Transcript fundamentals of cathodic protection and its application

Engineers India Limited
FUNDAMENTALS OF CATHODIC
PROTECTION AND ITS APPLICATION
R. CHAUDHURY
N. S. BHATTACHARYA
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1
MECHANISM OF CORROSION
•
CORROSION IS LOSS OF METAL BY ELECTROCHEMICAL
OF METAL WITH ITS ENVIRONMENT.
•
ALMOST ALWAYS ELECTROCHEMICAL
AQUEOUS SOLUTIONS.
•
IN ALL AQUEOUS ELECTROLYTES SUCH AS SOIL, WATER, METAL
ATOMS GO INTO SOLUTION AS METAL IONS.
•
FOR CORROSION (ELECTROCHEMICAL) REACTION TO PROCEED THE
FOLLOWING CONDITIONS MUST BE SATISFIED.
REACTIONS
REACTION
OCCUR
-
EXISTENCE OF ANODE & CATHODE.
-
EXTERNAL METAL PATH BETWEEN ANODE & CATHODE.
IN
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-
ANODE & CATHODE SHOULD BE IN CONTACT
WITH ELECTROLYTE.
-
ELECTROLYTE SHALL BE CONDUCTIVE TO
IONS.
-
POTENTIAL DIFFERENCE BETWEEN ANODE &
CATHODE.
•
AT ANODE METAL GOES INTO SOLUTION AS METAL IONS
(OXIDATION REACTION).
•
AT CATHODE METAL DEPOSITION OR REDUCTION OF
GASES OR CATIONS OCCUR. (REDUCTION
REACTION).
•
ANODIC SITES HAVE A MORE NEGATIVE POTENTIAL THAN
CATHODIC SITES IN THE SAME ELECTROLYTE.
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•
WHEN METAL ATOMS GO INTO SOLUTION AS
METAL IONS, ELECTRONS ARE RELEASED AT
ANODE WHICH MIGRATE TO CATHODE BY METAL
PATH WHERE IT IS CONSUMED BY REDUCTION
REACTION. THIS FLOW OF ELECTRONS GIVES RISE
TO D.C. CURRENT. THUS THE MAGNITUDE OF
CURRENT
WHICH
IS
CALLED
CORROSION
CURRENT IS DIRECTLY RELATED TO MASS OF
METAL GOING INTO SOLUTION BY FARADAY’S LAW.
•
PASSAGE OF ELECTRONS THROUGH EXTERNAL
METAL PATH FROM ANODE TO CATHODE MEANS
FLOW OF CONVENTIONAL CURRENT FROM ANODE
TO CATHODE THROUGH ELECTROLYTE. THUS ANY
CORROSION PHENOMENON IS ASSOCIATED WITH
PASSAGE OF D.C. CURRENT FROM ANODE TO
ELECTROLYTE.
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CORROSION REACTIONS
AT ANODE
M  M +z + Ze
AT CATHODE
(Acid)
2 H + +2e  H2
(Acidic solutions)
O2+ 4H+ + 4e 2 H2O
(Neutral
or Basic solutions)
O 2+ 2H2O+ 4e 4OH -
For Iron
Fe  Fe+++2e
Fe ++ + 20H-Fe (OH)2
4Fe (OH)2 + O2 + 2H2 O  4Fe (OH)3
Fe(OH)3  FeOOH+H2O (Hydrated Ferric oxide, Red Rust)
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CORROSION RATE
UNIFORM ATTACK
CORROSION RATE EXPRESSED AS mm/yr OR mils/year (mpy) OR
mg/dm2/day(mdd). THE ABOVE UNITS REPRESENT AVERAGE RATE OF
METAL PENETRATION OR WEIGHT LOSS OF METAL EXCLUDING ANY
ADHERENT OR NON ADHERENT CORROSION PRODUCTS.
FOR STEEL IN SEA WATER RATE IS
FOR STEEL IN SOIL RATE IS
= 0.13 mm/yr.
= 0.021 mm/yr.
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CLASSIFICATION ON BASIS OF CORROSION RATE
a)
 0.15 mm/yr (5mpy) – EXCELLENT CORROSION
RESISTANCE.
b) 0.15-0.5 mm/yr (5-20 mpy) –GOOD
c)
0.5 – 1.0 mm/yr (20-50 mpy) - FAIR
d)  1.0 mm/yr (50 mpy)-UNACCEPTABLE.
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CORROSION PREVENTION METHODS FOR METAL AGAINST
SOIL / ELECTROLYTE CORROSION
COATING
•
CATHODIC PROTECTION
COATING
•
COATING BY COAL TAR ENAMEL OR POLYETHYLENE IS PRIMARY
METHOD OF EXTERNAL CORROSION PROTECTION.
•
PROTECTION BY COATING ALONE IS NOT RECOMMENDED DUE
TO RAPID ATTACK OF METAL AT COATING HOLIDAYS (AREAS OF
COATING DEFECTS) DUE TO LARGE ANODIC CURRENT DENSITY
AT DEFECTS.
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•
THEREFORE COATING IS ALWAYS SUPPLEMENTED WITH CATHODIC
PROTECTION WHICH ESSENTIALLY PROTECTS EXPOSED METAL AT
COATING DEFECTS.
•
CONJOINT USE OF COATING AND CATHODIC PROTECTION TAKES
ADVANTAGE OF MOST ATTRACTIVE FEATURES OF EACH METHOD OF
CORROSION CONTROL.
•
APPLICATION OF COATING DRASTICALLY REDUCES CATHODIC
PROTECTION CURRENT REQUIREMENT SINCE C.P. CURRENT IS
REQUIRED AT DEFECT AREAS (HOLIDAYS) OF COATING.
•
CURRENT REDUCTION IS IN ORDER OF 2000-100 FOLDS DEPENDING
ON TYPE OF COATING AND ELECTROLYTE.
