Nace CP 1, Руководство по эксплуатации

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RP0176-2003
40
NACE International
The final current density (75 mA/m
2
) is obtained from Table
A1.
The initial current calculation required 175 anodes, the
mean current density requirement required 99 anodes, and
the final current calculation required 146 anodes. For this
application the proper number of anodes to use would be
175 anodes. However, the difference between this value
and the calculated mean current requirement of 99 anodes
is significant. Another anode shape that provides less vari-
ation between the three requirement values is often more
economical to install.
Dwight’s equation is valid when 4L/r
≥
16; for anodes when
4L/r
<
16 or for anodes that do not approximate cylindrical
shapes, equations such as Crennell’s (McCoy’s) (see Equa-
tion [D9]) or other versions of Dwight’s may better predict
the actual current output of the anodes. Theoretically, for a
deep sea submerged cylindrical anode, a more nearly cor-
rect equation would be as shown in Equation (D8):






−






=
1
r
2L
ln
L
K
ρ
R (D8)
However, the first version is more widely used in CP
practice.
Notes:
(a) For practical designs and to ensure adequate current
to protect the structure during the life of the anode, the
length (L) and radius (r) should be selected to show the
condition of the anode when it is nearly consumed. For an
elongated anode, the change in length may be ignored.
(b) If the structure potential rises above the minimum pro-
tection potential of -0.80 volt (Ag/AgCl [sw]), E becomes
less than 0.25 V. This decreases anode current output and
increases anode life.
(c) The anode net weight must be sufficient to provide the
calculated current for the design life of the system, in accor-
dance with the actual consumption rate of the anode mater-
ial selected (see Appendix B and Table B1).
(d) Crennell’s (McCoy’s) formula is shown in Equation
(D9)
40
:
A
315 . 0
R
ρ
=
(D9)
where:
R = resistance in ohms
ρ
= resistivity of water in ohm-cm
A = area of exposed anode in cm
2
________________________________________________________________________
Appendix E: Typical Method for Calculation of Galvanic Anode Current Output Using Design Slope and
Maintenance Current Density
This method is proposed as an alternative to calculation of
the initial, mean, and final current densities (Appendix D). It
affords the advantage of being first principles based such
that undisclosed over-design, as may be inherent to the
preceding method, may be avoided. This CP system
design for the seawater-exposed sections of an uncoated
structure using the design slope follows these steps
41
:
1. Determine the required design life for the structure, T.
2. Based on the anode material to be used and the appli-
cation environment, select a value for the anode consump-
tion rate, k (Note: a different symbol for anode consumption
rate is used elsewhere).
3. Specify the water resistivity,
ρ
. For large structures that
span zones of varying resistivity, separate designs may be
executed for different regions of the structure.
4. Based on prior experience or on experimental data at
the site of interest, select values for the design slope, S,
and for the maintenance current density, i
m
, which results
from that design slope. Again, separate designs may be
executed for different regions of the structure.
5. Determine the value of the right-hand side of Equation
(E1). This value then describes the anode size and shape
alternatives that may be used to achieve the selected
design, as dictated by the left-hand side.
TkS i w R
m a
=
(E1)
where:
R
a
= resistance of single anode to remote seawater
(for example, Equation [D1]) in ohms
w = weight of a single anode in kg
i
m
= maintenance current density in A/m
2
T = design life in years
k = anode consmption rate in kg/A-yr
S = design slope in ohm-m
2
(many or all of these
terms are defined elsewhere)
6. Iteratively determine a suitable anode for the applica-
tion:
(a) Select a candidate anode, and from its dimensions
calculate its resistance, R
a
, and its mass, w.