a 35 años

If current output of a galvanic anode of any given weight is known, its approximate useful life can be calculated. This calculation is based on the theoretical ampere-hour per pound of the anode material, its current efficiency, and a utilization factor. The utilization factor may be taken as 85%—meaning that when the anode is 85% consumed, it will require replacement because there is insufficient anode material remaining to maintain a reasonable percentage of its original current output.

For magnesium, anode life may be determined by the following expression (efficiency and utilization factor expressed as decimals).

Magnesium Anode

Life (years)=0.116 × Anode Weight (pounds) × Efficiency × Utilization Factor

Design Current (amperes) .

For zinc anodes, anode life may be determined in similar manner by the following expression:

Zinc Anode

Life (years)= 0.0424 × Anode Weight (pounds) × Efficiency × Utilization Factor

Design Current (amperes) .

As an example, assume that a 32-lb magnesium anode is producing 0.1 A at 50% efficiency and that a 30-lb zinc anode is producing 0.1 A at 90% efficiency. Compare the expected operating lives at the 0.1 A output.

Magnesium Anode Life (years)= 0.116 × 32 × 0.50 × 0.85

0.1 = 15.8 yr,

Zinc Anode Life (years)= 0.0424 × 30 × 0.90 × 0.85

0.1 = 9.7 yr.

These calculations reflect the difference in theoretical ampere hours per pound charac-teristic of the two materials. Although anode costs may fluctuate with the metal market, zinc is typically less expensive than magnesium. Graphical design information has been developed for typical anode types. An example is shown inFigure 9.3.

20 30 40 70 100 200 300 400 700 1000 soils from a 17 lb. anode, 10 ft. from an 8" bare pipeline, at two different pipe-to-soil potentials.

GALVOMAG p/s = −0.70 volts p/s = −0.85 volts

Current Output of GALVOMAG^® and High Purity H-1 Anodes.

Figure 9.3 Magnesium anode design curves. (Corrpro Companies Inc.)

DESIGN CONSIDERATIONS

To provide a better understanding of the differences in performance between zinc and magnesium installations it may be helpful to first identify typical types of installa-tion practices. A general rule-of-thumb says that zinc anodes are better used in the lower soil resistivity (below 1500 ohm-cm) and magnesium anodes are better in the higher resistivity soils (between 1500 and 10,000 ohm-cm). This rule is not universal and will depend on the application. This will be illustrated by the examples presented below.

Well-coated pipeline sections typically have low current requirements and will po-larize easily to a volt or more. Either zinc or magnesium anodes may provide sufficient current for full protection, but zinc anodes may provide full protection with much less current being wasted. This is illustrated in the following example.

Assume a well-coated section of pipeline having a native or static potential to a copper sulfate reference electrode (CSE) of−0.7 V, an effective resistance-to-earth of 2 ohm at the anode installation site and that 75 mA is necessary to shift the pipeline potential to

−0.85 V. Also assume that the soil resistivity at the anode installation site is 1500 ohm-cm.

The installation circuit resistance needed to raise the potential to−0.85 V using zinc anodes is calculated by determining the driving voltage (1.1 V− 0.85 V) or 0.25 V and dividing it by the current requirement. This calculation is performed using Ohm’s Law, 0.25 V/0.075 A= 3.3 ohms. The calculated 3.3 ohms minus the effective resistance of the pipeline, estimated at 2 ohms, leaves 1.3 ohms for the resistance of anodes and lead wires. Following the procedures outlined in Chapter 7, it can be calculated that five 1.4 × 1.4 × 60-in zinc anodes (30 lb) surrounded with 50–50 gypsum-bentonite backfill in 8-in diameter holes at 15-ft spacing will have a resistance of 1.21 ohms in the 1500 ohm-cm soil. With 0.03 ohms allowed for lead wire resistance, total resistance is 1.24 ohms which is within the 1.3-ohm design allowance.