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•
METAL DISSOLUTION
FOLLOWING :
•
HIGHLY CONDUCTIVE SUB SOIL WATER
•
PRESENCE OF SULFATE IONS AND CHORIDES IONS
IS
ACCELERATED
DUE
TO
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CATHODIC PROTECTION
THIS IS AN ELECTROCHEMICAL TECHNIQUE.
•
BY VIRTUE OF THIS, WHEN PROPERLY DESIGNED, THIS IS
MOST EFFECTIVE METHOD OF CORROSION PREVENTION.
•
CATHODIC
PROTECTION
BEING
ELECTRO-CHEMICAL
TECHNIQUE, ARRESTS ALL FORMS OF CORROSION (UNIFORM
ATTACK, GALVANIC, PITTING, CREVICE, MIC, STRESS
CORROSION CRACKING) EXCEPTING H2 DAMAGE.
•
MOST EFFECTIVE METHOD OF CORROSION CONTROL BRINGS
CORROSION RATE TO ZERO.
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•
LEAST COSTLY METHOD FOR PROTECTING IMMERSED
STRUCTURES IN SOIL AND AQUEOUS MEDIUM AGAINST
CORROSION.
•
“HAS PROVED ITS RELIABILITY IN NUMEROUS APPLICATIONS
AND DESIGNS.”
•
BY COMPARISON WITH VALUE OF ASSET PROTECTED COST
OF CATHODIC PROTECTION (INSTALLED + OPERATING) IS
LOW.
•
THE METHOD CONSISTS OF SUPPLYING ELECTRONS FROM
EXTERNAL SOURCE TO THE CORRODING METAL SO AS TO
CONVERT ALL ANODIC SITES OF CORRODING METAL TO
CATHODE WHERE BY ONLY REDUCTION REACTION OCCURS
BY CONSUMING THE SUPPLIED ELECTRONS.
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• THE MAGNITUDE OF EXTERNAL CURRENT IS SUCH AS TO
DEPRESS THE METAL POTENTIAL TO NEGATIVE SIDE TO PREVENT
DISSOLUTION OF METAL AND CURRENT DIRECTION BEING
OPPOSITE TO CORROSION CURRENT OFFSETS THE CORROSION
CURRENT, BRINGING DOWN CORROSION RATE TO ZERO (FIG. 6.0 &
FIG 7.0). CONVENTIONAL CURRENT DIRECTION IS FROM THE
ELECTROLYTE TO THE PROTECTED METAL (CATHODE)
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METHODS OF APPLYING CATHODIC PROTECTION
•
SACRIFICIAL ANODE
•
IMPRESSED CURRENT
•
IN SACRIFICIAL ANODE SYSTEM ELECTRONS ARE GENERATED BY
CORRODING A BASE METAL WHICH IS MORE ANODIC TO
PROTECTED STRUCTURE.
EXAMPLE : Zn, Mg, Al (Fig. 8).
•
IN IMPRESSED CURRENT SYSTEM, THE ELECTRONS ARE SUPPLIED
FROM A D.C. POWER SOURCE (RECTIFIER UNIT, BATTERY BANK
ETC.). IN ELECTROLYTE THE CURRENT IS FED THROUGH A NONCONSUMABLE ANODE (FIG. 9).
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CATHODIC PROTECTION REACTIONS
AT CATHODE :
O2 + H2+ 4e  4OH2H2O + 2e - H2  + 20H - ( At high Potential)
Ca++ + HCO3- + OH - H2O + CaCO3  (Calcareous scale)
Mg + + 2 H -

MgOH (Calcareous scale)
At Anode (Impressed Current System)
2H2O  O2 + 4H + + 4e.
2Cl-  Cl2  + 2 e
At Anode (Galvanic Anode)
Zn  Zn++ + 2e (Dissolution of Zinc)
OR
Al  Al
+++
+ 3e. (Dissolution of Aluminum)
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PROTECTION CRITERIA
•
STEEL STRUCTURES EXPOSED TO SOIL, FRESH WATER AND SEA
WATER ARE FULLY CATHODICALLY PROTECTED WHEN THEIR
INTERFACE POTENTIAL TO ELECTROLYTE WHEN MEASURED WITH A
COPPER –COPPER SULFATE REFERENCE ELECTRODE IS MINIMUM0.850 VOLTS. FOR SOIL / FRESH WATER AND -0.800V w.r.t. Ag/Agcl IN
SEA WATER.
•
MEASUREMENT TAKEN BY PLACING THE ELECTRODE IN THE
ELECTROLYTE AND MEASURING THE POTENTIAL BETWEEN
STRUCTURE AND ELECTRODE WITH A HIGH RESISTANCE
VOLTMETER ( SEE FIGURE).
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•
THE POTENTIAL REFERRED TO ABOVE IS THE INTERFACE
POLARISATION POTENTIAL AND DOES NOT INCLUDE ohmic IR
DROP IN SOIL AND ACROSS COATING.
•
WHERE LARGE IR DROP ERROR IS SUSPECTED TO BE INCLUDED
IN MEASURED POTENTIAL e.g. HIGH RESISTIVITY SOIL, POOR
COATING OR BARE STRUCTURE DRAWING LARGE CURRENT ONLY
POLARISED INTERFACE POTENTIAL TO BE MEASURED.
•
THIS IS DONE BY TEMPORARILY SWITCHING ‘OFF’ THE C.P.
CURRENT AND MEASURE POTENTIAL WITHIN 1 SEC FROM
SWITCHING OFF. POTENTIAL SO MEASURED IS TERMED AS
INSTANT ‘OFF’ POTENTIAL (SEE FIGURE). TYPICAL SWITCHING
CYCLE IS 12 SEC ‘ON’, 3 SEC ‘OFF’.
