Using the values from Table 52 for theoretical ampere hours per newton 

 and current efficiency, along with an 85 percent utilization factor for the 

 three anode materials, the above expressions may be simplified to: 



m • r 1.105 W in - 2 on 



Magnesium L = = • 10 " (4) 



7- r 0.767 W , n -2 , n 



Zinc L = = • 10 z (5) 



..... T 2.826 W in - 2 r ^ 



Aluminum L = = • 10 * (6) 



where L is the anode life in years, W the anode weight in newtons, and 

 I the anode current in amperes. 



As may be noted, Equations (4), (5], and (6) may also be used for 

 calculating anode bed life where L is the anode bed life in years, W the 

 total anode weight in newtons (all anodes), and I the anode bed current in 

 amperes . 



(7) Deep-Well Anode Beds . Some mention should be made regarding 

 deep-well anode beds, as in recent years they have attracted much interest 

 for impressed current systems, primarily on pipelines. Such installations 

 can be very useful if conditions permit. In the case of pipelines the well 

 may be in the pipeline right of way, avoiding the requirement for additional 

 right of way for conventional surface type anode beds. A deep- well anode 

 bed, usually 60 to 120 meters (200 to 400 feet) in depth, can be described 

 as one in which the anodes are placed in remote earth by drilling straight 

 down or by using an existing hole such as an abandoned water well. For 

 pipelines this accomplishes the same general result obtained by locating a 

 conventional surface type anode bed laterally several hundred feet from the 

 pipeline. Advantages of a deep-well anode bed include small surface space 

 needed (little or no additional right of way), probably less interference 

 problems, and frequently lower anode-to-soil resistance than with conven- 

 tional anodes. Disadvantages include great difficulty or impossibility of 

 repair, necessity to prevent contamination of underground potable water 

 sources, difficulty in determining soil resistivity at depths of several 

 hundred feet, and expense of installation. 



(8) Application of Calculation Methods . With the preceding back- 

 ground on design considerations, some examples follow to show how designs 

 may be worked out for several types of subsurface structures. Professional 

 consultation is advisable before finalizing plans for any cathodic pro- 

 tection installation. Each location has specific problems which must be 

 recognized and considered if the installation is to be effective and 

 reasonably trouble free. 



c. Example Project . A part of the waterfront facility consists of a 

 steel sheet-pile bulkhead (see Fig. 104 for cross section). It shows a 

 typical seawater cross section, illustrating the various zones of exposure. 

 The waterside of the bulkhead should be provided with cathodic protection 

 as soon as possible to prevent further loss of steel caused by the corrosive 

 action of the contacting seawater. Average water resistivity is 20 ohm- 

 centimeters. The soil side of the bulkhead is also to be provided with 

 cathodic protection at an early date. Average soil resistivity is 500 ohm- 

 centimeters. A minimum of 20 years life for the waterfront facility is 

 anticipated. ' 



368 



