Nace CP 1, Course Manual

Page: 411 / 535

RP0193-2001
4 NACE International
3.3.3.3 If deep groundbeds are considered,
resistivities should be analyzed using procedures
described by Barnes
3
to determine conditions on a
layer-by-layer basis. On-site resistivity data can
be supplemented with geological data including
subsurface stratigraphy, hydrology, and lithology.
Sources for geological information include water
well drillers, oil and gas production companies, the
U.S.
Geological Survey Office,
(2)
and other
regulatory agencies.
3.3.3.4 Cathodic protection current require-
ments can be estimated using test anode arrays
simulating the type of groundbed planned. Test
currents can be applied using suitable sources of
direct current.
Test groundbeds can include
driven rods, anode systems for adjacent cathodic
protection
installations, or other temporary
structures that are electrically separated from the
tank being tested.
Small-diameter anode test
wells may be appropriate and should be
considered if extensive use of deep anode
groundbeds is being considered.
Figure 2
illustrates a temporary groundbed for current
requirement testing.
3.3.4 Stray Currents
3.3.4.1 The presence of stray earth currents
may
result in cathodic protection current
requirements that are greater than those required
under natural conditions.
Possible sources of
stray current include DC-operated rail systems
and mining operations, other cathodic protection
systems, welding equipment, and high-voltage
direct current (HVDC) transmission systems.
3.3.4.1.1 Field tests to determine whether
stray currents are a concern include those
that
provide tank-to-electrolyte and
structure-to-electrolyte potential measure-
ments on adjacent structures, earth gradient
measurements, and current flow measure-
ments on tank piping and safety grounding
conductors.
3.3.4.1.2 Possible interference effects
caused by adjacent cathodic protection
systems should be determined by
interrupting the current output using a known
timing
cycle. Structure-to-electrolyte
potentials and other parameters should be
monitored over a minimum 24-hour period in
areas where dynamic stray currents or
transient effects are expected to be a
concern.
Recording instruments can be
used for this purpose. Figure 3 illustrates
stray current corrosion.
3.3.4.1.3 Cathodic protection designs
should incorporate every practical effort to
minimize electrical interference on structures
not
included in the protection system.
Predesign test results can be analyzed to
determine the possible need for stray-
current control provisions in the cathodic
protection system.
___________________________
(2)
U.S. Geological Survey Office, P.O. Box 25046. Federal Center, Denver, CO 80225.