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CALCULATION CATHODIC PROTECTION SYSTEM FOR PIPELINE

Under Ground Surface Pipe Area

The surface pipeline to be protected, as follows:

Length pipeline (L): 4.875 km

Pipeline Diameter (d) : ∅ 12 inch = 323.85 mm = 0.32385 m

Total surface to be protected (A) = π * d * L

A = 4957.33 m2

Demand of Current

Total demand current to given formula:

Where :

= Demand Current (Amp)

= Surface area to be protected (m2)

= Current density per m2 (mA)

= Coating breakdown factor

Therefore:

mA = 0.992 Amp

Resistance of Magnesium Anode for Kp 4+500

Rmg : Magnesium anode resistance (Ohm)

Lmg : Length of prepacked magnesium anode 32lbs (m) 0,9 m

Dmg : Diameter of prepacked magnesium anode 32lbs (m) 0,195 m

ρ : Layer Soil resistivity data at depth from survey (Ohm cm) 5500 ohm.cm

Rmg = 25.3739 ohm

Driving Voltage Anode

Net Driving Voltage The net driving voltage between Magnesium anode of -1.6 V

(Cu/CuSO4 reference) is 0.75 V (1.6 - 0.85).

Close circuit potential (Volt) of magnesium is -1.6 volt

Current of anode For Kp 4+500

Ia : Magnesium anode current output (Amp) = (Ec - Ea) : Rmg

Ipf : Required current

Ec : Protective potential (Volt) -0,85 Volt

Ea : Close circuit potential (Volt) -1,6 Volt

I = -0.85 – (-1.6) : 25.3739

I = 0.0296 A

Sacrificial anode requirement

Mass Metode

Total Sacrificial Anode weight to be given formula:

Where:

Y = Design Live Period (year)

S = Consumption rate (kg/amp-year)

= demand current (Amp)

f = Utility factor

So, total weight;

kg

N = 13.68 ≈ 14 Pcs

Current Output Method for Kp 4+500

Ip = Current Protective

Ia = Current Anode

N = 2.6 ≈ 3 Pcs

Lifetime of anode

Current capacity = mass anode : Consumtion rate

Current capacity = 14.5 kg : 8 kg/amp.year = 1.8125 A.Y

Life of anode = Current Capacity (A.Y) x N x f : IR

Life of anode = 1.8125 A.Y x 14 x 0.8 : 0.992 A

Life of anode = 20.46 Years > 20 Years

Total Anode Required

The total Anode required is 40 Pcs, weight is 32 lbs or 14.5kg for each. Uses Current Method for better distribution.

cathodic protection system for internal tank

Basic calculation for cathodic protection system for internal tank

Data:

Atmospheric Storage Tank:

- length : 11 m

- Diameter (D) : 38 m

- high of water level (L) : 9 m

Current Density (CD) : 55 mA/m²

Consumption Rate of Aluminum Anode : 3.5 kg/Amp Year

life time design : 0.2 tahun

Utility Factor : 0.8

Calculation:

1 Surface Area (SA)

            - SA bottom = p x D²            = 3.14 x 38²                   = 1134.1 m²
                                         4                                      4
            - SA wall = p x D x L                    = 3.14 x 38 x 9 = 1074.4 m²

Total Surface Area = 2208.5 m²

2 Current Requirement (Ip)

Ip = SA x CD

= 2208.5 m² x 55 mA/m²
= 121467.5 mA = 121.5 Amp

3 Weight of Anode (W)

W_Al       = Ip x Y x 3.5 kg/Amp Year
                                U
            = 121.5 Amp x 0.2 years x 3.5 kg/Amp Year
                                           0.8

= 106.3 kgs

4 Number of Aluminium Anode (N)

Proposed Aluminium Anode type 120 ADFB - Net weight = 12 kg/ea

N_Al = W_Al : 12 kg

= 106.3 kgs : 12 kg

= 8.86 pcs » 9 pcs

Total = 9 pcs Aluminium Anode type 120 ADFB

TEST FACILITIES AND TEST POST LOCATIONS

In order to monitor cathodic protection, we must be able to contact the metal of the subject pipeline or structure.

There was a time when contact was achieved by driving a steel rod into the ground from above the pipeline, and making temporary contact by piercing the coating. The main reason for discontinuing this practice was the physical damage that was possible to the pipe metal itself, and that damage that was caused to the coating.

