Bonding conductors for static electricity dissipation

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oranjeep

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2002 NEC, Article 250.70

"...listed pressure conectors, listed clamps, or other listed means."

I am a consultant trying to save the client money
on grounding and bonding a pneumatic conveying system of zinc galvanized stainless steel tubing that conveys pharmeceutical waste dust and debris in a R&D facility. The tubing, all together laid end to end, is probably a mile long, and has heat shrinked rubber couplings used as fittings to connect one tube to the next. There are hundreds of these couplings, and the tubing ranges from 2.5" diameter to 4".

The tubing can be penetrated to bond a wire to it,but we would rather not mess with the integrity of the system-we cannot afford any leaks (which is why they used the heak shrink) Also, any protrusions on the inside of the pipe are unacceptable.

Because of the sensitivity of the material inside the pipes, exothermic or regular welding is not an option either, due to heating of the pipe. I have already discussed this with the client.

The only option we have is clamps. There are several kinds of clamps.

Refering to the NEC, which clamps are "listed?"
Where are they listed? I know we can use the standarf water pipe clamp, but at ~$40 a clamp, and the weight of hundreds of these clamps,
and the obtrussivenes of them, I would rather try to find another way of doing this while still satisfying the code.
Can we use worm clamps, since all we are dealing with here is static electricity dissipation?
How about BURNDY abjustable ground straps? (FCI markets them)?
 

don_resqcapt19

Moderator
Staff member
Re: Bonding conductors for static electricity dissipation

I've seen flat braided tinned copper used for this purpose. In the application that I have seen the braid is run on the outside surface of the coupling, but placed under the coupling at each end. When the coupling clamp is tightened, it provides the contact between the pipe and the braid. The couplings used were a rubber coupling with a hose type clamp holding the coupling to the pipe. In your case, with heat shrink coupling, I don't know if this would work and it would require that the couplings be replaced.
I don't think that this is covered in the code, so I see no reason not to use the worm gear hose clamps and some type of conductor to provide this static bonding. You would have to make sure that there are no issues with dissimilar metals.
Don
 

oranjeep

Member
Re: Bonding conductors for static electricity dissipation

Thanks. I Read a little more in the NFPA 77
about static electricity and bonding. It is ok to use the simple worm clamps, since it is just static electricity. The rubber coupling is only 8-12" long, so I can jump them with several copper braids. The galvinization will definitely have to be scrubbed off while the braided wire is clamped down to the steel underneath. Then galvinization will have to be reapplied. I thinks copper braided wire will work the best because it will lay flat against the pipe- making the geatest area of contact-and wont be as much of an obstacle to the clamp being torqued down. Thanks again.
 

Len_B

Member
Re: Bonding conductors for static electricity dissipation

The galvinization will definitely have to be scrubbed off while the braided wire is clamped down to the steel underneath. Then galvinization will have to be reapplied.
Worm clamps and flat braid wiil work but this may become a problem when/if sections are disconnected for cleaning/maintenance. A procedure will have to be in place to cover the reassembly especially the re-galvanizing.

Because of the sensitivity of the material inside the pipes, exothermic or regular welding is not an option either, due to heating of the pipe.
Are any of the materials conveyed classified as "Hazardous"?
Is this a negative or positive pressure system?

Usually small leaks and protrusions(e.g. a bonding screw) in a negative pressure system would not be a big problem, especially for a dry powder waste material. Ambient air inleakage could be a problem in high humidity conditions---if the conveyed material is hygroscopic it could absorb moisture and agglomerate or stick to duct walls(and bonding screw protrusions).

Len
 

oranjeep

Member
Re: Bonding conductors for static electricity dissipation

What is a negative/positive pressure system?
I do know this. That thing (the house vacuum that the tubes are attached to) is very strong.
The guy opened on of the end caps in on of the 35 rooms the system serves and the thing nearly sucked me into the wall!

The debris it is picking up is most likely human skin, hair (even though it is an area that you have to gown up in) and biotech particles.

Perhaps not enough to cause a major explosion if their is build up in the pipes, but the fact that any construction at all (like disassembling the vacumm system to clean it out) will cause them to have to revalidate, becaus they are making ingestible drugs in there.
 

Len_B

Member
Re: Bonding conductors for static electricity dissipation

Negative pressure system = vacuum, what you have described. Most pneumatic conveying systems are negative pressure/vacuum systems because of all the unavoidable leakage problems. Small leaks(and you WILL have them!) will simply suck in a small amount of air. In a positive pressure system, should a leak occur, you would have product/powder blowing all over the place(for this and other reasons they are much less common).
I'm a little surprised that an oversight like this occured in a pharma plant knowing how crazy they usually are about GMP's, etc.
It sounds as if the only thing you can economically do at this point is to go via the clamp and braided wire method(welded studs or tabs, or duct connectors with bonding provisions, on each duct section would sure have been better in the long haul). Sorry, not much help...

Len
 

oranjeep

Member
Re: Bonding conductors for static electricity dissipation

They installed this system some time ago.
I am not sure static electricity in pneumatic systems was even an issue when they installed it. I am not sure, nor knowledgeable enough to know. Thanks for the advice on the worm clamps and regalvanizing.
 
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