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•
FOR LARGE BARE STRUCTURE DRAWING HIGH CURRENT, -0.850 V
CRITERIA IS CONSERVATIVE FOR SUCH CASES MINIMUM 100 MV
POTENTIAL SHIFT IS RECOMMENDED AS PROTECTION CRITERIA
SHIFT = INSTANT ‘OFF’ POTENTIAL - NATURAL POT
(Cu/CuSO4)
(Cu/CuSO4)
•
CATHODIC PROTECTION / CORROSION POTENTIALS ARE ALWAYS
REFERRED TO WITH RESPECT TO A REFERENCE ELECTRODE AGAINST
WHICH VALUES HAVE BEEN MEASURED
•
FOR ρ = 100-1000 ohm-M
Minimum protection potential = -750mV
For ρ > 1000 ohm-M
Minimum protection potential = -650 mV
High pH SCC in potential range -650 mV to -750 mV
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OVER PROTECTION
•
EXCESSIVE CURRENT LEADS TO VERY HIGH CATHODE POTENTIAL WHICH
HAS FOLLOWING DETRIMENTAL EFFECTS.
•
DAMAGE TO COATING BY SAPPONIFICATION (SOFTENING OF COATING) AT
POTENTIAL MORE NEGATIVE THAN –2.0 VOLTS ( Cu/CuSO4). High pH
•
HYDROGEN GENERATION AT POTENIAL MORE NEGATIVE THAN -1.2V (OFF)
Cu/CuSO4 LEADING TO BLISTERING AND DISBONDMENT OF COATING.
•
WASTAGE OF ENERGY.
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CATHODIC PROTECTION CURRENT REQUIREMENTS
PROTECTIVE CURRENTS ARE DETERMINED EMPIRICALLY OR BY
TESTING. A COATED SURFACE REQUIRES MUCH LESS CURRENT
THAN BARE SURFACE.
• CURRENT REQUIREMENT IS GOVERNED BY THE CORROSION RATE
OF THE METAL IN THE GIVEN ELECTROLYTE WHICH IN TURN IS
DETERMINED BY CORROSIVITY OF ENVIRONMENT, TEMPERATURE, O2
CONTENT, FLOW ETC. ACIDIC SOLUTIONS REQUIRE VERY HIGH
CURRENT.
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TYPICAL CURRENT REQUIREMENTS FOR CATHODIC
PROTECTION OF STEEL (BARE SURFACE)
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TYPICAL CURRENT REQUIREMENTS FOR COATED STEEL
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CATHODIC PROTECTION COMPONENTS
FOLLOWING ARE THE MAJOR COMPONENTS OF
CATHODIC PROTECTION SYSTEM :
•
D.C. POWER SOURCE (FOR IMPRESSED CURRENT SYSTEM)
•
ANODES
•
CABLES
•
BACKFILL
•
REFERENCE ELECTRODES (PERMANENT & PORTABLE)
•
JUNCTION BOXES
•
POTENTIAL MEASUREMENT TEST POINTS
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•
CORROSION PROBES / COUPONS
•
POTENTIAL RECORDER
•
ZINC GROUNDING CELLS
•
INSULATING JOINTS / FLANGES
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D.C. POWER SOURCE
•
AUTOMATIC POTENTIAL CONTROL OR CONSTANT VOLTAGE / CONSTANT
CURRENT TRANSFORMER RECTIFIER UNIT (TRU) OF FOLLOWING :
25A / 25V D.C.
50A / 50V D.C.
75A / 75V D.C.
50A / 75V D.C.
100A / 12V D.C.
•
FOR CROSS COUNTRY PIPELINES OR UNDERWATER STRUCTURES AUTO
POTENTIAL CONTROL UNITS ARE RECOMMENDED.
•
FOR PLANT STRUCTURES CONTANT VOLTAGE / CONSTANT CURRENT
UNITS ARE RECOMMENDED.
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ANODE MATERIAL
SACRIFICIAL  ZINC, ALUMINUM ALLOY, MAGNESIUM
IMPRESSED CURRENT ANODES
1. GRAPHITE
2. SILICON-CHROMIUM-IRON ALLOY
3. METAL OXIDE COATED TITANIUM (DSA ANODES)
4. PLATINISED TITANIUM
5. PLATINISED NIOBIUM
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SACRIFICIAL ANODES
MAGNESIUM, ZINC, ALUMINIUM.
•
SACRIFICIAL ANODES GENERATE THEIR OWN D.C. CHARGES AND
REQUIRE NO EXTERNAL D.C. POWER SOURCE.
•
SACRIFICIAL ANODES ARE MADE OF METALS WHICH ARE MORE
ELECTRONEGATIVE THAN PROTECTED METAL.
•
THEY HAVE FIXED DRIVING VOLTAGE TO PROTECTED METAL WHICH
IS IN RANGE OF 0.6-0.25 VOLTS.
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•
CLOSE CIRCUIT POTENTIAL OF VARIOUS SACRIFICIAL ANODES ARE
AS FOLLOWS :
MAGNESIUM
ZINC
ALUMINIUM
•
- -1.5 to -1.7V (w.r.t. Cu/CuSO4)
-
-1.05V (w.r.t. Cu/CuSO4)
- -1.05-1.1V (w.r.t. Cu/CuSO4)
ANODES ARE ALWAYS PRE-PACKED IN CHEMICAL BACKFILL OF 25%
GYPSUM, 70% BENTONITE, 5% Na2SO4.
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MOST COMMONLY USED ANODES ARE
- Fe-Si-Cr CYLINDRICAL
- METAL OXIDE COATED TITANIUM TUBULAR
SEMI CONSUMABLE AND NON-CONSUMABLE ANODES HAVE HIGH
RESISTANCE TO ACID (pH 0.5-2) PRODUCED AT ITS INTERFACE.
2H2O
 O2+4e+4H+
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SACRIFICIAL ANODE Vs. IMPRESSED CURRENT SYSTEM
EACH TECHNIQUE HAS ITS OWN WEAKNESS AND STRENGTH.
ADVANTAGE OF SACRIFICIAL ANODE TECHNIQUE :
-
OPERATE INDEPENDENTLY OF SUPPLY OF
ELECTRICAL POWER
-
SIMPLE TO INSTALL
-
CANNOT BE INCORRECTLY ATTACHED TO
STRUCTURE
-
NO CONTROL FUNCTION TO BE EXERCISED.
