Transcript 4.1.5 CETP Instructor PowerPoint Template

4.1.5
Determining the Proper Corrosion Protection
Systems for Underground ASME Tanks and
Buried Distribution Lines
Corrosion of underground tanks and piping is a significant
problem. However, the corrosion process is to a great
extent prevented by using a corrosion protection system.
In this module you will learn to identify:
(1) The cause of corrosion
(2) Methods and procedures used to protect buried tanks and
metal distribution lines from corrosion
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 1
Identifying the Cause of Corrosion
Corrosion is an electrochemical reaction between a metal and its
environment.
This damaging reaction occurs between the metal surface on the
underground ASME tank or metallic piping and the soil that
surrounds it.
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 1
Methods and Procedures Used to Protect Buried
Tanks and Metal Distribution Lines
Methods commonly used to protect buried tanks and metal
piping from the effects of deterioration by corrosion are:
1. Coatings
2. Insulated fittings
3. Impressed current (cathodic protection), typically,
sacrificial anode(s)
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 1
Methods and Procedures Used to Protect Buried
Tanks and Metal Distribution Lines
Coatings— Specially formulated coatings protect buried tanks and
metal pipe against corrosion.
Manufacturers typically coat underground tanks as part of the
manufacturing process to ensure a long and safe life of the buried
tanks.
Because tanks may be damaged during transport or installation,
additional coating may need to be done at the plant or customer
location to repair “holidays”. (A holiday is a void in the protective
coating that exposes the metal surface of the tank.)
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 1
Requirements for Properly Applying & Maintaining
Tank Coatings
•
Properly Cleaned Surface
•
Proper Priming and Coating Materials
•
Proper Application of Coatings
•
Proper Handling and Storing of Materials
•
Proper Handling of Coated Surfaces
•
Thorough Inspection and Repair of Holidays
Pipe and tank coatings provide the principle and most effective
deterrent to underground tank and steel pipe corrosion. A
properly manufactured and applied coating will serve to provide
approximately 99% of the protective needs of buried metal
structures.
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 2
Methods Used to Protect Buried Tanks
Insulating Fittings – Insulating fittings are used when buried
metallic distribution lines (especially copper tubing) are
connected to underground tanks to isolate various parts of the
pipeline system and for quality control in cathodic protection
systems.
Figure 1. Insulating
Dielectric Union
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 2
Methods Used to Protect Buried Tanks
Impressed Currents
Sacrificial Anodes – Sacrificial anodes are used where
•
Current requirements are low
•
Metal structures to be protected are usually well-coated
•
Localized protection is required, and
•
Relatively low soils having low electrical resistance exist.
When a sacrificial anode is buried and connected to an
underground tank (Figure 2), a galvanic cell is developed.
The electrical current produced by the galvanic cell provides
cathodic protection to the tank in areas where the coating is
defective.
4.1.5 Student Book  © 2004 Propane Education & Research Council
Pages 2 & 3
Methods Used to Protect Buried Tanks
“ative”
N
S
oilB
ackfiled&
S
o
l pedA
w
ayr
f om
Top
oD
f om
efor
D
ainage
r
C
adW
eld
LeadW
e
ir
(toinside&
o
t pod
f om
e)
C
adW
eld
onTankM
anua
f cu
t rer
S
uppe
il dS
r
t ucu
te
r
(N
otonTankS
hel)
“cotch-Lok”Tw
S
ist
W
eC
ir
onnector
joinsanodeandcad
(
w
eldleads;pr
ovides
C
uC
uS
Ohalfceltest
pon
i)
t
4
A
nodeLead
W
e(
ir
toinside
&
o
t pod
f om
e)
S
and
TankS
upport
F
li
A
nodeB
ag
Figure 2. Galvanic Cell
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 4
Methods Used to Protect Buried Tanks
Corrosion engineers typically use two factors to determine
sacrificial anode requirements for buried steel tanks:
•
Mass of the tank, typically expressed in pounds
•
Surrounding soil characteristics, primarily soil
electrical resistance
When selecting sacrificial anodes for large steel
underground tanks or steel piping systems, or when
anode installations in a particular area are not
producing satisfactory measured voltage outputs, it is
recommended that a consulting corrosion engineer be
contacted.
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 4
Methods Used to Protect Buried Tanks
Figure 3. Sacrificial Anode Requirements for Typical Underground Tanks
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 5
Methods Used to Protect Buried Tanks
Impressed Currents
Rectifiers — Rectifiers may be used for cathodic protection of
large underground tanks and steel piping systems. The rectifier
is an electrical device which changes alternating current (AC)
into direct current (DC) to provide cathodic protection for large
tanks and buried metallic piping systems.
4.1.5 Student Book  © 2004 Propane Education & Research Council
Page 5
Time to See If You Got the Key Points of
This Module…
• Complete the Review on page 6.
• See if you are ready for the Certification
Exam by checking off the
performance criteria on page 7.
4.1.5 Student Book  © 2004 Propane Education & Research Council
Pages 6 & 7