Design and Installation Aspects of Cathodic Protection

Design and Installation Aspects of Cathodic Protection
Design and Installation Aspects of Cathodic Protection

Design and Installation Aspects of Cathodic Protection on Storage Tanks & Pipelines for safe operation of the plant.

Design Aspects of Cathodic Protection
  • Sacrificial Anode System
  • Sacrificial anodes design calculations centre around two basic requirements:
  • There must be enough sacrificial anode material to last the design life.
  • The current output of the system must be sufficient to initially polarize and then maintain protection throughout the design life.
  • Design optimization is about meeting these two requirements at the lowest cost i.e. with the least amount of sacrificial anode material
  • Maintenance cost is also a contributing factor
Steps in designing sacrificial anode system
  1. Establish soil resistivity
  2. Determine current density requirement on the basis of soil resistivity and type of soil
  3. Electrical continuity of structure
  4. Select requirement of electrical isolation
  5. Select suitable anode material on the basis of soil resistivity and soil type
  6. Calculate total mass of anode required for design life
  7. Calculate anode resistance to deliver output current
  8. Calculate current output of each anode and its weight
  9. Calculate total number of anodes required to satisfy mass and their configuration that is horizontal or vertical
  10. Consider facilities for monitoring performance
  11. Proper project drawing & specifications
Current requirement for steel for 3LPE coating
 
Soil Resistivity Current density µA/m²
< 10 ?-m for marshy soil 50
10 ?-m to 100 ?-m for normal 25
> 100 ?-m dry high resistivity 15
Safety margin of 30% to be provided on the above current densities
•         Step 1: Surface Area
          Sa = pd l
•         Step 2: Current Requirement
           It = Sa x Cd x 1.3
           It = Protective current requirement(Amp)
        Cd = Protective current density (A/m2) l = length of the pipeline(meter)
           d = diameter of pipeline(meter)
          Sa = Surface Area (m2 )
          Safety margin is taken 1.3
Anode weight requirement
  • Step 3 : Anode weight requirement
                            W = It x Cr x L
                                     U x E
                            W = It x L
                                Ca X U X F
                            W = Total Anode Weight (Kg.)
                              It = C.P. current requirement in Amp
                             E = Efficiency of anode
                             Cr= Consumption Rate of Anode in Kg/Amp-year
                             Ca= 1/Cr = Theoretical capacity of anode in Amp-year/Kg L = Design Life in years
                              U = Anode Utilisation factor
Calculation of total circuit resistance
  • Step 4: Total Circuit Resistance
            Calculations Rt = Ra.re + Rc + Rs.re
                                  Rt = Total resistance
                             Ra.re =  Resistance of anode to remote earth  (?)
                                  Rc = cable resistance between anode to structure (?)
                              Rs.re = Resistance of structure to remote earth (?)
Anode to remote earth resistance
  • Vertical prepackaged single anode by Dwight’ s modified the equation

Equation (1) When L>>d, soil resistivity is homogeneous and anode is not located well below grade or t<L . Equation( 2) when depth t to the top of anode is equal to L
 
Vertical Anode Bed
Vertical Anode Bed
Anode to remote earth resistance
  • Resistance of single horizontal anode
  • When depth t < < L and L >> d then
  • Also used for resistance of multiple horizontal anode bed if s< 2La and s> La and t< La and sorrounded by continuous coke breeze. Here L is the length of anode            bed and La is the length of anode
  • L - Length of anode
  • d – Diameter of anode
  • t -  Depth of top of  anode
  • Rah is resistance of anode to remote earth
  • r- Soil resistivity
  • When t >> L, Rah =0.159 x ρln ( 2L/d)