-
NO OVER PROTECTION
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-
UNIFORM ELECTRODE POTENTIAL ACROSS
STRUCTURE
-
CAN
BE
USED
IN
CLOSED
FLAMABLE VAPOURS / LIQUIDS
SYSTEMS
CONTAINING
MOST SEVERE LIMITATIONS OF SACRIFICIAL ANODE TECHNIQUE :
-
SMALL DRIVING VOLTAGE AND HENCE
RESTRICTED USE TO LOW RESISTIVITY
ENVIRONMENT (SEA WATER, FRESH WATER, SALINE SOIL,
MARSH, ETC.) OR SYSTEMS REQUIRING
LOW
PROTECTION
CURRENT IN HIGH RESISTIVITY ENVIRONMENT (e.g. VERY WELL
COATED STRUCTURE OR STRUCTURES HAVING SMALL SURFACE
AREA).
-
ECONOMICAL FOR LOW PROTECTION CURRENT.
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-
NOT SUITABLE FOR LONG LIFE (10-20
SOILS HAVING CO3-, HCO3- OR PO4-. BEST FOR
SOILS HAVING Cl- OR SO4--.
-
SMALL CURRENT THROWING CAPACITY.
YEARS)
IN
DISADVANTAGES OF IMPRESSED CURRENT SYSTEM
-
NEED FOR AN EXTERNAL RELIABLE POWER
SUPPLY
-
OVER PROTECTION
-
DIFFICULTY IN ACHIEVING UNIFORM
POTENTIAL PROFILE IN COMPLEX SHAPED
STRUCTURES.
-
CANNOT BE USED IN CLOSED SYSTEMS
CONTAINING FLAMMABLE VAPOURS.
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CABLES
-
COPPER CONDUCTOR TO HAVE LESS VOLTAGE
-
HEADER CABLES ARE 25-50mm2 SINGLE CORE
-
MEASUREMENT CABLES ARE 4-6mm2 SINGLE
-
ANODE CABLES ARE 6-10mm2 SINGLE CORE
DROP
CORE
ALL CABLES EXCEPTING ANODE TAIL CABLES ARE ARMOURED WITH
PE OR PVC INSULATION & PVC SHEATH.
ANODE TAIL CABLES ARE UNARMOURED WITH XLPE OR HMWPE OR
EPR OR PVDF INSULATION WITH PVC OR HMWPE OR CSPE
SHEATHING FOR ACID RESISTANCE.
TYPE OF INSULATION
DEPENDS ON SEVERITY OF ENVIRONMENT.
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REFERENCE ELECTRODES (See Figure)
..
STABLE NON-POLARIZABLE ELECTRODES WITH
STABLE ELECTRODE POTENTIAL
-
CONSISTS OF HIGH PURITY METAL ELEMENT IN
THE SOLUTION OF ITS OWN SALT
-
MOST COMMONLY USED ELECTRODES ARE
-
SATURATED COPPER-COPPER SULFATE
(Cu/CuSO4)
-
SILVER-SILVER CHLORIDE (Ag/Agcl)
-
SILVER-SILVER CHLORIDE / SATURATED
Kcl. (Ag/Agcl/Kcl)
-
HIGH PURITY ZINC
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MOST IMPORTANT PROPERTY OF REFERENCE
ELECTRODES ARE
-
STABLE POTENTIAL OF ELECTRODE
IRRESPECTIVE OF EXTERNAL CURRENT (±5mv)
-
LOW TEMPERATURE CO-EFFICIENT
-
LOW INTERNAL RESISTANCE (< 200 ohms)
FOR FRESH WATER AND SOIL COPPER-COPPER SULFATE ELECTRODE
ARE USED.
FOR SEA WATER Ag-Agcl OR ZINC ELECTRODES ARE USED.
FOR BRACKESH WATER, SALINE SOIL (cl- > 300 ppm) Ag-Agcl/SAT Kcl
OR ZINC IS USED.
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PERMANENT REFERENCE ELECTRODES IN SOIL APPLICATION ARE
SURROUNDED IN CHEMICAL BACKFILL (BENTONITE + GYPSUM) TO
PREVENT DRYING UP OF SURROUNDING SOIL.
SOIL HAS TO BE THOROUGHLY WATERED BEFORE INSTALLING
ELECTRODE.
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CORROSION VOLTMETER
–
THESE ARE TO MEASURE STRUCTURE TO ELECTROLYTE POTENTIAL
– DUE TO HIGH EXTERNAL RESISTANCE IN CIRCUIT, THEY HAVE HIGH
INTERNAL RESISTANCE (> 10 K)
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TEST POINTS (See Figure)
TEST POINTS ARE INSTALLED AT REGULAR INTERVALS OR AT DESIRED
LOCATIONS TO SERVE FOLLOWING FUNCTIONS :
-
TERMINATION
OF
MEASUREMENT
CABLES
STRUCTURE AND REFERENCE ELECTRODES
-
CONNECTION OF SACRIFICIAL ANODES
-
BONDING OF DIFFERENT STRUCTURES
-
CONNECTION OF GROUNDING CELLS ACROSS
-
CONNECTION OF PROBES / COUPONS
FROM
I.F/I.J
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CORROSION PROBES COUPONS (See Figure)
COUPONS ARE INSTALLED TO MEASURE INSTANT ‘OFF’ POTENTIAL
MEASUREMENT AND ARE PLACED AT CRITICAL LOCATIONS AS
FOLLOWS :
-
AREAS OF MARGINAL PROTECTION
-
LOCATION OF LARGE IR DROP
-
INTERFERENCE LOCATION
-
HIGHLY CORROSIVE / AGGRESSIVE SPOTS
WITH COUPONS, T/R UNIT INTERRUPTION IS NOT REQUIRED.
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CATHODIC PROTECTION OF ONSHORE PIPELINES
BY IMPRESSED CURRENT FOR PERMANENT PROTECTION (~ 30 YEARS)
BY SACRIFICIAL ANODES FOR TEMPORARY PROTECTION (< 2 YEARS)
DURING CONSTRUCTION TILL PERMANENT PROTECTION IS IN
PLACE.
