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1300 CP For Tank Bottoms (Underside) Author: P.F. (Paul) Offermann

Abstract This section covers sacrificial anode and impressed current cathodic protection systems for the underside of tank bottoms. It discusses soil tests, foundation types, design parameters, and cost analysis. Step-by-step design examples are given for protecting single tanks, and larger systems that protect many tanks are discussed briefly. (For very large systems, a cathodic protection contractor should be consulted.) Retrofits for existing tanks are discussed, along with their limitations. The ETD Materials Division or Mechanical and Electrical Systems Division can provide additional support. Discussions of the corrosion mechanisms and whether or not cathodic protection is needed can be found in Section 720 of this manual and in the Tank Manual. Internal cathodic protection of tanks is discussed in Section 1600 of this manual. Contents

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1310 General Information

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1311 Galvanic Systems 1312 Impressed Current Systems 1320 Soil Testing (Predesign Phase)

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1321 Soil Resistivity 1322 Soil Analysis 1323 Structure-to-Soil Potential Measurements 1330 Types of Foundation

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1331 Sand or Dirt Foundations 1332 Sand and 80-mil High Density Polyethylene (HDPE) Membrane 1333 Concrete Foundations 1334 Asphalt Foundations 1340 Design Parameters

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1341 Sacrificial Anode Cathodic Protection Systems

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1342 Impressed Current Cathodic Protection Systems 1343 Current Required for Protection 1344 Allowance for Soil Gradient 1345 Anode Consumption Rate 1350 Cathodic Protection Design Procedures

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1351 Design Procedures 1352 Example 1: Design of a Galvanic Anode System for a Tank Without HDPE Membrane (Sand or Earth Foundation) 1353 Example 2: Design of an Impressed Current System 1354 Example 3: Tank with HDPE Membrane (New Tank or Retrofit) 1355 Example 4: Deep-Well Anode Cathodic Protection Design

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1360 Cathodic Interference

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1370 Cost Analysis

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1380 References

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1310 General Information This section describes two types of systems for external cathodic protection of tank bottoms. Figure 1300-1 is a flow diagram showing the necessary steps in the design of these systems.

1311 Galvanic Systems Galvanic systems typically consist of a ring of magnesium anodes buried around a tank. They require no external power source and are essentially maintenance free for the life of the system (typically 20 years). These systems are limited, by economics, to tanks less than 40 feet in diameter and to soils with resistivities of less than 5000 ohm-cm. Section 1352 contains a design example.

1312 Impressed Current Systems Impressed current systems for individual tanks consist of anodes (graphite, Duriron, platinum, or steel) placed either in a circle around the tank or sandwiched between a high density polyethylene (HDPE) membrane and the bottom of the tank. Larger deep- well systems with many anodes stacked vertically in a hole bored several hundred feet deep can also be used to protect many structures at once, but again this design is not covered in detail here. Impressed current systems require an external power source and a rectifier, along with periodic inspection and maintenance. Usually, impressed current systems are used for large tanks, for high resistivity soils, or for protection of many structures at once. Section 1352 contains a design example for a 100-foot diameter tank on 1800 ohm-cm soil.

1320 Soil Testing (Predesign Phase) 1321 Soil Resistivity Soil resistivity measurements are used both to evaluate soil corrosiveness in an area and as a parameter for anode ground bed design for cathodic protection. See Section 1700 for further discussion of soil resistivity. Soil resistivity may be classified as follows [1]: Resistivity Range, ohm-cm

Corrosion Activity

0-2,000

severe

2000-10,000

moderate

10,000-30,000

mild

>30,000

unlikely

For existing tanks, resistivity tests should be performed at a minimum of four locations around each tank at depths of 5 feet and 10 feet (Figure 1300-2). This can be done using the Wenner four-pin method described in Section 1700. Tests should be

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Fig. 1300-1 Flow Diagram for Design of External Cathodic Protection for Tank Bottoms

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Fig. 1300-2 Soil Resistivity Measurement Schematic

performed during the wet periods of the year. If tests cannot be performed during wet periods, soil samples should be taken for analysis. For new tanks with sand or special backfill material the resistivity of the backfill should be measured using soil boxes in the “as found” and “saturated” states, and a soil analysis of the backfill should be performed as described in Section 1700. For new tanks with a membrane, resistivities should be measured, but a soil analysis is not required.

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1322 Soil Analysis Soil analysis is often a useful test of whether corrosion rates will be high enough to make cathodic protection necessary. Determination of aggressive ions, such as chlorides and sulfates, and measurements of pH and saturated resistivity are necessary for further corrosion analysis. If selected backfill is used, it should be analyzed along with the native soil; contaminants in the native soil can leach into the selected backfill and cause it to become corrosive. The following should be used in evaluating soil analysis data. Allowable Range Constituent

Corrosive

Very Corrosive

pH

5.0 - 6.5

300 ppm

>1000 ppm

Sulfates

>1000 ppm

>5000 ppm

1323 Structure-to-Soil Potential Measurements Potential measurements (Figure 1300-3) in which a reference electrode is placed in the soil at grade along a structure are widely used to determine if outside sources of stray dc current (electrolytic corrosion) or galvanic corrosion are harming the structure. More details on this test are given in Section 1700. To provide accurate data on existing tanks, potential measurements should be taken at the four locations where the soil resistivity measurements were taken. If the potentials on one side of the tank are significantly different from those on the opposite side, stray currents are indicated. Corrosion will tend to occur on the side with the lowest negative potentials. If these measurements indicate no outside stray currents and no connection to a more noble metal such as copper, then the locations of highest negative potentials are the most anodic and the most likely to corrode.

1330 Types of Foundation This section deals with cathodic protection design for single tank installations. If a large existing or new tankfield is to be cathodically protected, deep-well anodes are usually the most economical method because they can protect large areas. Deepwell anode design is beyond the scope of this manual. It is best to have a cathodic protection contractor handle analysis and design of deep-well anode systems.

1331 Sand or Dirt Foundations Placement of a sand or dirt foundation on the existing grade does not present a unique problem for placement of the anodes. The anodes may be closely distributed around the perimeter of existing tanks using drilled anode holes 10 to 15 feet deep (Figure 1300-4). Anodes can also be placed in the native soil under the tank prior to placement of the sand and steel bottom. This is recommended.

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Fig. 1300-3 Potential Measurement Schematic

Fig. 1300-4 Vertical Anode Cathodic Protection System

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1332 Sand and 80-mil High Density Polyethylene (HDPE) Membrane If an HDPE membrane is being placed under the new tank bottom, a remote or distributed anode design cannot be implemented because current cannot pass through the membrane to the steel bottom. Cathodic protection is usually not used for these tank bottoms. If it is required, anodes should be placed above the membrane in the backfill prior to placement of the sand base and new tank bottom (Figure 1300-5). Steel plates 1/8 inch thick, skip welded together, make a good anode. A reference electrode should also be installed at the center of the tank to monitor the cathodic protection system. For this type of installation, an impressed current cathodic protection system is more effective than a galvanic anode system. This is due to the limited space, high soil resistivities and large current requirements for a bare steel bottom. Fig. 1300-5 Cathodic Protection System with HDPE Membrane Beneath Tank

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1333 Concrete Foundations Cathodic protection is not necessary for tanks on concrete foundations. The concrete is relatively impermeable to water. Even if water does get in, the alkalinity of the concrete helps passivate the steel and prevent corrosion.

1334 Asphalt Foundations Asphalt foundations can present unique problems for corrosion control systems. Asphalt is essentially nonconductive and, as long as it remains intact, minimizes corrosion deterioration. As the asphalt degrades, water and chemical constituents that are aggressive to the tank bottom may enter. The result is corrosion. Cathodic protection may aid in stopping corrosion when the asphalt is deteriorated; however, if the asphalt pad is still intact, and poor drainage is allowing water to enter between the asphalt and steel, cathodic protection will not be effective. The condition of the asphalt can be determined at the same time coupons are cut from the tank bottom for inspection. Current requirement tests can also give an indication of the condition and continuity of the asphalt.

