presentacion de proteccion catodica diseños calculos

1. Cathodic protection design

2. Cathodic protection prevents corrosion by converting all of the
anodic (active) sites on the metal surface to cathodic (passive)
sites by supplying electrical current (or free electrons) from an
alternate source
Cathodic protection shall be design with due to environment
condition, neighbouring structures and other activities
The general design procedure for both sacrificial anode and
impressed current systems is similar
Cathodic protection design

3. 1. Initial considerations
Modifications to the structure to incorporate requirements are best
made at the early design and pre-construction phase of the
structure
For underground structures it may be necessary to visit the
proposed site, or for pipelines the proposed route, to obtain
additional information on low-resistivity areas, availability of
electric power, and the existence of stray dc current or other
possible interaction.
It is common practice for a survey to be made before design

4. This survey is often combined with a study to establish economic
justification for the recommended anti-corrosion proposal while the
principal data necessary for design (chemical and physical) are
also collected
If the structure already exists, measurement of existing structure-to-
soil potentials is essential to give valuable information as to which
areas are anodic and which are cathodic
If the new structure, the design of a cathodic protection system
should include the calculation of:
Current demand
Resistance to earth of the anodes
Quantity and location of anodes
Electrical supply requirements
Test and monitoring facilities

5. 2. Drawings and material specifications
Engineering drawings:
• To establish the size and shape of the structure to be
protected in order to design an effective cathodic protection
system
Site drawings:
• To check all other metallic structures in the vicinity presence
may affect the operation of the system being designed
Material specifications:
• To establish material and surface conditions, particularly the
presence and quality of protective coatings.

6. 3. Site survey
To establish the actual environmental condition
• Media characterizations
• water chemistry
• pH measurement
• soil chemistry
• Current requirement test
• Availability power supply

7. Water chemistry:
Samples of water should be analyzed for pH value, amount of
aggressive anions such as chloride, sulfate and other chemical
constituents, and resistivity value
Water containing such chemical constituents can affect the
current requirement necessary for protection
pH measurement:
To validate of the acidity or alkalinity of the media (soil or water)
Soil pH is measured of the pH value of water contained in the soil
pH range: 0 – 7 being acidic, 7 being neutral & 7 – 14 being
alkaline

8. Soil chemistry
• Soil resistivity
It is important used in the design of cathodic protection for
underground structures.
Soil resistivity closely related to soil corrosivity
Higher resistivity being associated with low corrosivity
• Chemical constituents
Chemical constituents such as sulfide, sulfate, chloride and
others should be analyzed prior to cathodic protection system
design
• pH measurement

9. • Made of Plexiglas
• Rounded corners for easy cleaning
• Current plates are made of stainless steel
• Potential pins made of brass and easily
removed
Soil box
4-Pin Soil Resistance Meter
Model 400 by NILSSON
Control Panel Features

10. Resistivity of soil or water by soil box
method using an earth resistivity
measuring set, such as Nilsson Model
400 Soil Resistance Meter.
Schematic diagram for media resistivity measurement
P1 P2
C1 C2

11. L
L
WD
R


Notes: W, D, L in cm
R = resistance
= resistivity
P1 & P2 = potential connection
C1 & C2 = current connection
Schematic diagram soil box

12. Corrosivity ratings based on soil resistivity

13. Current requirement test
• The current requirement should be determined at the site
being to install cathodic protection system
• Need to install a temporary cathodic protection system
• The current supplied and the structure – to – electrolyte
potential results will be used to establish the required current
to protect the structure
Availability power supply
• Need AC power supply for ICCP
• Near AC power supply at site for cost saving (instead install
new ac power supply)

14. Temporary cathodic protection system for determining
current requirements
DC milliammeter
Rectifier
Adjustable resistor
+
-
Temporary anode
Structure to be protected
Soil surface
v
DC voltmeter
Cu/CuSO4

15. 4. Current density requirements
• It can be measured from a temporary cathodic protection
system
• The current densities shall be used for steel, stainless
steel and other metallic materials
• The total amount of current required is determined by
multiplying the required current density by the area of the
structure to be protected
• In the case of well coated structure, the amount of current
required can be two orders of magnitude less than the
current required of the same uncoated structure

16. 5. Selection type of cathodic protection systems
• For selecting between SACP or ICCP systems is based on
feasibility and cost factors.
• Cost factors include operating, maintenance, and
appropriate replacement
• Feasibility factors for example the systems required small
stable current (normally consider protection by SACP) or
large current (protection by ICCP)

17. 6. Sacrificial anode design
• Determination of the total current required either from
actual current requirement measurements or by multiplying
a typical current requirement by the surface area of the
structure to be protected
• Calculation of the individual anode current, Ia (A), required
to meet the current demand, Ic (A), is followed Ohm’s law

18. Ohm’s law
a
o
a
o
a
o
c
a
c
R
E
R
E
E
I
N
I





)
(
.
Where,
N = number of anodes
= the design protective potential 0.8V (relative to
Ag/AgCl/seawater reference electrode, accepted for carbon
and low-alloy steel
= the design closed circuit potential of the anode (V)
Ra = the anode resistance (Ohm)
o
c
E
o
a
E

19. Calculation anode resistance
The anode resistance, Ra (ohm), to be used shall be based on
the applicable formulas.
Anode type: Long slender stand-off (L 4r)