REQUIREMENTS PUT BY CATHODIC PROTECTION
CONTINUOUS LENGTHWISE ELECTRICAL CONTI-NUITY OF PIPELINE
AND ITS BRANCHES. FLANGED / SOCKET JOINTS TO BE BRIDGED.
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ELECTRICAL ISOLATION OF PIPELINE FROM ALL UNITS /
FACILITIES WITH LOW RESISTANCE GROUNDINGS AND
OTHER EXTRANEOUS INSTALLA-TION.
A SUITABLE SITE FOR ANODE BED.
IMPRESSED CURRENT SYSTEM
EACH PIPELINE IS PROTECTED BY A SERIES OF IMPRESSED
CURRENT CATHODIC PROTECTION INSTALLATION TERMED
AS C.P. STATION OR RECTIFIER STATION.
NO. OF STATIONS AND THEIR INTERVAL DECIDED BY
-
COATING QUALITY AND SOIL RESISTIVITY
-
PIPE DIAMETER
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-
MAXIMUM AND MINIMUM ALLOWABLE
POTENTIAL OF PIPE AT C.P. STATION AND POINT FARTHEST
FROM IT.
-
PIPE METAL RESISTANCE / METER (/M)
-
PROTECTION CURRENT DENSITY (mA/M2)
SPAN FOR GOOD QUALITY COATINGS = 35-40 KM
SPAN FOR POOR COATING / BARE PIPE = 5-10 KM
COMPONENTS OF EACH RECTIFIER STATION / C.P. STATION (See
Figure)
-
D.C. POWER SOURCE
-
ANODE AND ANODE BED
-
PERMANENT REFERENCE ELECTRODES AT
DRAINAGE POINT
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-
D.C. AND MEASUREMENT CABLING
-
ANODE AND CATHODE JUNCTION BOXES
-
INTERMEDIATE TEST STATIONS AT AVERAGE 1 KM INTERVAL.
-
COUPONS AT VULNERABLE LOCATIONS
D.C. POWER SOURCE
-
AUTOMATIC POTENTIAL CONTROLLED TRANSFORMER-REFTIFIER
OF RATING 50A / 50V D.C. OR 25A / 25V D.C.
-
BATTERY BANK WITH POTENTIAL CONTROLLER (CPPCM) 48V / 50A,
24V / 25A.
-
POSITIVE OF D.C. POWER SOURCE CONNECTED TO ANODE BED,
NEGATIVE TO PIPE THROUGH JUNCTION BOX (See Figure).
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ANODE AND ANODE BED
-
ANODES ARE Fe-Si-Cr OR METAL OXIDE COATED TITANIUM.
-
EACH ANODE BED CONSISTS OF 10-20 NOS. OF ANODES.
-
NO. OF ANODE DECIDED BY TOTAL CURRENT DEMAND, ANODE BED
RESISTANCE TO EARTH (< 0.5-0.6), DESIGN LIFE, CONSUMPTION
RATE OF ANODE.
-
ANODES ARE LAID IN PARALLEL IN FOLLOWING CONFIGURATION
(See Figure)
-
SHALLOW VERTICAL
-
SHALLOW HORIZONTAL
-
DEEP VERTICAL
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-
ANODE TO ANODE SPACING = 3-5 M
-
TYPE OF CONFIGURATION IS DECIDED BY SOIL
STRATA, WATER TABLE, SOIL RESISTIVITY
MAXIMUM ALLOWABLE ANODE BED RESISTANCE
EARTH = 0.5-0.6 )
TO
ANODE BED SHOULD BE ELECTRICALLY REMOTE
FROM
PROTECTED PIPELINE AND OTHER
NEARBY FOREIGN
STRUCTURES TO MINIMISE
INTERFERENCE ON FOREIGN
STRUCTURE AND
OVER PROTECTION OF PORTECTED
STRUCTURE
EACH ANODE PRE-PACKED IN COKE BACKFILL
TO
EXTEND LIFE OF ANODES BY TRANSFERRING
MOST OF
ANODE ELECTROCHEMICAL REACTION
TO COKE /
ELECTROLYTE INTERFACE
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Electrical remoteness achieved when potential to remote earth ≤ 0.5 volts
Ur (volts) = Iρ / 2πr,
Ur = potential to remote earth at location with distance r (meters) from anode.
I = anode current (amps)
ρ = Soil resistivity (ohm-m)
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COKE BED ALSO REDUCES ANODE RESISTANCE AND ALLOYS
UNIFORM CURRENT DISSIPATION.
PERMANENT REFERENCE ELECTRODE
–
MINIMUM 2 NOS. (1 WORKING + 1 STANDBY) COPPER-COPPER
SULFATE PERMANENT REFERENCE ELECTRODES ARE PLACED
NEAR DRAINAGE POINT OF EACH C.P. STATION.
–
ELECTRODES ARE PLACED AT 3 O’CLOCK OR 9 O’CLOCK POSITION
AT BURIAL DEPTH OF PIPE.
–
SEPARATION OF ELECTRODE TO PIPE WALL IS <150mm.
–
PERMANENT ELECTRODES ARE INSTALLED TO GIVE FEED BACK
SIGNAL TO T/R FOR AUTO POTENTIAL CONTROL.
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–
MEASUREMENT CABLE FROM REFERENCE ELECTRODE AND PIPE
ARE TERMINATED INSIDE T/R PANEL CORROSION VOLTMETER.
– LIFE OF ELECTRODES 8 YEARS.
SLEEVE ( CASING) PIPES
a)
Uncoated sleeves b) Coated sleeves
a) Uncoated Sleeves
- Metallic contacts between carrier and sleeve to be
costs( Resistance with contact<10 m- ohms
avoided at all
- Robust and reliable spacers to be installed
- Metallic contact with uncoated sleeve will render several metres
carrier pipe unprotected on each side
of
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SLEEVE ( CASING) PIPES
a)
Uncoated sleeves b) Coated sleeves
a) Uncoated Sleeves
- Metallic contacts between carrier and sleeve to be avoided at all
costs( Resistance with contact<10 m- ohms
- Robust and reliable spacers to be installed
- Metallic contact with uncoated sleeve will render several metres of
carrier pipe unprotected on each side
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In event of metallic contact and ingress of water inside annular
space carrier pipe will not get C.P current due to screening.
