Saturday, 26 January 2013
Attenuation curves
Pipe-to-soil voltages graphs are sometimes called 'attenuation curves'. They are similar measurements to those we make when carrying out close interval potential surveys and are ground potentials
The very word 'attenuation' infers that it is the pipe metal potential that varies and this is
incorrect. Could one of our NACE or Institute members please explain this?
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Reply 1
ReplyDelete• Attenuation in the magnitude of current and the driving voltage is caused by the resistance of metllic pipeline. The potential (voltage) is highest at the location of ground bed, as we go away from this location, along the pipeline the potential will decrease (attenuate) to a value where it falls below the acceptable level, so another ground bed must be located at this point. The mathematical equation of the attenuation heplps us find the optimum separation between two ground beds. The pipe to soil potential decreases, in one to one correspondence with the potential drop along the pipe line.
Reply 2
ReplyDelete• when doing CIPS you can observe the attenuation of the pipe potential as you walk away from the GB (Logarithmic potential decrease) and compare this measured value to the calculated attenuation constant during design. if the potential difference as you walk away increases much (if the potential is much lower than the point near the GB) then you have a large attenuation.
Reply 3
ReplyDelete• Can you explain how this can be, when the resistance of the pipeline is so low that if you apply ohms law the resulting maths are ridiculous? NACE give a list of pipeline metal resistances and an example I can remember is that 1 mile of 24" dia 1/2 wall thickness steel pipeline has a resistance of 0.006 Ohms.
If the half-cell is the zero potential against which you are measuring the pipeline potential along it's length, then you can apply Ohms law. If the half-cell is not a reference potential then you cannot make attenuation calculations.
Reply 4
ReplyDelete• Dear Roger,
yes you are right about that but the pipe resistance is not the only factor to be considered for attenuation calculation. According to NACE :
"The shape of the attenuation curve is governed by the relative values of the
structure longitudinal resistance (Rs) in ohms per unit length and the structure
leakage resistance to remote earth (R_L) in ohms per unit length"
then Attenuation = SQRT(Rs/R_L)
The leakage resistance (R_L) per unit length for a bare structure can be estimated
by
R_L=(ρ/2πL)ln(L^2/td)
where: ρ = soil resistivity (Ω-cm)
t = depth of burial (cm)
D = diameter of pipe (cm)
L = unit length of pipe (cm)
when using a coated pipe the calculation has another formula including leakage resistance of coating (Ω).
Practically when your pipe has good coating (the usual case nowadays) the attenuation is very small you may start to observe it after 20-30 km of survey
Reply 5
ReplyDelete• The NACE proposal seems to ignor the laws of electricity. We are measuring voltages using the only metering system available to both science and field work and we must therefore look at the MEASURING CIRCUIT to evaluate the displayed voltage.What happens in the electro chemical and transfer of charges, All of which I have studied in depth with nuclear chemists and electro chemists from several universities cannot be applied to the measurements that we record.
Reply 6
ReplyDelete• We are actually measuring a whole load of potentials in series, and the meter shows the combined voltage of all of these.
http://www.rogeralexander1938.webspace.virginmedia.com/cpn/ProcHTML/proc6.htm
It is simple to try... get a load of dry cell batteries and stick them in a tube (like one the big torches you can buy). We are trying to measure the voltage of ONE of the dry cells from the total of five. it cannot be done even if you know the voltage of one of the others (your reference electrode) because they are in a measuring circuit with too many variables to make calculation and computerization possible.
Reply 7
ReplyDelete• dear Roger,
i coudn't read the information in your link in detail for now. But again from NACE:
"A potential measurement with the reference placed
on grade will be a mixed potential of all the individual surface potentials within
the influence of the reference electrode. The corrosion potential consists primarily
(about 90%) of the polarized potentials on the pipe surface falling inside a 120°
angle from the reference electrode plus a voltage drop component due to corrosion currents in the earth between the reference and the respective anodes and cathodes on the pipe."
i coundn't understand the relationship between pipe potential measurement and dry cell example. returning to our original subject attenuation isn't a reading error but a physical combination of resistances blocking enough current to reach the pipe segment at a far location resulting in lower polarization
Reply 8
ReplyDelete• The resistances are constant and it is the potentials of the ground itself that are subject to the electrical flux that you describe. I have been party to many international studies on this subject and it is well understood and documented.
Your assumption of the 120 degree angle is totally unacceptable in every respect. You can copy and paste the link into your address bar on your browser and it will work. It is the conversion from MIME to HTML and back that screws up links... sorry about that.
Reply 9
ReplyDelete• Dear Roger,
the link worked but i had no time at that moment to read in detail sorry if misunderstood. Now i checked your website again, there are lots of useful information. the digital oscilloscope seems interesting to me as i have tried to build one myself.
the assumption of potential reading for 120 degree angle is made by NACE and i cannot claim or prove it is not working. what i am sure is i can read the damaged area pipe potential when i am exactly or a few meters around it, so practically i can say it is working. (and i am sure NACE has a proven or acceptable record for that)
Based on this idea since the trailing wire and the pipe is plain conductor, no matter how far you are from the anode or test station you will be measuring the 120 degree affected area (%90 according to NACE) , it is the same to install a test station at that location or carrying a 1 km long cable to here from somewhere else. (cable and pipe resistance in long distance measurements will have negligible effect in reading as long as you have high input impedance voltmeter)
Reply 10
ReplyDeleteGood luck helping Roger understand, although I don't believe he was seeking help when he started this discussion.
