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APPENDICES
APPENDIX A
CONTROL OF INTERFERENCE CURRENTS ON FOREIGN STRUCTURES
A.1 General
A.1.1 Interference from cathodic protection systems arises where a foreign structure intersects the direct current path between the anode and cathode. Where the current enters the structure the effect is cathodic. Where it leaves the structure the effect is anodic, and the rate of corrosion at that position may be increased.
A.1.2 Interference may be detected by a change in the potential of the foreign structure when the system current is interrupted. The result of this test indicates whether the foreign structure is being subjected to an increased or a decreased corrosion hazard.
A.1.3 Where a foreign structure is sited adjacent to a protected immersed structure, but not electrically bonded to it, interference can occur. Two common foreign structure types are as follows:
a) A discrete movable structure, such as a moored ship.
b) A buried or immersed pipeline or metal sheathed cable adjacent to the protected structure or its anode system.
A.1.4 Galvanic anodes used in immersed systems do not usually cause interference to other structures.
A.1.5 Interference problems are more probable with impressed current systems because of the electrolyte voltage gradients usually associated with the anodes.
Note:
When marine structures are cathodically protected, adequate precautions shall be taken to avoid interference effects when using impressed current and also to ensure that danger does not arise through the production of sparks when ships, barges, etc., make or break electrical contact with the protected structure.
Marine conductors (protective pipes through which wells are drilled) are often closely packed in the conductor bay area. Care shall be taken that adequate current densities are available for the protection of the marine conductors. Full electrical continuity may not always be provided and special measures may be required to ensure this.
Electrical interference effects are negligible with sacrificial-anode systems, as these anodes are placed much nearer to the protected structure than to any unprotected steelwork and because their driving voltage is usually much lower than that of impressed-current system groundbeds.
A.1.6 Telluric current and induced alternating currents can be of particular importance on transmission pipelines. The effects and remedies are dealt with in Clauses A.7 and A.8.
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XAMPLE OF CURRENT PICK-UP BY NEIGHBORING STRUCTURE FROM CATHODIC PROTECTION SYSTEM
THE UNPROTECTED PIPELINE BECOMES CATHODIC AT THE POINT WHERE THE CURRENT IS PICKED UP (A), AND ANODIC AT THE POINT WHERE THE CURRENT EVENTUALLY LEAVES THE LINE (B), RESULTING IN PROTECTION AT ’A’ AND CORROSION AT ’B’ SINCE POINT ’B’ BECOMES ANODIC, THE PIPE-TO-SOIL POTENTIAL MEASURED WITH A COPPER/COPPER SULPHATE ELECTRODE BECOMES MORE POSITIVE WHEN THE RECTIFIER IS SWITCHED ON. A CHANGE TO POSITIVE IN POTENTIAL OF MORE THAN 50 MILLIVOLTS WHEN THE RECTIFIER IS SWITCHED FROM ’OFF’ TO ’ON’ INDICATES AN OBJECTIONABLE DEGREE OF STRAY CURRENT CORROSION
Fig. A.1
A.2 Notifying Owners of Other Structures for Interference Testing
A.2.1 It is essential, throughout the installation, testing, commissioning and operation of a cathodic protection system, that notice of actions proposed be given to all organizations and owners having buried metallic pipes, cables or other structures in the near vicinity of the installation.
These notices are intended to ensure that information becomes available to enable a system to be installed in such a manner that interference is kept to a minimum and that enough information is given to other organizations to enable them to determine whether corrosion interference is likely.
A.2.2 After commissioning tests of the system have been completed, notification shall be sent to all organizations who have indicated that they have structures likely to be affected by the operation of the system. The following information should be supplied at least one month before the date proposed for interference tests:
a) the anticipated current at which each rectifier or sacrificial anode will be operated during interference tests;
b) an indication of structure/soil potentials along the primary structure before and after the application of protection;
c) dates for the tests.
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At the time of the tests, there shall be available to all participants, suitably scaled plans showing the layout of the primary and secondary structures at the test locations together with the locations of the cathodic protection installations, drawn up from information supplied by both the operator and the other interested parties.
A.2.3 If, at any time after the cathodic protection system has been brought into regular service, it is found necessary to alter the system substantially, details of the proposed amendments should be sent to all organizations having buried metallic structures near the revised cathodic protection system.
A.3 Interference Testing
A.3.1 Stage at Which Interference Tests shall be Made
At least one month shall be allowed, if required, for the owners of nearby structures to examine details of the proposed system and to respond so that the operator may arrange for interference tests. Tests shall be made within three months of switching on the cathodic protection.
A.3.2 Tests to assess interference
The changes in structure/electrolyte potential due to interference will vary along the length of the secondary structure and a negative potential change at any point will often indicate the presence of positive changes at other parts of the structure. For most metals, only positive potential changes are liable to accelerate corrosion. The usual object of interference testing is, therefore, to find the areas where the potential change is positive, to locate, by testing a number of positions, points at which the potential change locally reaches a maximum and to assess each maximum value with sufficient accuracy. In the case of discontinuous structures (such as mechanically jointed pipelines) it is essential that each discontinuous section shall be treated as a separate structure for testing.
Quantitative assessment of probable damage is difficult because any current discharge from a foreign structure is difficult to measure, and the surface area from which it discharges is difficult to estimate, particularly in built-up metropolitan areas where there is a multitude of underground services. The extent of foreign structure testing therefore depends on a number of factors, including the following:
a) The relative positioning of the protected structure and foreign structures.
b) Soil resistivity variations.
c) Electrical conductivity per unit length of all structures concerned.
d) Anode current.
e) Condition of coatings on all structures concerned.
Field experience and application of the above factors enables estimation of the likely degree of interference, and the extent of foreign structure testing required. The interference caused by the electrical gradient around the protected structure usually only extends for a radial distance of a few meters from the structure. However, the extent of interference caused by the electrical gradient field around the anode may extend for some hundreds of meters, with the result that foreign structures up to a kilometer or more away may be affected.
In certain cases, negative changes of potential in excess of the level that would be needed for cathodic protection may adversely affect the structure or its coating. The current used for the test shall be the maximum required during normal operating conditions to give the level of protection required on the protected structure. A test current below the anticipated current required during normal operation may not bring about the maximum changes in potential on secondary structures. The criterion is the magnitude of the change of potential of the secondary structure (see Note) with respect to its electrolytic environment that occurs when the cathodic protection is switched on or when the sacrificial anodes are connected. This change is usually equal in magnitude to, but has the opposite sign from, the change occurring when the protection is switched off.
Note:
When, as is normally the case, the positive terminal of the meter is connected to the reference electrode, the potentials measured are usually negative and a change in the
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positive direction will be indicated by a reduction in the meter reading.
