understanding cp system

7/31/2019 Understanding Cp System

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CATHODIC

PROTECTION


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Cathodic Protection

Definition

Cathodic protection is an electrochemical technique to controlthe corrosion of a metal surface by making that surface the

Cathode of an electrochemical cell. (NACE RP-0169 control ofexternal corrosion on under ground / submerged metallic pipingsystem)

Definition in terms of Polarization

Cathodic protection is defined asPolarization of all cathodicsites to a potential equal to the potential of the most active(most negative) static anodic site.


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Concept of CP

Corrosion of metal occurs due to electrochemical corrosioncell composed of anodic areas, cathodic areas, electrolyte anda metal path.

At anodic areas, corrosion current flows from metal intosurrounding electrolyte and metal corrodes.

At cathodic areas, current flows from the electrolyte on tostructure and metal is protected.

Thus, if every part of exposed metal could be made to collectcurrent, it will not corrode because entire surface will beCathodic. This is basic concept f CP.


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Concept of CP

In Terms of Polarization

Corrosion current flows as a result of a potential differencebetween the anodic and cathodic sites.

If the potential difference between the anodic and cathodicsites can be made equal to zero, than no current will flow.

This can be accomplished by polarizing cathodeelectronegatively so that the polarized cathodic potential is

equal to static anodic potential.


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Eliminating Corrosion Cell


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CP - How it Works

Corrosion occurs where current discharges from metal toelectrolyte.

Objective of cathodic protection is to force Direct Current toflow from a source onto all surfaces of the pipeline to make itsentire surface cathodic.

The current is discharged from an underground structurecalled ground bed / anode.

Current flow is adjusted so it overpowers corrosion current

discharging from all anodic sites on pipeline. Previously anodic areas now behave Cathodic and are

protected.


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Forcing Current on Pipeline


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Types of Cathodic Protection

There are two basic methods of achieving CathodicProtection:

Galvanic Anodes Cathodic Protection (SACP)

Impressed Current Cathodic Protection (ICCP)


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CP with Galvanic Anodes

CP with sacrificial anodes uses the electrochemical natureof metals as show in galvanic series.

A dissimilar corrosion cell is established to counter act

corrosion cells existing on pipe surface, by connecting avery active metal to the pipeline.

In practice most used sacrificial anode material aremagnesium, zinc and aluminum alloys.

Sacrificial anode corrodes in order to produce the CPcurrent and protects pipeline.


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Galvanic Anode Applications

Galvanic anodes are generally used where small currents arerequired (typically less than 1 ampere).

In applications where soils resistivity is low enough (typically

less than 10,000 ohm-cm). To protect only few feet of pipe (termed as hot spot) at

specific points e.g. repaired leak or to correct stray currentinterference.

May also be used also for electrical grounding at pumpingstations & across insulating joints.


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Types of Galvanic Anodes

Galvanic anode materials commonly used are:

Magnesium

Zinc

Aluminum (primarily used in marine applications. It has highercurrent capacity than Mg or Zn but passivates rapidly)


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Galvanic Anode Characteristics

Magnesium & Zinc are most widely used materials forgalvanic anodes. Typical characteristics are given in tablebelow

Characteristics of Anodes Mg ZnSpecific Gravity 1.94 7.0

Density lbs / ft3 121 440

Amp-hour per pound 1000 372

Pounds per Amp per Year 8.7 23.5

Current Efficiency 50 80

Actual Amp-hour per pound 500 335

Actual pounds per Amp per Year 17.4 26


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Galvanic Anode Installations

Single Package Anode Installation

The simplest installation involves burying of a singlepackaged anode at a leak point or for distributed anode

installations along a pipeline.


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Galvanic Anode Installations Multiple Packaged Anode Installation Several Mg or Zn anodes may be connected to a header wire which is

terminated in a test point for periodic monitoring of output current /voltage.


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Galvanic Anode Installations

UnpackagedAnode

Installation Mg / Zn anodes

also availableunpackaged.These are

installed withprepared backfillin buriedinstallations.


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Calculating Anode Life

If current output of a galvanic anode of given weight is known,its approx useful life can be calculated.