 1
.
4
ln
.
.
2 r
L
L
Ra


Note:
1- This equation is valid for anodes with minimum distance 0.30m from protection object.
However for anode-to-object distance less than 0.30m but minimum 0.15m the same
equation may be applied with a correction factor of 1.3.
2- For non-cylindrical anodes: where c(m) is the anode cross sectional periphery

2
c
r 
Where,  is media resistivity (ohm cm)
L is length of anode (cm)
r is equivalent radius of anode (cm)
R is anode resistance (ohm)

20. Anode type: Short slender stand-off (L 4r)












































2
2
2
1
2
2
1
1
2
ln
.
.
2 L
r
L
r
L
r
r
L
L
Ra


Anode type: Plate anode (Long flush mounted hull or bracelet
anodes (L  width, L  thickness)
S
Ra
.
2

 Where, S is mean length of anode sides (cm)
S
Ra
.
4

 If the flat plate anodes are close to the structure
or painted on the lower face
2
b
a
S

 , where b  2a

21. Anode type: Plate anode (Short flush-mounted hull, bracelet and
other types)
A
Ra

315
.
0

Where, A is exposed area of the anode (cm2)

22. Calculation of total anode weight
Total required anode weight, mTA,(or mass) based on the average
total current demand, Ic, is calculated according to the following
equation:
a
c
TA
C
U
T
I
W
.
8760
.
.

Where, T is lifetime (yr)
Ic is total current demand (A)
U is utilization factor for the anode
Ca is anode capacity (Ah/kg)
8760 is # hours/yr

23. Calculation of consumption of anode
hour
Ampere
factor
n
Utilizatio
x
weight
Gross
rate
n
Consumptio



24. Calculation of number of anodes
Where, N is number of anode
Id is current demand (ampere)
Tis design life (year)
Ca is anode capacity (Ah/kg)
Wa is anode weight (kg)
8760 is # hours/yr
p
d I
A
I .
 Where, A is surface area of structure to be protected (m2)
Ip is current density required (ampere)
a
a
d
C
W
T
I
N
.
.
8760
.


25. Example: Calculating number of anodes for a buried steel
pipeline
1. If we assume the pipeline length to be 100 meters, and the
O.D. of the pipe to be 0.17 m. The area to be protected is the
outside area of the pipe. We will assume the pipeline is
uncoated, but coating will alter the calculations. The area is:
53.4 m2.
2. Tables of current density requirements have been found to be
in the range of 10 - 60 mA/m2. (F.W. Hewes, Cathodic
Protection Theory and Practice, V. Ashworth and C.J.L.
Booker, eds., Wiley (Horwood), Chichester, West Sussex, p.
226, 1986.) For our example we will assume a current density
requirement of 40mA/m2.

26. 3. Current demand
mA
m
mA
x
m
I
A
I p
d 2136
/
40
4
.
53
. 2
2



4. The output for zinc anodes is 810Ah/kg, and the efficiency is
normally taken as 90%. Thus, the useful output of zinc is
729 Ah/kg
5. Design life to be protected is 20years
6. Total anode weight:
kg
Ah
x
yr
hrs
x
yrs
x
A
C
U
T
I
W
a
c
TA
/
810
9
.
0
/
8760
20
136
.
2
.
8760
.
.


kg
WTA 514
 for protecting 100m of pipeline

27. 7. Every meter required:
anodes
kg
W
W TA
a 14
.
5
100
514
100



8. Therefore, number of anodes required should satisfy both of
the following:
100
14
.
5
514



a
TA
W
W
N anodes

28. OR
100
87
.
99 

N anodes
yr
Ah
x
kg
yrs
x
yr
hr
x
A
C
W
T
I
N
a
a
d
/
729
14
.
5
20
/
8760
136
.
2
.
.
8760
.


NOTE: 100 ANODE REQUIRED FOR PROTECTING 100M
PIPELINE & EACH METRE SHALL BE INSTALLED ONE ANODE
WITH WEIGHT OF 5.14KG

29. 7. Impressed current design
Three steps shall be taken in designing of impressed current
cathodic protection:
I) Total current
II) Total resistance
III) Voltage and rectifier

30. I. Total current
Same as for sacrificial anode cathodic protection system
Determination of current requirement from the actual
current measurement or by multiplying a current by the
surface area of the structure to be protected
II. Total resistance
The major factor in the determination of the total circuit
resistance is the anode-to-electrolyte resistance
It is also known as "ground bed resistance," and this is
often the highest resistance in the impressed current
cathodic protection system circuit

31. III. Voltage and rectifier
Using the total circuit resistance and the current required,
the appropriate voltage for the rectifier is then calculated as
below:
R
I
E 
Where,
E is required voltage
I is required current
R is total circuit resistance

32. Equivalent circuit

33. The total circuit resistance is:
R = Rc+ + Rc- + Rs + RE + RA
Rc+ + Rc- is The resistance of the positive and negative cables will
be dependent on the length and cross sectional area of
the conductor
RS : The resistance of structures such as platforms may be ignored.

34. RE : The cathode to electrolyte resistance may be calculated
using ohms law:
I
E
R 
E is the change of the structure-to-electrolyte potential to achieve
cathodic protection (usually 1/3 to 1 Volt) and I is the total
current requirement in amperes.
RA : The anode-to-electrolyte resistance will be dependent on the
shape, number, and spacing of the anodes used, and the
electrolyte resistance