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.
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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

Cathodic protection is important

An idiots guide to cathodic protection

What the heck IS cathodic protection in the first place???

Cathodic protection is an electrical way of stopping rust.

Rust is chemical and electrical. Metal dissolves in some solutions and gives off electricity. Metal can be 'plated' onto other metal electrically.
All 'batteries' work on this principle and everyone knows that batteries drive loads of the things we use daily.
Not many people know that our gas and oil comes to us through pipes that are inclined to rust, but are protected by 'cathodic protection'.
Some people know that metal boats are protected by cathodic protection, and have seen lumps of metal attached to hulls for this purpose. These lumps of metal dissolve in the water and give off electricity which prevents the hull from rusting.
When you put two different metals in contact and submerge them in liquid (or wetness) one of the metals dissolves and discharges an electrical current into the liquid. The liquid (or damp material) is the 'electrolyte' and gets 'charged up' with electricity. It's 'electrical potential' is increased.
Electricity works by 'pressure' and anything with a higher 'pressure' gives off electricity to anything with a lower 'pressure'.
The electrolyte is then at a higher electrical 'pressure' than the metal that is not dissolving and so the electricity passes into it.
The metal that is dissolving is the 'anode' from which the electrical current passes into the electrolyte and the other metal is the cathode into which the current passes because the electrical pressure must be balanced out. (everything tries to equalise).
The dissolving metal is sacrificed to prevent the subject metal from corrosion, and this method is known as 'sacrificial cathodic protection'.
There are limits to which sacrificial cathodic protection can be used but the same principle can be used by causing a manufactured electrical pressure which is 'impressed' into the electrolyte. The electricity is then 'drained' out of the subject metal....... boat hull or pipeline.... and this interferes with the natural tendency of the metal to dissolve....or rust!

Impressed current cathodic protection

Electricity is generated by a sort of pumping action which causes it to flow backwards and forwards in 'waves', but this is no use for our purposes so we have to get it going in one direction through a circuit known as a 'rectifier'. At the same time we can control the amount of current by transforming it, so the apparatus is know as a transformer-rectifier.
A transformer-rectifier can be regarded as an electrical pump which is sucking the electricity out of the pipeline (etc) and pumping it into the ground (or sea ... or swamp... or wherever else you want to pump it).
The effect of this is amazing. It stops rust! And it's cheap!
But there are some snags.
Because it's so good, it gets installed .... then ignored...... well most people don't even know it exists... and because it's cheap some people don't think it's important.


But it 's life and death to some.

The villagers in the picture are gathering water from outside a flowstation in Nigeria. A pipeline in Nigeria leaked petrol and local people collected the petrol in cans and washing up bowls and the site drew hundreds of women and children until the petrol was accidentally ignited.... cooking up to 1000 people.

Cathodic protection IS important. A couple of years before this incident a pipeline in the USSR exploded and blew a train off it's tracks, killing many and causing ecological devastation. This was thought to be caused by corrosion.

by : Roger Alexander (my great Teacher)

Newsflash 5th December 2000 *** Natural gas spewing in Texas MONT BELVIEU, Texas (AP) - A pipeline ruptured and released a potentially explosive cloud of natural gas, forcing evacuations of about 40 homes and the rerouting of airplane flights around the area. Several minor injuries were reported Monday night when the pipeline, owned by Channel Industries Gas Co., blew open near Houston Raceway Park. The blowout was felt and heard as far away as Baytown, more than 10 miles to the south. There was no fire, said Baytown police Sgt. Keith Dougherty. However, residents of the immediate area were told to evacuate and flights east of Houston were kept at least miles from the site as a precaution, said Texas Department of Public Safety spokesman Richard Vasser. Full article at: http://www.infobeat.com/fullArticle?article=405225991

INTERNATIONAL STANDARD ISO 15589-2

Foreword

ISO 15589-2:2004(E)


ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.

The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.

ISO 15589-2 was prepared by Technical Committee ISO/TC 67, Materials, equipment and offshore structures
for petroleum, petrochemical and natural gas industries, Subcommittee SC 2, Pipeline transportation systems.