The change recorded shall be that change clearly seen to be due to the switching on of the cathodic protection unit, not more than 15 s being allowed for the instrument to indicate the resulting change of structure electrolyte potential before the reading is taken. If there are also fluctuations of potential due to the effects of stray currents from other sources, then only those changes caused by the switching on of the cathodic protection unit shall be recorded. Several observations shall be made at each point and compared. In marginal cases, the number of observations shall be increased and examined for consistency. This is of particular importance at positions where tests indicate that the changes in the positive direction on the secondary structure are locally at a maximum. The position of the reference electrode is often important. It is important to synchronize the measurement of structure/electrolyte potential with the switching. This can be done by providing radio, or other, communication between the individual who is making the actual measurement and the one who is controlling the cathodic protection unit, the cathodic protection being switched on and off alternately by hand. Alternatively, the cathodic protection unit can be switched on and off at agreed regular intervals by means of a suitable time-switch. The change in structure/electrolyte potential resulting from the cathodic protection shall be measured at a sufficient number of points, generally working outward from the anode of the protection system, and with the spacing being sufficiently close, to give an overall picture of the distribution of structure/electrolyte potential change. Detailed attention shall be given to crossing points or points of close proximity between the primary and secondary structures and to regions where the change produced has been found to be in the positive direction. Where more than one cathodic protection unit is installed on a particular structure, the combined effect shall be ascertained. Arrangements shall be made for all units which cause an appreciable effect at the position of tests, to be switched on and off or connected and disconnected simultaneously.
The protection current measured at each rectifier during interference tests, and the finally agreed currents to be employed as a result of any remedial measures, should be notified to all organizations attending the tests and all authorities who have indicated that they have structures likely to be affected by the operation of the system.
A.3.3 Tests after remedial measures have been applied
Further testing may be required after agreed remedial measures have been applied. If, after providing bonds between two structures or fitting sacrificial anodes in order to reduce interference, the structure/electrolyte potential of the secondary structure is found to be appreciably more negative than that measured with the cathodic protection switched off during the initial interference testing, this will normally be sufficient indication that the mitigation procedure is achieving its purpose. The criterion shall be the change of the structure/electrolyte potential between the original condition with the cathodic protection switched off and the final remedied condition with the cathodic protection operating, switching and bonding or anode connection being carried out quickly to minimize any effects of variations from other sources.
It may happen that the initial structure/electrolyte potential of the secondary structure is more negative than the potential of the primary structure. For example, a galvanized steel structure without applied cathodic protection, even when its structure/electrolyte potential is changed in the positive direction due to the effect of a nearby cathodic protection system, may be more negative than a cathodically-protected lead or ungalvanized steel structure. Under these circumstances, an adverse effect cannot easily be offset by bonding since, owing to galvanic action between the primary and secondary structures, the structure/electrolyte potential of the latter would be made more positive, the effect being larger than any beneficial effect due to the cathodic protection. Bonding could be made effective only by making the primary structure more negative, for example, by increasing the total protection current or moving one of the groundbeds closer to the point where it is proposed to bond. Alternatively, the secondary structure could be protected by a separate cathodic protection system, possibly by installing sacrificial anodes connected to the secondary structure. In exceptional cases it may be found possible, by special agreement between the parties, to accept structure/electrolyte potential changes on the secondary structure greater than the accepted normal limit and thereby avoid the need for remedial action.
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A.4 Criteria for Limiting Corrosion Interaction
A.4.1 General
Any current flow that makes the potential of a metal surface more positive with respect of its surroundings is liable to accelerate corrosion. The structure/electrolyte potential is therefore used as the basis of assessment. Positive structure/ electrolyte potential changes are the more important.
A.4.2 Limit of positive structure/electrolyte potential changes for all structures
The maximum positive potential change at any part of a secondary structure, resulting from interference, shall not exceed 20 mV. Subsequent experience has provided no indication that corrosion damage occurs when this limit is respected. However, in many circumstances, particularly if the existing conditions provide a measure of cathodic protection at the relevant part of the structure, higher potential changes could be tolerated.
Some structures are inherently more resistant to stray currents. For example, for cables having a good extruded-plastics coating, the danger of damage to the coating in making test connections for interference testing is probably a greater risk than that due to interference. Any interference associated with cable systems is likely to be manifested at the nearest earthing facility. Should the secondary structure be provided with independent cathodic protection, then the owners of the secondary structure may agree to accept a greater positive potential change on this structure, provided that its potential remains more negative than the value given in protection criteria.
A.4.3 Negative changes of structure/electrolyte potential
Large negative changes may, however, occur if the groundbed of an impressed current cathodic protection system is unduly close to a secondary structure. Structure/electrolyte potentials more negative than -2.5 V shall be avoided on buried structures.
A.5 Control of Interference
A.5.1 General
Where testing of a cathodic protection installation indicates that there is interference at a level which may result in corrosion of the foreign structure, control of the interference may be achieved by taking the following actions:
a) Installing galvanic anodes or an impressed current system on the foreign structure.
b) Bonding the foreign structure to the primary structure through a current controlling resistance, if appropriate.
c) Insulating the foreign structure.
d) Using distributed cathode points to reduce the average potential shift on a poorly-coated protected structure.
e) Relocating foreign structures away from the interfering field.
f) Reducing the cathodic protection system current.
Where the foreign structure is electrically discontinuous, as may occur on a cast iron pipeline with elastomer ring joints, some bonding of the high resistance joints may be necessary before the above measures can be adopted.
In practice, it is sometimes found that reducing the system current can reduce the interference on foreign structures to a level acceptable to all concerned, while maintaining a satisfactory level of protection on the protected structures.
A.5.1.2 Mitigation of interference on a fixed immersed foreign structure is generally achieved by either bonding the two structures, or by installing a separate cathodic system on the foreign structure to protect it in its own right.
With a discrete movable foreign structure, such as a ship berthed at a cathodically protected steel-piled wharf, bonding is usually avoided because of the following factors:
a) A movable structure will usually have its own cathodic protection system.
b) A movable structure is usually well coated; when bonded to a large fixed immersed structure
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system, excessively negative potentials may result on the movable structure, causing coating disbondment.
c) A movable structure will normally only remain near the fixed immersed structure for a short period, i.e. during loading and unloading, and as a result the total interference effect is minimal.
d) The making and breaking of temporary bond connections may result in the generation of sparks or arcs which can be hazardous at installations handling flammable materials.
A.5.2 Control by the use of galvanic anodes
Interference may be controlled by installing galvanic anodes on the foreign structure, to make the potential at least as electro-negative as that which existed prior to the interference.
Note:
Advantages and disadvantages of this method are as follows:
a) Advantages:
i) The owner of the foreign structure has control over the anodes, and thus can be assured of their continued operation.
ii) Galvanic corrosion or problems arising from complex stray currents shall not occur.
b) Disadvantages:
i) If interference is great, the foreign structure is bare or the soil is of high resistivity, the limited driving voltage of the galvanic anodes may not provide sufficient protection current.
ii) The performance of the anodes requires to be monitored.
iii) Galvanic anodes sited in areas where there are steep potential gradients resulting from a foreign cathodic protection system, may accentuate the pick-up of stray current which may discharge at the remote side of the structure, and cause corrosion.
A.5.3 Control by the use of impressed current cathodic protection
In special circumstances, interference can also be controlled by installing impressed current cathodic protection on the foreign structure.
Note:
Advantages and disadvantages of this method are as follows:
a) Advantages:
i) The owner of the foreign structure has control over the anodes, and thus can be assured of their continued operation.
ii) Galvanic corrosion or problems arising from complex stray currents shall not occur.
iii) If interference is significant, the foreign structure bare, or the soil resistivity high, the high driving voltage of impressed current cathodic protection may be required to provide adequate protection.
iv) In stray current areas, an impressed current cathodic protection system is less likely to accentuate stray current pick-up than a galvanic system.
b) Disadvantages:
i) Interlocks between the interfering cathodic protection system and the second cathodic protection system may be required to control adverse effects shall failure of the first cathodic protection system occur.
ii) The interference suppression current will require to be monitored for proper operation.
iii) Power supplies for the transformer/rectifier may be difficult to arrange at isolated locations.
iv) The impressed current interference suppression system may itself cause additional interference, and it may require registration with, or approval by, the relevant Authority.