Most pipelines now have provision for contact through electrical conductors connected to the subject metal in a variety of ways.

The most common is a process known as cadwelding, which can result in a low resistance, permanent, electrical bond between the copper conductor and the steel of a pipeline.

The disadvantage of this type of connection is that it needs considerable skill to achieve a good connection, in some field conditions. The joint must be carefully inspected and the integrity of the coating must be tested before backfilling.

We are all aware that copper and brass are more noble than steel which will tend to disolve if coupled together in an electrolyte. Cadwelding introduces such a ' bi-metalic coupling' to the surface of the pipe and care must be taken that all metal is separated from the electrolyte by a chemically impervious, electrically resistant coating. This coating must be compatible tho the pipeline coating and to the insulation on the copper conductor cable.

The use of copper conductor cables also introduces the possibility of a bi metalic coupling if its insulation is not perfect. It should be remembered that the voltage that we are measuring is between the potential of copper in a saturated solution of its own salts, and steel in the local environmental electrolyte.

If the conductor to the pipe is severed, then the voltage that we measure will be that between copper in a saturated solutionof its own salts and copper in the salts that are present in the local environment. This voltage will be very low, as the difference between the potentials, will be small.

Readings can be very confusing however as they are sometimes affected by the cathodic protection current. Charges will be passing onto the broken copper tail which is still attached to the steel of the pipe, due to the galvanic activity, and this will cause a variation in the potential of the ground in the immediate vicinity. The extent of this area of influence depends on the area of contact between the copper and the electrolyte, and the resistance of that electrolyte.

If the conductor is not completely severed, it will definitely draw currentfrom the ground, and this will have a significant effect on the measured voltage if the insulation damage is close to the electrode position.

The contact between the conductor and the subject metal must have a low electrical resistance, as it may be used for measuring current.

The best test facility is direct contact with the pipeline at a riser but there are many sections of pipeline which are buried with no riser.

There was a period when some test posts were connected to two conductors which contacted the pipeat two locations exactly 100m apart. The purpose was to enable current direction readings to be taken, but although I saw several attempts to do this, I never saw it done successfully, or was never able to obtain meaningful readings myself. I read and understand the theory behind this type of measurement, but the application seems impractical in field work.

The electrical resistance of the pipeline itself is extremely low, for example a 4" dia. steel pipeline is 0.141 ohms per mile and a 24" dia. pipeline is an incredibly low 0.0161 ohms per mile.(Peabodies) If we are dealing with other structures such as storage tanks we can never consider the resistance of the metal itself, as a significant feature in cathodic protection calculations.

It therefore follows that the POTENTIAL of the pipe metal does not vary significantly, over a 2km section of continuous welded steel pipeline, with the diameters quoted. This matter was debated during the application of over-the-pipeline potential surveys, conducted in the UK, on high pressure, welded steel, gas mains.

I was part of a team that carried out the field test which resolved this matter, on a 2km section of 24" dia. welded steel, coal tar enamel coated, buried pipeline. This pipeline was protected by impressed current cathodic protection which was switched on continuously during these tests.

The negative pole of a high resistance voltmeter was connected to the test post conductor terminal at the top of a test post at location A.

A standard copper/copper-sulphate electrode was placed in a fixed position at location A.

A reel of armature wire was used to connect the electrode to the positive pole of the high resistance voltmeter and the reading was noted.

The wire was reeled off the spool and used to make contact with a standard copper/copper-sulphate electrode at location B, which was 2km distance from location A.

A changed voltage was noted on the meter, which was still connected to the test post at location A.

The change of standard electrode positions had significantly altered the recorded voltage.

The armature wire was then used to connect the negative pole of the voltmeter to the distant test post at location B.

The positive pole of the voltmeter was then reconnected to the electrode at location A.

The voltage on the meter was identical to the first reading.

Altering the position of contact to the pipeline, by a distance of 2 km, had no detectable influence on the voltage measured.

The meter was taken to location B and connected between that test post terminal and electrode B.

The voltage recorded was identical to the second voltage of the test, confirming that the location of the electrode is the only significant feature.

The positive pole of the meter, at location B, was then connected to the armature wire which was connected to the electrode in the fixed position at location A.

The voltage recorded was identical to the first voltage recorded confirming, once more that the point of contact to the pipeline has no detectable effect on the recorded voltages.

The discussions culminating in this test, resulted in a re-appraisal of test post locating within the operating company. It was decided that fewer test posts were needed, and that the priority importance was access to the test post locations.