ISO 15589 consists of the following parts, under the general title Petroleum and natural gas industries —
Cathodic protection of pipeline transportation systems:

— Part 1: On-land pipelines

— Part 2: Offshore pipelines


Introduction

Pipeline cathodic protection is achieved by the supply of sufficient direct current to the external pipe surface,
so that the steel-to-electrolyte potential is lowered to values at which external corrosion is reduced to an
insignificant rate.

Cathodic protection is normally used in combination with a suitable protective coating system to protect the
external surfaces of steel pipelines from corrosion.

External corrosion control in general is covered by ISO 13623.

Users of this part of ISO 15589 should be aware that further or differing requirements may be needed for
individual applications. This part of ISO 15589 is not intended to inhibit alternative equipment or engineering
solutions to be used for the individual application. This may be particularly applicable where there is innovative
or developing technology. Where an alternative is offered, any variations from this part of ISO 15589 should
be identified.

Deviations from this part of ISO 15589 may be warranted in specific situations, provided it is demonstrated
that the objectives expressed in this part of ISO 15589 have been achieved.

More... you can download the file

http://www.4shared.com/file/142724789/8ca67b26/ISO15589-2-2004forOR.html

thks for all

bajakz

Examples of Design for Cathodic Protection Systems

From Estimated Exposed Surface Area

Estimating current requirements from expected exposed surface is always subject to
error. There are many factors, which affect the results.
Consider:
• Total surface area in contact with soil or other electrolyte.
• Dielectric properties of any protective coating.
• Factors which may damage a protective coating during installation.
• Expected protective coating life under service conditions.
• Expected percentage coverage by protective coating.
• Past experience with coating applicators and construction contractors.
• Current density required for cathodic protection of the metal(s) in the
environment.

In the end, the expected current requirement depends on calculating the area of
exposed metal in contact with the electrolyte and multiplying it by the “best estimate”
of current density for the conditions present.
There is an alternate approach for coated electrically isolated structures (pipes, under-
ground storage tanks, etc.) where there is data available on existing cathodic protection
systems.

The approach requires reliable local data on:
• Expected leakage conductance (Siemens/unit area) in 1000 ohm cm. soil for a
class of coating (epoxy, polyethylene tape, etc.) and type of service
(transmission pipeline, gas distribution, fuel tank).
• Soil resistivity in the service area.
• Structure to soil potential shift required to produce polarization needed to meet
cathodic protection criteria. This is the immediate change in potential of an
isolated structure measured to a point at “remote earth” when cathodic
protection is applied.

The value is not a criteria for protection. However, under a given set of operating and exposure conditions, a potential shift will provide a good estimate of current needed to meet accepted criteria.

The approach is best understood by using an example.

Example 5.1

A gas utility is planning to install 3049 meters (10,000 feet) of 5.1 cm (2 inch) coated
steel distribution mains in a new development. The average soil resistivity in the area
is 5,000 ohm cm. The corrosion engineer wishes to estimate the approximate current
required to cathodically protect the pipes.
Experience in the utility has developed the following data on cathodic protection
current requirements:
Average leakage conductance G for distribution type service is 2.14 × 10−3S/m2in
1000 ohm cm soil.
Average potential shift measured to “remote earth” to achieve protection is −0.250
volt.
Calculations:

Total surface area of the proposed pipe.
As=πd L = (5.1 × 3.1416/100) × 3049 = 488 sq. meters

Estimated leakage conductance of new pipe in 1000 ohm cm soil.
g = G × A = 2.14 × 10−3×488 = 1.04 Siemens
Since resistance = 1/conductance
Resistance to remote earth = 1/1.04 = 0.96 ohm

Estimated resistance to remote earth in 5000 ohm cm soil. (Resistance is directly pro-
portional to resistivity).
0.96 × 5 = 4.8 ohms
Estimated current to shift pipe potential to remote earth −0.250 volt. From Ohm’s
Law (I = E/R)
0.250/4.8 = 0.052 A.

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CATHODIC PROTECTION BASIC PRINCIPLES

The presence of anodes and cathodes in a structure can be caused by micro or macro influences.
On the micro scale, they may be due to:

Heterogenieties in alloy structure.