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A.5.4 Control by bonding
Interference may be controlled by bonding the foreign structure to the primary structure, or by connecting the foreign structure directly into the impressed current cathodic protection system. In the former case, an appropriate resistor is inserted in series with the bond to control the current flow to the level required to just offset interference. In the latter case, a diode and a resistor are inserted into the impressed current circuit.
Diodes are used to prevent the following problems from occurring:
a) Where the foreign structure and the primary structure are of dissimilar metals, interruption to the cathodic protection current can lead to galvanic corrosion.
b) Where structures are located in a stray current area, current flow from one structure to the other can affect the overall stray current flow.
Notes:
1) This is particularly important where stray current drainage from more than one structure is involved.
2) Advantages and disadvantages of the bonding method are as follows:
a) Advantages:
i) It is an economical solution where the structure access points are close together.
ii) The existing bond may be capable of automatically coping with an increase in output of the cathodic protection unit.
b) Disadvantages:
i) Where the foreign structure is remote from the cathodic protection installation, bonding may lead to galvanic corrosion and stray current problems shall failure of the cathodic protection system occur.
ii) The owner of the foreign structure does not have control of both ends of the bond.
iii) The bond requires monitoring to ensure it is not accidentally broken, cut, disconnected or fused.
Before bonds are installed, approval must be obtained from the affected parties.
A.6Fault Conditions in Electricity Power Systems in Relation to Remedial and/or Unintentional Bonds
There is a possible risk in bonding a cathodic protection system to any metalwork associated with the earthing system of an electricity supply network, whether by intention or not. This is particularly important in the vicinity of high-voltage sub-stations.
Bonds between metalwork associated with an electricity power system (e.g. cable sheaths) and cathodically-protected structures, can contribute an element of danger when abnormal conditions occur on the power network. The principal danger arises from the possibility of current flow, through the bonds, to the protected structure, due to either earth-fault conditions or out-of-balance load currents from the system neutral.
The current, together with the associated voltage rise, may result in electric shock, explosion, fire or overheating and also risk of electrical breakdown of coatings on buried structures. Such hazards shall be recognized by the parties installing the bond and any necessary precautions taken to minimize the possible consequences. The rise in temperature of conductors is proportional to i 2t, where i is the fault current and t its duration. Conductors, joints and terminations shall be sufficiently robust, and of such construction, as to withstand, without deterioration, the highest value of i2t expected under fault conditions. For extreme conditions, duplicate bonding is recommended. Precautions shall also be taken against danger arising from the high electro-mechanical forces which may accompany short-circuit currents.
It is difficult to ensure that current-limiting resistances comply with the foregoing requirements; their insertion in bonds through which heavy fault current might flow shall therefore be avoided as far as possible. If they are used, it is essential that they be carefully designed for the expected conditions.
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Bonds and any associated connections shall be adequately protected from damage or deterioration.
A.7 Telluric Current
A.7.1 Geomagnetic field variations associated with the ionospheric currents establish large-scale systems of electric currents within the earth by a process of electromagnetic induction. The global pattern of these currents flowing near the surface of the earth is known to be extremely complex due to factors such as the wide range of electrical conductivities of different strata.
A.7.2 The frequency of the fluctuations has been recorded to be a matter of one per several hours.
A.7.3 A pipeline of considerable length being positioned favorably can pick-up and discharge telluric currents. If the current picked-up is considerable in comparison with the total current applied for corrosion prevention (which can be the case with a very well-coated pipeline in high-resistivity soil) the effect of telluric current on such a system will become noticeable and may have to be corrected.
A.7.4 A number of countermeasures can be taken to combat ill effects from telluric currents. Sectioning the line by insertion of insulation flanges or joints will reduce long line current flow. Installation of discharge points by providing zinc or magnesium anodes at strategic locations will reduce the risks of corrosion at discharge of current at coating imperfections.
A.7.5 It is strongly recommended to obtain expert advice if a case is found suspect.
A.8 Alternating Current Effects
A.8.1 Alternating currents induced in pipeline systems running parallel with power lines especially, have no influence on the corrosion of the cathodically protected lines but can generate voltages that require mitigation.
A.8.2 It is not uncommon that pipelines and power transmission systems share a right of way. Rules and regulations exist for guidance on earthing of the power transmission system and the distances to be maintained between these and the pipeline(s) in question.
A.8.3 If routing the pipeline close and parallel to an overhead high-voltage system cannot be avoided, a study should be conducted by experts to determine which sections of the pipeline are influenced by a short circuit to earth and to what extent.
A.8.4 Some of the main distances that should be maintained are:
- during the construction of a pipeline, it should be separated from the vertical projection of the nearest highvoltage line by at least 10 m for safety reasons;
- valve stations, safety releases, etc., projecting above the ground are not to be installed within 30 m;
- between the pipeline and the earthing pit of the transmission tower the minimum distance shall be additional 3 m for a max. earth-fault current of 5 kA, plus 0.5 m for every additional kA.
A.8.5 Special attention shall be paid to the cathodic protection of pipelines and the overvoltage protection for rectifiers:
- At valve stations a steel net buried around the valve and electrically bonded to the pipelines may be required for the protection of personnel.
- It is generally advisable to discuss special requirements with the local power company or the labor inspectorate; especially during construction of a pipeline restrictive regulations may be imposed by the local power company.
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INTERFERENCE. SOME OF THE CURRENT FLOWING FROM THE ANODE BED TO THE PROTECTED LINE COLLECTS ON THE FOREIGN LINE, FLOWS ALONG IT TOWARD THE CROSSING (FROM BOTH SIDES), AND THEN DISCHARGES THROUGH THE SOIL TO THE PROTECTED LINE. DAMAGE IS INFLICTED ON THE FOREIGN LINE IN THE NEIGHBORHOOD OF THE CROSSING.
Fig. A.2
INTERFERENCE (RADIAL CURRENT FLOW). WHEN A STRUCTURE LIES IN A REGION OF HEAVY CURRENT DENSITY, SUCH AS THE TANK SHOWN CLOSE TO THE ANODE BED, IT MAY PICK UP CURRENT AT "A" AND DISCHARGE IT TO EARTH AT "B", WITH RESULTANT DAMAGE AT THE DISCHARGE AREA. SOMETIMES, BUT NOT OFTEN, AN ISOLATED METALLIC STRUCTURE LYING NEAR A PROTECTED LINE CAN UNDERGO THE SAME KIND OF DAMAGE.
