Sparking- does Copper wire resting on forged Alloy steel shaft cause? 2

[alwayslearning]

Dear All,

1) Does a copper cable (9 mm od- made of multi core fine drawn wires) riding freely on a rotating eequipment shaft (made of Forged Alloy Steel) lead to sparking?


2) Is it possible that the above cable (riding freely on the rotating shaft) can cause any grooves or damage to the shaft.

Let me explain this "riding freely" more clearly: We tied one end of the copper cable to the turbine casing and let the cable (about a foot long) rest freely on the shaft of a condensing steam turbine shaft to effectively "ground" the
shaft to eliminate the static charge generated. Having done this we in the plant are going through a debate about the safety aspect.

Is it at all possible that a spark is generated by the contact of the copper cable and shaft? Where in literature can i find a mention about such a possibility? Does NFPA mention any possibility of spark occurence due to contact of copper and steel etc.?

Any help would be most appreciated. Thanks

 

[rnd2]

This is not a great deal different to resting a copper conductor on a rotating weldment. If there is sufficient voltage yes there will be a spark.

 

[alwayslearning]

Dear Rnd2,

I ought to have been clearer. Having read your answer, I believe there should be 3 parts to my question:

1) Does the copper cable riding freely on the shaft create spark due to friction alone?

2)Is it possible that the soft copper cable can create grooves in the shaft?

3)Is it possible that there is an electrical spark due to occasional lifting (for whatever reasons) of the copper cable from the shaft? The voltage difference can be up to 20 volts (measured using oscilloscope).

I thank you for your response. It will be interesting to see response from other members. Thanks

 

[rnd2]

1) No

2) Given the light load only over a very long period of time

3) 20 volts will get you a spark. Bear in mind there are oxides on the steel surface always in a state of change. The copper is trying to wear the oxides off with friction so the oxide may be expected to thin in places. Natural corrosion by air-borne moisture allows more oxide to form, but need not be in the same place where it was removed. Therfore you may expect electrical insulation variations ranging from fully earthed to sometimes times non-conducting. The system will be "making and breaking" a circuit apparently with a voltage potential of 20 volts. When just the right conditions present, a spark with an intensity dependent upon the current available will occur.

 

[alwayslearning]

Dear RND2

Thanks again for your post. My queries on your reply as below:

>1) No

Why do you believe that the friction may not lead to sparking. Any reason to believe so, other than (the way i am thinking) that that the frictional energy may not heat up the surface sufficiently to issue forth a spark. Do you, perhaps, suggest that copper is "non-sparking"..? On this point we have arguments and we have reaasons to believe that strictly speaking copper is not "non sparking" etc.. Your comments please.

>2) Given the light load only over a very long period of time

Why the copper will damage the steel? I belive that copper being a softer material will be the first to wear out. If at all copper can mark steel (even after a long long period) then i would like to know the mechanism by which such action is possible.

>3) 20 volts will get you a spark. Bear in mind there are oxides on the steel surface always in a state of change. The copper is trying to wear the oxides off with friction so the oxide may be expected to thin in .....

a) Thanks for this explanation. But, as long as copper is in contact I think there is no danger of electrical sparking. Only when the copper cable makes a "recontact" (i.e. touches the surface after briefly being lifted up "automatically") then this possibility exists. Am I correct to believe this.

b) We did not measure the current in the circuit. I would, sometime soon, measure the current in the circuit to get an idea about the energy level of the spark. In the meantime, I am not sure what causes more harm; i.e. high voltage, or high current, or a product of the two?

I would appreciate if you can throw a bit of light on how this thing happens. I am thinking that high voltage = high charge accumulation, consequently when there is a discharge through a good conductor, there will be high current? Is this logic correct?

Or is it possible that there is a low voltage but the current still could be high? Under what circumstance such a thing will be possible.


Thanks a lot in advance.

Regards

 

[rnd2]

>1> Copper is non sparking ie particles of it will not ignite in air. Place a piece of copper in front of a grinder and you will find it impossible to make it spark.
>2> The same way any sharp knife edge dulls even if it is cutting soft material. The process can be very slow but it is real enough. I agree with you that the copper will be the first to wear out but that does'nt change the fact that the copper will also be wearing the steel albeit much slower.
>3>a Strictly speaking, the copper is being "lifted up" each time it travels over iron oxide that provides sufficent insulation to break the circuit.
b A static electrical discharge is normally associated with weak current build up. However the discharge curent can be extremely high and an excellent example of this is a lightning bolt.
I think that you should ask these sort of questions over in one of the electrical forums.