1340 Design Parameters This design section addresses the considerations necessary in choosing between a galvanic and impressed current system. The advantages and limitations of both systems are as follows:

1341 Sacrificial Anode Cathodic Protection Systems Advantages • • • • • • • • • •

No external power required No regulation required Easy to install Minimum of cathodic interference Anodes can be readily added Minimum of maintenance Uniform distribution of current around periphery of tank Installation can be inexpensive (if done during construction) Minimum right-of-way/easement costs Efficient use of protective current

Limitations

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Limited driving potential

•

Lower/limited current output (typically 1–2 amps maximum)

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Installation can be expensive (if done after construction)

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Can be ineffective in high-resistivity environments (generally greater than 5000 ohm-cm)

•

Practical only for small tanks (typically less than 40 feet in diameter)

•

Useful only for protecting one tank at a time.

•

Difficult to monitor effectiveness

1342 Impressed Current Cathodic Protection Systems Advantages • • • • • • •

Can be designed for wide range of voltage and current High ampere year output available from single ground bed Large areas can be protected by single installation Variable voltage and current output Applicable in high-resistivity environments Effective in protecting uncoated and poorly-coated structures Can be routinely monitored for effectiveness

Limitations • • • • • •

Can cause cathodic interference problems Subject to power failure and vandalism Requires periodic inspection and maintenance Requires external power Monthly power costs Overprotection can cause coating damage

Various design parameters and site information are necessary to design the system, including design current densities, design life of the structure and cathodic protection system, soil resistivity, tank configuration, and adjacent foreign structures.

1343 Current Required for Protection The current required for cathodic protection depends on the resistivity of the soil in contact with the tank bottom and the surface area of metal exposed to the electrolyte. Anything that increases the corrosion rate also increases the current required for protection. Typical current densities for bare steel tank bottoms range between 1.0 and 2.0 mA/sq ft of steel surface area. These current densities should be used with caution when estimating the current required for cathodic protection. For optimum design, the current required for cathodic protection should be calculated using the results of current requirement tests. These tests can only be performed on existing tanks and are conducted using a temporary anode bed (ground bed) and an appropriate source of direct current (Figure 1300-6). See Section 1700 for a further discussion of current drain tests. The temporary ground bed is typically positioned vertically in the soil, 5 to 10 feet from the outside

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Fig. 1300-6 Current Requirement Test Setup

perimeter of the tank. Depending on the current required, the power source can vary from a 12-volt storage battery to a 300-amp welding unit. Factors affecting current requirement tests include the following: • • •

Location and configuration of ground bed with respect to the tank Polarization effects Distribution of tanks to be protected from one dc source

Basically, current requirement tests are conducted by forcing a known amount of current to flow from the temporary anode bed through the electrolyte (soil) to the structure to be protected. The degree of protection at various locations around the structure is evaluated using potential measurements. This testing allows approximation of the current required to protect the structure. The usual criterion for protection is that potential readings be at least minus 0.85 volts versus a Cu/CuSO4 reference electrode. See Section 1100 for a further discussion of cathodic protection criteria. A reference electrode installed under the center of the tank bottom would be necessary for an accurate determination of the potential at that point. Since a reference electrode under the center of a tank bottom is not usually available, measurements are made using a reference electrode in the soil at the periphery of the tank. It is necessary to estimate the potential drop in the soil between the edge and the center of the tank. See Section 1700 for more information on the potential profile across a tank bottom.

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1344 Allowance for Soil Gradient The potential drop in the soil under a tank depends on a number of factors, including total current picked up by the tank bottom, location of the anode, soil resistivity, and variation of soil resistivity with depth. Under average conditions with a single remote anode, one millivolt per foot is usually considered a reasonable voltage gradient. If the anode is located fairly close to the tank so that the tank is in the anode field, the soil under the edge of the tank nearest the anode will be considerably more positive than the soil under the edge most remote from the anode, and the tank bottom will therefore be more negative with respect to soil at the near edge than at the remote edge. Under these conditions, the potential drop in the soil between the edge nearest the anode and the center of the tank will be considerably larger than the potential drop between the remote edge and the center of the tank. In this case, a reasonable estimate of the potential of the soil under the center of the tank can be made if it is assumed that the voltage gradient is one millivolt per foot from a point on the edge of the tank the same distance from the anode as the center of the tank. In a distributed anode system with anodes spaced around the periphery of the tank, the potential drop in the soil under the tank will be considerably more than one millivolt per foot. In this case, it is possible to estimate the voltage gradient in the soil under the tank by measuring the gradient due to the anode field on the side of the anodes away from the tank, and adding that gradient to the arbitrary one millivolt per foot under the tank. See Section 1700 for more information on soil voltage gradients. On new installations a permanent reference cell should be placed under the center of the tank, because this is the most difficult area of the tank to protect.

1345 Anode Consumption Rate The anode material selected for a particular design has a great impact on the system economics. The consumption rate of an anode determines the design life of the cathodic protection system. Consumption rate data are given in Figure 1300-7 for both galvanic and impressed current anodes. For galvanic systems in soil, magnesium anodes are almost always preferable to zinc or aluminum because their higher driving potential provides greater current output.

1350 Cathodic Protection Design Procedures This section presents procedures and examples for effective and economical cathodic protection system design for storage tanks. The procedures presented are sequential. Their purpose is to optimize the design and size the components of the cathodic protection system so that the engineer can select the most advantageous system. This process is also represented in the flow diagram given as Figure 1300-1. These procedures consist of a predesign phase and a design phase, and are followed by installation and post-installation testing. See Section 1200 for additional information on impressed current ground beds and cables.

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Fig. 1300-7 Anode Consumption Rates and Driving Voltages Consumption Rates for Various Anodes, Lb/amp-yr(1) Material

Driving Voltage ∆E, Volts

Theoretical

Actual

% Efficiency

Magnesium

8.75

17.5

50

0.9(2) 0.7(3)

Zinc

23.5

25.8

90

0.25

Aluminum

6.46

7.0–8.0

90

0.25

Galvanic

Impressed Durichlor 51

0.25–1.0

Graphite

0.5–2.0

(1) These values are for anodes surrounded by select backfill materials. (2) Galvomag Alloy (3) H-1 Alloy

•

Predesign phase. During the predesign phase (See Section 1320), basic information is obtained on the structure and its environment through field tests of soil resistivity, potential measurements, soil analysis, etc., and through evaluation of the corrosion control performance of other installations in the general area. The predesign phase determines the viability of cathodic protection as a means of corrosion control.

•

Design phase. In the design phase cathodic protection system components are selected. Initial iterations in the design phase are tentative. Technical and economical life cycle costs are then calculated for the system components, and the various alternatives are compared. Following design analysis, plans and specifications are developed. The system is then installed.

•

Post-installation testing. Following installation, the engineer must conduct further field surveys to ensure that the protection criteria selected are satisfied. With an impressed current system, additional field tests must be conducted to ensure that no stray-current corrosion problems exist. Finally, the procedure requires preparation of a cathodic protection system routine maintenance program.

1351 Design Procedures A generic approach for designing each type of cathodic protection system is detailed below:

Tank Without HDPE Membrane

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Conduct current requirement test (preferred), or

2.

Estimate current required by calculating surface area and estimating current density of 2.0 mA/ft2.

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Use soil resistivity data and current requirements for determining size and number of anodes. a.

Galvanic (magnesium anodes) — Resistivity 1.0 A

4.

Calculate number of anodes (See Section 1352, Equation 1300-5).

5.

Calculate anode design life (See Section 1352, Equation 1300-6).

6.

If galvanic, provide layout and anode spacing (Figure 1300-4).

7.

If impressed current, provide anode spacing and sizing of rectifier (See Section 1352, Equation 1300-7).

8.

Provide drawings and specifications.

9.

Provide engineering cost estimate.

Tank With HDPE Membrane 1.

Conduct soil resistivity tests on sand/backfill foundation material for anode bed design.

2.

Calculate steel bottom surface area.

3.

Calculate current requirement using estimated current density of 2.0 mA/ft2 of bottom surface area.

4.

Determine minimum rectifier voltage.

5.

Size minimum rectifier current and voltage (see Section 1352, Equation 1300-7).

6.

Select standard rectifier size.

7.

Provide design drawings and specifications.

8.

Perform cost analysis.

9.

Provide engineering cost estimate.

The following two simple examples illustrate the fundamentals of the cathodic protection design procedure for tank bottoms. The formulas and considerations required for complex system design are beyond the scope of this manual. Where necessary, the references listed will explain the design formulas.