Mg Anode Life (years)

= { 0.116 Anode Weight (lbs) Current Efficiency UtilizationFactor } / Design current

Example:

Assume 32lb Mg anode producing 0.1A at 50% efficiency.Calculate expected life:

Mg Anode Life (yrs) = (0.116320.500.85) /0.1 = 15.8 years


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CP With Impressed Current

In galvanic anode systems, a higher energy metal is used tosupply protection current.

An impressed current CP system uses an external DC power

source to supply the protection current. The current is distributed to pipeline through an underground

current discharging structure (anode ground bed) which iselectrically connected to T/R.

Most common power source is a transformer rectifier, whichconverts AC to low voltage DC supply.


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Components of ICCP

An ICCP system has following typicalcomponents:

Source of DC power supply

Ground Bed

Object to be Protected (pipeline/structure)

Connecting Wires / Cables

Test Points


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DC Power Source

A DC power source can be any of the following:

Transfer Rectifiers

DC Batteries

Thermo Electrical Generators

Motor Generators

Wind Generators

Solar Cells etc.


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Characteristics of a Rectifier

A T/R unit typically consists of the following:

A transformer to step down AC line voltage to low AC on thesecondary.

A tap arrangement permits selecting range of output voltage

A rectifier (silicon diode) converts AC to DC. External housing for outdoor mounting.

A wide range of rectifiers are available for varying dc out putvoltage, in small increments.

Voltage output typically range from 10 to 50 volts. Current output range from 10 to many hundred amps.


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Anode Ground beds

Materials

Materials popular for anode beds include graphite, highsilicone iron, mixed metal oxide and steel scrap.

Graphite anodes are available in various sizes, but 3-in by60-in rods are most common for pipeline use and are suppliedwith insulated Cu leads (HMWPE).

High silicon cast iron anodes normally contain b/w 14~15%silicon plus other alloying elements.

Mixed metal oxide films are thermally applied to cores ofprecious metals e.g. Ti or Nb. These have excellentconductivity, resistant to acid environment and have lowconsumption rates.


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Anode Ground

Beds


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Connecting Wires

CP system components must be duly connected.

Connecting wires should be of pure copper and have leastinternal resistance.

Insulation should be at least 600v rating & suitbale for directburial. HMW-PE insulated wires widely used.

Anode cables are at +ve potential w.r.t. earth and willdischarge current (corrode), if not perfectly insulated.

Acceptable connection methods include soldering, powderwelding (thermite), phos-copper brazing, crimp type couplingand spilt bolt coupling to avoid heat damage to cableinsulation.


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ICCP Ground Bed Installation

Typically two anode constructions are used i.e.

Vertical anode installations

Horizontal anode installations

No. of anodes required to attain a certain ground bedresistance are typical selected from charts.

Important factor in selecting no. of anodes is desired anodelife. To reduce current density per anode, no. of anodes can

be increased, increasing anode life. Carbonaceous backfill is around anode is of very low

resistivity and reduces anode to earth resistance.


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ICCP Ground Bed Installation


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Object to be protected

The underground object may be any one of following:

A cross country pipeline.

A distribution pipeline.

A piping network

A steel structure.

A storage tank

An RCC foundation

A ships hull etc.


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A Backfill

Backfill around anode serves two basic purposes:

Increases effective size of the anode to obtain lower anode toearth resistance.

Bears main consumption rate from current discharge.

Materials suitable for this requirement are:

Coal coke breeze.

Calcined petroleum coke breeze.

Natural or man-made graphite part icles.


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Test Points

Test Points or Stations are used for connecting a volt meter toallow for potential testing.

Test points provide contacts for reference electrode as well as

test leads to underground pipeline.

Test point should be installed at inaccessible points ofstructures e.g. paved areas, concrete slabs.

Any accessible location can be used as a test point. A valve pit,a low level drain pit, an exposed crossing can be used as testpoint.


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Design Basis of CP System

German Standard

The CP system design is in accordance with German StandardAFK (Arbeitsgemeinschaft fr korrsionssch = German corrosioncommittee) Recommendation 9.

British Standard

The system also complies with British Standard BS 7361-91 (PartI) Cathodic Protection code of practice for land & marineapplications.


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Current Demand

Current demand for CP is based on surface area of RCCfoundations at plant site, which is 180,989 m2.

As per AFK Rec. 09, RCC absorbs 10mA/m2. So total

current required is approximately 1800 Amperes.