The best form of test post, for a steel pipeline consists of a steel bar welded directly onto the pipeline metal and protruding through the surface of the ground directly above. This is protected by encasement in a concrete block, which includes a vertical 4" dia.pipe filled with the local ground material. The standard electrode would always be placed in the top of 4" pipe for the purposes of periodic voltage measurements.

Measuring the effects (CATHODIC PROTECTION)

Measuring the effects

Cathodic protection has been extremely cost effective since first used, but there have been instances throughout its history, where corrosion failures have occurred in spite of it's use.

The problem is to measure the effectiveness of cathodic protection. Corrosion is electro-chemical and this suggests that electrical metering can be used for short term monitoring.

The simplicity of the circuit of a single corrosion cellwould tend to suggest that there is a simple means available to make the required measurement. Standard reference electrodes have a recognised and known potential which can be used as an electrical datum point against which to measure other potentials, in a laboratory.

We normally measure VOLTAGES which are the differences between two potentials.

This causes confusion because the readings are commonly called "potentials", where in fact, either of the two potentials can be regarded as zero and the other will be either higher or lower. The meter will show positive or negative values according to the polarity of the connecting conductors.

Cathodic protection theory dictates that the metal must be reduced to below its corrosion potential IN RELATION TO A STANDARD REFERENCE POTENTIAL. These potentials can be measured in a laboratory where it is possible to control all elements of the circuit, but it has proved impossible, so far, to measure the required potential in field work.

The problem with field measurements, is that the earth at one location has never exactly the same potential as the earth at another location. In a laboratory, the electrolyte is contained in an electrically insulated container and the currents are all in closed circuit and related to the corrosion reaction. The potential of the electrolyte can be measured at the reaction interface by the use of a glass capillary containing an inert, but conductive, electrolyte such as agar-agar gel. This cannot be achieved in field work, although a close approximation has been achieved by Dr Prinz of Rhurgas, in Germany.

Field readings taken and analysed in the established way are not related to each other, except through the low resistance of the pipeline itself. This is easily demonstrated by a simple calculation base on Ohms law as follows.

Take any sample readings from a typical pipe-to-soil "potential" cathodic protection survey and work out the amount of current that must be passing through the pipeline between any two cathodic protection test facilities. It will be found to be ridiculously high, to the extent of being unbelievable.

For example, a span of 24"diameter steel pipeline ten miles long will have a resistance of about 0.001 Ohms, depending on the wall thickness, and the readings at either end of the span might be -0.950 volts and -1.250volts. This would not cause alarm, and would be plotted on an 'attenuation curve', without too much comment. However, calculation shows that, if the 'half-cell' (electrode) is truly a reference, then there is a volts drop of 0.300 over the ten mile span. This seems reasonable until it is realised that with such a low resistance there must be 300 amps passing through the pipeline. Something is quite clearly wrong with the measuring system or the theories.

This has not been seen as a major problem until pipelines became so widespread and numerous that reliability became a major industrial consideration. Even now there seems to be little concern with this subject until a failure causes financial losses. The public at large are not even aware that the inadequacy of present technology could result in an unforeseen disaster.

As far as I'm concerned, the more fucking pipelines that blow up the better. I'll put my price up.

Before going any further it is necessary to imagine electricity and this has been likened to water pressure, with containers connected by pipes to allow current to flow.

The pressure is caused by the height of the water in each container and not the weight. The water will fill any connecting tube and then the pressure downwards will be greater in the vessel which has the highest level. The reason for this is obviously due to the imbalance between the pressures in the two containers and electrical potentials have the same tendency when connected by conductors.

This is fine when visualising a simple circuit such as a single corrosion cell or a dry cell battery connected through a light bulb, but in a cathodic protection circuit, or when corrosion takes place on a pipeline we have no means of measuring each separate cell in this way.

If we examine the technique that is used in the laboratory then it becomes clear that provision has been made to eliminate outside influences in this 'open circuit measurement'.

This is not possible in cathodic protection field work, and yet laboratory derived theories are applied to readings obtained in the field.

It can be seen that it is impossible to measure the pressure differences in each cell by making a single connection to the common reservoir at the bottom. However it would be possible to stop the flow of water from the highest level in the small vessels by adding a supply of water from a higher level.

However, it can be seen that the pressure measurement in such a system would need to be between the lowest water level and the highest water level in the whole system. This would be a much greater voltage (Vp in the drawing) than that required to stop the flow in the single cell with the biggest differential.