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1352 Example 1: Design of a Galvanic Anode System for a Tank Without HDPE Membrane (Sand or Earth Foundation) The tank is 30 feet in diameter. Soil resistivities measured around the tank range between 1000 and 1800 ohm-cm.

Current Requirements Current requirement tests were performed and the estimated current to provide protection is 1.0 ampere. If current requirements were calculated using a current density of 2.0 mA/ft2 the estimated current would be 1.41 amperes. Ground Bed Resistance. Having determined current requirements from field testing we next calculate a maximum allowable ground bed resistance. The total series equivalent electric circuit must be considered (see Figure 1300-8). Kirchoff’s Voltage Law can be used to develop a relationship for estimating total circuit resistance. Ea – Ec I = --------------------------------R a + Rw + Rc ∆E = ------Rt (Eq. 1300-1)

where: I = current required for cathodic protection, amps Ea = anode-to-electrolyte (soil) open circuit potential, volts (Figure 1300-8) Ec = structure-to-electrolyte (soil) polarized potential, volts (Figure 1300-8) Ra = anode-to-electrolyte (soil) resistance, ohms Rt = maximum ground bed resistance, ohms Rw = anode lead wire resistance, ohms Rc = structure-to-electrolyte (soil) resistance, ohms ∆E = driving voltage, volts Another expression of Equation 1300-1 is: ∆E R t = ------I For this example a 32-pound prepackaged high potential magnesium anode has been selected for the design. The driving voltage ∆ • E for this anode is 0.9 volts (see Figure 1300-7).

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So, using Equation 1300-2, the maximum permissible resistance Rt for 1.0 amps is: 0.9 Rt = ------- = 0.9 ohm 1.0 (Eq. 1300-2)

For galvanic cathodic protection the anode lead wire resistance (Rw) is usually small and can be neglected. The structure-to-electrolyte resistance for uncoated structures is also so small it can be neglected. Hence, for most cases Rt = Ra. Fig. 1300-8 Cathodic Protection Equivalent Electric Current

Number of Anodes The ground bed resistance of single and multiple anodes in parallel has been derived by Dwight [2]. These equations can be used to determine the number of anodes required to satisfy the maximum allowable ground bed resistance. In simplified form, the resistance of one anode (R1) or of any number of vertically-placed anodes (Rn) can be calculated using the following equations: 8L 0.0052ρ R 1 = -------------------- ln ------ – 1 d L (Eq. 1300-3)

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0.0052ρ 8L 2L R n = -------------------- ln ------ – 1 + ------- ln ( 0.656n ) nL d S (Eq. 1300-4)

where: ρ = soil resistivity, ohm-cm L = length of backfill column, ft (see Figure 1300-9) d = diameter of backfill, ft (see Figure 1300-9) Rn = combined resistance of n anodes, ohms n = number of anodes S = center-to-center spacing between anodes, ft Equation 1300-5 is a simplified form of Equation 1300-4 and can be used if S is greater than 6L and n is less than 15. If S is not greater than 6L or n is not less than 15, then Equation 1300-5 must be used. 8L 0.0052ρ n = -------------------- ln ------- – 1 d Rn L (Eq. 1300-5)

From Figure 1300-9, The maximum ground bed resistance Rn is 0.9 ohm. Therefore, the number of anodes required for the system is: 8 ( 1.91 ) 0.0052 ( 1800 ) n = --------------------------------- ln ------------------ – 1 0.667 0.9 ( 1.91 ) =11.6 anodes (use 12) It follows that twelve 32-pound high potential magnesium anodes are required to keep total circuit resistance below 0.9 ohms. Once the number of anodes required for current output has been determined we then must determine if the anodes will provide the required design life, 20 years in this case. Wn D = --------QI a (Eq. 1300-6)

where: D = design life, years W = weight of anode, lb. Use 32 lb (from Figure 1300-9). n = number of anodes. Use 12. Q = actual consumption rate, lb/amp-yr. Use 17.5 (from Figure 1300-7). Chevron Corporation

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Ia = current required for cathodic protection, amps. Use 1.0. then: ( 32 ) ( 12 ) D = ---------------------------- = 21.9 years ( 17.5 ) ( 1.0 ) Fig. 1300-9 Magnesium Anode Dimensions and Shipping Weights

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These calculations show that a design utilizing twelve 32-lb magnesium anodes will provide the desired current output for the proposed 20-year design life. If the design life (L) is not met the engineer would rearrange Equation 1300-6 and solve for n to achieve 20-year life. Lives for various sizes of magnesium anodes are shown as a function of current output in Figure 1300-10. The engineer must now produce design drawings (Figure 1300-4) and specifications for equipment. An engineering cost analysis is developed later in this section. Fig. 1300-10 Magnesium Anode Life Versus Current Output

1353 Example 2: Design of an Impressed Current System The tank is 100 feet in diameter with a current requirement of 8.32 amperes and soil resistivity between 1000 and 1800 ohm-cm. This design differs from that of the galvanic system in having no fixed voltage, but a variable voltage derived from an external dc rectifier. Therefore, anode resistance to earth constitutes a significant role in the design. In this case 3-inch by 60-inch graphite anodes will be placed and backfilled in 10-inch diameter, 10-ft deep, vertically augured holes (typical hole dimensions). Subject to economic analysis, a total anode bed resistance (Rn) of less than 2 ohms is typically desirable to provide lower energy costs. Therefore, the number of

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anodes required to provide a maximum 2-ohm anode bed can be calculated by substituting the following data into Equation 1300-5 and solving for n: ρ = soil resistivity, ohm-cm. Use 1800 (given field measurement). L = length of backfill column, ft. Use 7 (from Figure 1300-11). d = diameter of backfill, ft. Use 10 in = 0.833 ft (from Figure 1300-11). then, solving Equation 1300-5: n = 2.14 anodes (use 3) Now, will three anodes provide the needed 20-year design life at the required current level? Using Equation 1300-6, where: W = weight of anode, lb. Use 25 (from Figure 1300-11). n = number of anodes. Use 3. Q = actual consumption rate, lb/amp-yr. Use 1.5 (from Figure 1300-7). Ia = current required for cathodic protection, amps. Use 8.32. then: ( 25 ) ( 3 ) D = ---------------------------- = 6.0 years ( 1.5 ) ( 8.32 ) Since the three anodes will not meet the required design life, additional anodes must be used. To determine the minimum number of anodes required for a 20-year life we will rearrange Equation 1300-6 and solve for n: DQI a ( 20 ) ( 1.5 ) ( 8.32 ) n = ------------- = --------------------------------------W 25 = 9.98 anodes Now, both conditional statements—bed resistance and design life—have been satisfied. Size the rectifier by calculating the final bed resistance (R) with the 10 anodes required for design life using Equation 1300-3. ( 0.0052 ) ( 1800 ) R n = -------------------------------------( 10 ) ( 7 )

(8)(7 ) ln ---------------- – 1 0.833

= 0.429 ohms Equation 1300-3

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Rectifier voltage output can now be determined using Ohm’s Law: V = I Rt (Eq. 1300-7)

where: I = 8.32 amperes Rt = Rn = 0.429 ohm then: V = (8.32)(0.429) = 3.57 volts Therefore, the minimum rectifier output to provide adequate current and voltage is a 3.57-volt, 8.32-ampere unit. Other resistances, such as cable resistance (Rw), have not been considered and should be determined in the final analysis. Whenever possible, the rectifier rating should be determined by test after the ground bed is installed. See Figure 1700-10. Fig. 1300-11 Graphite Anode Dimensions and Shipping Weights (For reference only. Consult the anode manufacturer for specific instructions.)

Figure 1300-4 shows a typical layout drawing with anodes placed around the perimeter of the tank 10 feet from the outside wall. The drawing also shows a No. 8 HMW/PE cable connecting the anodes around the perimeter. The calculated length of No. 8 cable around the perimeter is approximately 380 feet. The resistance of No. 8 cable is 0.000822 ohm/ft (See Figure 1300-12), resulting in a resistance (R) equal to 0.312 ohms.

