Nace CP 1, Руководство по эксплуатации

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RP0285-2002
NACE International 9
and tank by the use of dielectric isolating unions
and bushings.
6.2.3.3 Electrical isolation of the piping can be
accomplished by the use of flange isolation kits,
dielectric bushings, or dielectric unions rated for
the proper operating pressure and compatible with
the product being stored in the tank.
Use of
dielectric unions underground should be avoided if
at all possible.
Dielectric unions should remain
exposed for future inspection and maintenance.
6.2.3.4 When required by local codes and
regulations, the tank shall be grounded to protect
against damage due to lightning.
This must be
accomplished without compromising the CP
design.
6.2.4 Galvanic Anode Selection
6.2.4.1 The three most common types of galvanic
anodes that are effective in soil environments are
standard-potential magnesium (ASTM B 843,
16
UNS
(5)
M11632), high-potential magnesium
(ASTM B 843, UNS M15102), and high-purity zinc
(ASTM B 418,
17
UNS Z13000). The selection and
use of these anodes are based on the current
requirements for the structure to be protected, the
soil conditions, and the temperature of the
structure to be protected.
6.2.4.2 The current output from each type of
anode depends greatly on the soil conditions, the
anode shape, and the net driving potential of the
anode.
6.2.4.3 When high-purity zinc anodes are
employed, care shall be exercised to ensure the
anodes meet the requirements of ASTM B 418,
Type II anode material.
18
The purity of the zinc
greatly affects its performance as a galvanic
anode for soil applications.
6.2.4.4 Zinc anodes shall not be used when the
temperature of the anode environment is above
49°C (120°F). The high temperature can cause
the anode to assume passive characteristics. The
presence of some chemicals in the soil, such as
carbonates, bicarbonates, and nitrates, may also
affect the performance of bare zinc as an anode
material.
6.2.4.5 Galvanic anode performance is enhanced
with special backfill material.
A mixture of 75%
gypsum, 20% bentonite, and 5% sodium sulfate is
typically used with magnesium anodes. Either
75% gypsum, 20% bentonite, and 5% sodium
sulfate or a mixture of 50% gypsum and 50%
bentonite can be used with zinc anodes.
6.2.4.6 The anodes should be supplied with
adequate lead wire attached. Lead wire shall be
at least 4 mm
2
(#12 AWG
(6)
) solid wire with TW
(thermoplastic insulated wire) or equivalent oil-
and water-resistant insulation.
6.2.5 Galvanic Anode Installation
6.2.5.1 The CP anodes shall be installed around
the tank in a manner that allows optimal current
distribution. Anodes should be placed close to or
below the elevation of the bottom of the UST. If
multiple UST installations are spaced closely
together, installation of additional anodes between
the tanks and above the center line of the tanks
may be required to provide adequate current
distribution to the upper surfaces of the UST.
6.2.5.2 The anode lead wires shall be installed
with sufficient slack to avoid possible damage due
to settlement of surrounding soil.
6.3 Impressed Current Systems
6.3.1 This subsection recommends procedures for
designing impressed current CP systems.
In the
design of a CP system, the following factors shall be
considered:
(a) recognition of hazardous conditions prevailing at
the site and selection and specification of materials
and installation practices that will ensure safe
installation and operation;
(b) all applicable regulatory codes;
(c) selection and specification of materials and
installation practices that will ensure dependable,
economical operation of the system throughout its
intended operating life; and
(d) selection of proposed installation to minimize
stray currents.
6.3.2 Electrical Continuity
6.3.2.1 All structures to be protected must be
electrically continuous.
Bonds may be required
between piping and tanks and, in some cases,
from tank to tank.
Electrical conduits, hydraulic
lifts, and utility piping, such as water and gas
piping, must be investigated for isolation or
continuity as required.
___________________________
(5)
Metals and Alloys in the Unified Numbering System (latest revision), a joint publication of the American Society for Testing and Materials
(ASTM) and the American Society of Automotive Engineers Inc. (SAE), Warrendale, PA.
(6)
American Wire Gauge (AWG): A particular series of specified diameters and thicknesses established as a standard in the United States
and used for nonferrous sheets, rods, and wires. Also known as the Brown and Sharpe Gauge.