Comparing electrical pressure with that of water is a good starting point, but it is better to imagine electricity as simply a pressure which can pass through conductors, and is restricted by resistances.

Imagine trying to measure the gas pressure within a cylinder. We must allow that pressure to act on a meter which will guage the pressure. This action will consume some of the gas within the cylinder and it is the passage of the gas which makes it possible to measure the pressure itself.

The same rule applies to electrical pressure and this used to cause considerable inaccuracy in voltage measurements until the digital meters made it possible to measure voltages while drawing very little current.

Back to the gas cylinder and imagine measuring the pressure with a guage which draws very little gas. We still have the problem that this pressure has to be compared to something. In the case of gases, we can related this pressure to atmospheric pressure, displayed in such a way that we can imagine its effect on our senses. We are aware that we are all subject to atmospheric pressure and the effect of increasing the pressure on the human body can be felt, when swimming under water, for example. We use our muscles to compress stale air which is then exhaled and can feel the current of air through our nose and mouth. Gas pressure is therefore part of our lives with which we are familiar. We can use this experience to imagine electrical pressure, which has similar qualities.

Everything has an electrical potential (pressure) which has the tendency to equalise on contact with another item of a different potential. It is this tendency which causes current to flow and allows us to make.

In the same way that chemical reactions can give off gas, and increase the pressure within a cylinder, for example, chemical reactions can cause an electro-motive-force (EMF) which increases the electrical pressure, or potential, on one side of the reaction.

In order to measure the electrical pressure of this reaction we must complete a measuring circuit with a low resistant electrically conductive path. The whole measuring circuit reaches equilibrium with a small amount of current flowing depending on the requirement of the meter.(in the case of digital meters, the current required to make the measurement is very small).

In the case of measuring the voltage of a dry cell battery, we connect a voltmeter between the poles of the battery and the voltage is the difference in electrical pressure caused by the chemical reaction at the interface between the electrolytic paste and the inner surface of the metal container and the electrode which serves as the positive pole of the battery.

The technique is simple because it is possible to confine the path of the current to that of the measuring circuit and each element of this circuit can be evaluated. Voltage drops can be measured around the circuit, using independent meters and measuring current can be detected by magnetic field and other techniques.

Natural corrosion cells are much different, as they can be physically minute or large. Large corrosion cells can contain micro-cells within the same area where anodic areas completely surround cathodes or vice-versa. When studying such cells, we are not able to separate the component parts, and the measurements have come to be known as 'open circuit measurements'.

This type of measurement involves connections to the electrolyte as well as the metal and this requires the use of an electrode. There is a danger that this will introduce another EMF into the circuit, by the reaction between the electrode and the electrolyte. We therefore use an electrode in a solution of its own salts, which has a known reaction EMF. We can then make a connection between the electrolyte in the cell and the earth electrolyte, in the hopes that there will be no electrical disturbance to the measuring circuit.

In the laboratory, this disturbance is prevented by the use of a glass capillary filled with inert gel, which is used as a conductor from the reaction interface to the reference electrode. The reference electrode is a metal in a saturated solution of its own salts, as this has a known reaction potential. Reference electrodes are related to each other by known voltages and are used as international standards. Without this consistency it would be impossible to evaluate the reaction, develop theories or design cathodic protection systems etc.

Unfortunately, it became the practice to apply the same principles in cathodic protection field work. It seems that many thought that the electrode could be regarded as a reference against which other potentials can be established. They thought that pipe to soil voltages were pipeline metal potentials which could be plotted against a fixed potential supplied by the use of the 'reference electrode'. There are still remnants of this concept in cathodic protection practice today, which are manifest in 'attenuation curves' etc., which are used by some in the design of CP systems.

This subject can now be studied in greater detail by computer modeling which makes it much clearer that the fixed potential is normally that of the pipeline metal, and the variation in the measured voltage is due to the different potentials elsewhere in the measuring circuit.

Imagine that we require to know the voltage of two dry cell batteries which are arranged in parallel. That is to say that each is in connection with a common conductor to the positive pole and another common conductor to their negative poles. Both conductors would carry equilibrium current according to the reaction within each battery and the voltage between the two conductors could be measured by connecting a meter between the two. Unless the two cells are separated, it is impossible to evaluate the voltage of each battery. Even this is not as complex as the expectancies of cathodic protection monitors.