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Fig. 1300-12 Cable Resistance Table

Since the cable is looped and both ends terminate at the rectifier the actual resistance is R/2 or 0.156 ohms. If the cable resistance is added to R the total circuit resistance is 0.585 ohm. Therefore, the new voltage requirement is 4.87 volts. Sizes for various rectifiers can be obtained from manufacturers’ catalogues. Standard rectifiers are available in various outputs. The type of rectifier (i.e., air-cooled, oil-cooled, explosion-proof) must be determined for cost analysis. For the impressed current system a standard 8-volt, 12-ampere rectifier powering ten graphite anodes with coke breeze backfill, placed vertically around the tank perimeter should provide the necessary design life. A conceptual drawing of a typical installation is shown in Figure 1300-4. High-silicon cast iron anodes could also be used for this installation. Design information is given in Figure 1300-13.

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Fig. 1300-13 High Silicon Cast Iron Design Information

1354 Example 3: Tank with HDPE Membrane (New Tank or Retrofit) This design concept is rather unique for cathodic protection. Since the anode must be placed between the 80-mil HDPE membrane and the steel tank, location and placement of the anode for proper current distribution is critical. Because of the high resistivity of the backfill, analysis indicates that the use of an impressed current cathodic protection system will provide a more adequate current distribution than a galvanic anode system. A typical design layout for this system is shown in Figure 1300-5. The anodes suitable for this installation are steel plates placed within the backfill. From predesign data, soil resistivity values on saturated backfill averaged 50,000 ohm-cm. In this example a 100-foot diameter tank will be used for analysis.

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The steel bottom surface area is 7854 ft2. Using a design current density of 2.0 mA/ft2 the estimate current required for cathodic protection is 15.7 amperes. The anode in this design is a circular plate of 1/8-inch steel. Steel plate 1/8-inch thick is much thicker than required, but is used to withstand construction vehicle traffic during placement of backfill. The resistance of the anode placed in this environment can be calculated using Equation 1300-8. ρt ( 0.002734 ) R = --------------------------------- + 0.10 A (Eq. 1300-8)

where: R = resistance of soil backfill ρ = soil resistivity, ohm-cm. Use 50,000 (given measurement). t = thickness of backfill, in. Use 6 (assumed in this example. See actual foundation design). A = area of tank to be protected, ft2. Use 7854. Note 0.1 ohm is used to compensate for resistance between anode and soil, resistance between tank bottom and soil, and cable resistances. then: ( 50, 000 ) ( 6 ) ( 0.002734 ) R = ---------------------------------------------------------- = 0.204 ohms 7854 Now the rectifier voltage can be calculated using Equation 1300-7: V = IRt = 15.7 (0.204) = 3.21 volts Therefore the minimum rectifier output to provide adequate current and voltage is 3.21 volts at 15.7 amps. A cost analysis is provided later in this section.

1355 Example 4: Deep-Well Anode Cathodic Protection Design The use of deep-well anodes has proven very effective in protecting tank bottoms in tank farms. This type of system may be combined with surface anode beds. A deepwell anode system allows the designer to incorporate remote anodes in limited areas. An anode hole is bored to depths of 100 to 400 feet. Typical hole diameters range between 6 inches and 12 inches. Impressed current anodes, such as graphite or Durichlor 51, are lowered to the bottom and spaced accordingly above each other in the deep-well (Figure 1300-14). The number of anodes required is calculated as described in the previous section. There are limits on the number of anodes that can

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be placed into a deep well. Typically, 15 anodes is the upper limit for each well. See Section 1200 for further discussion of deep-well ground beds. Fig. 1300-14 Deep - Well Anode

The designer should be aware of subsurface geological strata in the area where the deep-well anode is to be located. Areas having surface strata of high resistivity and deeper strata of low resistivity (where the anodes are to be placed) may cause shielding of the current from the deep anode to the steel tank bottom. See Figure 1300-12. The design of deep-well anode systems requires more detailed analysis than is presented in this manual.

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1360 Cathodic Interference Cathodic protection interference is the undesirable current pickup and discharge from a buried structure caused by cathodic protection applied to a tank or tank farm. It is only a problem for the impressed current cathodic protection system, with its higher operating voltage. The current pickup on an adjacent unprotected structure will discharge at some point to the protected structure, resulting in corrosion deterioration of the unprotected structure. A more detailed description can be found in Section 1750. In the case of tanks and tank farms, transfer piping or underground electrical systems may be subjected to interference. Potential testing following the application of cathodic protection can establish the degree of interference. Mitigative procedures can be extensive and may include bonding of the affected structure to the cathodically protected structure or diode blocking of current to the protected structure. Mitigation techniques are generally not straightforward and are outside the scope of this section. The engineer should be aware of the possibility of interference on adjacent structures.

1370 Cost Analysis The simplest method of choosing the best of several available alternatives is to determine which has the lowest cost. For a fair evaluation, the alternatives must be compared over equivalent life cycles. They must also be providing similar corrosion protection. The most economical system is the one which has the lowest present worth. The following examples (Figure 1300-15) provide guidelines on estimating the costs of several galvanic anode and impressed current systems. They need to be corrected for your specific use.

1380 References

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1.

Corrosion Control. Air Force Manual No. AFM88-9. United States Air Force, 1962.

2.

Dwight, H.B. “Calculations of Resistance to Ground.” AIEE Transactions, Volume 55, 1319 – 1328 (1939).

3.

Sunde, E.D. Earth Conduction Effects in Transmission Systems. D. Van Nordstrand Company, New York, 1949.

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Fig. 1300-15 Design Example Cost Analysis (1 of 2) DESIGN EXAMPLE 1: SAND FOUNDATION—GALVANIC ANODE Tank Diameter = 30 ft. Anode Diameter = 50 ft. (10 ft. from tank) Anode Circumference = 157 ft. Material

Unit

Total

12 - 32 lb Anodes with 10 ft. #12 Wire

$102.00/ea.

$1,250.00

12 - Splice Kits

$25.00/ea.

$300.00

1 - Test Station

$18.00/ea.

$20.00

$0.25/ft.

$50.00

$80.00/ea.

$1000.00

$3.00/ft.

$600.00

177 ft. - #8 HMW/PE Cable Installation 12 - Augured Holes 10 ft. × 12" Dia. 177 ft. - Trenching/Backfill Total Installed Cost

$3,220.00

Maintenance Inspection (1 manhour/month) x (12 months/year) x ($25.00/manhour) =

$300.00/yr

Annual Corrosion Survey

$200.00/yr

DESIGN EXAMPLE 2: SAND FOUNDATION—IMPRESSED CURRENT Tank Diameter = 100 ft. Anode Diameter = 120 ft. (10 ft. from Tank) Anode Circumference = 377 ft. (Add 20 ft. to Rectifier) Material

Unit

Total

10 - 3" x 60" Graphite Anodes

$132.00/ea

$1,320.00

1 - 8 volt 12 amp Rectifier

$560.00/ea.

$560.00

10 - Splice Kits

$25.00/ea.

$250.00

$0.25/ft.

$100.00

$80.00/ea.

$800.00

$3.00/ft.

$1,191.00

397 ft. - #18 HMW/PE Cable Installation 10 - Augured Holes 10 ft. X 12" Dia. 397 ft.- Trenching/Backfill Total Installed Cost

$4,200.25

Maintenance Inspection

$300.00/yr

Annual Corrosion Survey Power Consumption 4 volts x 8.32 amps = 33.28 watts

$200.00/yr

33.28w x (8760 hr/year) x (1kw/1000w) x 0.05/Kwhr =

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Fig. 1300-15 Design Example Cost Analysis (2 of 2) DESIGN EXAMPLE 3: HDPE MEMBRANE BENEATH TANK—IMPRESSED CURRENT Tank Diameter = 100 ft. Steel Plate Diameter = 97 ft. Material

Unit

Total

$1.53/ft.2

$11,300.00

300 ft. - #8 HMW/PE Cable

$0.25/ft.

$75.00

4 - Cadweld connections

$10.00/ea.

$40.00

$1000.00/ea.

$1,000.00

$200.00/400 ft.2

$3,695.00

$6.50/ft.

$1,950.00

4 - Cadweld connections

$50.00/ea.

$200.00

1 - Rectifier

$500.00/ea.

$500.00

7390 ft.

