Groundpath For An ESD Problem

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tomc5

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It was suggested to me at the Grounding & Bonding forum that I might gain some guidance elsewhere, so I am reposting my situation here with additional detail:

On our hydraulic test benches we use a glass tube, float type flowmeter (a rotameter) 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 inside this glass tube when the flow is stopped and pipe begins to drain. In an effort to dissipate the static charge, today we tried adding a ground wire between the metal base of the flowmeter and the test bench electrical power ground wire. This had no effect.

I began to question the integrity of the earth ground at the electrical service panel, but now I'm struggling to understand the theory of what we are observing.

Is it fair to say that the arc is created because there ultimately is an existing 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 test bench 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. I believed we should try to obtain a very low resistive path to ground, on the order of 25 ohms, in keeping with NEC ground rod criteria. It was suggested to me that a resistive ground path on the order of a million ohms is sufficient to drain off static charges. This sounds enormous to me and I don't don't have a sense that such a high value could preclude arcing (but I am not a EE).

So I am pleading for explanations and advice here. Does it make sense to strive to obtain the lowest possible resistive ground path? Or something else? If Code sections do not apply to my situation, is there a way to determine what a reasonable resistive or impedence value should be in order to preclude my arcing problem?

By the way, I have talked to many rotameter manufacturers and received no guidance on this problem at all.

Thanks,
TC
 
Re: Groundpath For An ESD Problem

tom,
you are confusing earth ground as though it has something to do with your static problem. you actually have a power source on the work bench. your return path to disapate the static charge has nothing to do with the earth which we refer to "ground". your return path may be to the pump frame --not earth...
 
Re: Groundpath For An ESD Problem

A resistance of a million ohms should prevent arcing. I also agree with Charlie's post.

Please describe the flowmeter, and the glass tube, and your application a little better. Is this a commercial meter you are using, or are you trying to design a flowmeter?

Do you have 2 sections of steel pipe separated and insulated by the glass tube? If that is the case, you need to bond the two sections of steel pipe together. Basically a jumper across the glass tube.

Steve
 
Re: Groundpath For An ESD Problem

Originally posted by tomc5: It was suggested to me at the Grounding & Bonding forum that I might gain some guidance elsewhere, so I am reposting my situation here with additional detail:
Click to expand...
When I made that suggestion, I had intended to suggest that you look beyond the Mike Holt website, that you look for information sources related to the fuels industry. All of the "Topic Areas" on this Forum, from ?Electrical Calculations / Engineering? to ?Grounding Versus Bonding? to ?Safety? are read, and commented upon, by the same membership. You might get lucky and have someone familiar with your situation give you some valuable information. You are most welcome to try. But I think you?ll have better luck asking persons whose livelihood comes from other than the design, installation, and inspection of electrical power transmission, distribution, and utilization equipment. We just don?t do ?static? that often.
 
Re: Groundpath For An ESD Problem

Charlie and Steve-
I know my own ignorance of terminology is preventing me from providing a clear explanation of the advice I need. Hopefully here is more detail for you:

The flowmeter is ~ 30 years old. It consists of a glass tube which is contoured in a long conical shape, 25 inches long. Inside this tube is a "float" (actually it is a steel plug which is lifted by the velocity of the fluid flow; its elevation in the tube corresponds to the fluid flow in terms of gpm.) The tube has a fitted metal base on top and bottom of the tube which in turn, is connected to steel pipe by flared type fittings (there is metal to metal contact between the metal bases and the flared tube ends). The flow meter is secured to the test bench frame (I previously referred to it as a chassis) by means of screws through the frame into the top and bottom metal bases. The test bench also include a 440v, 3-phase motor and control circuit which drives the pump which develops the fluid flow. There is a green grounding wire fastened to the test bench frame which runs back to the local fused disconnect switch panel. I have establihed with an ohm meter that continuity exists between the flowmeter top and bottom base and this ground wire attached to the tests bench frame. Additionally we attached an extra #10 wire between the flowmeter base and the ground wire. In this way, I think we were we were accomplishing the bonding that Steve mentioned, however I think the bond already existed by virtue of the way the bases are mounted to the test bench frame. Arcing still occurs.

So then, here I have arcing inside this glass tube, which means to me that the static charge is building up on the inside glass surface to a high enough value that it finally discharges to the metal base. Am I wrong to think that the charge ultimately finds it way back to "earth" by means of the ground wire? Isn't the ground wire eventually connected to a ground rod driven into the earth?

Thanks again, I appreciate your time and thoughts.
Tom
 
Re: Groundpath For An ESD Problem

It finds its way back to earth the same way it does when you walk across a carpet and touch something conductive. The conductive object does not necessarily have to be "grounded" for the charge to transfer, ie a doorknob. So your reasoning about grounding is faulty.