                                                 L

  • Also used for single vertical anode where t >>L

Anode to remote earth resistance

  • Resistance of multiple horizontal anode where continuous coke breeze is not there:
            Rgbh = Rah
                          F N
Where, Rah = Single anode to earth resistance
             N = Number of anodes in parallel
             F = Crowding factor due to multiple anodes
             F = 1 + 0.318 x ρ ln 0.656N
             S x Rah Where,
             S = Anode spacing
Horizontal Anode Bed
Horizontal Anode Bed
Pipe to earth and cable resistance
  • Pipe to remote earth resistance Rpre = r" /As
  • r" = Specific coating resistance in Ω-m² @ given soil resistivity
  • ρ As= Surface are of pipe = 3.14 x d x L
  • r" =  10Ω-m² for excellent coating @ 1000 ?-cm soil
  • Cable resistance: As per Table provided by supplierFor 12 AWG copper conductor cable Rc = 0.005314 Ω/m
Calculation of system life & number of anodes
  • Step 5 : Calculation of system life in years
                        Wt = Weight of single anode
                          U = Utilization factor of anode
                          Ia = Anode output current
                           E = efficiency of anode
                          Cr = Consumption rate of anode in Kg/Amp-year
                          Ca = Theoretical capacity of anode in Amp-year/Kg
  • Step 6 : Number of anodes required on weight basis
          Number of anodes = W/Wt, W is the total weight and Wt is the weight of individual anode
          Number of anodes required on total current basis
          N = Icp/Ia Icp and Ia are total CP current required to anode output current Number of anodes must satisfy both weight and current
System driving voltage & current output of anode bed
  •  System driving voltage Ecp = Eap –  Esp Ecp = Driving potential
           Eap = Anode polarization potential – 1700 mV for Mg Esp = Structure protective potential – 850 mV
           Ecp = 1700 – 850 = 850 mV
  • Current output of anode
             Ia = Ecp/Rt
  • Area of pipeline to be protected by each anode A = Sa / N
            Sa = Total surface area to be protected N= Number of anodes
  • Anode is spaced uniformly for protecting pipeline to cover all pipeline
Example of sacrificial anode system design
  • Consider the following data :
            Pipe diameter 32.4 Cm, 6 Km long, FBE coated pipeline Soil resistivity 5500 ohm-cm
            Current density 30 µA/m²
            Specific coating resistance r" = 10Ω-m² Design life 15 years
  • Total CP current = 3.14 x .324 x 6000 x 30
                                                  1000
                                      =      183 mA
  • Considering pre-packaged 17 lb (7.70 Kg) (17 D3) Magnesium anode of size 30” long, 6” dia, placed at 1.2 meter depth from grade the resistance of anode
                               Rah = 0.159 x ρ  (ln 2L)  , horizontal anode is considered                                                                                                       
                                             L                  d
                                       =        26.5 Ω
  Pipeline resistance Rp =  r„  =10??-m²
                              Sa   3.14 x 0.32 x 6000 m²
                                       = 1.64 Ω
Sacrificial anode design
 
  • Anode output current Ia = Ecp Rah+ Rp
                                                   = 1700-850 /26.5 + 1.64  =30 mA 
  • Number of anodes on the basis of current requirement :
            N = Icp = 183 = 6.1 or 6 anodes
                   Ia      30
  • Anode current density ia = Ia/A , Ia is anode output current and A ia area of anode
  • Area of anode = 2 (W + H) X L = 2 ( 3.5” + 3.75”) X 26” = 2.6 ft²
          ia = 30 mA/2.6 ft² = 11.5 mA/ft²
          W,H and L are width, height and length of anode
         From current density vs current capacity or consumption graph at 11.5 ma/ft², current capacity is 300 A-hr/lb or current consumption is                 1/300 A-hr/lb = .0034 lb/A-hr or .0034 x 8760 lb/A-yr or 29.78 lb/A-y
 
Anode current density vs elecrochemical capacity

Sacrificial anode design
  • Anode weight requirement  W = Icp  x Cr  x L/ U x E

                                                           =  0.183 A x 29.78 lb/A-y x 15 yr 0.85 x 1

                                                           =   96.17 lb

  • Number of anodes per weight requirement N = W/Wt = 96.17/17 = 5.65 or 6 anodes
  • To meet current requirement 6 anodes are required