Remedial action( in case of metallic contact)
-Provide supplementary protection to carrier outside sleeve
- Insert zinc wire inside annular space for protection of
carrier(
in case of water ingress)
OR
- Fill annular space with insulating material
-b) Coated Sleeve pipe
- Metallic contact will not jeopardise protection of carrier pipe
outside sleeve
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However in case of annular filled with water ,carrier will not receive CP
current ,even when there is no metallic contact.
In case of metallic contact in conjunction with water ingress, as in case of
uncoated sleeve ,carrier will not get C.P because of shielding effect (
screening)
Remedial action
If metal contact in conjunction with water ingress,
insert zinc wire in annular space and weld to carrier
OR
Fill annular space with insulating compound
Electrolytic contact but no metal contact
-No action required for uncoated sleeves
-For coated sleeves same as for metal contact(see above)
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OPERATION OF C.P. SYSTEM
THE C.P. SYSTEM DOES NOT INVOLVE ANY MOVING PART AND
HENCE REQUIRE MINIMUM MAINTENANCE AND OPERATION.
-
MAJOR OPERATION IS IN THE D.C. POWER
SOURCE WHICH ARE TO BE OPERATED
POTENTIAL CONTROL MODE FOR ONSHORE
PIPELINE.
IN
AUTO
FOR DETAILED OPERATION AND MAINTENANCE OF
UNIT, BATTERY BANK, MANUFACTURER
OPERATION / MAINTENANCE MANUAL SHOULD
BE REFERRED.
T/R
-
DURING OPERATION THE INTERNAL REFERENCE
SET POTENTIAL SHALL BE MAINTAINED AT
VALUE PRESET DURING COMMISSIONING.
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IN CASE POTENTIAL AT FARTHEST POINT
FROM
RECTIFIER OR ANY OTHER LOCATION
FALLS
BELOW -0.950 (Cu/CuSO4), INTERNAL
SET
POTENTIAL TO BE
INCREASED.
-
-
CHECKS TO BE MADE THAT EXTERNAL
POTENTIAL AT DRAIN POINTS
MEASURED BY
PERMANENT
REFERENCE ELECTRODES
MATCHES WITH INTERNAL SET VALUE.
-
INTERNAL SET POTENTIAL SHOULD BE
MAINTAINED AT SUCH VALUE TO
OBTAIN -0.950
(MINIMUM) AT ALL
MEASUREMENT POINTS.
-
POTENTIAL AT DRAINAGE POINT SHALL NOT
EXCEED 2.0V (ON) AND -1.2V (OFF)
WITH RESPECT
TO Cu/CuSO4.
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-
INDIVIDUAL ANODE CURRENT OUTPUT VALUE
SHOULD NOT EXCEED RATED OUTPUT.
FOR Fe-SiCr ANODES, THIS IS 3.5A AND SUBSEA ANODES 20
AMPS.
-
FOR ANY MALFUNCTION IN THE T/R UNIT OR
CPPSM MODULE FOR BATTERY BANK,
MANUFACTURER OPERATION MAINTENANCE
MANUAL TO BE REFERRED (ALSO SEE FAULT
LOCATION).
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MAINTENANCE & MONITORING OF C.P. SYSTEM AND ITS
PERFORMANCE
OBJECTIVE & AIM OF MONITORING
–
TO ASSESS LEVEL OF CATHODIC PROTECTION BY MEASURING
PIPE TO ELECTROLYTE POTENTIAL.
–
TO INSPECT AND CHECK PERFORMANCE OF VARIOUS
COMPONENTS OF THE SYSTEM e.g. T/R ANODES, JUNCTION
BOXES, CABLES ETC.
–
TO RECTIFY ANY DEFICIENCY OBSERVED DURING MONITORING
IN OVERALL PERFORMANCE OF SYSTEM WHICH MAY BE EITHER
INADEQUATE PROTECITON LEVEL OR ANY FAULT / MALFUNCTION
OF EQUIPMENT.
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–
C.P. SYSTEM REQUIRES MINIMUM ROUTINE MAINTENANCE, IF
ROUTINE MONITORING IS UNDERTAKEN WHICH ENABLE
TIMELY DETECTION AND RECTIFICATION OF FAULTS.
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MONITORING PROGRAM
–
T/R UNIT PERFORMANCE CHECKS
–
ANODE PERFORMANCE CHECK
–
POTENTIAL MEASUREMENT
– INSPECTION OF EQUIPMENTS
–
CONTINUITY CHECKS FOR
GROUNDING OF EQUIPMENTS.
–
CLOSE ORDER POLENTIAL SURVEY (INTENSIVE SURVEY).
BONDING
AND
ELECTRICAL
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T/R UNIT PERFORMANCE CHECK
ONCE EVERY 15 DAYS
–
RECORDING OF AC VOLTAGE AND CURRENT
– RECORDING OF DC VOLTAGE AND CURRENT
–
CHECK DC CURRENT IS EQUAL TO SET CURRENT VALUE
–
CHECK AND RECORD D.C. VOLTAGE LIMIT SETTING. SETTING
SHOULD BE ADEQUATE TO CATER FOR ANY POSSIBLE
INCREASE IN FUTURE OF EXTERNAL LOAD RESISTANCE A
MARGIN OF 25% IS ADEQUATE.
–
RECORD ANY INCREASE / DECREASE IN D.C. VOLTAGE WITH
SUBSEQUENT DECREASE / INCREASE IN D.C. CURRENT
VALUE.
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ANODE PERFORMANCE CHECK
ONCE EVERY 6 MONTHS
– TOTAL RESISTANCE OF GROUP OF ANODES (T/R D.C. VOLTS 
T/R D.C. AMPS).
