Location of Other Structures in the Area. As the presence of
interfering structures is very important in the design of cathodic protection
systems, a site survey should be performed to locate any such structures.
Buried structures such as pipelines which have risers, valves, etc., in the
area can be traced using electronic tracers. The presence of buried
structures in the area that do not have any surface indications are difficult
to locate. This factor makes the maintenance of accurate activity records
showing all buried structures and utilities extremely important.
Availability of ac Power. The availability of ac power at the site
should be determined as the cost of installing new ac power lines may be an
important factor in determining whether a sacrificial or impressed current
system is the most practical for a specific application.
Current Requirements. The most desirable means of determining the
amount of current required for protection of a structure is to measure the
actual amount of current required to achieve protection through installation
and operation of a temporary cathodic protection system as shown in Figure 8.
For the design of cathodic protection systems to be installed in conjunction
with the installation of a new structure the current requirements can be
estimated from following Tables 1 and 2. Table 1 gives typical current
requirements for uncoated steel structures. The values are for coated steel
structures and include allowance for typical coating efficiencies.
The total amount of current required is determined by multiplying
the required current density by the area of the structure to be protected. As
can be seen by comparing Table 1 with Table 2, the currents required to
protect a well coated structure can be two orders of magnitude less than the
current required to protect the same structure if it is uncoated. The reduced
amount of current required for the protection of well coated structures
reduces the cost of protection as well as reducing other problems such as
Choice of Sacrificial or Impressed Current System. The decision
between using sacrificial anode or impressed current cathodic protection
systems is based upon two major factors, feasibility and cost. Often, a
cathodic protection system using both methods is designed for a given
structure and the systems are directly compared in order to select the most
appropriate type of system. Economic analysis to determine the least cost
system should include considerations of mission life, operating costs,
maintenance costs, and cathodic protection system replacement costs as
appropriate. In general, systems with small stable current requirements (0.5
A or less per 100 lineal feet of structure) are more likely to be protected
using sacrificial anode type systems. Those structures with larger current
requirements (1 A or more per 100 lineal feet of structure), or where the
current requirements vary considerably with time, are more likely to be
protected using impressed current systems. Other advantages of sacrificial
anode systems are the lesser amount of required maintenance and reduced levels
distances. Impressed current systems are generally used where amounts of
current larger than can be supplied by a sacrificial anode system are
required. This may be due to high electrolyte resistivity which limits
sacrificial anode output, or to high current requirements associated with
protection of large or poorly coated structures.