ground that, in a few places, it would discharge the static?
 
I was up 1/2 the night trying to think of the word braided. That's what was used on doors and covers.
Maybe for the flexibility rather than a lot of small wires. What was really interesting was Lexan covers. You can get Lexan with an ESD (Electrostatic Dissipating ) coating. The only way to tell is it is printed on the paper that is stripped off. It is really funny to see plastic doors and covers with a ground strap on them.
I think adding grounds in a few places is the way to start. I don't have a technical backround for this , just what I have seen. Static is a surface charge and has to be drained from the surface. I think the closer the drain( ground) to the surface with the problem , the better.
 
6sunset6 said:
I think the closer the drain( ground) to the surface with the problem , the better.
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Since static electricity is a high voltage, extremely low current phenomenon (except for lightning, of course), the shortness or thickness of the drain conductor is not really important, except to the extent that it also affects RF noise or energized surface issues.
If the static charge is on top of an insulating surface, the only ways to deal with it are either providing a conductive coating or screen or increasing the conductivity of the air using a humidifier or ion generator.
 
NFPA 77 is your friend. It is not a standard, per say, but a recommended practice.

The key is to eliminate the static before it is generated, if at all possible. Comprehensive grounding, ionized air if you can localize the generation, etc...

Avoid non-conductive piping, rubber boots coupling rigid structures together are especially problematic.
 
Lost_RFTech said:
NFPA 77 is your friend. It is not a standard, per say, but a recommended practice.

The key is to eliminate the static before it is generated, if at all possible. Comprehensive grounding, ionized air if you can localize the generation, etc...

Avoid non-conductive piping, rubber boots coupling rigid structures together are especially problematic.
Click to expand...

I like that approach, find out what exactly is the source of the static energy and then tackle that problem instead of trying to fight all the symptoms.
 
Lost_RFTech said:
NFPA 77 is your friend. It is not a standard, per say, but a recommended practice.

The key is to eliminate the static before it is generated, if at all possible. Comprehensive grounding, ionized air if you can localize the generation, etc...

Avoid non-conductive piping, rubber boots coupling rigid structures together are especially problematic.
Click to expand...

The KEY is actually understanding HOW static electricity is generated. In the case of dry material transport the generation method is called the 'separation' during the transport or even in the rotary kiln as the material tumbles and falls away from the buildup as it rotates. As it was mentioned, elevated humidity is a good way to minimize static buildup, but in this case - where the objective is to heat and then keep the cement dry - the injection of steam into the kiln would be counterproductive. The rotary kiln itself is insulated due to the fire-brick lining, so that gives another barrier to the solution. The construction of the kiln itself can be modified where the mortar used to attach the bricks to the lining is infused with conductive carbon or otehr high temperature metallic particles, or embedded metallic tinsels that extends from the conductive wall into the material/fire chamber. INternally, ionizers can be inserted into the combustion air supply lines. The dry line transports are usually operated by pneumatic transfer, in those cases the transporting air can be ionized.
Externally, humidity control by atmoizing nozzles or steam injection intot he air works well. The kiln shell should be grounded at the rollers with brushes. A simple berillyum/bronze strap will do. Transport pipelines should be intentionally grounded at every suport point or if there are long spans a grounding wire can be wound along the entire length of the pipe and connected to the pipe with a grounding strap at every 10'-15'.
 
You probably cannot avoid generating static electricity in this type of operation. Grounding/bonding conductors for static dissipation are usually sized for mechanical integrity - currents are very low and physical damage is often a major concern. As stated above, 1 million ohms is usually adequate to control static buildup and is standard practice. In practice, 100 million ohms may suffice. Yes, there should also be an electrical connection to ground. Otherwise the bonded mass can build up sufficient electrical charge to arc to ground (earth), possibly through electronic gear.

Non-conducting (high resistivity) materials cannot be effectively grounded or bonded, and wrapping ground wires/screens/plates/etc on or around them may make the situation much worse by constructing a capacitive storage system that will permit higher voltages and more severe static discharges.

Dry cement transferred into a large storage tank (or silo) will likely result in a highly charged non-conducting mass in the tank that cannot be readily controlled and will be subject to surface discharges, which may without aggressive means. If the tank is electrically isolated, as it may be if on load cells, the voltage will increase until it can flash over the isolated portion to the nearest ground. A large tank would represent a large capacitance, and damage to sensitive electronic would be a given under proper circumstances. At minimum, a good electrical connection should be made between the tank supports, presumably metal, and the base of the load cells, also presumably metal, to minimize damage from electrical discharge. Additional precautions may also be required.

Also as mentioned above, get a copy of NFPA 77 and go through it thoroughly. Unlike many standards, this recommended practice has much explanatory material that will help you deal with the issues in a rational way. There are many examples which are similar to what you are dealing with. Good luck.
 
Yes, let's add sprinklers to a cement plant. NEXT .... :D

No, the 'tinsel' is the best you can do. Dangle a bunch of very-well grounded, very fine wires near the product, to 'capture' static and bleed it off. Do this at the END of the run.

I say "end," because the static is playing a role in keeping the powder / air mix in a 'fluid' state. No static = feed problems.
 
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