MIL-HDBK-1004/10
resistance that is encountered is sometimes sufficiently different from the
calculated value to require adjustment or modification of the system. The
actual anode-to-electrolyte resistance can also be determined by actual field
measurements.
a) Anode First Method. In this method of determining anode-to-
electrolyte resistance, the anodes are installed as designed and the actual
resistance between the anode or anode bed and the structure to be protected is
measured. This measurement includes both the anode-to-electrolyte resistance
and the structure-to-electrolyte resistance and can be used to determine the
required driving potential so that the proper power supply can be ordered.
This is the most accurate method of sizing the needed rectifier and should be
used where practical.
b) Power Supply First Method. In this method, the power supply is
ordered based upon the calculated circuit resistance and is installed and
connected to the structure. The anodes are installed as planned, but one at a
time. The total circuit resistance is calculated based upon the actual power
supply output in amperes and volts. If additional anodes are required in
order to achieve the desired anode-to-electrolyte resistance, they can be
installed at this time at a relatively low cost since the equipment required
for installation is on site and excavations for the anode lead cables are
open.
Effect of Backfill. Backfill is very important and is usually used
6.2.1.6
to surround impressed current anodes in order to reduce anode-to-electrolyte
resistivity, to increase porosity around the anodes to insure that any gasses
formed during operation will be properly vented, and to reduce polarization
effects and reduce localized dissolution of the anode. Under favorable
circumstances, the anode-to-electrolyte resistivity can be reduced to one-half
through the use of backfill. In extremely low resistance environments such as
seawater, graphite and high silicon cast iron anodes can be used without
backfill; otherwise, impressed current anodes should always be used with
backfill. In high resistivity environments where the use of backfill is
impractical, graphite anodes should not be used. High silicon chromium
bearing cast iron (HSCBCI) anodes can be used with or without backfill in most
instances. The cost of using backfill should be evaluated on an economic
basis with the reduction in the power requirements or the number of anodes
required being the cost reduction factors. If the resistivity of the backfill
is less than one-tenth the soil resistivity, then the voltage drop through the
backfill becomes negligible.
a) Thus, the effective diameter of the anode is the diameter of
the backfill rather than the diameter of the anode itself. As can be
evaluated through calculation of anode-to-electrolyte resistance, this can
result in a significant reduction in anode-to-electrolyte resistance which can
be useful in reducing the number of anodes required, the required driving
potential, or both. Backfill for impressed current anodes is carbonaceous
material from several sources. It can be either coke breeze (crushed coke),
flake graphite, or round particle petroleum coke. Experience has shown that
round particle calcined petroleum coke has many advantages over coke breeze
made from coal. Specification "Loresco DW-2" or equal should be used for
surface anode beds and Loresco DW-3 or equal for deep anode beds. Because the
material can be pumped and has good porosity and particle-to-particle contact,
round particle petroleum coke backfill is the most desirable material and its
higher cost will be justified for most installations, particularly for "deep
anodes."
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