If we take two batteries and half bury them in an electrolyte with their positive poles exposed and connected, we have two corrosion cells in closer condition to those found on a pipeline. A circuit drawing of this arrangement will show that current will pass through the ground to equalise the pressures caused by the interface reactions within each battery.

We must now try to evaluate the reaction within each battery using a high resistance voltmeter and an electrode. We cannot break the circuit or separate the batteries but connections can be made to the metal or the electrolyte or both. It will be seen that we are only capable of measuring voltages across various spans of the circuit, and cannot establish a reference within that circuit. The laboratory techniques cannot be applied to these conditions as there are too many variables which are impossible to evaluate.

If we increase the number of half buried batteries connected together, we improve the similarity to a pipeline, but in order to be more realistic, we must include some which have their positive poles buried. The complexity of the situation is now apparent and what seemed to be a simple measurement, now seems almost impossible.

A circuit diagram of the complex arrangement will show that a different voltage will be measured with every new position of the electrode, and this is born out in cathodic protection field practice. It is especially obvious on pipelines which are not connected to cathodic protection systems and which have poor coating.

The different voltages are due to the variety of potentials at each pole of the voltmeter. These can be caused in many ways, as described later, but it is important to realise that they are all components of the voltage shown on the meter. It is possible to eliminate them in the laboratory but not in the field, therefore they must be evaluated and considered in the analysis of survey results.

The problem is even more complex when cathodic protection is introduced as this is an additional voltage which is superimposed over all the others. Being designed to drain charges from the whole of the pipeline, it has an effect on the equilibrium of all the other electrical influences. However, the dynamic effects of an impressed current system can be removed by taking voltage measurements immediately after the system has been switched off. This cannot be achieved where sacrificial anodes are used, unless they have a special facility designed for this purpose at construction stage.

The voltages obtained between the pipeline metal and a randomly placed electrode have a certain amount of value when compared to others obtained from connections to the same pipeline. This is because of the very low electrical resistance in this part of the corrosion and cathodic protection circuits.

EXPERIMENT HOW TO UNDERSTAND VOLTAGE OF CATHODIC PROTECTION SYSTEM

It is possible to make a model of electrical components to demonstrate further complications involved in DC electrical fields of this nature.

Dangerous Coating Breakdown Factor

Explosion of gas pipeline because wrong protection the pipeline. this because a little bit coating breakdown. Just a very little bit but give more lose
Cathodic Protection System will protect pipeline from this thing.
i will show you why coating breakdown will destroy the gas pipeline
dangerous of coating breakdown factor
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basic cathodic protection system for presentation, dasar proteksi katodik

basic cathodic protection system for presentation, dasar proteksi katodik
Presentation of cathodic protection system, basic about cathodic protection system if any question, please contact me (08565305351 - bajakz@gmail.com)

CONCEPT CATHODIC PROTECTION SYSTEM

The basic concept of cathodic protection is that the electrical potential of the subject metal is reduced below its corrosion potential, and that it will then be incapable of going into solution, or corroding.

This mechanism has been defined by many scientists and has become established beyond dispute. Indeed the principles of corrosion reactions are used in the design and construction of expendable and re-chargeable batteries and accumulators which play such a major part in modern life.

A battery that is 'dead' has no energy left and does not corrode any further. Likewise a car battery on charge does not corrode, in fact in this case the reaction is reversible, and energy is 'pumped back in'.

However, a battery has a very carefully composed electrolyte which has qualities to ensure a predictable reaction with the other components of the battery. We know that the corrosion within a battery can be controlled very accurately, by external electrical input, as this technique is in common use with rechargeable batteries which are nowadays controlled by computers which balance the reaction equilibrium to suit their own power demands.

Unfortunately a cathodic protection system is not composed of simple elements in the way that batteries are, because the electrolyte is the ground itself. This electrolyte is uncontrollable and has an almost infinite variety of qualities. The chemical composition and electrical conductivity can span a vast range, as can the temperatures and pressures to which the reaction is subjected.

Cathodic protection of such subjects as ships hulls and storage tank bases is relatively simple as the electrolyte is likely to be almost homogeneous, but as the size of the structure increases, it extends through different electrolytes and the reaction at each interface varies.

Offshore oil rigs, for example have different temperatures and pressures at the sea bed to those at the surface, and a study of this situation has shown that it has a substantial influence on corrosion.

