Nace CP 1, Course Manual

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RP0169-2002
4 NACE International
________________________________________________________________________
Section 4: Piping System Design
4.1 Introduction
4.1.1 This section provides accepted corrosion control
practices
in the design of an underground or
submerged piping system. A person qualified to
engage in the practice of corrosion control should be
consulted during all phases of pipeline design and
construction
(see Paragraph 1.3). These
recommendations should not be construed as taking
precedence over recognized electrical safety practices.
4.2 External Corrosion Control
4.2.1 External corrosion control must be a primary
consideration during the design of a piping system.
Materials selection and coatings are the first line of
defense against external corrosion. Because perfect
coatings are not feasible, cathodic protection must be
used in conjunction with coatings.
For additional
information, see Sections 5 and 6.
4.2.2 New piping systems should be externally coated
unless thorough investigation indicates that coatings
are not required (see Section 5).
4.2.3 Materials and construction practices that create
electrical shielding should not be used on the pipeline.
Pipelines should be installed at locations where
proximity to other structures and subsurface formations
do not cause shielding.
4.3 Electrical Isolation
4.3.1 Isolation devices such as flange assemblies,
prefabricated joint unions, or couplings should be
installed within piping systems where electrical isolation
of portions of the system is required to facilitate the
application of external corrosion control.
These
devices should be properly selected for temperature,
pressure, chemical resistance, dielectric resistance,
and mechanical strength.
Installation of isolation
devices should be avoided or safeguarded in areas in
which combustible atmospheres are likely to be
present. Locations at which electrical isolating devices
should be considered include, but are not limited to, the
following:
4.3.1.1 Points at which facilities change
ownership, such as meter stations and well heads;
4.3.1.2 Connections to main-line piping systems,
such as gathering or distribution system laterals;
4.3.1.3 Inlet and outlet piping of in-line measuring
and/or pressure-regulating stations;
4.3.1.4 Compressor or pumping stations, either in
the suction and discharge piping or in the main
line immediately upstream and downstream from
the station;
4.3.1.5 Stray current areas;
4.3.1.6 The junction of dissimilar metals;
4.3.1.7 The termination of service line
connections and entrance piping;
4.3.1.8 The junction of a coated pipe and a bare
pipe; and
4.3.1.9 Locations at which electrical grounding is
used,
such as motorized valves and
instrumentation.
4.3.2 The need for lightning and fault current
protection at isolating devices should be considered.
Cable connections from isolating devices to arresters
should be short, direct, and of a size suitable for short-
term high-current loading.
4.3.3 When metallic casings are required as part of
the underground piping system, the pipeline should be
electrically isolated from such casings.
Casing
insulators must be properly sized and spaced and be
tightened securely on the pipeline to withstand insertion
stresses without sliding on the pipe. Inspection should
be made to verify that the leading insulator has
remained in position. Concrete coatings on the carrier
pipe could preclude the use of casing insulators.
Consideration should be given to the use of support
under the pipeline at each end of the casing to
minimize settlement.
The type of support selected
should not cause damage to the pipe coating or act as
a shield to cathodic protection current.
4.3.4 Casing seals should be installed to resist the
entry of foreign matter into the casing.
4.3.5 When electrical contact would adversely affect
cathodic
protection, piping systems should be
electrically isolated from supporting pipe stanchions,
bridge structures, tunnel enclosures, pilings, offshore
structures, or reinforcing steel in concrete. However,
piping can be attached directly to a bridge without
isolation if isolating devices are installed in the pipe
system on each side of the bridge to electrically isolate
the bridge piping from adjacent underground piping.
4.3.6 When an isolating joint is required, a device
manufactured to perform this function should be used,
or, if permissible, a section of nonconductive pipe, such
as plastic pipe, may be installed. In either case, these