Fig. A.3
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ADJUSTMENT OF CROSSING BOND. THE COPPER SULFATE ELECTRODE IS PLACED BETWEEN THE TWO LINES AT THE POINT OF CROSSING. THE RESISTOR IS THEN ADJUSTED BY TRIAL AND ERROR UNTIL THERE IS NO CHANGE IN THE POTENTIAL OF THE FOREIGN LINE WITH RESPECT TO THE ELECTRODE WHEN THE RECTIFIER IS TURNED ON AND OFF. IT IS HELPFUL TO MEASURE THE "SHORT-CIRCUIT" CURRENT BETWEEN THE TWO LINES FIRST, USING THE "ZERO-RESISTANCE AMMETER" CIRCUIT DIAGRAMMED IN FIG. A.5
Fig. A.4
ZERO-RESISTANCE AMMETER. TO DETERMINE THE CURRENT WHICH WOULD FLOW THROUGH A "SOLID" OR ZERO-RESISTANCE BOND BETWEEN TWO STRUCTURES, THE CIRCUIT ILLUSTRATED IS USED. THE CURRENT FROM THE BATTERY IS ADJUSTED UNTIL THE POTENTIOMETER (OR HIGH-RESISTANCE VOLTMETER) READS ZERO. THEN THE CURRENT INDICATED BY THE AMMETER I IS THE SOUGHT FOR VALUE. THERE ARE INSTRUMENTS AVAILABLE WHICH INCORPORATE THIS COMPLETE CIRCUIT WITHIN THEMSELVES
Fig. A.5
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AN OBSCURE CASE. MUCH OF THE CURRENT COLLECTED ON THE FOREIGN LINE FLOWS TOWARD THE CROSSING, WHERE IT CAN BE SAFELY HANDLED WITH A SIMPLE BOND. SOME OF IT, HOWEVER, FLOWS IN THE OPPOSITE DIRECTION AND IS DISCHARGED OVER A RELATIVELY LARGE AND REMOTE AREA. SUCH A SITUATION DOES NOT ARISE OF TEN AND PROBABLY DOES LITTLE DAMAGE IN ANY CASE BECAUSE OF THE LARGE DISCHARGE AREA. IT CAN BE AVOIDED BY PROPER ANODE BED PLACEMENT AND REMEDIED BY THE USE OF AUXILIARY DRAINAGE ANODES. THE BEST SOLUTION IS THE INSTALLATION OF ONE OR MORE MAGNESIUM ANODES AT THE POINT OF EXPOSURE. THE COLLECTED CURRENT, INSTEAD OF FOLLOWING THE BOND BACK TO THE PROTECTED LINE, FLOWS TO EARTH BY WAY OF THE MAGNESIUM ANODES
Fig. A.6
CURRENT TRANSFER BETWEEN PARALLEL LINES. IF THE DELTA (DIFFERENCE BETWEEN ON AND OFF READINGS) IN THE POSITION SHOWN IN SOLID LINES IS APPRECIABLY GREATER THAN THAT IN THE DOTTED POSITION, THEN THERE IS CURRENT TRANSFER FROM THE FOREIGN LINE TO THE PROTECTED LINE. WHEN THE POINT OF WORST EXPOSURE IS LOCATED, A BOND SHOULD BE INSTALLED. A REPEAT SURVEY MUST THEN BE MADE TO DETERMINE THE LENGTH OF SECTION WHICH THE BOND WILL PROTECT, AND OTHER BONDS INSTALLED IF REQUIRED
Fig. A.7
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FOREIGN LINE WITH DRESSER COUPLINGS. THE INSTALLATION OF A BOND AT THE POINT OF CROSSING WILL AVERT THE DAMAGE THERE, BUT THERE WILL STILL BE DAMAGE DONE AT THE MECHANICAL JOINTS, BY CURRENT BYPASSING THEM THROUGH THE EARTH. THIS CAN BE REMEDIED BY BONDING THE JOINTS OR BY THE USE OF AUXILIARY MAGNESIUM ANODE DRAINAGE
Fig. A.8
APPENDIX B
MEASUREMENT OF SOIL RESISTIVITY
B.1 General
B.1.1 There are a number of methods for measuring soil resistivity (see Fig. B.1), the most common is the "Wenner 4-pin method".
B.1.2 The equipment required for field resistivity measurements consists of a hand-driven earth-tester (vibroground equipment), four metal electrodes, and the necessary wiring to make the connections shown in Fig. B.1. Terminals shall be of good quality to ensure that low-resistance contact is made at the electrodes and the meter.
B.1.3 The Wenner four-electrode method requires that four metal electrodes be placed with equal separation in a straight line in the surface of the soil to a depth not exceeding 5% of the minimum separation of the electrodes. Watering in moderation around the electrodes is permissible to ensure adequate contact with the soil. The electrode separation should be selected with consideration of the soil strata of interest. The resulting resistivity measurement represents the average resistivity of a hemisphere of soil of a radius equal to the electrode separation.
B.1.4 A voltage is impressed between the outer electrodes, causing current to flow, and the voltage drop between the inner electrodes is measured. Alternatively, the resistance can be measured directly. The resistivity, ( is then:
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()cminaaRcmπρ2.,=Ω
().5.191ftinaaR=
Where:
a = electrode separation, and
R = resistance, .Ω
Using dimensional analysis, the correct unit for resistivity is ohm-centimeter.
The depth of the electrodes should not exceed the value a/20.
B.2 Field Procedures
B.2.1 Select the alignment of the measurement to include uniform topography over the limits of the electrode span. Do not include large nonconductive bodies such as frozen soil, boulders, concrete foundations, etc., which are not representative of the soil of interest, in the electrode span. Conductive structures such as pipes and cables shall not be within "½ a" of the electrode span unless they are at right angles to the span.
B.2.2 Select electrode spacings with regard to the structure of interest. Since must pipelines are installed at depths of from 1.5 to 3 m, electrode spacings of 1.5 and 3 m are commonly used. The "a" spacing shall equal the maximum depth of the interest. To facilitate field calculation of resistivities, spacing of 1.58 and 3.16 m, which result in multiplication factors of 1000 and 2000, can be used.
B.2.3 Impress a voltage across the outer electrodes, causing the current to flow. Measure the voltage drop across the inner electrodes and read the resistance directly and record.
B.2.4 Make a record of electrode spacing, resistance, date, time, air temperature, topography, drainage, and indications of contamination to facilitate subsequent interpretation.
B.2.5 It should be recognized that subsurface conditions can vary greatly in a short distance, particularly where other buried structures have been installed. Surface contamination tends to concentrate in existing ditches with surface runoff, appreciably lowering the resistivity below the natural level.
B.2.6 To evaluate contamination effects when a new route is being evaluated, soil samples can be obtained at crossings of existing pipelines, cables, etc., or by intentional sampling using soil augers.
Notes:
1) Other field resistivity measurement techniques and equipment are available. These commonly use two electrodes mounted on a prod that is inserted in the soil-at-grade in an excavation or a driven or bored hole. The two-electrode technique is inherently less accurate than the four-electrode method because of polarization effects, but useful information can be obtained concerning the characteristics of particular strata.
2) Risk and error must be arbitrarily selected to allow determination of the number of measurements. A risk of 5% of an error greater than 100 . cm should be suitable for most situations. The error limit should be about 10% of the anticipated mean resistivity.
B.3 Frequency of Measurement
B.3.1 The frequency of measurement is dependent on the purpose of the soil survey. Common practice for a general pipeline route survey is to record values at an average of 1 km intervals, however, where the purpose is to locate corrosion "hot-spots", measurements every 100-150 m may be necessary.
B.3.2 Where a variation between two successive readings is greater than the ratio 2:1, intermediate readings shall be included.
B.3.3 Where pipelines are existing, measurements are taken to one side of the pipeline and at right angles to it at minimum distance equal to the electrode spacing "a" from the pipeline.