        Life of anode L =  Wt x U x E = 17 lb x 0.85 X1 = 16.17

  • For 6 Km line first anode is put after 500 meter and then at each 1 Km interval horizontally at a depth of 1.2 meter and 6th at 500 meter from end for uniform distribution.
Potential Attenuation along pipeline

Impressed current CP system Dseign
  • Power supply voltage Eo = Icp (Ra.re+Rs.re+Rc) + Eb where
            Icp = Cathodic protection current
        Ra.re = Groundbed to remote earth resistance Rs.re = Structure to remote earth resistance Rc = All cable resistance
            Eb = Back voltage = ? Ea.p + ? Es.p + Eg = 2 V
           ?       Ea.p & ? Es.p are change in polarization of anode and structure due to cathodic protection current & Eg is the difference in      corrosion potential between an               anode and structure
 
Shallow anode ground bed
 
Deep anode ground bed
 
Deep well anode bed
 
Anode bed locations
  • Low soil resistivity area at soil resistivity of <50 Ohm m
  • Moist and not drained  soil
  • Anode bed should be remote to the structure for better distribution
  • Anode bed should not be in a vicinity of foreign pipeline or foreign materials to avoid interference
  • Anode bed configuration as horizontal, vertical, distributed or deep well depends upon current distribution, attenuation, land area availability and level of interference
  • Type of anodes such as high silicon iron, graphite or MMO anodes are chosen on the basis of current requirement, soil resistivity, economy and design life
Remoteness of anode bed
  • Anode bed should be located at remote location from structure for ideal current distribution. The current is maximum at drain point and reduces as moving away from drain point. This is due to a property that is called attenuation that is loss of current due to high longitudinal resistance of pipe , poor coating or high soil resistivity. Potential is also affected due to current distribution.

Remoteness of anode bed
The key issues surrounding remoteness:
  1. Remote earth potential (IR) of the Anode Ground Bed as this will determine the pipeline voltage shift to the minimum protection level that is - 0.85 V with respect to Cu/CuSo4 half cell
  2. Voltage rise in earth with respect to remote earth at a distance x from the anode which is given by following formula
    The maximum voltage rise should not be more than 0.5 V where:
    I = current delivered (by the anode) to the earth (A)
    ρ   = average resistivity of the earth (ohm-m)
    L = Depth of anode bed (Active length in meters)
    Xr= distance (meters) from the centre of the anode to the point xVr= the voltage rise at x (volt) with respect to remote earth
 
