Design of Cp1.PDF-3

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Design Aspects & Features of CP Systems8yAnand KulkarniDirecto0 Corrosion Control Services (8) Pvt Ltc!.


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Design Aspects & Features of CP SystemsByAnand Kulkarni

Directo~ Corrosion Control Services (B) Pvt Ltd

Galvanic CP SystemBy electrically connecting a more active metal electrode (anode) to the buried steelstructure intended to be protected (cathode), corrosion gets transferred from the steelstructure to the anode. Corrosion of the anode causes a current to flow in the circuitresulting in cathodic protection of the buried steel structure.The choice of the anode will determine the efficacy of the galvanic CP system. The threemain types of galvanic anodes widely used for providing protection are zinc, magnesiumand aluminum.

vZincFrom a historical perspective, zinc was the first galvanic anode used for protecting steelsince the nineteenth century. This metal tends to passivate if it is not pure. Zinc findsextensive use for protection of ship hulls and offshore pipelines. Because of its lowelectrochemical potential, zinc is used for protection in low resistivity soils. When a zincanode is used in soils, it is surrounded by a special backfill to lower its groundingresistance.Characteristics of zinc anodes:Rate of consumption, kg IA. year 11.8Potential, volts to CSE -1.1

JMagnesiumMagnesium is a useful galvanic anode in both soil and fresh water. When used in soils, it issurrounded by a special backfill to lower its grounding resistance. Its high electrochemicalpotential makes it suitable for higher resistivity soils, unlike zinc. However, it is susceptibleto self- corrosion which reduces the available current.Characteristics of Magnesiun;lAnodes:Rate of consumption, kg/A.yearPotential, volts to CSE

7.9- 1.50 to - 1.55


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JAluminumAluminum passivates easily and hence it is used as a galvanic anode only in the form ofalloys containing zinc and other elements. Its rate of self-corrosion is higher than zinc,although much lower than magnesium. With its low rate of consumption and anelectrochemical potential that is slightly more electronegative than zinc, aluminum is theobvious choice for protecting steel in offshore applications.Characteristics of Aluminum anodes:Rate of Consumption, kg /A.yearPotential, volts to CSE 3.1 to 3.4- 1.10 to -1.15

Anode BackfillBefore installing zinc or magnesium anodes in soil, it is a standard practice to surround itwith a chemical backfill. This chemical backfill is typically a mixture of gypsum (CaS04),bentonite clay and sodium sulphate in the proportion 75%, 20% and 5% respectively.The main reason that the galvanic anode is surrounded by such a chemical backfill prior toits installation in soil is to reduce its resistance to earth and hence enhance its currentdischarging ability. Additionally, a bare anode installed directly in soil would suffer fromlocal corrosion caused by variations in the composition of soil Leading to its non-uniformconsumption. The chemical backfill helps in avoiding this problem resulting in a moreefficient discharge of current. Also, the chemical backfill keeps the immediate environmentaround the anode moist, which aids conduction of current.

Impressed Current CP SystemIn contrast to the galvanic CP system, the impressed current CP system is an electrolyticcell designed to include a dc power source for providing power and impressed currentanodes for discharging the CPcurrent.Impressed Current AnodesAs opposed to galvanic anodes, impressed current anodes discharge the current providedby an external power source. It is important, therefore, that the impressed current anodematerial does not get consumed at a rapid rate. Also, its resistance should not get tooaffected by its consumption or polarization. Finally, it is important that the anode materialis economical as the design life of these anodes is quite high.There is a wide choice of impressed current anodes available today thanks to continual


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Some of these anodes are: Scrap Iron anodes High silicon cast iron anodes Mixed - metal oxide coated titanium anodes Platinized titanium / niobium anodes Magnetite anodes Polymer cable anodesScrap Iron anodes:These anodes are typically seen in the form of "pipes", "rails" etc. Scrap iron anodes wereused in earlier times and continue to be used for current drainage surveys that are carriedout to estimate current demand prior to implementing CPo With a rate of consumption of

about 10kg/ A year, these anodes are the economical choice for relatively short designlives.High Silicon cast iron anodes:

These are the most widely used anodes in impressed current CPsystems with long designlives. These iron anodes, available in a cylindrical rod type shape, for pipeline CP systemcontain 14% to 15% of silicon in addition to other alloying elements such as chromium.Because of the inclusion of silicon, the rate of consumption of these anodes, at about 0.2-0.3 kg/A.year, is quite low.Mixed -metal oxide (MMO) coated titanium anodes:

These anodes are composed of a highly conductive mixed metal oxide film thermallyapplied to a titanium substrate. These oxide films have a very low rate of consumption,about 1 mg/A.year. Additionally, they are chemically stable and resistant to acidicelectrolytes. Mixed metal oxide anodes although much newer than high silicon cast ironanodes, have been used extensively in deep well and other configurations.Platinized titanium/niobium anodes:These anodes form an oxide film when treated as impressed current anodes in fresh andsalt-water. The consumption rate of these anodes is very low, of the order of 10mg/A.yearMagnetite Anodes:

These are steel-based anode with a low rate of consumption, about0.02kg /A.year. These anod,esare not commercially available in India.Polymer Cable Anodes:


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ARROW HEAD INDICATESDIRECTIONS OFCURRENT FLOW THRUELEMENT

DCpower sourcesThe dc power source in an impressed current CP system is a vital element that suppliesthe current needed for protecting the steel structure. Commonly used DC power sourcesare now discussed.Transformer Rectifier Units (TRU):A TRU is the most commonly used dc power source in impressed current CPsystems. Thisequipment uses ac supply as input which is transformed and rectified to provide a dcoutput. TRUs can be manually controlled or automatically controlled. The latter TRUs nowuse thyristors for the auto-control feature. Both single -phase and three-phase TRUs areavailable. Single-phase TRUs are generally used for onshore applications and three-phaseTRUsare typically used for offshore applications.

TRANSFORMER RECTIFIER UNIT CIRCUIT DIAGRAMAC1 PHASESUPPLYI I

1----- ---+- -t- -I GROUNDED SHIELDI r--J --1 fI '-]" __,-. ,,-,p,,., -/ I. L'" 'I"" I"".c",\-"''' ,/.,111:~i,~'~r'rt . ; I II Il I ,;' ~" ~: II I:I ._- - I', I II " . I

'I' f-- GROUNDI SHUNT ,11, ! I CONNECTION FORInitl"-! .n 'r'1"ECTIA"""'NETGROUND BEDTO PIPELINE

Additionally, there are dc power units working on both AC and DC supplies. These unitswork on AC supply when AC power is available. When it is not, the unit derives DC powerfrom batteries that are charged during the period that the AC power is available.Thermo Electric Generators (TEGs) and Closed Cycle Vapour Turbogeneartors (CCVTs):TEGs use multiple thermocouples that work on the principle that a temperature differencebetween its ends produces an emf that is used for supplying CP current. TEGs aresuitable for applications where CPpower requirements are low, say under lOOW. The gas


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Solar Power Units:Solar power units make use of high efficiency solar cells that absorb solar energy andconvert it into electricity. Specially designed batteries are used to store the electricalenergy thus produced so that dc power can be made available at nights or during cloudydays. These units are suitable for CP applications in areas where sunshine is plentiful andthe CP power requirement is very small, say under 5 KW.Ground BedsA ground bed is a group of buried anodes that discharge the current needed for CP of astructure. For a cathodically protected pipeline, current flows to the pipeline because ofthe difference in the potentials of earth (which is relatively positive) and the pipeline(which is relatively negative). This difference in potentials between the earth and pipelinecan be created using two different types of ground beds. Remote ground beds Close ground beds

Remote ground beds:Current discharged from the anode ground bed will set up a potential gradient in thesurrounding soil. This potential gradient will be the highest close to the ground bed andwill diminish as one moves away from it until a point is reached where the potentialgradient is zero or almost zero. This point is termed as 'remote earth'. The area betweenremote earth and the anode ground bed is called as the 'area of influence' of the groundbed.Similarly, the current flowing to the pipeline will set up a potential gradient around it. Thispotential gradient will diminish as one moves away from the pipeline, and an 'area ofinfluence' circumscribed by 'remote earth' will also exist for the pipeline. A remote anodeground bed is located far enough from the pipeline such that the areas of influencesurrounding the pipeline and the ground bed remain more or less distinct. The currentdischarged by such a ground bed flows into 'remote earth' and on to the pipeline resultingin cathodic protection of the pipeline. Remote ground beds are typically used forprotecting large lengths of pipelines.