In the grounding vs bonding forum I misunderstood you to say that that the arcing was on the outside of the glass. Since you now state that the flowmeter is ~30 years old I question why this arcing is a problem now? Is it bothering anything?

-Hal
 
Re: Groundpath For An ESD Problem

Tom,
Have you changed anything that could of caused the static build-up? In some cases it is not possible to prevent this problem. I ran across an air force document a while back (I can't find it now) that talked about a static problem in jet fuel transfer piping. This was a bonded, all metal piping system and there was still a static problem at high flow rates.
Don
 
Re: Groundpath For An ESD Problem

Hal, Don-
"why now ?" is the really true mystery and we are struggling with that question as well. We think it may be related to a change in our hydraulic oil specification, one that is now less conductive then its predesssor. We cannot now revert to the original oil.

I'm also thinking that maybe the glass has become etched and now represents a high resistive path, but that gets into my original question about what is the appropriate resistive value for a path for a static discharge.

"Is it hurting anything?" Fortunately our oil is non-flammable, but I have been told with confidence that on one occasion the glass was shattered. We have plexi shields over the glass, but it is still a safety concern. Also too, I can no longer replace a broken glass tube. And of course I'll only be able to justify a budget to refit the flowmeters with a newer type after I explain what is going on with the present arcing problem.

Tom
 
Re: Groundpath For An ESD Problem

Tom, I am not personally an expert in this area, but I found a Unified Facilities Criteria (UFC) which is a Department of Defense document that deals with what you are experiencing (albeit dealing with jet fuel). Some quotes from the text:
9.3. Preventing Static Electricity. It is not possible to completely eliminate static electricity. Use the following precautions to reduce the magnitude of charge and therefore the possibility of sparks:

9.3.1. Connect a static bonding wire between two components before making or breaking a connection and before working on flanged connections insulated from one another by nonmetallic insulating materials. When vehicles or aircraft are grounded, attach grounding wires to the vehicle or aircraft before bonding to the grounding rod. This is especially important for operations involving fuel transfers (fueling, defueling, loading, and unloading).
9.3.2. Avoid surface agitation by limiting the initial fill rate into a fuel storage tank to less than 0.91 meter (3 feet) per second. Maintain this flow rate until the floating roof or pan is afloat and the fill pipe is completely submerged, or until the fill pipe is completely submerged in all other tanks. NOTE: Wait thirty minutes after loading or unloading an aboveground fuel tank before allowing anyone on it.
9.3.3. Personnel will ground themselves to the tank by making firm contact between the tank and back of the bare hand or by holding a coin in the bare hand before opening access covers or inspection holes. Also, ground sampling devices to the tank before opening the sampling well. 9.4. Relaxation (Release) of Electrostatic Energy. Fuels are poor conductors. Static dissipater additive (SDA) is added to JP-8 to improve conductivity. JP-5 lacks an additive, so designs rely on time delays to dissipate the charge (i.e., certain refueling equipment has relaxation tanks to delay movement and relax electrostatic charges).
9.4.1. Fuel components, such as hydrocarbons and chemicals, permit a flow of electrons. When there is electrostatically charged fuel in a tank or pipe, the mutual repulsion of like charges in the fuel and their attraction to the opposite charge on the tank or pipe causes a current flow (i.e., positive ions
may be attracted to the F/S while negative ions are attracted to the aircraft tank) The bonding wire provides a return path for the ions to equalize.
9.4.2. Most electrostatic problems can be prevented through design, bonding, and adhering to safety procedures. It is important to remember to initially fill tanks and F/Ss slowly.
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Not sure if this helps, but if it does help, I can send you the document in whole (pdf) if you send me a PM with your email address. That being said, you might consider hiring someone who deals with this on a regular basis.
 
Re: Groundpath For An ESD Problem

The only thing not bonded together is the "float" which I suppose would be a metallic object which is free in the center of the cone.

How you could discharge static charge built up on the float is a mystery to me unless you could keep the flow meter full after flow stops for a short time to allow the charge to equalize. Or maybe change the piping so it is full all the time.

Jim
 
Re: Groundpath For An ESD Problem

TC, Two other reference sources both apply to electrostatic protection and related bonding. NFPA 77 defines electro-statics as a source of personnel and environmental hazards. Air Force TM5-811 and AFM 88-9 describes interfacing concerns with aircraft facility and ground bonding mechanics of fuel areas. NASA-STD-P023 also has reference to Class "S" electrostatic bonding requirements that specifies DC bond resistance requirements. (25k-1 meg-ohm) The Class "S" para 4.5 gets into line fluid static and 350V max range allowed. Hope this helps.

Key in electrostatic grounding on Google for NASA and other DOD sites that cover static areas.

rbj, Seattle
 
Re: Groundpath For An ESD Problem

Tom, I tried to send you the files, but the email was rejected. I think it might be because they are too large. I just found out that they can be accessed online for download web page . Good luck.

Dave
 
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