Cathodic Protection Training Course


Module 7

Interference



Proximity of foreign structures.

Foreign structures include pipelines, and anything that can cause a disturbance to the natural electrical equilibrium of the ground itself.


The most commonly quoted is railways which are driven by DC charges. These are carried in overhead cables and return via the rails which are earthed.

This means that there are surges of DC charges in the ground itself which will take the least line of electrical resistance to complete their particular circuit.

Remote earth is a resistanceless conductor and the charges in remote earth will not cause a detectable surge. However, the charges are drawn from the rails and the energy drained from these will cause a momentary potential gradient in the electrical path from remote earth.

If there is a steel pipeline in this path then it will offer a lower resistance path to the returning current, but if the coating is perfect then it will not.

In order to affect the corrosion status of the pipeline there must be a point or points of entry to the pipeline metal and point or points of discharge from the metal into the ground. Where the current enters the pipe metal it will stop corrosion but where it leaves it will accelerate corrosion.

However, it will cause errors in the voltage measurements whether or not it is detrimental to the pipeline corrosion performance.

Accellerated corrosion is readily prevented by the exact placement of sacrificial anodes which must be provided with a method of continuous monitoring.

This is simple to carry out providing you have a clear perception of the 'electrical picture'.

We can construct a computer model of each situation and simple add the data gathered by our specialised survey methods.
This will show the exact paths of the various currents and the exact position in which to place the sacrificial anode in each case.
It will allow us to see the performance of each anode and to balance the impressed current cathodic protection systems to reach a 'protected' equilibrium.
It will allow us to recocile voltage measurements which might otherwise be taken to indicate problems.
It is sometimes advantageous to use recording voltmeters and osciliscopes to examine the causes and likely effects of some of these ground surges.
It is now possible that we can control the effects of this 'noise' very efficiently using electronic means, which have been developed by NASA and are available to us through our members in Italy, Sobrel.
The Alexander Cell certainly displays the effect of these features on the corrosion reaction and is therefore the only known method of short term monitoring for electronic corrosion control.


Other Interference possibilities.

The generation of AC electricity by mechanical means requires that energy is applied to rotating magnets which induce a potential into coils of conductor wire. These coils discharge current through carbon brushes into cables which then are at a higher electrical potential than the metal to which they are connected.

The magnet then charges the next coil in turn and the first returns to it's basic potential value.

In countries such as the UK, generating companies are required by law to supply electrical energy at exactly 50 cycles per second. That means that each conductor is charged and discharges it's energy 50 times per second.

The amount of energy carried by each conductor depends on how far apart the 'potential' of each conductor is forced away from 'zero'by the engine supplying the energy.

For a 50th of a second this is DC charge. There is then a mechanical arrangement that forces the current to flow in the other direction until the cable is charged negatively in relation to original 'zero'.

The whole generator system is balanced to maintain the zero at an equal potential to the ground on which the generator is based.

Field generators are sometimes unbalanced and cause the potential of the ground to alter. This can be manifest as a steady fixed potential increase of decrease which can be plotted around the earthing system of the generator itself or it can be in a waveform the effects of which can be detected up to half a mile away in high resistance grounds.

Overhead high energy conductors (high tension wires) opperate in the UK at a potential difference of 400,000 volts and have a field of of influence that can effect buried pipeline that run parallel to them.
This can be seen in fluctuations in the voltages measured in pipe-to-soil surveys.
In one such instance the voltages on the meter swung from 0.600v to 2.13volts in an irregular pattern over a period of 20 minutes. This was in a location where three pipelines ran parallel to two high tension pylon runs rated at 400,000 and 232,000 respectively for lengths of several miles.

I reported this matter to the operators of the pipelines but learned that sections of each of the pipelines had been replaced as they took no action.

I was unable to offer assistance in this matter as I had no money to set up a business. I was shut out in the cold by established service companies despite a letter from the chief executive of one of the operators instructing that my services be engaged.


Basic interference testing and resolution

The first step is to acquire all available drawings and maps of the area.
Local planning authorities, electricity suppliers, gas and water suppliers an the national survey authorities are usefull starting points.
Satelite imaging is now a valuable rescource to give a factual picture on which to base all the information (in the abscence of the dynamic project which incorporates all this information)

The second step is to add all of the historical data available from cathodic protection records to a schematic of the area in the form of a circuit diagram.

The areas of high ground potential should be shaded in various degrees of green allowing for areas in which corrosion might be found should be shaded in various degrees of red. It is convenient to relate all of these voltages to to the pipeline metal being zero.(deep red)
It follows that a groundbed area of 5 volts or more would be deep green and that any metal in this area would be protected by the current tending to enter at every interface.
This metal would then be charged and shown as an equal voltage to the part of the groundbed profile through which it passed.
If this is the subject pipeline it would be immediately drained of charges by the negative connection to the transformer-rectifier, thus being at zero volts.
Any sacrificial anode attached to the subject pipeline in the area of influence of the impressed current ground bed would be protected depending on it's nobility in relation to the pipeline metal. It is important to recognise that in an area of over 1.6 volts the ground potential would show a depression in potential with the impressed current switched on and this would reverse to a peak when the current is on. This feature is easily recognised in a two-half-cell survey grid plot with the TR switching.

A foreign conductor passing through the subject groundbed area(very green) will pick up charges and discharge in areas where the ground potential is lower(shaded red)
Their presence will cause the ground potential to be higher (greener or less red) than surrounding ground. This will be detectable on a CIPS or DCVG survey but the cause might not be apparent until a proper two-half-cell survey is conducted with that particular groundbed switching.

It is best practice to notify all services in the area of interference testing and to invite their representation. However, it is never essential to have their co-operation, which is just as well because you will find that most of them have no idea what it is all about. Even worse is that some of them pretend to know what they are doing but their procedures are not logical and are unproductive.
Don't let them confuse you! Keep a clear electrical picure in your head based on the data that you gather. Believe your meters and trust your own expertise. Electricity does bullshit!
If you haven't established all the facts then your electrical picture will be unclear. Gather more facts!!! Add more data to the schematic circuit and the real picture will become clear.
If you can get the co-operation of other operators in the area then it will be very useful to get them to switch their own equipment at certain stages of your investigation.
You should only do this when you want to confirm your own picture and can predict the likely outcome of their switching. You should explain your reasoning to them, before the switching begins. If the results are not as expected you will look a fool if you do not have alternative paths of enquiry, so make sure that have covered everything.
At this stage you should have remedial action in mind.