2 - 1/8" thick Steel Plate

1 - 12 volt, 20 amp Rectifier Installation 7390 ft.2 - Steel Plates Set 300 ft. - Welding

Total Installed Cost

$18,766.00

Maintenance Inspection

$300.00/yr

Annual Corrosion Survey Power Consumption 4 Volts x 20 amps = 80 watts

$200.00/yr $35.04/yr

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Abstract This section covers sacrificial anode and impressed current cathodic protection systems for the underside of tank bottoms. It discusses soil tests, foundation types, design parameters, and cost analysis. Step-by-step design examples are given for protecting single tanks, and larger systems that protect many tanks are discussed briefly. (For very large systems, a cathodic protection contractor should be consulted.) Retrofits for existing tanks are discussed, along with their limitations. The ETD Materials Division or Mechanical and Electrical Systems Division can provide additional support. Discussions of the corrosion mechanisms and whether or not cathodic protection is needed can be found in Section 720 of this manual and in the Tank Manual. Internal cathodic protection of tanks is discussed in Section 1600 of this manual. Contents

Page

1310 General Information

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1311 Galvanic Systems 1312 Impressed Current Systems 1320 Soil Testing (Predesign Phase)

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1321 Soil Resistivity 1322 Soil Analysis 1323 Structure-to-Soil Potential Measurements 1330 Types of Foundation

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1331 Sand or Dirt Foundations 1332 Sand and 80-mil High Density Polyethylene (HDPE) Membrane 1333 Concrete Foundations 1334 Asphalt Foundations 1340 Design Parameters

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1341 Sacrificial Anode Cathodic Protection Systems

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1342 Impressed Current Cathodic Protection Systems 1343 Current Required for Protection 1344 Allowance for Soil Gradient 1345 Anode Consumption Rate 1350 Cathodic Protection Design Procedures

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1351 Design Procedures 1352 Example 1: Design of a Galvanic Anode System for a Tank Without HDPE Membrane (Sand or Earth Foundation) 1353 Example 2: Design of an Impressed Current System 1354 Example 3: Tank with HDPE Membrane (New Tank or Retrofit) 1355 Example 4: Deep-Well Anode Cathodic Protection Design

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1360 Cathodic Interference

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1370 Cost Analysis

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1380 References

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1310 General Information This section describes two types of systems for external cathodic protection of tank bottoms. Figure 1300-1 is a flow diagram showing the necessary steps in the design of these systems.

1311 Galvanic Systems Galvanic systems typically consist of a ring of magnesium anodes buried around a tank. They require no external power source and are essentially maintenance free for the life of the system (typically 20 years). These systems are limited, by economics, to tanks less than 40 feet in diameter and to soils with resistivities of less than 5000 ohm-cm. Section 1352 contains a design example.

1312 Impressed Current Systems Impressed current systems for individual tanks consist of anodes (graphite, Duriron, platinum, or steel) placed either in a circle around the tank or sandwiched between a high density polyethylene (HDPE) membrane and the bottom of the tank. Larger deep- well systems with many anodes stacked vertically in a hole bored several hundred feet deep can also be used to protect many structures at once, but again this design is not covered in detail here. Impressed current systems require an external power source and a rectifier, along with periodic inspection and maintenance. Usually, impressed current systems are used for large tanks, for high resistivity soils, or for protection of many structures at once. Section 1352 contains a design example for a 100-foot diameter tank on 1800 ohm-cm soil.

1320 Soil Testing (Predesign Phase) 1321 Soil Resistivity Soil resistivity measurements are used both to evaluate soil corrosiveness in an area and as a parameter for anode ground bed design for cathodic protection. See Section 1700 for further discussion of soil resistivity. Soil resistivity may be classified as follows [1]: Resistivity Range, ohm-cm

Corrosion Activity

0-2,000

severe

2000-10,000

moderate

10,000-30,000

mild

>30,000

unlikely

For existing tanks, resistivity tests should be performed at a minimum of four locations around each tank at depths of 5 feet and 10 feet (Figure 1300-2). This can be done using the Wenner four-pin method described in Section 1700. Tests should be

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Fig. 1300-1 Flow Diagram for Design of External Cathodic Protection for Tank Bottoms

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Fig. 1300-2 Soil Resistivity Measurement Schematic

performed during the wet periods of the year. If tests cannot be performed during wet periods, soil samples should be taken for analysis. For new tanks with sand or special backfill material the resistivity of the backfill should be measured using soil boxes in the “as found” and “saturated” states, and a soil analysis of the backfill should be performed as described in Section 1700. For new tanks with a membrane, resistivities should be measured, but a soil analysis is not required.

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1322 Soil Analysis Soil analysis is often a useful test of whether corrosion rates will be high enough to make cathodic protection necessary. Determination of aggressive ions, such as chlorides and sulfates, and measurements of pH and saturated resistivity are necessary for further corrosion analysis. If selected backfill is used, it should be analyzed along with the native soil; contaminants in the native soil can leach into the selected backfill and cause it to become corrosive. The following should be used in evaluating soil analysis data. Allowable Range Constituent

Corrosive

Very Corrosive

pH

5.0 - 6.5

300 ppm

>1000 ppm

Sulfates

>1000 ppm

>5000 ppm

1323 Structure-to-Soil Potential Measurements Potential measurements (Figure 1300-3) in which a reference electrode is placed in the soil at grade along a structure are widely used to determine if outside sources of stray dc current (electrolytic corrosion) or galvanic corrosion are harming the structure. More details on this test are given in Section 1700. To provide accurate data on existing tanks, potential measurements should be taken at the four locations where the soil resistivity measurements were taken. If the potentials on one side of the tank are significantly different from those on the opposite side, stray currents are indicated. Corrosion will tend to occur on the side with the lowest negative potentials. If these measurements indicate no outside stray currents and no connection to a more noble metal such as copper, then the locations of highest negative potentials are the most anodic and the most likely to corrode.

1330 Types of Foundation This section deals with cathodic protection design for single tank installations. If a large existing or new tankfield is to be cathodically protected, deep-well anodes are usually the most economical method because they can protect large areas. Deepwell anode design is beyond the scope of this manual. It is best to have a cathodic protection contractor handle analysis and design of deep-well anode systems.

1331 Sand or Dirt Foundations Placement of a sand or dirt foundation on the existing grade does not present a unique problem for placement of the anodes. The anodes may be closely distributed around the perimeter of existing tanks using drilled anode holes 10 to 15 feet deep (Figure 1300-4). Anodes can also be placed in the native soil under the tank prior to placement of the sand and steel bottom. This is recommended.

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Fig. 1300-3 Potential Measurement Schematic

Fig. 1300-4 Vertical Anode Cathodic Protection System

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1332 Sand and 80-mil High Density Polyethylene (HDPE) Membrane If an HDPE membrane is being placed under the new tank bottom, a remote or distributed anode design cannot be implemented because current cannot pass through the membrane to the steel bottom. Cathodic protection is usually not used for these tank bottoms. If it is required, anodes should be placed above the membrane in the backfill prior to placement of the sand base and new tank bottom (Figure 1300-5). Steel plates 1/8 inch thick, skip welded together, make a good anode. A reference electrode should also be installed at the center of the tank to monitor the cathodic protection system. For this type of installation, an impressed current cathodic protection system is more effective than a galvanic anode system. This is due to the limited space, high soil resistivities and large current requirements for a bare steel bottom. Fig. 1300-5 Cathodic Protection System with HDPE Membrane Beneath Tank

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1333 Concrete Foundations Cathodic protection is not necessary for tanks on concrete foundations. The concrete is relatively impermeable to water. Even if water does get in, the alkalinity of the concrete helps passivate the steel and prevent corrosion.

1334 Asphalt Foundations Asphalt foundations can present unique problems for corrosion control systems. Asphalt is essentially nonconductive and, as long as it remains intact, minimizes corrosion deterioration. As the asphalt degrades, water and chemical constituents that are aggressive to the tank bottom may enter. The result is corrosion. Cathodic protection may aid in stopping corrosion when the asphalt is deteriorated; however, if the asphalt pad is still intact, and poor drainage is allowing water to enter between the asphalt and steel, cathodic protection will not be effective. The condition of the asphalt can be determined at the same time coupons are cut from the tank bottom for inspection. Current requirement tests can also give an indication of the condition and continuity of the asphalt.

1340 Design Parameters This design section addresses the considerations necessary in choosing between a galvanic and impressed current system. The advantages and limitations of both systems are as follows:

1341 Sacrificial Anode Cathodic Protection Systems Advantages • • • • • • • • • •

No external power required No regulation required Easy to install Minimum of cathodic interference Anodes can be readily added Minimum of maintenance Uniform distribution of current around periphery of tank Installation can be inexpensive (if done during construction) Minimum right-of-way/easement costs Efficient use of protective current

Limitations

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•

Limited driving potential

•

Lower/limited current output (typically 1–2 amps maximum)

•

Installation can be expensive (if done after construction)

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•

Can be ineffective in high-resistivity environments (generally greater than 5000 ohm-cm)

•

Practical only for small tanks (typically less than 40 feet in diameter)

•

Useful only for protecting one tank at a time.