– THIS RESISTANCE HAS TO BE MAINTAINED CONSTANT. IF FOUND
TO INCREASE WATERING OF THE ANODE BED REQUIRED.
DECREASE IN RESISTANCE SELDOM OCCURS.
– INDIVIDUAL ANODE CURRENT BY CLAMP METER OR SHUNT IN
AJB IS A CHECK WHETHER ANY ANODE HAS GONE OUT OF
CIRCUIT OR HAS FAILED.
– DISCONNECTED ANODE WILL SHOW HIGHER RESISTANCE OF THE
ANODE BED AND OF TOTAL GROUP.
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POTENTIAL MEASUREMENT
–
ONCE EVERY 1 MONTH AT ALL TEST POINTS (TEST STATIONS)
INSTRUMENTS
–
HIGH RESISTANCE VOLTAMETER
–
PORTABLE Cu/CuSO4 REF. ELECTRODE
CONNECTION SCHEME (REF. FIGURE)
–
ENSURE THAT ELECTRODE IS IN CONTACT WITH ELECTROLYTE
ONLY AT THE LOWER CERAMIC PLUG (TIP) NO OTHER PART OF IT
SHOULD BE EXPOSED TO ELECTROLYTE.
–
ONLY STABLE POTENTIAL VALUES INDICATED BY VOLTMETER TO
BE RECORDED.
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THE REFERENCE ELECTRODE MUST BE KEPT VERTICALLY ABOVE
PIPE TO MINIMISE IR DROP ERROR (See Figure).
ONCE IN 3 MONTHS, MEASURE INSTANT ‘OFF’ POTENTIAL
MEASUREMENT BY SWITCHING ‘ON AND ‘OFF’ C.P. CURRENT BY
THE IN-BUILT CURRENT INTERRUPTOR (12 SEC ‘ON, 3 SEC ‘OFF’).
ENSURE GOOD CONTACT OF ELECTRODE TIP WITH SOIL ENSURE
THAT TIP IS NOT PLACED ON ROCK, GRAVEL OR OTHER HIGH
RESISTANCE MATERIAL e.g. PLASTIC SHEETS, PLANTS, ETC.)
SUFFICIENTLY WATER THE SOIL BEFORE PLACING THE ELECTRODE.
MEASURE POTENTIAL IN 2V D.C. RANGE.
MEASURE IN 20V.
IN CASE OF OVERLOAD,
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FOR CHECKING ACCURACY OF MEASUREMENT, MEASURE BOTH IN
2V AND 20V RANGE. MEASURED VALUES SHOULD BE SAME IN
BOTH RANGES.
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MAINTENANCE & CALIBRATION OF Cu/CuSO4 ELECTRODE
TO SAFEGUARD AGAINST MEASUREMENT ERROR, IT IS IMPORTANT
THAT REFERENCE ELECTRODE IS PREPARED, CALIBRATED AND
MAINTAINED PROPERLY.
PREPARATION OF ELECTRODE AND CALIBRATION
PRIOR TO EVERY ROUTINE MEASUREMENT THE REFERENCE
ELECTRODE SHALL BE FRESHLY PREPARED AND CALIBRATED /
MATCHED AS PER
MANUFACTURER INSTRUCTION MANUAL
-
ALWAYS TWO ELECTRODES SHALL BE
PREPARED. ONE FOR USE AND OTHER TO BE
KEPT FOR CALIBRATION OF WORKING
ELECTRODE.
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-
BETWEEN TWO SUCCESSIVE ROUTINE
MEASUREMENTS, ELECTRODE SHALL BE
EMPTIED OF CuSO4 SOLUTION, RINSED
THOROUGHLY AND DRIED.
-
AT THE END OF EVERY DAY DURING USE,
ELECTRODE SHALL BE KEPT IMMERSED IN A
BEAKER OF DISTILLED WATER WITH ONLY
CERAMIC PLUG IMMERSED IN WATER.
-
ELECTRODE SHALL NEVER BE IMMERSED IN
WATER CONTAINING > 300 ppm CHLORIDE
IONS.
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JUNCTION BOXES, CABLES AND CONNECTIONS
JUNCTION BOXES TO BE INSPECTED INTERNALLY AND
EXTERNALLY ON ANNUAL BASIS TO CHECK :
-
TIGHTNESS AND CLEANLINESS OF CABLE
CONNECTIONS AND CABLE GLANDS
-
DAMAGE OF PAINT
-
INTERNAL CLEANLINESS
-
ANY BURNT OUT SHUNT TO BE REPLACED
IMMEDIATELY.
-
ALL CABLE TERMINATIONS SHALL BE CHECKED
FOR THEIR TIGHTNESS.
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INSPECTION OF C.P. EQUIPMENTS
T/R UNITS (ONCE EVERY 6 MONTHS)
–
CLEAN & TIGHTEN BOLTS OF ALL CURRENT CARRYING CONDUCTORS
–
CLEAN VENTILATING SCREEENS
–
CALIBRATE PANEL METERS WITH PORTABLE METERS
– CHECK ALL PROTECTIVE DEVICES
–
FIND OVERALL EFFICIENCY OF UNIT BY ACTUAL A/C & D/C WATTS
MEASUREMENTS.
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INSPECTION
OF
UNDERWATER
ELECTRODES AND CABLES
ANODES,
REFERENCE
–
ALL UNDERWATER C.P. EQUIPMENTS SHALL BE INSPECTED BY
DIVERS ONCE IN 6 MONTHS FOR THEIR INTEGRITY. HOLDING CLAMPS
SHALL BE TIGHTENED IF NECESSARY.
–
ALL MARINE GROWTH SURROUNDING ANODE AND REFERENCE
ELECTRODE SHALL BE MANUALLY CLEANED BY DIVERS.
–
ANY ABNORMALITY SHALL BE REPORTED AND RECTIFIED BY DIVERS.
