Earth Ground for ESD

[tomc5]

I am trying to improve a static charge buildup problem I am experiencing with a section of glass tube which is incorporated in a steel pipeline. The piped fluid is a non-conductive non-flammable oil and flow velocity is approx 5 ft/sec. We observe arcing when the flow is stopped and pipe begins to drain. I am looking to install grounding rings at each end of the glass section to dissipate the charge to an earth ground.
Can anyone advise on what specifications or standards tell me what ground impedence I must have when dealing with this type of static charge ?
Thanks,
Tom

 

[don_resqcapt19]

Re: Earth Ground for ESD

Tom,
If the static discharge is taking place inside the glass pipe, I doubt that any grouning or bonding would make any difference. In any case, I don't think a connection to earth would be of any use. You could try bonding around the glass section. The IEEE Green book says a conductive path of 1,000,0000 ohms or less will prevent the build up of static charges in most cases.
Don

 

[petersonra]

Re: Earth Ground for ESD

Since you mention the fluid is nonconductive, chances are that grounding rings will be of little use in reducing static in the nonconductive glass.

Consider this. If the glass segment is between two steel flanges you might be able to run a stainless steel wire between the two flanges on each end of the glassed segment inside the glass.

 

[tomc5]

Re: Earth Ground for ESD

Don-
A million ohms!?*#@!
I thought for sure any static charge grounding would want a LOW impedence to prevent a high buildup?

The idea of a million ohm ground path per IEEE is counter to what I would expect per NEC in the way of a 25 ohm ground. What explains this enormous difference??

Peter-
At this point, I am not allowed to install anything inside the fluid flow path.

By the way, I'm not an inspector, electrician or a EE, I just need your help!

Thanks for your interest.
Tom

 

[petersonra]

Re: Earth Ground for ESD

The 25 Ohm thing has little to do with dissipating static electricity.

You can easily dissipate static charges with a fairly high impedance because the static only builds up on things that are basically insulators.

The problem in this case is the nonconductive nature of the glass and the fluid it contains. the only reliable way I can think of to eliminate the sttaic charge is to introduce some kind of conductor inside the line.

 

[tomc5]

Re: Earth Ground for ESD

I think I misunderstood the comma in Dons post, is that one million or 10 million ohms? In either case, I'm struggling to understand the theory.

Is it fair to say that the arc is created because there ultimately is a path to ground, from the glass surface - to the connected steel pipe - then somewhere to earth? This ultimate path must have a high resistive value, sufficient to permit static charge to build until it finally arcs over. I'm thinking if the steel pipe where perfectly isolated from earth, I would observe no arc at all. On the other hand I want to improve the earth path to try to drain the static charge before it can build up to a point of arcing.

If IEEE suggests that 1 million ohms (or 10 million ohms?) are sufficient to dissipate static charge, I would like to try that route before I install a conductor inside the glass.

So can you tell me between what two points would this 1 million ohms (or 10 million ohms?) apply?
Is this tested and measured the same way as the 25 ohm NEC requirement?

Thanks guys,
I appreciate your help.
Tom

 

[hbiss]

Re: Earth Ground for ESD

As I recall this is a problem also with dust collection systems in woodworking shops, actually worse because the dust can be explosive.

Anytime a nonconductive fluid or gas passes through a nonconductive pipeline the friction causes a static charge to build up on the pipe. Take a look at the plastic piping used for a central vac system. It's usually covered with dust that has been attracted to the surface of the pipe by the static charge.

I don't know what the purpose of your glass section is, is it just to monitor the liquid? If so I would think winding a small gauge wire spirally around the glass section and connecting it at one or both ends to the metallic pipe should solve your problem.

There is no need to introduce any resistance here. Normally you would want a resistance, usually in the megohm range in series with any personal grounding device such as a wrist strap. The reason for this is for safety, to limit the current flow should the person come in contact with a voltage yet still drain off any static charge that may accumulate. I think that's what some of you guys were thinking about.

-Hal

 

[charlie b]

Re: Earth Ground for ESD

Originally posted by tomc5: I thought for sure any static charge grounding would want a LOW impedance to prevent a high buildup?

I think you may note that Don said 1,000,000 ohms or less.. If there is a path for current to flow, and if the resistance of that path is not extraordinarily high (i.e., more than a million ohms), then the static charge will find its way to planet Earth, and be discharged. However, please note that so long as there is flow in the pipe, there will be a source for static charge (see discussion below).

. . . static charge buildup problem . . . with a section of glass tube . . . the piped fluid is a non-conductive . . . oil . . . .

One of the original experiments that led to the discovery of many electrical principles involved rubbing a (non-conductive) glass rod with a (non-conductive) silk cloth. The cloth imparted a static charge to the rod. You may be looking at a similar situation. What I am saying is that you should not be confused by the phenomenon occurring with a non-conductive fluid in the pipes.

The problem you are facing relates to the non-conductive nature of glass. The ?rubbing? of oil against the glass may be imparting a charge to the glass. But once there, the charge might just stay there. Charge will not easily be conducted off the glass, and onto some other surface. The glass might be in intimate contact with the metal pipe, and the metal pipe might already be in intimate contact with planet Earth. But the charge is not being conducted from glass to pipe to dirt, because the glass is not a good conductor. So adding any type of grounding conductor (e.g., from the pipe to a ground rod) will not change the situation. Nor will bypassing the glass section with an internal or external connection between the metal sections on either side of the glass section.

You are certainly outside the realm of the electrical power industry. The NEC will not help you with this problem. There may be a member of this Forum who knows how to handle the problem, but I am not that member. Have you tried speaking with other members of your own profession? Surely someone else has seen this situation before, and has already figured out a solution. Good luck.

 

[tomc5]

Re: Earth Ground for ESD

Thanks all for your help and input.
Hal- your analogy to dust collection systems is interesting, this gives me another area to explore.

What about grounding for aircraft refueling or tanker refilling? Is there a specific mention of ground impedence that you could refer me to. (I realize this does not apply to my situation but I am trying to relate to the principles that apply there.)
Thanks again,
TC