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#27 Design of Cathodic Protection System

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While designing the cathodic protection system, we must know which type of cathodic protection system is needed. (galvanic or impressed current). Condition at the sites sometimes dictates the choice. However, when it is not clear, the criteria used most widely is based on current density and soil resistivity. If the soil resistivity is low (less than 5000 ohm-centimeters) and the current density requirement is low (less than 1milliampere per square foot), a galvanic system can be used. However, if the soil resistivity or current density requirement exceeds the above valves, an impressed current system should be used.

A) Galvanic cathodic protection system design: The following 8 steps are required when designing Galvanic cathodic protection systems;

1) Review soil resistivity: The site of the lowest resistivity will likely be used for anode location to minimize anode to electrolyte resistivity. In addition, if resistivity variations are not significant, the average resistivity will be used for design calculations.

2) Select Anode: Galvanic anode are usually either magnesium or zinc. Zinc anodes are used in extremely corrosive soil (resistivity below 2000 ohm-centimeters). Data from commercially available anodes must be reviewed. Each anode specification will include an anode weight, anode dimensions, and package dimensions (anode plus backfill). In addition, the anode driving potential must be considered. The choice of the anode from those available is though arbitrary, but the design calculations will be made for several available anodes. and the most economical one will be chosen.

3) Calculate Net driving potential for anodes: The open-circuit potential of standard alloy magnesium anodes is approximately -1.55 volts to a copper-copper sulphate half cell. The open-circuit potential of high manganese magnesium anodes is approximately -1.75 volts toa copper -copper sulphate half-cell. a) The potential of iron in contact with soil or water usually ranges around -0.55 volt relative to copper-copper sulphate. when cathodic protection is applied using magnesium anodes, the iron potential assumes some value between -0.55 to -1.0 volt, depending on the degree of protection provided. In highly corrosive soils or waters, the natural potential of iron may be as high as -0.82 volt relative to copper-copper sulphate. From this, it is evident that -0.55 volt should not be used to calculate the net driving potential available from magnesium anodes. b) A more practical approach is to consider iron polarized to -0.85 volt. On this basis, standard alloy magnesium anodes have a driving potential of 0.70 volt (1.55 -0.85) and high potential magnesium anodes have a driving potential of 0.90 volts (1.75-0.85). For cathodic protection design that involves magnesium anodes, these potentials 0.70 and 0.90 volts, should be used depending on the alloy selected.

4) Calculate the number of anodes needed to meet ground-bed resistance limitations: The total resistance (Rt) is given by the equation;

R(t)=R(a) + R(w) + R(c),

where R(a) is the anode to electrolyte resistance, R(w) is the anode lead wire resistance & R(c) is the structure to electrolyte resistance.

The total resistance also can be found by using equation;

R(t) = E/I , where E is the anode’s driving potential, I is the current density required.

R(c) = R/A , where R is the average coating resistance in ohms per square feet, and A is the structure surface area in square feet.

Anode-to-electrolyte resistance can then be calculated from equation,

R(a) = R(t) -R(c)

which gives the maximum allowable grounded resistance; this will dictate the minimum number of anode required (as number of anodes decreases, grounded resistance increases). To calculate the number of anodes required, we use the following equation:

N={(0.0052)(P)/(Ra)(L)} (ln 8L/d -1)

where, N is the numbers of anodes, P is the soil resistivity in ohms, Ra is the maximum allowable ground bed resistance in ohms, L is the length of back fill column in feet, d is the diameter of backfill column in feet.

5) Calculate a number of anodes for system life expectancy: Each cathodic protection system will be designed to protect a structure for a given number of years. To meet this life requirement, the number of anodes must be calculated using the equation.

N= {(L*I)/49.3 W

where L is the expected lifetime in years, W is weight (in pounds) of one anode, and I is the current density required to protect the structure (in milliamperes)

6) Select the number of anodes to be used: The greater value of the above two will be used as the number of anodes needed for the system,

7) Select ground bed layout: When the required number of anodes has been calculated, the area to be protected by each anode is calculated by the equation:

A= A(t) /N

where, A is area to be protected by one anode, A(t) is total surface area to be protected , N is total nos of anodes to be used.

8) Calculation life cycle cost for proposed design: NACE standard RP-02 should be used to calculate the system design process for several different anode choices to find the one with minimal life cycle cost.

9) Prepare plans and specifications: When the design procedure has been done, several plans and specifications can be completed.

B) Impressed current cathode protection system design: Thirteen steps are required when designing impressed current cathodic protection systems.

1) Review soil resistivity: As with galvanic systems, this information will contribute to both design calculations and the location of anode groundbed.

2) Review current requirement test: The required current will be used throughout the design calculations. The calculated current required to protect 1 square foot of bare pipe shall agree with the values in Table 1:

EnvironmentCurrent density (mA/sqft)
Neutral soil0.4 to 1.5
Well aerated neutral soil2 to 3
Wetsoil1 to 6
Highly acidic soil3 to 15
soil supporting activesulphate reducing bacteria6 to 42
Heated soil3 to 25
Stationary fresh water1 to 6
Moving fresh water containing dissolved oxygen3 to 15
Seawater3 to 10
Table 1

The table gives an estimate of current, in milliamperes per square foot, required for complete cathodic protection. That value, multiplied by the surface area of the structure to be protected (in square feet) gives the total estimated current required. Caution should be used when estimating, however, as under- or overprotection may result.