B.3.4 For general pipeline route surveys, it is common to undertake a series of readings covering soil above the pipe, soil at pipe depth, soil immediately below the pipe, i.e. 1/2D, D and 2D where D
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is depth of the pipe.
B.3.5 For groundbed location surveys it may be necessary to measure to greater depths and spacings of up to say 30 m are recorded depending on the variation of soil resistivity with depth.
B.3.6 In very dry/high resistance surface soils or where large numbers of measurements or measurements to considerable depths may be required (e.g. for deepwell groundbeds) resistivities may be taken using electromagnetic induction techniques.
B.4 Presentation of Results
Results are presented in ohm-cm and graphically with resistivities (log scale) versus distance (linear) for a pipeline route (Fig. B.2). Where resistivities have been taken to various depths the layer resistivity between each depth can be calculated using the "Barnes Method"*.
* See control of pipeline corrosion; A.W. Peabody, P 90-91, NACE.
B.5 Criteria and Interpretation
Soil corrosivity assessment may be based on either one or a number of parameters.
BS 7361 quotes a form of assessment based on resistivity as:
up to 1000 ohm-cm - severely corrosive
1000 to 10000 ohm-cm - moderately corrosive
10000 ohm-cm and above - slightly corrosive
Often a "global index" technique is used and three useful Indices are given in Table B.1.
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RESISTIVITY MEASUREMENT METHODS
Fig. B.1
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SOIL RESISTIVITY GRAPH
Fig. B.2
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TABLE B.1 - SOIL CORROSIVITY ASSESSMENT INDICES
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APPENDIX C
MEASUREMENT OF ELECTRODE RESISTANCE
C.1 General
In some cases the voltages involved when testing earth electrodes may present a risk of shock and care should be exercised to take the necessary precautions.
Once installed, electrodes are almost certain to be connected, either deliberately or fortuitously, to other items in contact with the general mass of earth. For a new installation it is generally possible to arrange for a measurement to be made before the electrode is so connected and is still electrically isolated. For existing installations it is not permissible to disconnect earth electrodes unless the installation is also disconnected from all sources of power. The problem can sometimes be solved by installing multiple electrodes so that, with one disconnected for testing, the remaining electrodes provide an adequately low resistance.
The accuracy of measurement is subject to a number of features which should be borne in mind when assessing the implications of the value obtained. Apart from seasonal variations and trends in soil resistivity, an electrode is influenced by the presence of other conducting items in the ground, such as cables, pipes and foundations, as well as other electrodes connected together. A value obtained with existing installations, although not of great accuracy, may nevertheless provide useful information on the stability of the earthing.
For a new installation, a measurement should provide better information than a calculated value based on a measured value for the soil resistivity, because any unknown inhomogeneity in the soil is taken into account.
C.2 Measurement of Earth Electrode Resistance
Measurement of the resistance to earth of an earth electrode is not necessarily a simple matter. While certain fairly simple rules can be laid down, circumstances frequently arise which make it necessary to modify them. The resistance of an earth electrode is unique in that only the terminal provided by the electrode itself is definite, the other terminal of the resistance being theoretically at an infinite distance. In practice a measurement has to be made which includes the greater part, say 98%, of the total resistance. There is no point in striving for a high degree of accuracy with such a measurement since, within the volume of such a resistance, there may be considerable non-uniformity in the soil and other disturbing features. An accuracy of 2% is more than adequate, and accuracies of the order of 5% are usually quite acceptable.
The best method of measurement is illustrated in Fig. C.1. A measured current is passed between electrode X, the one being tested, and an auxiliary current electrode Y. The voltage drop between electrode X and a second auxiliary electrode Z is measured and the resistance of the electrode X is then the voltage between X and Z divided by the current flowing between X and Y. The source of current and the means of metering either the current and voltage or their ratio are often, but not necessarily, combined in one device.
A = ammeter
V = voltmeter
MEASUREMENT OF EARTH ELECTRODE RESISTANCE
Fig. C.1
The accuracy of the measurement is influenced by the following considerations:
a) Distance between electrodes
The distance between electrodes X and Y has to be such that the resistance area of each, i.e. the area within which roughly 98% of its resistance lies, is independent of the other. If X is a
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simple rod or plate Y should be placed 30 m to 50 m from X, with Z about midway between. A reading should be taken, followed by two further readings with Z moved, say, 7 m nearer to X and then 7 m nearer to Y. If the three readings give values for the resistance which agree within the accuracy required, then the mean value can be assumed to be the resistance of X.
If the results do not agree, then Y should be moved further away and the procedure repeated. This whole procedure should be repeated until the three readings do agree.
The above procedure is not satisfactory when X has a low resistance, say, 1 Ω or less. This usually occurs when X is an extended electrode or is composed of a system of electrodes which cannot be measured individually; generally occupying a large area. This problem is usually solved by obtaining earth resistance curves.
To do this Y should be placed some arbitrary distance, usually some hundreds of meters, away from X and a series of measurements made with Z at various locations along the line X-Y. If the curve of resistance plotted against the position of Z has a substantially horizontal portion, this will give the required value of resistance. If the curve does not show such a horizontal section, Y has to be moved further away from X and the process repeated until a horizontal portion is obtained (see Fig. C.2). The horizontal portion does not necessarily occur where Z is midway between X and Y. As an example, in a test on a power station electrode system, ultimately found to have a resistance of 0.05, it was necessary to place Y 700 m away and the horizontal section of the curve was found for potential electrode distances of 70 m to 100 m. Ω
b) Interference by stray earth currents
Soil conduction is an electrolytic phenomenon and hence small dc. potentials arise between the electrodes, and stray ac. or dc potentials are picked up by the electrodes if there is a traction system in the area. Both of these forms of interference can be eliminated by testing with alternating current at a frequency different from that of the interfering power currents and their harmonics. This is usually achieved by using a frequency of about 60 Hz to 90 Hz.
If the source of power for the measurement is a hand driven generator, the frequency of the measuring current can be varied to obtain the best result. An alternating current instrument usually incorporates a synchronous rectifier or equivalent in its metering circuits so that it responds only to voltage signals of its own frequency.
c) Resistance of the auxiliary electrodes
The resistance of Y and Z are in series with the measuring and power supply circuits. These electrodes are often, for convenience, single rod electrodes which may have quite a high resistance, depending on the resistivity of the soil in which they are driven. Resistance at Y increases the power supply requirements needed to ensure an accurately measurable voltage between X and Z. Resistance at Z is in series with the voltage measuring circuit and may affect its accuracy. Information on the highest acceptable values of auxiliary electrode resistance are usually provided with instruments designed for earth resistance measurement.
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EARTH RESISTANCE CURVES
Fig. C.2
C.3 Measurement of Resistance of Earthing Conductor
The following types of instrument may be used to measure the resistance of an earthing conductor:
a) Direct reading DC ohmmeter incorporating a hand driven generator,
b) direct reading ohmmeter supplied from a battery, or
c) a.c. test set generally mains driven and incorporating a suitable transformer providing isolation.
Tests which give no quantitative result (e.g. bell or lamp tests) should never be used to prove the adequacy of earthing conductors.
Of the above instruments the first two differ only in the magnitude of the test current and they measure only the resistance of the conductor. A.C. mains testers can provide high test currents, but are usually limited to about 25 A because of the weight of the transformer.
An accuracy of about 5% in the measured value is desirable. To achieve such an accuracy with low resistance conductors, correct selection of the method of measurement is important and the instrument manufacturer’s instructions should be consulted to confirm the conditions under which it can be achieved.