The same procedure can be derived for magnesium anodes. The magnesium in-stallation circuit resistance needed to provide the initial 75 mA requirement will be the driving potential (1.55 − 0.85 = 0.70 V) divided by the estimated current requirement of 75 mA, equals 9.33 ohms. Subtracting the 2-ohm pipeline resistance leaves 7.33 ohms for anode-to-earth resistance and lead wire resistance. Using Chapter 7 anode bed resistance procedures, one 2× 2 × 60-in magnesium anode (20 lb each) with 75% gypsum, 20%

bentonite, 5% sodium sulfate backfill in 8-in diameter hole will give 4.80 ohms anode-to-earth resistance in the 1500 ohm-cm soil with 0.01 ohm allowed for lead wire resistance.

This totals 4.81 ohms, which is within the 7.33-ohm requirement.

As shown in the previous example, the zinc anode installation number of anodes is much larger. This is not, however, the complete analysis. In most cases, a well-coated pipeline will continue to polarize after initial requirements are met. The pipeline in the zinc anode example may polarize to a potential approaching the−1.1 V open circuit potential of zinc. This will result in the reduced current demand.

Assume a polarized potential of−1.05 volt. The zinc anodes will now have a driving potential of only 0.05 V (1.1 − 1.05 V) and the current output will be 0.05-V/3.24-ohms

circuit resistance = 0.0154 A. The magnesium anodes will have a driving potential of 0.35 V (1.55 − 1.2 V) and the current output will be 0.35 V/6.81-ohms circuit resistance= 0.0513 A. These final or stabilized currents can be used to determine the useful lives of the two installations.

The five, 30-lb zinc anodes will have calculated life as follows.

Zinc Anode Life (years)= 0.0424 × 5(30) × 0.90 × 0.85

0.0154 = 315 yr.

The single 20-lb magnesium anode will have a calculated life as follows:

Magnesium Anode Life (years)= 0.116 × 20 × 0.50 × 0.85

0.0513 = 19.2 yr.

The new calculated design life for each anode is at the efficiencies shown in the for-mula (90% for zinc, 50% for magnesium). At very low current densities, efficiencies will decrease and actual lives would be less than indicated. This is particularly true for magnesium anodes.

To evaluate these two installations in terms of cost per year of estimated life, the installed cost must be known. Zinc is less expensive than magnesium. Assuming a con-servatively high material and installation cost per anode, the five zinc anodes could cost $250 per anode to install or $1,250. The one magnesium anode with installation on the same basis could cost $350. The indicated cost per year for the zinc installation would be $1,250/315 years= $3.96 per year. Similarly, the indicated cost per year for the magnesium installation would be $350/19.2 years= $18.22 per year.

It may appear that the indicated life of 315 years for the zinc installation is beyond reasonable expectations when the usual design life of a pipeline is 20–50 yr. If the cost per year is based on a 20-yr pipeline life, the zinc anode installation cost per year would be $62.50 (although not consumed at the end of this period) while the magnesium cost per year remains at approximately $18. This analysis favors magnesium if the estimated protection current requirements do not change.

If current requirements increase (see the following discussion on regulation), the orig-inal zinc installation can continue to provide adequate protection, whereas replacement of the magnesium anode would be required in less than 20 yr. The magnesium anode replacement would require more anodes than were used originally in order to maintain adequate protection. This will bring the cost per year to roughly equivalent figures for zinc and magnesium for the example used in the 1500 ohm-cm soil. Chapter 15 provides more detailed analysis of life cycle cost.

The previous illustration indicates that on a long-term basis, zinc anode installations can be less expensive than magnesium when calculated on a simple cost per year basis.

In the example, it was shown that one magnesium anode discharged more than twice as much current as the five zinc anodes. This offers no advantage because zinc fully protects the line while excess current from magnesium is wasted.

Inserting resistance in series in the circuit can control the wasted current from mag-nesium. In the previous example, current wastage can be eliminated if sufficient resis-tance could be inserted in series with the magnesium anode to reduce its output to the point that the pipeline would remain protected. That current level would be the same as that obtained from the zinc anode (0.030 A). The driving potential would now be 1.55 − 1.05 = 0.4 V. Circuit resistance would be 0.4 V/0.030 A = 13 ohms. This value of 13 ohms minus the circuit resistance of the magnesium anode alone, 6.81 ohms (from the example above), leaves 6.52 ohms resistance to be inserted in the circuit. By reducing the current, the magnesium anode indicated life is increased to 33 yr, reducing the cost per year of life to $10.60, which is more in line with the zinc installation. This is further discussed below.

ANODE PERFORMANCE