•

Difficult to monitor effectiveness

1342 Impressed Current Cathodic Protection Systems Advantages • • • • • • •

Can be designed for wide range of voltage and current High ampere year output available from single ground bed Large areas can be protected by single installation Variable voltage and current output Applicable in high-resistivity environments Effective in protecting uncoated and poorly-coated structures Can be routinely monitored for effectiveness

Limitations • • • • • •

Can cause cathodic interference problems Subject to power failure and vandalism Requires periodic inspection and maintenance Requires external power Monthly power costs Overprotection can cause coating damage

Various design parameters and site information are necessary to design the system, including design current densities, design life of the structure and cathodic protection system, soil resistivity, tank configuration, and adjacent foreign structures.

1343 Current Required for Protection The current required for cathodic protection depends on the resistivity of the soil in contact with the tank bottom and the surface area of metal exposed to the electrolyte. Anything that increases the corrosion rate also increases the current required for protection. Typical current densities for bare steel tank bottoms range between 1.0 and 2.0 mA/sq ft of steel surface area. These current densities should be used with caution when estimating the current required for cathodic protection. For optimum design, the current required for cathodic protection should be calculated using the results of current requirement tests. These tests can only be performed on existing tanks and are conducted using a temporary anode bed (ground bed) and an appropriate source of direct current (Figure 1300-6). See Section 1700 for a further discussion of current drain tests. The temporary ground bed is typically positioned vertically in the soil, 5 to 10 feet from the outside

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Fig. 1300-6 Current Requirement Test Setup

perimeter of the tank. Depending on the current required, the power source can vary from a 12-volt storage battery to a 300-amp welding unit. Factors affecting current requirement tests include the following: • • •

Location and configuration of ground bed with respect to the tank Polarization effects Distribution of tanks to be protected from one dc source

Basically, current requirement tests are conducted by forcing a known amount of current to flow from the temporary anode bed through the electrolyte (soil) to the structure to be protected. The degree of protection at various locations around the structure is evaluated using potential measurements. This testing allows approximation of the current required to protect the structure. The usual criterion for protection is that potential readings be at least minus 0.85 volts versus a Cu/CuSO4 reference electrode. See Section 1100 for a further discussion of cathodic protection criteria. A reference electrode installed under the center of the tank bottom would be necessary for an accurate determination of the potential at that point. Since a reference electrode under the center of a tank bottom is not usually available, measurements are made using a reference electrode in the soil at the periphery of the tank. It is necessary to estimate the potential drop in the soil between the edge and the center of the tank. See Section 1700 for more information on the potential profile across a tank bottom.

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1344 Allowance for Soil Gradient The potential drop in the soil under a tank depends on a number of factors, including total current picked up by the tank bottom, location of the anode, soil resistivity, and variation of soil resistivity with depth. Under average conditions with a single remote anode, one millivolt per foot is usually considered a reasonable voltage gradient. If the anode is located fairly close to the tank so that the tank is in the anode field, the soil under the edge of the tank nearest the anode will be considerably more positive than the soil under the edge most remote from the anode, and the tank bottom will therefore be more negative with respect to soil at the near edge than at the remote edge. Under these conditions, the potential drop in the soil between the edge nearest the anode and the center of the tank will be considerably larger than the potential drop between the remote edge and the center of the tank. In this case, a reasonable estimate of the potential of the soil under the center of the tank can be made if it is assumed that the voltage gradient is one millivolt per foot from a point on the edge of the tank the same distance from the anode as the center of the tank. In a distributed anode system with anodes spaced around the periphery of the tank, the potential drop in the soil under the tank will be considerably more than one millivolt per foot. In this case, it is possible to estimate the voltage gradient in the soil under the tank by measuring the gradient due to the anode field on the side of the anodes away from the tank, and adding that gradient to the arbitrary one millivolt per foot under the tank. See Section 1700 for more information on soil voltage gradients. On new installations a permanent reference cell should be placed under the center of the tank, because this is the most difficult area of the tank to protect.

1345 Anode Consumption Rate The anode material selected for a particular design has a great impact on the system economics. The consumption rate of an anode determines the design life of the cathodic protection system. Consumption rate data are given in Figure 1300-7 for both galvanic and impressed current anodes. For galvanic systems in soil, magnesium anodes are almost always preferable to zinc or aluminum because their higher driving potential provides greater current output.

1350 Cathodic Protection Design Procedures This section presents procedures and examples for effective and economical cathodic protection system design for storage tanks. The procedures presented are sequential. Their purpose is to optimize the design and size the components of the cathodic protection system so that the engineer can select the most advantageous system. This process is also represented in the flow diagram given as Figure 1300-1. These procedures consist of a predesign phase and a design phase, and are followed by installation and post-installation testing. See Section 1200 for additional information on impressed current ground beds and cables.

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Fig. 1300-7 Anode Consumption Rates and Driving Voltages Consumption Rates for Various Anodes, Lb/amp-yr(1) Material

Driving Voltage ∆E, Volts

Theoretical

Actual

% Efficiency

Magnesium

8.75

17.5

50

0.9(2) 0.7(3)

Zinc

23.5

25.8

90

0.25

Aluminum

6.46

7.0–8.0

90

0.25

Galvanic

Impressed Durichlor 51

0.25–1.0

Graphite

0.5–2.0

(1) These values are for anodes surrounded by select backfill materials. (2) Galvomag Alloy (3) H-1 Alloy

•

Predesign phase. During the predesign phase (See Section 1320), basic information is obtained on the structure and its environment through field tests of soil resistivity, potential measurements, soil analysis, etc., and through evaluation of the corrosion control performance of other installations in the general area. The predesign phase determines the viability of cathodic protection as a means of corrosion control.

•

Design phase. In the design phase cathodic protection system components are selected. Initial iterations in the design phase are tentative. Technical and economical life cycle costs are then calculated for the system components, and the various alternatives are compared. Following design analysis, plans and specifications are developed. The system is then installed.

•

Post-installation testing. Following installation, the engineer must conduct further field surveys to ensure that the protection criteria selected are satisfied. With an impressed current system, additional field tests must be conducted to ensure that no stray-current corrosion problems exist. Finally, the procedure requires preparation of a cathodic protection system routine maintenance program.

1351 Design Procedures A generic approach for designing each type of cathodic protection system is detailed below:

Tank Without HDPE Membrane

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1.

Conduct current requirement test (preferred), or

2.

Estimate current required by calculating surface area and estimating current density of 2.0 mA/ft2.

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3.

Corrosion Prevention and Metallurgy Manual

Use soil resistivity data and current requirements for determining size and number of anodes. a.

Galvanic (magnesium anodes) — Resistivity 1.0 A

4.

Calculate number of anodes (See Section 1352, Equation 1300-5).

5.

Calculate anode design life (See Section 1352, Equation 1300-6).

6.

If galvanic, provide layout and anode spacing (Figure 1300-4).

7.

If impressed current, provide anode spacing and sizing of rectifier (See Section 1352, Equation 1300-7).

8.

Provide drawings and specifications.

9.

Provide engineering cost estimate.

Tank With HDPE Membrane 1.

Conduct soil resistivity tests on sand/backfill foundation material for anode bed design.

2.

Calculate steel bottom surface area.

3.

Calculate current requirement using estimated current density of 2.0 mA/ft2 of bottom surface area.

4.

Determine minimum rectifier voltage.

5.

Size minimum rectifier current and voltage (see Section 1352, Equation 1300-7).

6.

Select standard rectifier size.

7.

Provide design drawings and specifications.

8.

Perform cost analysis.

9.

Provide engineering cost estimate.

The following two simple examples illustrate the fundamentals of the cathodic protection design procedure for tank bottoms. The formulas and considerations required for complex system design are beyond the scope of this manual. Where necessary, the references listed will explain the design formulas.

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1352 Example 1: Design of a Galvanic Anode System for a Tank Without HDPE Membrane (Sand or Earth Foundation) The tank is 30 feet in diameter. Soil resistivities measured around the tank range between 1000 and 1800 ohm-cm.