Example of remote calculation
  • Let us consider a deep well anode of active depth 30 meter in a soil resistivity of 20 ?-m and current output of 10.52 A, then at a distance of 75 meter from anode the voltage rise Vr will be
                         10.52 A x 20?-m { ln {30 + √(30²) + (75²)}
                          2 x 3.14 x 30                             75
           =   0.435 V which is less than 0.50 V
  • This will be the voltage rise at that point with respect to remote earth
  • The remoteness of horizontal anode bed is achieved when a minimum distance of 100 meter is maintained for anode bed from pipeline. In case of deep well anode, depth of anode is kept sufficient to deliver ideal current distribution.
Current distribution & Attenuation
  • Factors affecting current distribution
  1. The anode ground bed should be at remote earth from structure
  2. Current distribution is improved when attenuation is small. Attenuation is more as pipeline longitudinal resistance increases with distance from the drain point. Since attenuation is large hence current distribution diminishes.
  3. The good coating improves current distribution as attenuation is small.
  4. An increased soil resistivity improves current distribution.
  5. Polarization improves with time and so attenuation also becomes small and current distribution improves.
Example of impressed current system design
  • Consider the following data
  • Pipeline length 3048 meter, OD 21.9 cm, ID 20.32 cm, coating effectiveness 99 % , Soil resistivity 4000 ?-cm, current requirement 21.5 mA/m² of bare pipe, pipe to ground resistance ( Rp.re) 0.3 ?
  • Surface area = 3.14 x .219 x 3048 = 2097 m²
  • Bare surface area = .01 x 2097 = 20.97 m²
  • Current required = 21.5 x 20.97 = 451 mA
  • Assuming coating efficiency to decrease, let us take current requirement as 5 x 451 mA that is 2.3 A (Coating efficiency 95 %)
  • Let us design anode ground bed by using vertical anodes of high silicon iron anodes of 21 Kg weight having consumption rate of 1 Kg/A-y and designing for 20 year life
  • Total weight required for 20 year life = 20 x 1 x 2.3 = 46 Kg
  • Number of anodes = 46/21 = 2.2 say 3 anodes of 21 Kg each
  • Anode ground bed resistance Ra.re =
Impressed current system design
  • ρ = 4000 ?-cm, L = 213.4 cm, d = 20.3 cm, S = 609.6 cm, N = 3 anodes
  • Ra.re =    4000                {  ln 8x213.4 _1} + (2x213.4 x ln 0.656x3
                        2x3.14x3x213.4        20.3                   609.6
                    = 3.9 Ω
  • Pipe to ground resistance Rp.re =  0.3?
  • Pipe lineal resistance Rp =ρsteel x L
                                                            A
  • ρsteel = 18 µ?-cm = .000018 ? -cm & L= 3048 meter = 304800 cm
  • A = 3.14  (OD² - ID² ) ; OD =  21.9 cm, ID = 20.32 cm
                    4
  • Rp =  .000018 x 304800   = .104?, One direction is .104/2=0.052Ω
                 785 x { ( 21.9 )² - (20.32)² }
  • Ground bed is installed in the middle of the pipeline, then longitudinal resistance will be half of this value that is 0.026Ω.
Impressed current system design
  • Cable resistance measurement
  • 6 AWG negative cable from rectifier to drain point- 15.24 meter
  • 6AWG positive cable from anode bed to rectifier- 60.96 meter
  • 6 AWG ground bed cable from anode (3) to junction box- 18.29 m
  • Total length = 85.3 meter
  • Cable resistance Rc = 85.3 meter x          1.322 ? / 1000 m = .11 ?
  • Total resistance Rt = Ra.re + Rp.re + Rp + Rc

                                         = 3.9 + .3 + .026 + 0.11 = 4 ? (approx)

  • Rectifier output = Icp x Rt + 2 V (Back voltage)
  • 2.3 A x 4 ? + 2 V = 11.2 V
  • The anode to earth resistance is high at 3.9 ? which limits current output from anode if anode further deteriorates. Anode to earth resistance should be less as much as possible. It can be reduced to less than 2 ? by increasing number of anodes to 5 or 6 which will also increase current required in case of anode deterioration.
Test stations
  • Test stations are provided along the structure for measurement of structure to soil potential, current and other parameters to monitor effectiveness of cathodic protection system. Test stations are provided essentially at following locations
  1. Cased crossings
  2. Foreign structure crossings
  3. Isolating joints
  4. Waterway crossings
  5. Road, rail and bridge crossings where casing is provided
  6. Galvanic anode installations
  7. Valve stations
  8. Stray current areas
  9. Rectifier installations
  10. High tension line parallel or crossing
  11. Vulnerable locations such as marshy areas or populated areas
  12. Any other location needed for CP testing

Test station connection scheme
 
Anode junction box and cathode junction box
Anode junction box and cathode junction box are provided to measure individual anode output current and pipeline current. Also anode output current and pipeline current van be adjusted through variable resistor. Anode resistance and ground bed resistance can also be measured
 
