pipeline cathodic protection

I assume you mean grounding rather than "drounding" and that HVAC does not mean Heating, Ventilating, Air Conditioning but rather a high voltage AC transmission line(s).

Without knowing what the pipeline made of, what it carries, and if it is above ground (how high) and how far it is from the AC lines (and what voltage they are), I will assume a general case for an above ground, metal pipeline.

Typically, the grounding would be to prevent the possibility of electric shock to people or animals. Cathodic corrosion is a DC phenomenon and shouldn't be an issue here involving the AC fields.

A good ground reference and a decent ohmeter should be fine, however AC voltage potentials could occur between separate metal parts in the proximity of the powerline if they are not in electrical contact with each other, or much further away if not grounded in some manner.

I think he should rephrase the question!!!! HVAC to me means Heating, Ventilating, Air Conditioning. If he is talking about cathodic protection, he should really look into Ni rods.

Jaime V.

venegas_jaime@hotmail.com

venegas_jaime@yahoo.com

Let's assume( please always revise what you post! ) he is talking of a Steel pipeline buried in soil --coated--wrapped--Cathodically Protected by making the pipeline- steel a Cathode in an EMF arrangement where the Anode is a Zinc/Nickel/Carbon rod buried somewhere to ensure that only Cathodic current leaks in thru the coat and wrap into the Steel of the pipeline! That way Pipeline does not corrode.

Now the Question.:

The Devil is above--a High Voltage A.C Transmission line Carrying 1000's amps in each of 3 phases say 6 meter above.

He asks : Any Problems?

I shall ask: Did you say " Crosses" at 60 to 90 degrees? If so ,don't worry--the HVAC line will induce no resultant harmful circulating currents in your pipeline under the coat and wrap.

If you say --"they travel parallel for miles" then too nothing should happen to your pipe--if the route soil is moist and conducting to create an EARTH SHIELD for the resultant field from Transposed/untransposed phases of the HV Power line

Voltage can be 33KV or 1000KV--what matters for the buried pipeline is Amperes.

In reply to MUKULMANT:

You replied to my post so I assume you are addressing me.

I don't understand your (please always revise what you post!) because that seems to apply to the original questioner in which case I fully agree that questions should be appropriately revised/updated bases on the responses. So now I am confused about who you are addressing but it really doesn't matter.

The original post is titled "pipeline cathodic protection". He asks how he can calculate grounding resistance. We seem to all agree on that part. Next he says "especially where the pipe crosses a high voltage AC transmission line(s). I think we can agree that that is what he means, but note the word "crosses". Finally he asks how he can measure the resistance between the pipeline or some part of it and/or its support structure if present, and the earth ground. I think we can agree on at least most of this interpretation.

Cathodic protection / protection from cathodic corrosion is an issue all by itself and can be accomplished in a number of ways depending on all the specifics and you mention one way in your assumption of a buried pipeline.

An often used method is imposing a DC current of typically 20-50 amps at 50 volts between the pipeline and the ground by placing a series of anodes in the ground along the pipeline and making the pipeline the cathode.

While AC induced corrosion certainly does occur, it is at a relatively low rate and becomes a significant concern when a pipeline and high voltage AC lines share a right of way, with either or both buried or not. We both seem to agree that crossing the electric line right of way tends to have minimal or no corrosive potential to a well grounded pipeline. You seem to be implying that the coat and wrap improve protection from induced circulating currents and that is false. The coat and wrap interfere with the grounding and do nothing to shield the pipeline from the AC induced fields.

Yes, in your "parallel for miles" example unbalanced loads in the phases can induce more currents in a pipeline and low resistance soil is a mitigating factor.

However we part company here. You say "voltage can be 33KV or 1000KV-what matters for the buried pipeline is amperes". Induced amperes in the pipeline of course, but you seem to be saying here (and with your "1000's of amps in each ...." in your "The Devil is above" line) that power line amperage has more effect than the volts. The opposite is true for inducing currents in a pipeline from high voltage AC transmission lines. The high voltages produce much greater electric fields than the amperage produces magnetic fields in their case. I certainly wouldn't go so far as to say "nothing should happen to your pipe" no matter how conducting the soil is if the pipe is insulated from it by a coat and wrap in a pipeline adjacent to transmission lines for miles.

