Nace CP 1, Course Manual

Page: 372 / 535

RP0388-2001
6 NACE International
5.4 Inspection and Handling
5.4.1 The DC power source shall be inspected to
ensure that internal connections are mechanically
secure and that no damage is apparent. Rating of the
DC power source shall comply with Paragraph 4.3.1
and the construction specifications.
5.4.2 Impressed current anodes shall be inspected for
conformance
to specifications for correct anode
material and size, length of lead wire, method of wire
connection, and damage to any portion of the
assembly.
Damage to anodes during handling and
installation must be avoided. All wire should be
carefully inspected for insulation defects, and damage
to insulation must be avoided.
If defects are not
repaired, the anode must be rejected. Damage to
insulation may result in premature failure of the wire.
5.5 Installation
5.5.1 The power supply should be installed so that the
possibility of damage or vandalism is minimized.
5.5.2 Construction practices shall conform to all
applicable local and national codes.
5.5.3 All power sources which provide AC service to a
CP system should have a disconnect built into the
equipment and shall also have a separate, external
disconnect.
Dedicated AC service will allow the
equipment to be isolated for maintenance and lockout.
5.5.4 The power supply should be installed at a
convenient height above the ground so that it may be
serviced
readily unless other factors such as
vandalism, flooding, etc., apply.
5.5.5 The positive cable and anode leads are
especially critical to the operation of the system. It is
imperative that insulation remains intact.
The entire
cable and all connections must be waterproof. The
cable must be protected from abrasion and sharp
objects.
There shall be no metal-to-metal contact
between the positive cable and the tank structure or
conduit. The positive cable(s) must be connected to
the impressed current anodes, and the negative
cable(s) must be properly connected to the tank.
5.5.6 Underwater splices on the positive cable should
be avoided unless required by in-service installation or
repairs to the system.
Connections between the
positive cable and anode leads must be mechanically
secure and electrically conductive. These connections
must be sealed to prevent moisture penetration.
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Section 6: Criteria for Cathodic Protection and Measurement Procedures
6.1 Introduction
6.1.1 This section describes the criteria for impressed
current cathodic protection along with measurement
techniques which, when followed, provide virtually
complete corrosion control on the internal, submerged
surfaces of steel water storage tanks.
6.1.2 The criteria in Paragraph 6.2 have been
developed through laboratory experiments and have
been verified by evaluating data obtained from
successfully operated impressed current cathodic
protection systems. It is not intended that the corrosion
engineer be limited to these criteria if it can be
demonstrated that corrosion control can be otherwise
achieved.
6.1.3 Measurement of the voltage between the tank
and a reference electrode placed in the water shall be
used
to determine whether adequate cathodic
protection has been achieved. This voltage is referred
to as tank-to-water potential.
6.2 Criteria
6.2.1 A negative, polarized tank-to-water potential of at
least -850 mV relative to a saturated copper/copper
sulfate reference electrode (CSE); or
6.2.2 A minimum of 100 mV of cathodic polarization
between the tank surface and a stable reference
electrode contacting the electrolyte. The formation or
decay of polarization can be measured to satisfy this
criterion.
CAUTIONARY NOTE: The 100 mV polarization shift
criterion is only applicable to steel water tanks not
having corrosion cells caused by connection to more
noble metals such as copper, brass, or passive
stainless steel (e.g., heated water tanks in which the
submerged heater tubes are constructed of copper or
brass,
and are electrically continuous with the
submerged steel tank).
6.3 Measurement Procedures
6.3.1 In order to observe the polarized tank-to-water
potential, the IR drop error must be eliminated from the
measurement. Among the methods used to eliminate
IR drop error are (1) measuring the tank-to-water
potential with the protective current instantaneously
interrupted and (2) interrupting the electrical connection
between the tank and a submerged coupon(s) and
immediately measuring the coupon-to-water potential.
6.3.2 A sufficient number of potential measurements
should be taken to determine that adequate protection