Anode BedVoltage


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Close Ground Beds:In contrast to remote ground beds, close ground beds are located in the vicinity of thepipeline section to be protected. Hence, a small section of the pipeline will pass throughthe 'area of influence' of the ground bed. The earth surrounding the pipeline section,being part of the 'area of influence', will be at a positive potential with respect to remoteearth. The difference in potential between this earth surrounding the pipeline and thepipeline is sufficient to effect flow of current to protect the small section of the pipelines.Close ground beds are used to protect limited sections of the pipeline by providing 'local'CP to areas of the pipeline in their immediate vicinity. Such ground beds are used in a'distributed' fashion to protect underground piping in congested areas, such as plants,where protection using remote ground beds may be unviable due to the shielding effectsof other structures in the vicinity.Types of remote ground beds:The three common remote ground beds in use are: Shallow horizontal anode bed Shallow vertical anode bed Deep well anode bed

The choice of remote ground bed would depend on many factors.Some of these main factors include: Soil resistivity Type of soil Availability of land for locating the ground bed

Carbonaceous backfillImpressed current anodes are surrounded by a carbonaceous backfill before they areinstalled in a ground bed. Carbonaceous backfill increases the effective size of the anodeso that its resistance to earth is lowered. Care should be taken to compact or tamp thebackfill so that it can withstand the consumption caused by current discharge. Thecommon types of carbonaceous backfill in use are Metallurgical coke breeze - this contains about 85% of fixed carbon besides ash,moisture and volatile matter. Calcined petroleum coke breeze - this contains 97% - 99% of fixed carbon. This


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Deep well ground bedDEEP WELL ANODE GROUND BED SCHEMATIC

COLlAR EPOXYCOATING ORP\lC PIPE

rA

\.GRAVEl

/~ANOOE

eVENT PIPEf+~~',CASING PIPEi .;J... ~... ,J(:..~ ...)'~;~TRALIZERSEC. - 'AA'

Test points I Test stationsTest points or test stations facilitate measurement and hence monitoring of electricalparameters related to a CP system. These include pipe-to-soil potentials (PSP), currentflowing through a pipeline, voltage across a pipeline section, potential and current ofanode etc. It would not be feasible to record these parameters by excavating the buriedpipeline each time a measurement is to be made. Test points, thus, provide a convenient"above ground" facility for enabling such measurements.The most commonly used test points are illustrated below.

TEST POINT - POTENTIAL MEASUREMENT

[-g'v~--(,L~-'ln_'_ --------------,1.. HALF CELL_- .,~- GROUND

MEASUREMENT__ .,.CABLE.


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TEST POINT - CURRENT MEASUREMENT

TEST POINT - CASED CROSSING

L~'lABLE j~_..JI l _ -- -I_.J ~_m_ ~- -~_~~ -r=-=-~-~

___ n_~ _

CARRIER .. CA SING

TEST POINT - FOREIGN PIPELINE CROSSING

CABLE


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CABLE

TEST POINT - IMPRESSED CURRENT DRAINAGE POINT

TEST POINT - AT INSULATING JOINT

[t-=tJg~~'io~TION-- ,------- I -t-WITH CLEAT I________~_--~~_l_ --THE~MITWELD

-1::..:=-_=-_=:-=-=--=---l;=:JJ'--\=-=-:-~--=~=~ -- -&-UNPROTECTED SIDE . '. PROTECTED SIDE

INSULA TlNG JOINTSURGE DIVERTER

TEST POINT-GROUNDING ANODE


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Design aspects of CP systems(A Pipeline Perspective)Once the need for applying cathodic protection is determined, one needs to consideravailable facts and requirements while planning the design of a CPsystem. These include: Dimensional details of structure needing CP: For instance if a pipeline is to beprotected. Data related to its diameter and length will be needed.

Design life of the CP system: This is an important criterion in planning a CPsystem. For a short design life, a galvanic CP system would be suitable, while asfor a longer design life, an impressed current CPsystem would be needed. Current requirement for ensuri~ CP: This factor depends upon the surface area ofthe structure needing protection, the type of coating on the pipeline, soilresistivity/composition and engineering expertise. Details of other structures that may cause shielding or unintended drainage of CPcurrent: This aspect is especially important while planning a CP system forunderground piping in a plant where the piping network is located in a congestedarea consisting of several other buried utilities. Interference posed by undergroundutilities not intended to be protected needs to be studied while designing a CP

system for underground piping in a congested area. Based on above aspects, determine the type of CP system that will be suitable galvanic or impressed current: Galvanic and impressed current CP systems aresuited for different applications. The techno-economic feasibility for both these CPsystems should be studied before making a choice. Determine anode configuration: Based on the current demand and currentdistribution needed for effecting adequate CP, a suitable anode configuration has tobe selected. Availability of external power for impressed current CP systems: This is animportant aspect while planning an impressed current CP system. For cross

country pipelines, it is even more important to study this aspect, as multiple CPsources may be neede~ to protect the pipeline. Adequate monitoring facilities.

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