3) Select anode. As with the galvanic system, the choice of the anode is arbitrary at this time; the economy will determine which anode is best. Table 2 gives common anode sizes and specifications. The anodes used most often are made of high-silicon chromium-bearing cast-iron (HSCBCI). When impressed current-type cathodic protection systems are used to mitigate corrosion on an underground steel structure, the auxiliary anodes often are surrounded by a carbonaceous backfill. Backfill materials commonly used include coal coke breeze, calcined petroleum coke breeze, and natural graphite particles. The backfill serves three basic functions: (a) it decreases the anode-to-earth resistance by increasing the anode’s effective size, (b) it extends the system’s operational life by providing additional anode material, and (c) it provides a uniform environment around the anode, minimizing deleterious localized attack. The carbonaceous backfill, however, cannot be expected to increase the groundbed life expectancy unless it is well compacted around the anodes. In addition to HSCBCI anodes, the ceramic anode should be considered as a possible alternative for long-term cathodic protection of water storage tanks and underground pipes in soils with resistivities less than 5000 ohm-centimeters. The ceramic anode consumption rate is 0.0035 ounce per ampere-year compared to 1 pound per ampere-year for HSCRCI anodes.

Anode weight (lb)Anode dimensions (in.)Anode surface size (in.)Package area (sq ft)
121*601.410*84
442*602.610*84
602*602.810*84
1103*604.010*84
Table 2

4) Calculate a number of anodes needed to satisfy the manufacturer’s current density limitations: Impressed current anodes are supplied with a recommended maximum current density. Higher current densities will reduce anode life. To determine the number of anodes needed to meet the current density limitations, use the equation

N= I / (A1*I1) equation 1

where N is number of anodes required, I is total protection current in milliamperes, A1 is anode surface area in square feet per anode, and I1 is recommended maximum current density output in milliamperes.

5) Calculate number of anodes needed to meet design life requirement. Below Equation is used to find the number of anodes:

N = (L*I) / (1000*W) equation 2

where N is number of anodes, L is life in years, and W is weight of one anode in pounds.

6) Calculate number of anodes needed to meet maximum anode groundbed resistance requirements. Below equation is used to calculate the number of anodes required:

Ra = (pK)/(NL) = pP/S equation 3

where Ra is the anodes’ resistance, ρ is soil resistivity in ohm-centimeters, K is the anode shape factor from Table 3, N is the number of anodes, L is length of the anode backfill column in feet, P is the paralleling factor from Table 4, and S is the center-to-center spacing between anode backfill columns in feet.

L/dK
50.0140
60.0150
70.0158
80.0165
90.0171
100.0177
120.0186
140.0194
160.0201
180.0207
200.0213
250.0224
300.0234
350.0242
400.0249
450.0255
500.0261
550.0266
600.0270
Table 3
NP
20.00261
30.00289
40.00283
50.00268
60.00252
70.00237
80.00224
90.00212
100.00201
120.00182
140.00168
160.00155
180.00145
200.00135
220.00128
240.00121
260.00114
280.00109
300.00104
Table 4

7) Select the number of anodes to be used. The highest number calculated by Equation 1, 2, or 3 will be the number of anodes used.

8) Select area for placement of anode bed. The area with the lowest soil resistivity will bechosen to minimize anode-to-electrolyte resistance

9) Determine total circuit resistance. The total circuit resistance will be used to calculate the rectifier size needed

a) Calculate anode groundbed resistance. Use Equation 3.

b) Calculate groundbed header cable resistance. The cable is typically supplied with a specified resistance in ohms per 100 feet. The wire resistance then is calculated from Equation 4:

Rw = ohms (L) / 100 ft equation 4

where L is the structure’s length in feet. Economics are important in choosing a cable, and may indeed be the controlling factor. To determine the total annual cable cost, Kelvin’s Economic Law can be used as shown in Equation 5

T = (0.0876)(I^2)(R)(L)(P) / E = (0.15) (S) (L) equation 5

Where T is total annual cost in dollars per year, I is total protection current in amperes, R is cable resistance in ohms per 1000 feet, L is cable length in feet, P is cost of electrical energy in kilowatt-hour, E is the rectifier efficiency expressed as percent, and S is the cable’s initial cost in dollars per foot.

c) Calculate structure-to-electrolyte resistance. Using Equation 6:

Rc = R/N equation 6

where R is the structure-to-electrolyte resistance, R is the coating resistance in ohms per square feet, and N is the coated pipe area in square feet.

d) Calculate total circuit resistance. To calculate the total resistance, RT , equation 7 is used

R(t) = R(a) + R(w) +R(c) equation 7

10) Calculate rectifier voltage. Equation 8 is used to determine voltage output (V ) of the rectifier:

Vrec = (I) R(t) (150%) equation 8

where I is total protection current in amperes, RT is total circuit resistance, and 150 percent is a factor to allow for aging of the rectifier stacks.

11) Select a rectifier. A rectifier must be chosen based on the results of Equation 8. Many rectifiers are available commercially; one that satisfies the minimum requirements of (I) and (Vrec) in Equation 8 should be chosen. Besides the more common rectifiers being marketed, a solar cathodic protection power supply (for d.c. power) may be considered for remote sites with no electrical power. Three factors should be considered when specifying a solar cathodic protection power supply are:

a) The cost of the solar cathodic protection power supply in dollars per watt of continuous power. The solar cathodic protection power supply’s much higher initial cost compared to selenium rectifiers operated by a.c. power.

b) The additional maintenance required for a solar cathodic protection power supply, mainly to keep the solar panels free of dirt deposits.

12) Calculate system cost. As with the galvanic cathodic protection system, the choice of anode for design calculation is arbitrary. When several anodes have been used in the design calculations, an economic evaluation should be done as recommended in NACE Standard RP-02.

13) Prepare plans and specifications.

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