The measured impedance or resistance, except for four-terminal measurements, is usually for a loop consisting of the conductor under test, the return conductor and probably some test leads. The resistance of the conductor under test is obtained by subtracting a separately measured value for the return conductor and leads from the loop resistance. In order that this process does not introduce too great an error, the resistance of the return conductor and leads should be as low as practicable. For a similar reason all connections should be made so as to have a low resistance.
The impedance of a ferromagnetic conductor varies with the current; with the sizes of conductor likely to be involved, the highest impedance generally occurs at currents in the region of 25 A to 50A. Measurement with such a value of current will provide a worst case value, since the magnetic effect decreases as the current increases to fault current values. If measurements are made with d.c, and a substantial part of the length of the conductor is of ferrous material, it is recommended that the measured value be doubled to take account of magnetic effects.
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APPENDIX D
CURRENT DRAINAGE SURVEY
D.1 General
D.1.1 The current drainage survey is a technique for determining the amount of current required to provide chathodic protection to a buried pipeline and the spread of protection from a single point or the current required to boost cathodic protection on a line with deteriorating coating.
D.1.2 To obtain relevant results, pipeline isolation equipment and monitoring facilities (test points) shall be installed before current drainage tests are carried out.
D.1.3 If the pipeline has been previously cathodically protected, historical data may be used to determine the current demand.
D.2 Method
D.2.1 A temporary cathodic protection system shall be set-up using a DC power sources (e.g. batteries, portable rectifiers, etc.) timer-units, test points and one (or more) groundbed(s) in a direct current circuit.
D.2.2 After full polarization is achieved, the output shall be adjusted to provide the required drain point potential, and then maximizing the output.
D.2.3 Then, the current shall be interrupted in a sequence of e.g. 40 seconds "on", 20 seconds "off".
D.2.4 The spread can be measured by taking structure-to-soil potentials away from the drain point until loss of protection occurs.
D.2.5 The location with the least "swing" determines the required minimum output of the final installation in such a way that the minimum acceptable swing is 300 millivolt negative once the final installation is activated.*
Notes:
1) The current requirements for coated pipelines is often estimated.
2) Current-drainage tests, although technically reliable, are generally, confined to investigations for complicated structures.
3) This technique may also be used to determine the average coating resistance of a pipeline coating.
D.3 An alternative method for determining the current required for cathodic protection is the measurement of the structure-to-electrolyte potential with stepped increase of impressed current. A graph is made showing the structure-toelectrolyte potential against the logarithm of the impressed current. The relationship is a straight line with a slight inclination at low currents; after a break point the curve continues as a straight line with a sharper rise at higher currents.
The break point indicates the current required to provide cathodic protection, see Fig. D.1. This method is often the only feasible one when cathodic protection is being considered for structures not fully accessible, e.g. oil well casings. As the measurements are taken in a short period, full polarization does not occur and the structure-to-water or structure -to-soil potential at the break point is not a measure of the potential required to provide protection.
The location of the half-cell in such an experiment is rather critical. A remote location should be found by moving the half-cell further away during which no significant change in potential is observed. This then is the remote potential. The potential after each current step increase shall be measured whilst interrupting the current for approximately one second and reading the instant ’off ’ potential. The time over which each current step increase is being applied shall be a constant (e.g. one or two minutes).
* The ratio swing observed/swing required equals test current applied/final minimum current required.
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RELATIONSHIP BETWEEN POTENTIAL AND IMPRESSED-CURRENT
Fig. D.1
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APPENDIX E
DETERMINATION IN-SITU OF THE REDOX POTENTIAL OF SOIL
E.1 General
This method covers the determination of the redox potential (reduction/oxidation) of soil tested in situ at a selected depth by measuring the electro-chemical potential between a platinum electrode and a saturated calomel reference electrode. The test is used to indicate the likelihood of microbial corrosion of metals by sulphate-reducing bacteria which can proliferate in anaerobic conditions. The redox potential is principally related to the oxygen in the soil, and a high value indicates that a relatively large amount is present. Anaerobic microbial corrosion can occur if a soil has a low oxygen content and hence a low redox potential. This standard requires the use of a calomel reference probe as defined in E.2.2. This is not intended to prohibit the use of the other established portable versions of reference probes, e.g. copper/ copper sulphate and silver/silver chloride (see note). In submitting reports the type of reference probe used and the correction factor applied to convert the measurement to the standard hydrogen electrode shall be given.
Redox potential may also be measured in the laboratory.
Note:
Where the other types of reference probes, e.g. copper/copper sulphate and silver/silver chloride, are used it is very important to note that their preparation and storage procedures are different from that required for calomel probes and the manufacturer’s instructions should be followed. Moreover copper/copper sulphate probes are not suitable in chloride contaminated soil or in alkaline environments when silver/silver chloride should be used. Copper/copper sulphate cells are also sensitive to heat, light and a wide variety of chemicals.
The correction factors for reference probes to convert the measurement to the standard hydrogen electrode are as follows:
Mercury/mercuric chloride 240 mV
Copper/copper sulphate 316 mV at 25°C
Silver/silver chloride 246 mV
E.2 Apparatus
E.2.1 Platinum probe of a design having two separate platinum electrodes embedded in the nose piece. Also a means of protection when not in use. The probe shall have a connecting lead permitting the inclusion of each platinum electrode individually in an electrical circuit. Each connection shall be separately identified.
E.2.2 Calomel reference probe, having a mercury/mercuric chloride reference electrode which can be refilled and with a connection to a porous ceramic junction. The calomel reference electrode shall be kept clean when not in use by being stored in a sealed container. The precipitation of crystals shall be prevented when not in use, particularly at the porous junction, by storing upright and closing the breather hole.
Note:
The platinum and calomel probes are often separate and the latter made of glass which can make field use sometimes difficult. The National Corrosion Service at the National Physical Laboratory (NPL)* has designed and supplies a robust redox probe that combines both the pair of platinum electrodes and a calomel reference probe mounted together behind a
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steeltipped nose cone that is fitted on the end of a steel tube which can be driven into fine-and medium-grained soils. The NPL redox probe is supplied complete with ancillary equipment for cleaning the electrodes.
* National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK.
E.2.3 Calibrated millivoltmeter. The total measuring range shall be at least 0 V dc to 2 V dc with a readability at least to 10 mV. The input impedance shall be not less than 106 and the polarity (positive or negative) shall be marked on the two input terminals.
The instrument shall include suitable insulated flexible electric cable and connectors for use with the probes.
The instrument shall be recalibrated at intervals not exceeding 2 years.
E.2.4 Installation equipment consisting of a soil auger, spade and trowel to excavate soil to test level, and, where soil is compact, a 1 kg hammer and spike.
E.2.5 pH measuring apparatus.
E.2.6 Disturbed sample container of glass or dense plastic, that can be hermetically sealed.
Note:
When a sample from the test location is required for microbiological examination, a glass container of a size suitable to hold about 500 mL will need to have been cleaned and sterilized by scalding with boiling water beforehand. Alternatively medically sterilized plastic bags may be used. Fill the container completely and minimize air voids.
E.3 Materials
E.3.1 Saturated solution of potassium chloride in a screw-topped plastic bottle either with pouring lip suitable for filling the reservoir of the calomel reference probe or a separate small dropper or syringe 500 mL is a suitable quantity.