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INTENSIVE SURVEYS
EVERY 5 YEARS
ITEMS OF TEST
-
CLOSE ORDER POTENTIAL SURVEY (ON AND OFF)
-
COATING RESISTANCE TEST
-
LINE CURRENT TEST
-
INSULATION JOINT TEST
-
PROTECTIVE CURRENT DENSITY ESTIMATION
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FAULT DETECTION
IF ABNORMAL VALUES OF POTENTIAL AND CURRENT
ARE NOTED, COMPARISON WITH EXPECTED OR
EARLIER VALUES WILL INDICATE THE NATURE OF
THE FAULT AS FOLLOW.
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APPLIED VOLTAGE ZERO OR VERY LOW,
CURRENT ZERO
–
FAILURE OF A.C. FUSE OR TRIPPING OF OTHER PROTECTIVE
DEVICE
–
FAILURE OF AC SUPPLY
–
FAILURE OF TRANSFORMER-RECTIFIER
NOTE :
APPROXIMATELY 2.0V MAY STILL BE INDICATED ON THE
VOLTMETER DUE TO THE BACK E.M.F. BETWEEN THE STEEL
STRUCTURE
AND
CABONACEOUS
BACKFILL
IN
THE
GROUNDBED.
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APPLIED VOLTAGE NORMAL, CURRENT LOW
BUT NOT ZERO
–
DETERIORATION OF ANODES OR GROUNDBEDS.
–
DRYING OUT OF SOIL AROUND GROUNDBEDS, OR SOME ANODES NO
LONGER IMMERSED.
–
ACCUMULATION OF ELECTROLYTICALLY PRODUCED GAS AROUND
ANODES.
–
DISCONNECTION OF SOME OF THE CONNECTIONS TO INDIVIDUAL
ANODES OF A GROUNDBED OR ANODE SYSTEM.
– DISCONNECTION OF PART OF THE PROTECTED STRUCTURE.
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APPLIED VOLTAGE NORMAL, CURRENT ZERO
–
SEVERANCE OF ANODE OR CATHODE CABLES.
–
FAILURE OF D.C. FUSE OR AMMETER OF TRANSFORMERRECTIFIER UNIT.
–
COMPLETE FAILURE OF GROUNDBED OR ANODE SYSTEM.
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APPLIED VOLTAGE AND CURRENT BOTH LOW
– CONTROL ON TRANSFORMER RECTIFIER UNIT SET TOO LOW.
–
TRANSFORMER OR RECTIFIER FAILING.
–
ELECTRICITY SUPPLY FAULT.
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APPLIED VOLTAGE AND CURRENT BOTH HIGH
–
BREAK IN CONTINUITY BOND OR INCREASED RESISTANCE
BETWEEN POINT OF CONNECTION AND POINT OF TEST.
–
GREATLY INCREASED AERATION OF SOIL AT OR NEAR POINT OF
TEST DUE TO BROUGHT OR INCREASED LOCAL DRAINAGE.
–
ALTERATION OF ENVIRONMENT CAUSING RAPID DEPOLARIZATION
OR INCREASE IN OXYGEN CONTENT OR WATER DUE, FOR EXAMPLE,
TO REDUCED LEVEL OF POLLUTION OR INCREASE FLOW.
–
FAULTY ISOLATION EQUIPMENT e.g. SHORT CIRCUITING OF AN
ISOLATION JOINT IN A PIPELINE.
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–
PROTECTED STRUCTURE SHIELDED OR OTHERWISE AFFECTED
BY NEW STRUCTURE.
–
FAILURE OF CATHODIC PROTECTION SYSTEM ON ANOTHER PART
OF THE SAME STRUCTURE OR ON A SECONDARY STRUCTURE
BONDED TO IT.
–
FAILURE OF CATHODIC PROTECTION SYSTEM ON ANOTHER PART
OF THE SAME STRUCTURE OR ON A SECONDARY STRUCTURE
BONDED TO IT.
–
DETERIORATION OF, OR DAMAGE TO PROTECTIVE COATINGS.
–
ADDITION OR EXTENSION TO BURIED STRUCTURE, INCLUDING
FORTUITOUS CONTACT WITH OTHER METALLIC STRUCTURES.
–
INTERACTION FROM ANOTHER CATHODIC PROTECTION SYSTEM.
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APPLIED VOLTAGE AND CURRENT NORMAL
BUT PIPELINE / ELECTROLYTE POTENTIAL
ABNORMALLY NEGATIVE
–
BREAK IN CONTINUITY BONDING AT POSITION FURTHER FROM THE
POINT OF APPLICATION THAN THE POINT OF TEST.
–
DECREASED AERATION OF SOIL OR ELECTROLYTE AT POINT OF
TEST.
–
REDUCTION IN RATE OF FLOW OF ELECTROLYTE.
–
SECONDARY STRUCTURES HAVE BEEN REMOVED OR HAVE BEEN
CATHODICALLY PROTECTED OR BONDS TO THEM BROKEN.
–
EFFECTS OF STRAY AC ON THE STRUCTURE.
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Electrical Earthing :
- Safety Earthing
- To mitigate effect of induced electrical voltage]
-
Safety earthing by polarisation cells or diode circuits or by
installing separate zinc electrodes in low resistivity back- fill
and not in direct electrical continuity with other earthing
systems.
For mitigation of AC induced voltage, this should be done at
locations where measured AC voltage to ground is
highest and where
pipeline is exposed & can be leveled by
personnel.
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Cost And Economics
-
C.P. System Cost is 0.1% Of Installed Cost Of Plant.
Economically Prudent To Install C.P. System Before
Or Latest Within 1-2 Years From Start
Up.
After 2 Years Leaks Are Unmanageable As
Plants.
Plant Start Up
Experienced In Many
C.P. System Cost Should Not Be And Cannot Be
With Leak Repair Cost.
Compared
In Absence Of C.P., Leak Repair Is A Recurring Cost
Entire Plant Operation.
Over
The
Leak Repair Is Cumbersome, Hazardous And Interferes With Plant
Operation.
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Thank You
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