Current Requirements Current requirement tests were performed and the estimated current to provide protection is 1.0 ampere. If current requirements were calculated using a current density of 2.0 mA/ft2 the estimated current would be 1.41 amperes. Ground Bed Resistance. Having determined current requirements from field testing we next calculate a maximum allowable ground bed resistance. The total series equivalent electric circuit must be considered (see Figure 1300-8). Kirchoff’s Voltage Law can be used to develop a relationship for estimating total circuit resistance. Ea – Ec I = --------------------------------R a + Rw + Rc ∆E = ------Rt (Eq. 1300-1)

where: I = current required for cathodic protection, amps Ea = anode-to-electrolyte (soil) open circuit potential, volts (Figure 1300-8) Ec = structure-to-electrolyte (soil) polarized potential, volts (Figure 1300-8) Ra = anode-to-electrolyte (soil) resistance, ohms Rt = maximum ground bed resistance, ohms Rw = anode lead wire resistance, ohms Rc = structure-to-electrolyte (soil) resistance, ohms ∆E = driving voltage, volts Another expression of Equation 1300-1 is: ∆E R t = ------I For this example a 32-pound prepackaged high potential magnesium anode has been selected for the design. The driving voltage ∆ • E for this anode is 0.9 volts (see Figure 1300-7).

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So, using Equation 1300-2, the maximum permissible resistance Rt for 1.0 amps is: 0.9 Rt = ------- = 0.9 ohm 1.0 (Eq. 1300-2)

For galvanic cathodic protection the anode lead wire resistance (Rw) is usually small and can be neglected. The structure-to-electrolyte resistance for uncoated structures is also so small it can be neglected. Hence, for most cases Rt = Ra. Fig. 1300-8 Cathodic Protection Equivalent Electric Current

Number of Anodes The ground bed resistance of single and multiple anodes in parallel has been derived by Dwight [2]. These equations can be used to determine the number of anodes required to satisfy the maximum allowable ground bed resistance. In simplified form, the resistance of one anode (R1) or of any number of vertically-placed anodes (Rn) can be calculated using the following equations: 8L 0.0052ρ R 1 = -------------------- ln ------ – 1 d L (Eq. 1300-3)

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0.0052ρ 8L 2L R n = -------------------- ln ------ – 1 + ------- ln ( 0.656n ) nL d S (Eq. 1300-4)

where: ρ = soil resistivity, ohm-cm L = length of backfill column, ft (see Figure 1300-9) d = diameter of backfill, ft (see Figure 1300-9) Rn = combined resistance of n anodes, ohms n = number of anodes S = center-to-center spacing between anodes, ft Equation 1300-5 is a simplified form of Equation 1300-4 and can be used if S is greater than 6L and n is less than 15. If S is not greater than 6L or n is not less than 15, then Equation 1300-5 must be used. 8L 0.0052ρ n = -------------------- ln ------- – 1 d Rn L (Eq. 1300-5)

From Figure 1300-9, The maximum ground bed resistance Rn is 0.9 ohm. Therefore, the number of anodes required for the system is: 8 ( 1.91 ) 0.0052 ( 1800 ) n = --------------------------------- ln ------------------ – 1 0.667 0.9 ( 1.91 ) =11.6 anodes (use 12) It follows that twelve 32-pound high potential magnesium anodes are required to keep total circuit resistance below 0.9 ohms. Once the number of anodes required for current output has been determined we then must determine if the anodes will provide the required design life, 20 years in this case. Wn D = --------QI a (Eq. 1300-6)

where: D = design life, years W = weight of anode, lb. Use 32 lb (from Figure 1300-9). n = number of anodes. Use 12. Q = actual consumption rate, lb/amp-yr. Use 17.5 (from Figure 1300-7). Chevron Corporation

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Ia = current required for cathodic protection, amps. Use 1.0. then: ( 32 ) ( 12 ) D = ---------------------------- = 21.9 years ( 17.5 ) ( 1.0 ) Fig. 1300-9 Magnesium Anode Dimensions and Shipping Weights

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These calculations show that a design utilizing twelve 32-lb magnesium anodes will provide the desired current output for the proposed 20-year design life. If the design life (L) is not met the engineer would rearrange Equation 1300-6 and solve for n to achieve 20-year life. Lives for various sizes of magnesium anodes are shown as a function of current output in Figure 1300-10. The engineer must now produce design drawings (Figure 1300-4) and specifications for equipment. An engineering cost analysis is developed later in this section. Fig. 1300-10 Magnesium Anode Life Versus Current Output

1353 Example 2: Design of an Impressed Current System The tank is 100 feet in diameter with a current requirement of 8.32 amperes and soil resistivity between 1000 and 1800 ohm-cm. This design differs from that of the galvanic system in having no fixed voltage, but a variable voltage derived from an external dc rectifier. Therefore, anode resistance to earth constitutes a significant role in the design. In this case 3-inch by 60-inch graphite anodes will be placed and backfilled in 10-inch diameter, 10-ft deep, vertically augured holes (typical hole dimensions). Subject to economic analysis, a total anode bed resistance (Rn) of less than 2 ohms is typically desirable to provide lower energy costs. Therefore, the number of

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Corrosion Prevention and Metallurgy Manual

anodes required to provide a maximum 2-ohm anode bed can be calculated by substituting the following data into Equation 1300-5 and solving for n: ρ = soil resistivity, ohm-cm. Use 1800 (given field measurement). L = length of backfill column, ft. Use 7 (from Figure 1300-11). d = diameter of backfill, ft. Use 10 in = 0.833 ft (from Figure 1300-11). then, solving Equation 1300-5: n = 2.14 anodes (use 3) Now, will three anodes provide the needed 20-year design life at the required current level? Using Equation 1300-6, where: W = weight of anode, lb. Use 25 (from Figure 1300-11). n = number of anodes. Use 3. Q = actual consumption rate, lb/amp-yr. Use 1.5 (from Figure 1300-7). Ia = current required for cathodic protection, amps. Use 8.32. then: ( 25 ) ( 3 ) D = ---------------------------- = 6.0 years ( 1.5 ) ( 8.32 ) Since the three anodes will not meet the required design life, additional anodes must be used. To determine the minimum number of anodes required for a 20-year life we will rearrange Equation 1300-6 and solve for n: DQI a ( 20 ) ( 1.5 ) ( 8.32 ) n = ------------- = --------------------------------------W 25 = 9.98 anodes Now, both conditional statements—bed resistance and design life—have been satisfied. Size the rectifier by calculating the final bed resistance (R) with the 10 anodes required for design life using Equation 1300-3. ( 0.0052 ) ( 1800 ) R n = -------------------------------------( 10 ) ( 7 )

(8)(7 ) ln ---------------- – 1 0.833

= 0.429 ohms Equation 1300-3

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Rectifier voltage output can now be determined using Ohm’s Law: V = I Rt (Eq. 1300-7)

where: I = 8.32 amperes Rt = Rn = 0.429 ohm then: V = (8.32)(0.429) = 3.57 volts Therefore, the minimum rectifier output to provide adequate current and voltage is a 3.57-volt, 8.32-ampere unit. Other resistances, such as cable resistance (Rw), have not been considered and should be determined in the final analysis. Whenever possible, the rectifier rating should be determined by test after the ground bed is installed. See Figure 1700-10. Fig. 1300-11 Graphite Anode Dimensions and Shipping Weights (For reference only. Consult the anode manufacturer for specific instructions.)

Figure 1300-4 shows a typical layout drawing with anodes placed around the perimeter of the tank 10 feet from the outside wall. The drawing also shows a No. 8 HMW/PE cable connecting the anodes around the perimeter. The calculated length of No. 8 cable around the perimeter is approximately 380 feet. The resistance of No. 8 cable is 0.000822 ohm/ft (See Figure 1300-12), resulting in a resistance (R) equal to 0.312 ohms.