Pipelines can be regarded as many interface reactions connected together in parallel. The metal element can be well defined, as this is specified to a high degree by the designers, as is the coating material.

However it is accepted that no coating can be perfect, and the faults, or 'Holidays' introduce the first indefinable variable to the system.

During the construction of a pipeline all possible measures are taken to detect and repair coating faults, so it follows that those remaining are undefined. It is possible to calculate the theoretical resistance of a perfectly coated pipeline, given the specification of the coating and dimensions of the pipeline, but it is impossible to calculate the actual resistance of the total pipeline.

The electrical current measurements, taken during routine cathodic protection monitoring, show that there is little resistance in the total coating (with faults) of a pipeline and this can be explained by the difficulty in quality control, during the construction period.

Undetected coating faults are the path of cathodic protection current and a perfect coating would prevent any output from the CP system. We therefore, know that there are many unspecified 'metal to electrolyte' interfaces present on an average pipeline.

The electrical resistance of the pipeline metal itself can be calculated, and is found to be very low. In fact the effect that the pipeline resistance has on the complex current paths and variation in potentials, is so small that it can almost be ignored.

The complication is due to each interface being capable of a different reaction, electro- motive-force (EMF) which cannot be measured as it is in parallel with all other EMF’s on the same section of pipeline. The magnitude of the current from each of these reactions is dependent on the earth resistance immediately adjacent to the interface, and the direction of all the resulting currents is the result of the combined effects of all the resistances and electrical pressures caused by all the EMF's.

Although it is simple to understand each corrosion cell and the mechanism of corrosion itself, the reality of applying the science, to the field, becomes immensely complex. This becomes more obvious when the circuit has been subject to computer modelling as discussed later.
To be effective, cathodic protection must reduce the metal at each single interface, to below it's corrosion potential. This is not too difficult to achieve, as each interface is part of the same metal structure, which has a very low electrical resistance. The difficulty is knowing when all the interfaces have been reduced to below their corrosion potential in relation to the electrolyte in their reaction vicinity. ( Don't forget, if we knew where each interface was we would repair them all!!!!)

OVER PROTECTION

There are several other problems, however, as too much current passing onto a steel surface can cause embrittlement, which under certain circumstances can be as detrimental as corrosion itself. This is manifest in such applications as the protection of the external surfaces of drill pipe casings, where a considerable amount of cathodic protection current is used.

CATHODIC DISBONDMENT

Another fear of 'over-protection' is that of cathodic disbondment of the coating. This happens when the coating manufacturers specifications are exceeded. Cathodic protection current passing onto the metal causes the release of hydrogen which disbonds the coating. In reality this is rarely a problem, and a careful study reveals why.

The current will only pass onto the metal at a coating fault, and the density of the current will depend on the size of the coating fault and the current locally available. As the current blows the coating from the metal, the volts drop at the interface will decrease, and equilibrium will be reached with a very small increase in additional disbondment.

If there is no coating fault, then no cathodic disbondment will occur as recognised in the British Standard Code of Practice for testing the coating manufacturers specification. This requires a specific size of coating fault on a steel coupon, to be subjected to an increasing voltage over a specified period. The test cannot be carried out on a coupon with perfect coating as the disbondment is observed under the coating at the edge of the fault.

It is logical to deduce that if cathodic disbondment is caused by current and that if all current is prevented by a perfect coating, then no disbondment will take place. This is not common sense, however, as many excavations have been dug in areas where high 'pipe- to-soil potentials' have caused concern about cathodic disbondment. In the event, it has proved the logic (above) and no disbondment has been found.

In one particular example voltages of over 5 volts had been recorded when the electrode was place on the surface above the buried pipeline which was subsequently excavated, at several spots, for examination. A coating fault was found at one location but no disbondment. The current passing onto the metal at this coating fault, caused a drop in the voltage of the electrode as it got nearer to the pipe. Whereas at the surface the reading had been over 5 volts, this reduced to 0.950 volts when the electrode could be placed close to the actual interface between the metal and the earth.

This simple drawing shows that the earth at the surface has a higher potential than the earth close to the pipeline at the coating fault, due to the current passing from 'mass earth' into the pipe metal.

At such site it is easy to plot the 'potential gradient' using a static electrode as a reference and a moving electrode to trace the potential isobars. As soon as the coating fault is fully exposed to the air, the gradient disappears completely, as the current stops. The meter then reads 5 volts, even with the electrode placed in the ground a few mm from the metal.

by : Roger Alexander

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