Abdul's description of attenuation is consistent with my understanding and training.
Roger,
When the copper sulphate electrode is moved to a new location along the pipe, the CP technician is now measuring the pipe-to-soil of a different section of pipe. It appears from your posts that you expect the measurement to remain unchanged because you are still connected to the same pipeline with low linear resistance.
I did not check the correctness of formulas posted by XXXXX, but:
"Attenuation curves" you see people using on charts could be predictions of current density or predictions of pipe-to-soil potential along a pipe based on the linear pipe resistance and the coating effectiveness. These calculations are widely published and applied and presented in NACE training. They can also be used in reverse to estimate coating effectiveness based on CIS test data.
You said "The very word 'attenuation' infers that it is the pipe metal potential that varies and this is incorrect."
Perhaps what you infer is incorrect.
You will agree that there is no reason that the pipe-to-soil potential measurement must be unchanged when the electrode is moving.
Anyways, take care,
Reply 11
ReplyDelete• XXXX XXXXX, you are correct, I have carried out this demonstration/experiment on site extensively on five continents over the past 30 years and the rules of electricity have not changed, but the accuracy of hand held instruments has improved dramatically.
I know that some people have suggested the 120 degree scan of the half-cell but they cannot give a lucid reason for this and it most definitely cannot be repeatedly demonstrated in the laboratory or field work. It is a fantasy.
Charges (or electrical energy) disperse from a high potential to lower potentials according the Kirchhoffs Laws of resistances in parallel and this can be safely modeled as the inverse square law of radiation through a variable medium. If the input of these charges to earth is close to the surface it forms a hemisphere of charges and there is no logical reason that this should be 120 degrees as there is no boundary except the change of medium from ground to atmosphere. The Wenner four pin earth resistance method is based on this concept.
The charges passing through the measuring circuit of a pipe to soil voltage measurement come from remote earth (we all know that this has no resistance as it can be regarded as an infinite number of resistances in parallel.)
It is when these charges reach the 'shells of resistance' approaching the coating fault that they form potential zones that we measure when using DCVG and CIPS. I was the inventor of DCVG and a leading member of the team that first developed CIPS. as I boringly remind everyone in these posts... har har har.
I have recently had access to the voltages measured on a 1500 km pipeline that had a total resistance of less that 5 ohms from end to end. The pipe to soil voltages on and off are displayed on hundreds of pages of graphs that have a straight baseline on the assumption that the Copper/copper-sulphate electrode is a reference electrode that can be regarded as zero while the pipe metal potential varies as xxxxx suggests.
In one section between test posts there is a drop of 0.6 v in less than 1 km...... that means over 100 amps has appeared from nowhere and goes to nowhere according to Ohms Law. I know this to be untrue and stupid and that is why I adopt a different approach and have raised this topic in the first place.
NACE have still not responded with a clear logical explanation in simple electrical terms suited to the instruments that we use and the people that use them.
Reply 12
ReplyDelete• When you have marked the exact spot for excavation of a coating fault there are procedures that can define the position of that fault on the pipeline and using the Alexander Cell, you can tell if the area is corrosive and if the cathodic protection systems are stopping that corrosion. I have detailed records of hundreds of excavations I have personally attended and I always take pipe to soil voltages using Cu/CuSO4 electrode as the excavation progresses towards the exact location of the coating fault. The voltages decrease..... that is the reading on the meter gets smaller... until the coating fault is fully exposed and the electrode is placed on the bare metal itself.
If you then place it on the ground anywhere within the excavation or tens of meters away from the excavation, the electrode is in remote earth and the meter reading will increase accordingly.
There is no way that this can happen if the electrode is a reference potential when used in pipe to soil measurements.... try it yourselves.... look at the three nails experiment and follow the logic as hundreds of Cathodic Protection Network students now have ... and then address this topic from the point of view of NACE.
Reply 13
ReplyDelete• I agree with Roger. I have the same experience with excavations and the RE readings that Roger mentions. As xxxxxxxxxxx mentions look at the situation in terms of the equivalent circuit of the impressed current cathodic protection system where all resistance and capacitative effect of coating (3 layer?) is accounted.
Regards
Reply 14
ReplyDelete• HI Roger,
I think you are probably taking the electrical analogy a little bit too far. While is correct to consider the pipe as a cable, the soil should not be considered as a perfect salt bridge.
In the CP case, polarization (accumulation of charge) is something desired. Consider the soil (and coating) as an inefficient salt bridge where some ionic current can be transmitted, but insufficient to avoid polarization.
The attenuation coefficient in the curves, is nothing but an extremely simplified estimation of how inefficient this salt bridge is.
I hope this has been helpful.
Regards,
I have tried to respond to this latest post but my reply is still awaiting publication so I am repeating the topic here to avoid commercial censorship.
ReplyDeleteThe above comments were mad by an assortment of people and I have shielded their names as they posted to public but did not specify that their posts could be re-published. I will acknowledge their contributions if the want.
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ReplyDelete