Cable to pipe connection
  • Cable to pipe connection should be mechanically strong and electrically conductive. Two methods can be adopted
  • Thermit welding
  • Pin brazing
  • Thermit welding :
  • Thermit welding is a process where cable conductor is connected to pipe through fusion process of superheated molten metal from chemical reaction of thermite. Thermite is a mixture of Fe?O?and Al when ignited produce heat and molten metal which join cable conductor to steel .
  • Pin brazing :
  • Pin brazing is a simple process of electric arc silver soldering where cable lug is soldered on pipe surface by producing an electric arc at the end of a silver pin by brazing gun. The arc also heats cable lug and pipe surface to soldering temperature. The melted soldering pin and flux solder cable lug on to the pipe
  • Pin brazing is more common as it has got advantages of easier procedures and comparatively low temperature than thermit welding
Thermit welding kit
 
 
Location of Cathodic protection stations
  • Cathodic protection stations are located evenly along the pipeline considering span of protection and CP rectifier current capacity
  • Power availability is another important factor such as grid supply or solar power cell
  • Approachability to the location for maintenance purpose
  • Security threats are a concern fearing vandalism .
  • Availability of land

Power sources for CP

  • Following power sources are used for cathodic protection need
  1. Grid power
  2. Solar cells
  3. Ni-Cd battery back up system

Type of insulation required in cathodic protection cables
Types of insulation required in corrosive environment such as at deep well anode and shallow anode bed where coke backfill is used :
  1. High Molecular Weight Polyethylene(HMWPE): The HMWPE insulation is majorly used in soil installations. Not resistant to chlorine, hydrochloric and petroleum hydrocarbon environments.
  2. HALAR/HMWPE layered Insulation: this is a dual insulations where the outer jacket HMWPE provides chemical resistance and di-electric insulation, the inner HALAR layer is a fluoro-copolymer which is resistant to chlorine, hydrochloric and petroleum hydrocarbon environments.
  3. KYNAR/ HMWPE: Kynar(polyvinylidene fluoride)/modified polyolefin is a dual jacketed insulation similar to Halar/polyethylene. provides mechanical protection to the wire as well as chemical resistance and dielectric insulation. resistant to chlorine, hydrochloric acid, sulfates, hydrogen sulfides, alkalis, other acids, petroleum-based chemicals, and chlorine gas.
Cable specifications
  • Anode and cathode header cable:
  • Shall be annealed, high conductivity,,650/1100V grade, PVC insulated and sheathed, armored, stranded copper conductor cable of 25 mm2
  •  Anode tail cable : 10 mm2
  • Reference cell cable: 4 mm2
  • Grounding cell/polarization cell: XLPE insulated stranded 25mm2copper conductor cable
  • Anode tail cable
Cathodic protection of plant piping, mounded bullets and storage tank bottoms
  • Why cathodic protection is required for plant piping, mounded bullets and storage tank bottoms
  • Integrity of buried plant piping carrying fire water, cooling water, waste water, raw water, product etc is very important for safe operation of the plant. The underground pipeline coating is not robust so cathodic protection is required to protect against corrosion.
  • Heat of welding destroys soil side bottom plate coating which is not repairable
  • Welding of steel plate results in micro structure changes in steel resulting in formation of corrosion cells and significant metal loss.
  • Coating alone cannot be relied to protect the tank bottom and mounded bullet from external corrosion.
  • Once leak arises, it is difficult to repair and environmental hazard is there.
  • Cathodic protection of plant piping, storage tank bottom and mounded bullet is challenging task since underground structures in plant are supported on RCC foundation and are connected through copper earthing system which not only drains cathodic protection current but also corrodes steel.
Plant piping cathodic protection anode configuration
 
design and installation aspects of cathodic protection
Deep anode ground bed

       1. Anodes and Installation: MMO tabular anodes In a vertical well todepths of 20--35 meter.

       2. Current Effectiveness:

  • 95% to 98% current being applied to incidental structures
  • only 2 to 5% of the current actually going to the piping systems.

