Nace CP 1, Руководство по эксплуатации

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RP0388-2001
NACE International 3
3.3 Cathodic Protection of Uncoated Steel Tanks
3.3.1 It is feasible to design a cathodic protection
system to provide complete protection for uncoated
submerged surfaces of steel tanks (see Section 4).
3.3.2 For existing steel tanks with submerged surfaces
that are not coated or that have coatings in poor
condition, it may be economical to rely entirely on
cathodic protection.
3.4 Economic Considerations
3.4.1 Coated Tanks
The cost of cathodic protection should be compared
with the cost of recoating and metal loss.
Cathodic
protection defers the need for coating repair and
recoating for considerable time.
This effectively
extends the life of the coating system. In addition,
cathodic protection prevents repair costs from metal
loss.
3.4.2 Uncoated Tanks
It is possible to prevent metal loss in steel tanks where
the surfaces are not coated or where the coating has
deteriorated to the point that the surfaces can be
considered uncoated. The cost of cathodic protection
should be compared with the cost of metal loss, service
disruption, reduction in design life, and water quality
degradation.
3.5 Operational Considerations
3.5.1 Taking the tank out of service for coating
maintenance or corrosion repair can be eliminated or
postponed
if the cathodic protection system is
designed and operated properly.
3.5.2 Two benefits of keeping the tank in continuous
service are system capacity and water availability for
fire protection.
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Section 4: Design of Impressed Current Cathodic Protection Systems
4.1 Introduction
4.1.1 This section outlines the elements that should be
considered when designing impressed current cathodic
protection systems for steel water storage tanks.
4.1.2 In the design of an impressed current system,
the following items must be considered:
(a) Design of the tank (accessibility, crevices, shielded
areas),
(b) Present and future condition of the coating and the
generic type of coating employed,
(c) Voltage and current capacity of the power source,
(d) Impressed
current anode material and
configuration,
(e) Impressed
current anode life and ease of
replacement,
(f)
Type of power source control,
(g) Hardware,
(h) Reference cell location(s),
(i)
Power costs,
(j) Possible evolution of hydrogen and ventilation
requirements,
(k) Possibility of vandalism, and
(l)
Compliance with ANSI/NSF-61
2
in the U.S., or
other governing standards outside the U.S.
4.1.3 Whenever possible, the design should be based
on standard components provided by manufacturers
regularly engaged in the production of impressed
current cathodic protection systems for steel water
storage tanks.
4.1.4 Selection of materials and system design should
be accomplished in a manner to provide the best
economic balance between the installation cost, the
maintenance cost, and the electrical power cost over
the design life of the system.
4.2 Information Required
4.2.1 Before undertaking the design of an impressed
current cathodic protection system, the following
information about the tank should be obtained:
(a) Dimensions of tank, including riser if tank is
elevated,
(b) Area of submerged steel surfaces (including
ladders, baffles, braces, struts, etc.),
(c) Chemical analysis of water,
(d) Water conductivity,
(e) Seasonal variation in water composition,
(f)
Frequency and rate of emptying and filling the
tank,
(g) Type, specification and condition of internal
coating,
(h) Condition of internal steel surfaces exposed to the
water,
(i)
Whether or not water is subject to freezing,
(j) Average and extreme water temperatures,
(k) Cost and availability of alternating current (AC)
power,
(l)
Flow patterns, recirculation, aerators, heaters, riser
pipes, and
(m) Other pertinent information, including age and
history of tank; and whether the tank is of welded,
bolted, or riveted construction; and area classification.