E.3.2 Jeweller’s rouge.
E.3.3 Colorless methylated spirits, 70% by volume with 30% by volume distilled water, in a screw-topped wide-mouth bottle, 500 ml is a suitable quantity.
E.3.4 Distilled water. Two differently marked wash bottles full for cleaning platinum electrodes. 500 ml is a suitable quantity for each bottle.
E.3.5 Paper tissues and absorbent-type surgical cotton wool swabs.
E.4 Procedure
Note:
Thorough cleanliness of the probes is essential for reliable results.
E.4.1 Assemble from the storage unit according to the manufacturer’s operating instructions the calomel reference probe, ensuring that the unit is full of a saturated solution of potassium chloride and that this moistens the porous junction. Remove any air bubbles in the potassium chloride solution by gently tapping the probe and remove excess fluid from the porous junction.
Note:
During use on site it is important to prevent precipitation of crystals at the porous junction. This may be done by keeping the probe between tests in distilled water in a wide-necked bottle with a rubber bung to ensure that the porous junction is kept moist.
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Clean and polish each platinum electrode. Initially smear the surfaces lightly with moist jeweller’s rouge and use gentle abrasive action with cotton wool swabs. Follow with a single wash using the methylated spirits. Afterwards wash thoroughly with distilled water. Lastly dry each electrode with clean paper tissues.
Note:
Where the platinum electrode is dipped into distilled water for washing the bottles will need separate identification to select the correct sequence when reused.
E.4.2 Connect the positive terminal of the millivoltmeter with the electric cable to one of the platinum electrodes and the negative terminal to the calomel reference electrode, but leave the circuit open. This circuit shall be considered to give positive readings.
E.4.3 Test shall always be made below the level of organic growth. A hole not less than 150 mm in diameter is needed to reach the selected level when using separate probes. A combined redox probe may be driven from the surface to the selected level in weak soil, otherwise it may be necessary to auger or dig a hole part way.
E.4.4 If the probes are separate install them about 100 mm apart in the hole. The platinum probe shall penetrate at least 100 mm to ensure full soil contact below any disturbed surface material. A combined redox probe shall be pressed into position sufficiently to obtain full soil contact on the electrodes.
E.4.5 Rotate the platinum probe about a quarter turn without letting air reach the probe. Close the electric circuit then take the reading as soon as the voltage becomes stable. It may be necessary to wait 30 s or more for stable conditions to be reached. Where the probes are separate turn the platinum probe one revolution under firm hand pressure to ensure good contact. Rotate the combined redox probe a half to one revolution.
Record the reading to the nearest 10 mV when the voltage is steady and record whether it is positive or negative.
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Note:
Very occasionally the current between the platinum electrode and the reference electrode will be in the reverse direction such as to require the connections to the millivoltmeter to be reversed. In this case the reading should be considered to be negative.
E.4.6 Transfer the electric circuit to the other platinum electrode, connecting it again to the positive terminal of the millivoltmeter, and repeat the procedure as specified in E.4.3. Record the reading to the nearest 10 mV and its polarity.
E.4.7 If the two readings differ by more than 20 mV remove the probes, reclean the platinum electrodes and re-install in a different position at the test site. Do not install the probes in the original position because oxygen will have penetrated and a false reading could result. Repeat the procedures as specified in E.4.5 and E.4.6.
E.4.8 Remove the probes and clean the electrodes taking note of the requirements of E.2.2 and E.4.1.
E.4.9 Place a disturbed sample from the position of the test in an hermetically sealed container.
E.4.10 Determine the pH of the sample by the method specified in Clause 9 of BS 1377: 1990 or ASTM G 51.
E.5 Calculations and Expression of the Results
The mean of the two acceptable readings and their sign shall be recorded as the potential of the platinum probe, Ep, to the nearest 10 mV. Calculate the redox potential, Eh (in mV), to the nearest 10 mV from the equation:
Eh = Ep + 250 + 60 (pH - 7)
Where:
Ep is the potential of the platinum probe (in mV) (may be a positive or negative value);
pH is the value of the acidity of an aqueous solution of the soil at the test position.
250 is the correction factor for a calomel reference probe to convert the measurement to the standard hydrogen electrode.
E.6 Test Report
The test report shall affirm that the test was carried out in accordance with this Standard and shall contain the following information:
a) The method of test used.
b) The mean value of the potential (in mV) of the two platinum probes to the nearest 10 mV.
c) The redox potential (in mV) to the nearest 10 mV.
d) The pH value.
e) The type of reference probe used in the test.
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APPENDIX F
INSPECTION OF CP INSTALLATIONS
F.1 General
Adequate inspection during the construction of corrosion control facilities on pipelines can make the difference between first class performance (to be had if inspection is truly effective) and a system that may perform poorly and which may require relatively high maintenance expenditures if ineffective or no inspection is used.
F.2 Cathodic Protection Installations
Inspectors responsible for cathodic protection installations must be fully familiar with all details of good cathodic protection construction practice as well as with the specific provisions of the installations being made.
In some instances field modifications may be necessary. This may occur, for example, if vertical anode installations were specified but rock is closer to the surface than expected. A field decision is then necessary to determine whether the type of anode installation may be changed from vertical to horizontal or if best results will be obtained by boring the rock. Occasionally what appears to be solid rock is actually a relatively thin layer with good soil underneath. The inspector must be qualified to evaluate all such situations when encountered. where major modifications appear advisable, he should check with the designer of the installation to be sure that the system performance will not be affected adversely.
At galvanic anode installations, one of the more particular points to be watched is the anode backfilling operation to be sure that there are no voids in the fill around the anodes. This can be a problem with packaged galvanic anodes placed vertically in augured holes. If the hole is small, it may be difficult to work earth backfill all around the anode package so that no voids backfill material can settle away from the anode, once the container has deteriorated, with probable reduction in anode effectiveness. Where anodes and chemical backfill are installed separately, the inspector must verify that the anode is centered in the hole or trench as specified and that the fill is so placed and compacted that no voids can exist.
All other details of galvanic anode installations must be verified by the inspector as being in accordance with design specification for such construction.
When inspecting impressed current groundbeds, anode placement and backfilling operations must be given careful attention to ensure installation at the design location and to avoid voids in special backfill (carbonaceous) which would tend to increase anode resistance and shorten life. Adequate compacting of carbonaceous backfill materials is important and the inspector must verify that this is done effectively but in a way that will not damage the anode proper or its connecting cable.
The most important single feature of impressed current groundbeds to be verified by the inspector is the insulation on all positive header cable, anode connecting cable and connections between the two. The inspector must be sure that no damaged cable insulation is backfilled without being repaired and that the insulation of all splices and tap connections is such that they will be permanently water-tight and such that no current leakage can occur. Likewise, the inspector must be sure that the cable trench has been so prepared that there are no materials in the trench bottom that can damage the cable insulation and that the backfill in contact with the cable is free of insulation damaging material.
When rectifier installations are involved, the inspector verifies that the rectifier unit has been placed at the specified location and in the specified manner, that all wiring is correct and that requirements of the serving power company and of local jurisdictional codes have been met. Grounding connections shall be checked.
Where power supplies other than rectifiers are used, the inspector must be familiar with the details of the specific power source specified in order that he may verify that the installation is being made properly.