      3. Shielding / Current Distribution:

  • Piping in close proximity to RCC foundations and earthed structures
  • current distribution not effective

      4. Design reliability:

  • Current distribution is a problem due to shielding by other structures in vicinity
  • Expensive proposition
  • Current requirement is huge due to other structures in the vicinity of structures
  • to be protected
  • Performance is not satisfactory as anode consumption is fast
Distributed shallow anode ground bed
Anodes and installation:
  • Mixed metal oxide tabular anodes distributed all around the structure placed around 3 to 5 meters from the plant piping and lateral separation of 10 to 30 meters.
Issues with this type of anode installation:
  • Difficult to distribute the anodes properly because of physical limitations.
  • Incidental structures pick up current between the anodes
  • Proper potential distribution is not achieved due to uneven current distribution
  • Highly vulnerable to breakdown and damages during plant maintenance
  • Prone to areas of poor coverage due to shielding and over polarization.
  • Accurate information on grounding system, concrete foundations, and all other buried metallic structures is required for proper system designing.
Long line linear anode system
Anode and installation:
  • Long runs of linear anode laid parallel to pipeline and in very close proximity to the piping being protected
Benefits of using long line linear anodes:
  • The current output is uniform and consistent across the entire system
  • Close proximity to the piping significantly limits current losses to other structures and current is mostly utilized to structure to be protected
  • Each anode segments output can be measured & controlled independently.
  • Can be adopted for new construction projects and for selected spot remediation
  • Accurate soil resistivity data is not required since the long anode length significantly reduces overall system resistance.
  • Anode placement is a function of pipe location and not soil resistivity.
  • Significantly reduces the total current requirements for the system
  • Minimizes the risk of third party damage
  • Reduces trenching required for buried cable.
  • Very cost effective installation when installed concurrently with the piping.
Configuration of linear anodes
Linear anodes with factory filled coke breeze are either polymeric or copper cored mixed metal oxide based or titanium based mixed metal oxide anodes. The copper cored MMO anode is frequently used for cathodic protection Purpose.

Protective design current density
Comparison of current density for different design:
Isolated buried pipeline with coal tar epoxy coating: 200µA/m2)
Not isolated & coal tar epoxy coated, protected with a combination of deep ground bed and distributed
shallow anodes: 10 to 20 ma//m2
Not isolated & coal tar epoxy coated, protected with long line linear anodes : 3 to 5 ma/m2
The current consumption of long line linear anode is 52 ma/m and suitable for 20 years of life and more.
 
Cathodic Protection of Pipelines with linear anodes

A typical layout of cathodic protection system using linear anode
 
Combination of shallow and deep anode ground bed
 
Long line linear anodes
 
Graph of potential with respect to distance
 
industrial safety training programs in Mumbai India
Off potential graph with respect to distance
 cathodic protection on pipelines
Installation of the Anodeflex System provides Cathodic Protection on every point without causing any under or overprotection
 
Conceptual design of CP for bullet tanks
civil & structural training in India
 
Conceptual design of CP for bullet tanks
 
Conceptual design for CP for above ground tank bottom

GROUNDING OF BULLET OR TANK
  • Each bullet or tank is required to be grounded at 2 locations
  • The grounding is done through XLPE insulated 25mm2Copper conductor cable
  • The grounding of the bullet to the earthing pits is done through a “dc-de-coupler device which blocks DC current but allows AC current to conduct.
  • DC voltage is blocked to a threshold value as per the-de-coupler selected but within limits

ELECTRICAL ISOLATION
 
Electrical isolation is required to prevent CP current drain through earthed structures and armour of instrument cable and imprive performance of CP system. Monolithic Isolation Joints or flange kit is provided to isolate the earthed structure. Surge diverter or spark gap arrestor or over voltage protector is provided to clear lightening fault or short circuit fault and induced AC voltage to < 5 KV

Spark gap arrestor/ over voltage protector across isolating joint
For Training and Consultancy Services Contact us on 022-62210100 or Email us on techsupport@marcepinc.com
 
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