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Corrosion Prevention and Metallurgy Manual

Fig. 1300-12 Cable Resistance Table

Since the cable is looped and both ends terminate at the rectifier the actual resistance is R/2 or 0.156 ohms. If the cable resistance is added to R the total circuit resistance is 0.585 ohm. Therefore, the new voltage requirement is 4.87 volts. Sizes for various rectifiers can be obtained from manufacturers’ catalogues. Standard rectifiers are available in various outputs. The type of rectifier (i.e., air-cooled, oil-cooled, explosion-proof) must be determined for cost analysis. For the impressed current system a standard 8-volt, 12-ampere rectifier powering ten graphite anodes with coke breeze backfill, placed vertically around the tank perimeter should provide the necessary design life. A conceptual drawing of a typical installation is shown in Figure 1300-4. High-silicon cast iron anodes could also be used for this installation. Design information is given in Figure 1300-13.

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Fig. 1300-13 High Silicon Cast Iron Design Information

1354 Example 3: Tank with HDPE Membrane (New Tank or Retrofit) This design concept is rather unique for cathodic protection. Since the anode must be placed between the 80-mil HDPE membrane and the steel tank, location and placement of the anode for proper current distribution is critical. Because of the high resistivity of the backfill, analysis indicates that the use of an impressed current cathodic protection system will provide a more adequate current distribution than a galvanic anode system. A typical design layout for this system is shown in Figure 1300-5. The anodes suitable for this installation are steel plates placed within the backfill. From predesign data, soil resistivity values on saturated backfill averaged 50,000 ohm-cm. In this example a 100-foot diameter tank will be used for analysis.

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The steel bottom surface area is 7854 ft2. Using a design current density of 2.0 mA/ft2 the estimate current required for cathodic protection is 15.7 amperes. The anode in this design is a circular plate of 1/8-inch steel. Steel plate 1/8-inch thick is much thicker than required, but is used to withstand construction vehicle traffic during placement of backfill. The resistance of the anode placed in this environment can be calculated using Equation 1300-8. ρt ( 0.002734 ) R = --------------------------------- + 0.10 A (Eq. 1300-8)

where: R = resistance of soil backfill ρ = soil resistivity, ohm-cm. Use 50,000 (given measurement). t = thickness of backfill, in. Use 6 (assumed in this example. See actual foundation design). A = area of tank to be protected, ft2. Use 7854. Note 0.1 ohm is used to compensate for resistance between anode and soil, resistance between tank bottom and soil, and cable resistances. then: ( 50, 000 ) ( 6 ) ( 0.002734 ) R = ---------------------------------------------------------- = 0.204 ohms 7854 Now the rectifier voltage can be calculated using Equation 1300-7: V = IRt = 15.7 (0.204) = 3.21 volts Therefore the minimum rectifier output to provide adequate current and voltage is 3.21 volts at 15.7 amps. A cost analysis is provided later in this section.

1355 Example 4: Deep-Well Anode Cathodic Protection Design The use of deep-well anodes has proven very effective in protecting tank bottoms in tank farms. This type of system may be combined with surface anode beds. A deepwell anode system allows the designer to incorporate remote anodes in limited areas. An anode hole is bored to depths of 100 to 400 feet. Typical hole diameters range between 6 inches and 12 inches. Impressed current anodes, such as graphite or Durichlor 51, are lowered to the bottom and spaced accordingly above each other in the deep-well (Figure 1300-14). The number of anodes required is calculated as described in the previous section. There are limits on the number of anodes that can

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be placed into a deep well. Typically, 15 anodes is the upper limit for each well. See Section 1200 for further discussion of deep-well ground beds. Fig. 1300-14 Deep - Well Anode

The designer should be aware of subsurface geological strata in the area where the deep-well anode is to be located. Areas having surface strata of high resistivity and deeper strata of low resistivity (where the anodes are to be placed) may cause shielding of the current from the deep anode to the steel tank bottom. See Figure 1300-12. The design of deep-well anode systems requires more detailed analysis than is presented in this manual.

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1360 Cathodic Interference Cathodic protection interference is the undesirable current pickup and discharge from a buried structure caused by cathodic protection applied to a tank or tank farm. It is only a problem for the impressed current cathodic protection system, with its higher operating voltage. The current pickup on an adjacent unprotected structure will discharge at some point to the protected structure, resulting in corrosion deterioration of the unprotected structure. A more detailed description can be found in Section 1750. In the case of tanks and tank farms, transfer piping or underground electrical systems may be subjected to interference. Potential testing following the application of cathodic protection can establish the degree of interference. Mitigative procedures can be extensive and may include bonding of the affected structure to the cathodically protected structure or diode blocking of current to the protected structure. Mitigation techniques are generally not straightforward and are outside the scope of this section. The engineer should be aware of the possibility of interference on adjacent structures.

1370 Cost Analysis The simplest method of choosing the best of several available alternatives is to determine which has the lowest cost. For a fair evaluation, the alternatives must be compared over equivalent life cycles. They must also be providing similar corrosion protection. The most economical system is the one which has the lowest present worth. The following examples (Figure 1300-15) provide guidelines on estimating the costs of several galvanic anode and impressed current systems. They need to be corrected for your specific use.

1380 References

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1.

Corrosion Control. Air Force Manual No. AFM88-9. United States Air Force, 1962.

2.

Dwight, H.B. “Calculations of Resistance to Ground.” AIEE Transactions, Volume 55, 1319 – 1328 (1939).

3.

Sunde, E.D. Earth Conduction Effects in Transmission Systems. D. Van Nordstrand Company, New York, 1949.

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Fig. 1300-15 Design Example Cost Analysis (1 of 2) DESIGN EXAMPLE 1: SAND FOUNDATION—GALVANIC ANODE Tank Diameter = 30 ft. Anode Diameter = 50 ft. (10 ft. from tank) Anode Circumference = 157 ft. Material

Unit

Total

12 - 32 lb Anodes with 10 ft. #12 Wire

$102.00/ea.

$1,250.00

12 - Splice Kits

$25.00/ea.

$300.00

1 - Test Station

$18.00/ea.

$20.00

$0.25/ft.

$50.00

$80.00/ea.

$1000.00

$3.00/ft.

$600.00

177 ft. - #8 HMW/PE Cable Installation 12 - Augured Holes 10 ft. × 12" Dia. 177 ft. - Trenching/Backfill Total Installed Cost

$3,220.00

Maintenance Inspection (1 manhour/month) x (12 months/year) x ($25.00/manhour) =

$300.00/yr

Annual Corrosion Survey

$200.00/yr

DESIGN EXAMPLE 2: SAND FOUNDATION—IMPRESSED CURRENT Tank Diameter = 100 ft. Anode Diameter = 120 ft. (10 ft. from Tank) Anode Circumference = 377 ft. (Add 20 ft. to Rectifier) Material

Unit

Total

10 - 3" x 60" Graphite Anodes

$132.00/ea

$1,320.00

1 - 8 volt 12 amp Rectifier

$560.00/ea.

$560.00

10 - Splice Kits

$25.00/ea.

$250.00

$0.25/ft.

$100.00

$80.00/ea.

$800.00

$3.00/ft.

$1,191.00

397 ft. - #18 HMW/PE Cable Installation 10 - Augured Holes 10 ft. X 12" Dia. 397 ft.- Trenching/Backfill Total Installed Cost

$4,200.25

Maintenance Inspection

$300.00/yr

Annual Corrosion Survey Power Consumption 4 volts x 8.32 amps = 33.28 watts

$200.00/yr

33.28w x (8760 hr/year) x (1kw/1000w) x 0.05/Kwhr =

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Fig. 1300-15 Design Example Cost Analysis (2 of 2) DESIGN EXAMPLE 3: HDPE MEMBRANE BENEATH TANK—IMPRESSED CURRENT Tank Diameter = 100 ft. Steel Plate Diameter = 97 ft. Material

Unit

Total

$1.53/ft.2

$11,300.00

300 ft. - #8 HMW/PE Cable

$0.25/ft.

$75.00

4 - Cadweld connections

$10.00/ea.

$40.00

$1000.00/ea.

$1,000.00

$200.00/400 ft.2

$3,695.00

$6.50/ft.

$1,950.00

4 - Cadweld connections

$50.00/ea.

$200.00

1 - Rectifier

$500.00/ea.

$500.00

7390 ft.

2 - 1/8" thick Steel Plate

1 - 12 volt, 20 amp Rectifier Installation 7390 ft.2 - Steel Plates Set 300 ft. - Welding

Total Installed Cost

$18,766.00

Maintenance Inspection

$300.00/yr

Annual Corrosion Survey Power Consumption 4 Volts x 20 amps = 80 watts

$200.00/yr $35.04/yr

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