You made no comment on his specific question: "my problem here is how this resistance is measured and with what intruments?" Was this an oversight if you were replying to him, or did you agree with my suggestion and therefore added nothing further?

Perhaps I misunderstood you. If I did please correct me.

Best wishes for the New Year!

Apologies to you Greg.

I clicked on the wrong post-your post. And I kept on writing as I would have to the original Questioner.

And I drifted out from the Original question.

He wanted how to measure Resistance.

Simple- a Megohmmeter, a Schering Bridge ,a Megger(British Classic).

But this does not solve his problem yet. What is his final issue?

I agree-we should stick to the posed Issue only. Rest -as posed!

And a happy and Prosperous 2007 to you Greg and all the CR4 Bloggers

Dear Mr Greg,

You describbe the phenomeno of a Cp Systems very well but I still do not have the answer I wanted, that is, "to calculate the grounding resistance when a pipeline and high voltage AC lines share a right of way, with either or both buried or not". In my case the pipe is buried, carries Natural Gas, is of high pressure (60 bars) and of heavy metal.

So please I really need help here.

While I you and all in CR4 the best of 2007

Grunding resistance for Cathodic protection maybe measured by spec meter or Thompson Bridge with up to three electrodes, with one located at least in the distance of 20 meters (60 feet) from the bondary of the grounding system (to have the almost "neutral- zero reference earth/ground")

WILL THE DC READINGS BE reliable with the devil above inducing huge AC Volts and Amps?

Some power distribution and telecom untiity companies may still use a megger but more often they now use a clamp on type meter for measuring the resistance to ground (earth).

Remember also that this grounding value will change with the season and the average moisture content of the ground. In one area I measured changes week to week as the rains came. Summer time droughts produced unacceptable high resistance values.

A Google search for "Ground Resistance Meter" yields numerous hits. I used a product by RCC but many excellent products are available; all of which are more convenient that the traditional megger.

For safety sake the metallic pipeline has to be grounded to earth but this now creates the potential for galvanic activity. Without these deliberate grounding straps, the insulated pipeline would have very high resistance to ground. In the order of megohms, in direct proportion to the insulating value of the non-conductive protective jacket surrounding the pipe.

From my own experience, I would expect the high voltage transmission authority to be very strict about permitting any buried pipeline to be placed in proximity to their lines and then it must always cross at right angles.

I found that pipelines buried in parallel to lower voltage distribution power lines to be much more troublesome. We had one area with oil pipes buried within two meters of the 27,600V underground distribution cables. There was always a leak somewhere due to corrosion currents induced from the AC power grid.

While it is true that AC will produce a slower corrosion rate than DC, it is not true that no corrosion takes place.

Long distance pipelines are often protected by passive zinc or magnesium sacrifical anodes. It is less expensive than active impressed current protection. In remote localities impressed current systems requires expensive generating facilites.

Calculating the resistance would be a very difficult exercise due to the sesonal variable connection to the water table in the earth.

Measuring it is not so difficult and you can easily place sufficient sacrificial anodes to properly polarize the pipe above the galvanic potential. These sacrifical anodes also server as the grounding points.

Arild

<in your "The Devil is above" line) that power line amperage has more effect than the volts. The opposite is true for inducing currents in a pipeline from high voltage AC transmission lines. >

<I found that pipelines buried in parallel to lower voltage distribution power lines to be much more troublesome. We had one area with oil pipes buried within two meters of the 27,600V underground distribution cables. There was always a leak somewhere due to corrosion currents induced from the AC power grid>

The opposite is true for inducing .. .let's discuss from fundamentals of Field theory and see what is more dangerous for the pipeline.

this a common phenonmenon in cathodic protection; this is called 'stray current' effect and this could be corrected applying corrective principle.

kayode

Dear Kayode, I would like you to tell me what you know about "stray currents" and their effects, mostly when it either AC or DC.

hanks for your help and time