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Anode GBs Distribution


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Deep Well Ground Beds

Construction

Each ground bed (150m deep) consists of:

32 Si Fe anodes (size 2"60) in 16 chains. Each chain has 02

anodes & armored cable 110 mm2.

Carbonaceous backfill around anodes which increase effectivesize of anode & bears main consumption.

Steel support construction, consisting of 2 steel angle parts,

connecting / lifting plates, anode carrying plates andcentralizers.

2 pieces of 50 mm dia ventilation / irrigation pipes.


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Chemical Composition (SiFe) Anodes

Si 14~15%

Mn 0.6~0.8%C 0.9~1.0%

Cr 4.0~5.0%

Fe Remainder


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Estimated Anode Life

Maximum current density = 0.5A / dm2

Anode weight /ground bed = 16 (223.8kg) = 760kg

Anode consumption (bare) = 0.3 Kg / A-year Anode consumption (in coke) = 0.1 Kg / A-year

Efficiency =60%

Life Time = 760 0.60= 25.4 years (at max. output)

(Theor.) 90 x 0.2


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Anode Junction Boxes

Anode JB are installed near the ground beds.

Anode JB is of stainless steel of 600600250 mm, supportedon 2 nos. of 100 mm SS pipes.

Positive cables of 16 anode chains from ground bed enteranode JB through support pipes.

A positive cable from T/R also runs to anode JB and isconnected with 16 anode chain cables.

16 shunts are provided for current measurement & 16variable resistors to limit current per anode to 5A.

A shunt enables to measure total anode bed current.


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Anode Junction Boxes


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Transformer Rectifier Units

Construction T/R units are DC power sources for ICCP system. 07 T/R units are installed, suited for outdoor location at 50C &

85% relative humidity, with sun roof.

TR units are oil cooled with components installed inside oiltank. Minimum oil level must be checked.

Above oil tank a control cabinet is provided with two lockabledoors for metering and regulation. Three 4 posit ion rotaryswitches select voltage from 0~50 V.

For installation of positive and negative cables, a JB is installedon one side of T/R. From T.R unit, 3 positive cables (95 mm2) run to 3 anode beds

and 35 mm2 negative cables run to negative bond boxes.


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Transformer Rectifier Units


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Negative Bonding Boxes

The bonding boxes are installed to hook the negative cablesup to pipelines with bonding cables.

Total 15 bonding boxes, specially designed for CP system for50 C and 85% relative humidity are installed.

Bonding box is made of stainless steel of 200300150 mm,supported on 100 mm dia. pipe.

Each negative main cable (95mm2) from T/R unit is connectedinto a negative bonding box.

All negative cables (35mm2) from to be protected pipelinesone terminated in the bond box.


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Test Points

Test points are installed at various locations of plant area topermit monitoring of pipe to soil potentials.

Total 90 test points are installed out which 30 are located atammonia plant and 60 on EPC plants.

Test points are specifically designed for CP system for 50C and85% relative humidity.

The 30 test points for ammonia plant are provided with 30permanent CuSO4 reference electrodes, due to concrete slab

at plant.


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Test Points

Each test point consists of:

A test post made of stainless steel of dimensions 100100200mm.

Cables (Cu 35 mm2) from pipelines to terminals inside the testpost.

Steel (St-37) test coupon bridged by means of 10 ohm resistorwith pipeline cable.

A CuCSO4

reference electrode (1 out of 5 test points).


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Test Points


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Test Coupons

Test coupons are installed to simulate a damage incoating of pipeline. This coating damage should becathodically protected.

Test coupon is made of Steel grade St. 37 andconstructed complete with 2 cables. One cable leadconnects with pipeline cable, via 10 ohm resistor andother lead used for potential measurement.

Distance b/w pipeline and test coupon is kept 50 to100 mm (max.) and b/w test coupon and permanentCuSO4 reference electrode also 50 to 100mm (max.)


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Reference Electrodes

Accepted criteria to determine cathodic protection is pipeto soil (metal to electrolyte) potential.

This is essentially voltage difference between the surface ofprotected pipe and a stable CSE.

Stelth reference electrodes are specially designed for use indry and sandy soils.

Stelth 2 stationary reference electrode (SRE-007-CUY) is usedfor permanent installations in soil. These are installed at 56locations

Stelth 3 portable reference electrode (SRE-010-CPY) is usedfor potential testing at ground level.


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Reference Electrodes

understanding cp system

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