 

[unclesyd]

I'll have to agree with and disagree with some of the comments in previous posts.

Copper wire rubbing on a steel can easily wear the shaft and itself not be worn. The very simple mechanism for this is that the iron oxides will embed in the copper and slowly wear the steel shaft. The right combination of iron oxides is call "jeweler’s rouge". Copper laps, even lead, are used to hold various abrasives to finish or polish harder parts.

An electric spark can easily damage a steel shaft by electro-erosion. If it sparks it can damage the shaft at any current level. If stray currents are being generated by the rotation of the shaft and have be grounded there is a problem with the equipment that needs to be corrected. The electrical charge is not being generated at this point. Indiscriminate ground of rotating components in equipment usually to trouble.

The "generator" of this current needs to eliminated or you will eventually loose some bearings.

All equipment I've seen start generating electricity only get worse.

Depending on the iron oxide mixture on the surface it can have conductivities all over the map.

 

[Tmoose]

http://www.sohreturbo.com/

John S. Sohre did a lot of work with rotating machinery.
Several years ago he seemed to focus full time on grounding shafts. Tom Sohre is running Sohre Turbomachinery now.

 

[EdStainless]

I am most worried about an aspect that Unclesyd mentioned, the arc damage to the shaft. This can be roughening by electro erosion. It can also cause localized marteniste transformation and significantly change the properties of the surface of the shaft. These spots could provide initiation points for fatigue.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be slowed down.

 

[rnd2]

alwayslearning

Consider a conductive copper or brass ring around the shaft. Earth from that.

 

[alwayslearning]

Dear rnd2,

Thanks for your last post. In my opinion that was a leap forward of imagination and yet seemingly so simple to implement.

Currently the machine is in operation so we have to live with the current mechanism. I am only worried that this suggestion may not be accepted by the OEM when we get an opportunity during next scheduled maintenance.

Unclesyd and EdStainless: Thanks for the posts. From your experience, have you come across any situation where such a thing occured.

Tmoose: How are the sohre brushes different from a copper wire. If copper wire can cause damage then can the metal brushes not cause them.. would be interesting to hear your experience.

 

[SMF1964]

You can also be looking at a galvanic corrosion mechanism. The steel is anodic with respect to the copper and will oxidize/corrode where contact occurs. Over time, this will produce a groove in the steel.

 

[alwayslearning]

Dear SMF1964,

I just thanked you for your post

If possible can you give me the galvanic potential of steel vs copper? ALso please educate me a bit on how the galvanic corrosion initiates etc..

Regards

 

[unclesyd]

I've seen several instances of spark erosion of shafts of different sorts.
Though arc damage is generally seen on bearings, if your are in the business long enough you will see where it works on the shaft.
The worst case came on a screw compressor where there was an air balanced seal and for some reason the arcing came from the Al labyrinth to the shaft. It had arched to the shaft at each of the labyrinth parts (3) deep enough that the shaft had to be reworked in that area. It apparently started on the inner partition and progressed to the outer most piece. It was noticed by an operator on the dark shift, a highly unusual act.
I remember an arcing problem on the shafts of a couple of high HP sleeve type motors again on to the outside where an Al dust shield was in close proximity to the shaft.

I would like to again emphasize the importance of turning the generator off instead of grounding the output.

 

[rnd2]

alwayslearning
It occurred to me that we do not know the nature of the electricity. Can we assume it is unwanted and from a 'stray' current source? I mean by this that we know it is 20 volts, but do not know if it is DC or AC current or if it is static generated. Now there's an unintended oxymoron
I mention this because it may be possible to cancel out the unwanted current by negatively matching the current so that the nett result is zero current flow.
The electrical forum may be able to provide you with most useful information in this regard.
Good luck with your project.

 

[Tmoose]

Re Sohre -
I believe Their products were developed specifically to handle shaft currents and similar problems with rotating machinery using both rolling element and plain bearings. I have no experience with their products. But I do have immense respect for John Sohre as a very practical and thoughtful man.