Oxide layer.

Difference in stress level.

Micro segregation, etc

On the macro scale, anodes and cathodes may be caused by:

Variation in oxygen availability.

Water composition.

Soil resistivity.

Bi-metallic couples.

Presence or otherwise of protective coatings, etc.

Corrosion results from an electrochemical reaction. It requires an anode, a cathode, a common electrolyte, and an electrical connection between the two zones. The corrosion process results in the flow of a small electric current from the anode to the cathode through the electrolyte. The magnitude of the current which is due to a number of factors is directly proportional to the metal lost due to corrosion. One ampere flowing for one year would result in the loss of 9 kg of steel from a corroding surface.

Freely flowing corrosion current from Anode to Cathode.

In recent years cathodic protection has found a general acceptance amongst engineers and structure owners as being a truly effective method of preventing corrosion under the ground or under the sea. It is now more common than not to find cathodic protection used on marine structures and on buried pipelines.

The concept of cathodic protection is straight forward. Corrosion occurs as the result of electrochemical reactions between zones of differing potential on a metal surface. Oxidation (corrosion) occurs at the anodic zone and reduction (no corrosion) occurs on the cathodic zone. Cathodic protection is achieved when an entire metal surface is converted to a cathodic zone.

The corrosion reactions at each surface may be described as:

Cathodic protection is achieved by supplying a current from an external source so that it reverses the natural corrosion currents and ensures that current is flowing through the electrolyte onto all of the metal surface requiring protection. This current flow causes a change in potential.

Freely corroding mild steel in seawater has a resultant potential between anode and cathode of approximately -0.50 to -0.60 volts compared to a silver/silver chloride reference electrode. When cathodic protection is applied, it will be noted that the surface potential of steel will change to more negative than -0.80 volts when measured relative to a silver/silver chloride reference cell. Thus by using this simple practical measurement, it is possible to determine whether corrosion has been completely eliminated or not.

The external current applied in cathodic protection may be generated from either of two methods, sacrificial anodes or impressed current systems.

GALVANIC ANODES
Sacrificial or galvanic anodes rely on the galvanic corrosion of a more reactive metal to produce current, e.g. aluminium anodes, zinc anodes or magnesium anodes.

Flow of corrosion current suppressed by protective current discharged from sacrificial anode.

Sacrificial anodes are most commonly used to protect metallic structures in electrolytes because of their simplicity of installation and maintenance free operation. Of the alloys available for sacrificial anodes, alloys of aluminium have proven to be the most economical in seawater or very low resistivity muds.

Knowing the total submerged and buried steel areas, the water resistivity and the required system life, a corrosion engineer can determine precisely what energy will be required to protect a structure and can design a galvanic system to suit the environmental requirements.

IMPRESSED CURRENT ANODES
Impressed current systems provide the same electric current as galvanic anodes by the discharge of D.C. current from a relative inert anode energised from an external D.C. power source such as a transformer rectifier or thermo electric generator. Impressed current system anodes include materials such as graphite, silicon iron, platinised precious metals and lead alloys.

Flow of corrosion current suppressed by protective current discharged from Impressed Current System.

Effective cathodic protection guarantees corrosion free existence. Providing the structure is maintained at a potential of -0.8 volts (or more negative) no loss of metal will occur at all during the life of the structure. As cathodic protection can be renewed or added to during the life of the structure, the maintenance of the desired potential is readily achievable. The efficacy of the system can be monitored by simple electrical measurements.

Cathodic protection apart from overcoming the more "normal" causes of corrosion, may be used to counter accelerated corrosion resulting from contact between different metals, from impingement by high velocity water, from the effects of sulphate reducing bacteria and from the effects of stray D.C. currents.

In fact, any metal such as scrap iron may be used as an impressed current anode. In cathodic protection practice, we choose to use either semi-permanent or permanent anode and very seldom non-permanent anode (such as scrap iron).

Examples of semi-permanent anodes are silicon/chromium/iron anode, lead/silver/antimony anode, graphite anode etc.
Examples of permanent anodes are mixed metal oxide anode, platinised titanium anode etc.