F.3 Test Points, Cased Crossings and Insulating Joints
Construction of test points must be inspected to ensure their installation where called for on
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construction plans, that connections to the pipeline are sound and well insulated, that color coding is observed, that wires are so placed in the ditch that backfill will not break them and that the specified terminal panel and housing is installed properly.
Insofar as corrosion control is concerned, cased crossings should be inspected as soon as they are installed. This is necessary to ascertain that the casing is electrically insulated from the carrier pipe and to permit correction of defects before backfilling.
Even though insulating joints may be installed as complete pre-assembled pre-tested units, inspection should include verification of installation at the locations indicated on the plans, that the joint is not so placed that it is subject to undue mechanical strains that could cause early failure of insulating material and that the joint is in fact serving its insulating function once welded into the line.
The inspector charged with inspecting the above features must be thoroughly familiar with the use of test points, cased crossings and insulating joints. He must be equipped with the necessary instrumentation to verify electrically the satisfactory performance of the various items prior to acceptance. He must have the authority to see that corrections are made should defects be found.
F.4 Coating Inspection
Pipeline coating inspection, rigorously applied, will result in the best practicable coating performance from the standpoint of maximum effective electrical resistance and maximum stability with time, which is what the money invested in coating is spent for. If pipeline is to be mill coated, inspection starts at the coating plant with close attention given to all phases of pipe cleaning, priming, coating, holiday testing, yard stacking and loading out for shipment by rail or truck. Additionally, material used are inspected for compliance with the coating specification and the manner in which they are stored and handled is checked for any possible adverse effect on applied coating quality. In the case of hot-applied enamels, the heating kettles are checked for proper temperature range and suitable charging and cleaning cycles. Where other coating systems are used, the application system is checked for continued satisfactory performance under conditions accepted as good practice for that type of coating system. Applied coating thickness is checked for compliance with the specification.
At the job site, inspection practices on mill coated pipe cover the unloading, hauling, stringing, joint coating, holiday testing, damage repair, lowering in and backfilling operations.
When coating application is over the ditch, the coating inspector will pay close attention to the pipe cleaning and priming machines to be assured that the prime coat will permit the best practicable bond with hot enamel or other types of field-applied coatings. Where hot-applied enamels are used, inspection of the critical dope kettle cleaning, charging and heating operations must be thorough. Materials handling techniques are inspected for assurance that materials are kept free of dirt and that wrappers particularly are kept dry. Coating machines are checked for specification coating thickness and smooth application of wrappers under correct tension with correct overlap. As with the mill coated pipe, holiday testing, damage repair, lowering in and backfilling operations are checked for adequacy.
With the varied items to be covered and the critical nature of this inspection, it becomes obvious that coating inspection responsibilities should be assigned only to personnel who are fully qualified by experience and training to do the job. Further, their responsibilities should be confined to this one inspection operation and they should have full backing by the owner to obtain immediate and positive correction in the event unsuitable practices are used in any phase of the coating operation. They must have fully adequate specifications covering the entire coating operation.
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APPENDIX G
INSTALLATION IN HAZARDOUS ATMOSPHERES
G.1 General
Flammable mixtures of gas, vapor or dust may develop wherever hydrocarbons or finely divided materials are handled, stored or processed. Such hazardous atmospheres may be ignited by an electric arc or spark. Unless proper precautions are taken all electrical installations, including cathodic protection systems, will introduce the danger of sparks and ignition.
Cathodic protection system that are to operate where flammable concentrations of gas or vapor may occur shall conform to the statutory and other safety regulations applicable to the particular structure and industry concerned, and approval shall be obtained in each individual case as appropriate. Some of the explosion hazards cathodic protection may cause and the measures needed to avoid them are described in G.2 to G.10.
G.2 Bonds
Intentional or unintentional disconnection of bonds across pipeline joints or any other associated equipment under protection or fortuitously bonded to protected equipment constitutes a hazard.
To avoid the hazard, bonds shall be installed outside the hazardous area or in a protected position to avoid an unintentional break. The cathodic protection supply shall be switched off or disconnected by means of a flameproof switch.
G.3 Isolating Joints
Intentional or unintentional short-circuit of isolating joints e.g. by tools, or breakdown due to voltage surges on the protected structure induced by lightning or electrical power faults constitute a hazard.
To avoid the hazard, isolating joints shall, if possible, be located outside the hazardous area. Where this is not practicable, measures shall be adopted to avoid arcing or sparking. These will include the use of resistance bonds fitted in a flameproof enclosure or located outside the hazardous area, an encapsulated spark gap or surge diverter, zinc earth electrodes connected to each side of the isolating joint or polarization cell connected across the isolating joint, or to earth. The surfaces of the isolating joint shall be insulated to prevent fortuitous short circuiting by tools.
G.4 Short-Circuits Between Points of Different Potentials
Unintentional short-circuits by fortuitous bridging of points of different potential, e.g. by metal scraps, odd lengths of wire, mobile plant constitute a hazard.
This hazard is difficult to foresee but may be limited by bonding all metalwork together to minimize the potential difference between different parts of the structure.
G.5 Disconnection, Separation or Breaking of Protected Pipework
Cathodically protected pipework will have a portion of the protection current flowing through it. Any intentional disconnection, separation or breaking of the pipework will interrupt the current flow and may produce arcing depending on the magnitude of the current.
During any modification, maintenance or repair of cathodically protected pipework, transformer-rectifiers that affect that section of pipework shall be switched off and a temporary continuity bond attached across any intended break. It is essential that the bond is securely clamped to each side of the intended break and remains connected until the work is completed and electrical continuity restored or until the area is certified as gas-free or non-hazardous.
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G.6 Electrical Equipment
All electrical equipment installed in a hazardous area shall be flameproof and certified for use in that area. To avoid the need for flameproof equipment, the equipment shall be located outside the hazardous area. Double pole switches shall be provided in each circuit entering a hazardous area to ensure that both poles are isolated during maintenance etc. (see also G.5). It is essential that any cables carrying cathodic protection current shall be installed in such a manner that disconnection cannot take place within the hazardous area without de-energizing the cathodic protection system. The cables also be adequately protected mechanically to prevent accidental breakage.
G.7 Test Instruments
The connection and disconnection of instruments used for measuring and testing cathodic protection systems may produce arcing or sparking. Where measurements are taken within the hazardous areas, the meter used shall be intrinsically safe. The test leads shall be connected to the structure before being connected to the meter. Alternatively, the area shall be tested and declared gas-free, allowing conventional instruments to be used. Consideration may also be given to the use of permanently installed reference electrodes and test leads with the cables taken outside the hazardous area where conventional instruments can be used.
G.8 Internal Anodes
Unintentional short-circuiting of impressed current anodes when the liquid level is lowered in plant under internal cathodic protection can constitute a hazard.
Arrangements shall be made to ensure that the circuit is automatically or manually isolated when the anode is not submerged, i.e. when the anode circuit becomes an open circuit.
G.9 Sacrificial Anodes
Whilst the normal operation of sacrificial anode not considered hazardous, there is a danger of incendive sparking if a suspended or supported sacrificial magnesium or aluminum anode or portion of anode becomes detached and falls onto steel. This risk is not present with zinc anodes.
G.10 Instruction of Personnel
In locations where any of the above hazards may occur, it is essential that operating personnel be suitably instructed, and durable warning notices shall be authoritatively displayed as appropriate. Suitable written procedures and work authorization permits shall be included in the operations manual




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