Static charge on plastic piping in Cl 2 Div 1

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winslowfam

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Hi...we have a client who is transporting an metal abrasive dust product that is left over from a blasting process for preparing metal surfaces for painting. They have decided to use schedule 160 PVC 6" piping for abrasive resistance to convey this product. The problem is that the area around the system dust collector is rated Cl 2 Div 1; and conveying through PVC piping develops static charge. We can wrap a copper braided conductor around the outside of the piping and bond that to ground, but what about the charge on the inside of the piping? Any ideas?
 
If the dust is abrasive, it will chew through PVC. A possible solution, albeit expensive, would be the use of steel tubing. It may reduce wear, and provide a solid grounding means.

Some installations use Stainless Steel to achive a higher wear resistance.

They should be consulting a materials handling vendor. There are numerous products, some have a spiraling, or lattice type ribbing on the interior of the tubing to reduce friction wear.
 
They were going to use a heavy steel but it would have been way to heavy for normal maintenance. They expect to replace these pieces on a regular basis. Plus it is already installed, from what we have heard. Can you think of a bonding method in a classified area for the interior of the PVC piping - if it is even a static producing situation - since the outside would be bonded to ground, it may not develop a static charge on the interior from the material flow?
 
winslowfam said:
Hi...we have a client who is transporting an metal abrasive dust product that is left over from a blasting process for preparing metal surfaces for painting. They have decided to use schedule 160 PVC 6" piping for abrasive resistance to convey this product. The problem is that the area around the system dust collector is rated Cl 2 Div 1; and conveying through PVC piping develops static charge. We can wrap a copper braided conductor around the outside of the piping and bond that to ground, but what about the charge on the inside of the piping? Any ideas?
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You have a problem not only on the outside, but on the inside as well unless the conveying means is pressurized nitrogen or other inert gas. What you describe is somewhat strange, since you are talking about 'metal abrasive dust'. Metal, should be conductive itself and should not be building up static charge.

Static discarges can occur even with metallic piping. Raising the humidity, slowing the speed of transfer, transporting it as liquid slurry, ionization are some of the methods the static buildup can be controlled.
 
who is worried about the potential for static build up?

there are calculations that can be made to determine whether static buildup is likely or not.

if it was me, I would be using metal pipe for this application since they know it will wear out due to abrasion, and it will last much longer than pvc.

most of the wear occurs at elbows or other changes in direction. material handling people sell special metal fittings that have extra thick pads at these points that can be periodically replaced.

if they insist on plastic, and insist on grounding it, about all you can do is run a bare copper wire down the pvc pipe to dissipate whatever static buildup there might be there.
 
Any granular material transported through a pipe will generate static electricity. You can not ground a non-conductor, such as plastic pipe unless it is "static dissipating", which is uncommon. Static will occur both on the inside and on the outside of the pipe, possibly resulting in very large (HIGH voltage) sparks! Best to refer to NFPA 77 and consult an expert. Well intentioned grounding efforts may make the situation far worse, since you can increase capacitance and make the sparks even more intense.
 
thank you for your comments. Who would you consider an expert who we could talk with to develop a grounding approach for this piping? A piping vendor told us to wrap the exterior of the piping with bare copper braided cable its full length and then bond that to ground. Any help is appreciated.
 
winslowfam said:
thank you for your comments. Who would you consider an expert who we could talk with to develop a grounding approach for this piping? A piping vendor told us to wrap the exterior of the piping with bare copper braided cable its full length and then bond that to ground. Any help is appreciated.
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There are a fair number of companies that sell this type of stuff that might be able to help you. maybe google dust collector?
 
sgunsel said:
Any granular material transported through a pipe will generate static electricity.
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I discovered this the hard way when I dropped a bag of flour in the kitchen, which, of course, covered the floor in flour. When I vacuum'd a big glob of it up I got static shocked to hell and back. Was a total surprise, I had no idea...
 
If you wrap the pipe with a grounded conductor, you create a very big capacitor that will not stop charging inside the pipe, but increase it. What is the metal. Hopefully it is not aluminum or other reactive metal, which could be an explosion hazard as well. I think you need to go with grounded metal pipe and beef up the corners with extra heavy pipe or use conductive rubber where the direction changes. If there are any combustible materials inside the pipe, you must consider expolsion prevention/venting, especially at any dust collector.
 
the material being conveyed is what is left on the floor after steel shot is used in a blast application to prepare large metal surfaces for coating. The steel shot will be vaccumed up off the floor along with all of the material blasted off of the large pieces of metal being prepared for coating, so there is metal dust as well.
 
winslowfam said:
Hi...we have a client who is transporting an metal abrasive dust product that is left over from a blasting process for preparing metal surfaces for painting. They have decided to use schedule 160 PVC 6" piping for abrasive resistance to convey this product. The problem is that the area around the system dust collector is rated Cl 2 Div 1; and conveying through PVC piping develops static charge. We can wrap a copper braided conductor around the outside of the piping and bond that to ground, but what about the charge on the inside of the piping? Any ideas?
Click to expand...

I had an evil thought.

Is this a bag type collector? You may well have more an issue with it than the piping.
 
Spend some time digging throught the latest versions of NFPA 654 and NFPA91. It will at least help you weed out the vendors that know nothing.
 
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