Page 462 - Pipeline Pigging Technology

Internal cleaning and coating

by removing rust or scale, cleaning may expose leaks that must be repaired
before coating.
Clear advantages of this system over traditional cleaning methods, such as
pigging or chemical washing, are numerous. Most important is its ability to
reach a NACE #1 or SSPC SP5 white-metal blasted finish, which eliminates any
contamination that may prevent bonding between the pipe and coating. The
cleaning particles produce a deep, angular anchor pattern that enhances the
coating bond. The nitrogen used to propel the cleaning particles also dries the
line and leaves it in an inert atmosphere to prevent flash corrosion. Most lines
can be cleaned very quickly, in about eight hours. Also, long sections of
pipelines can be cleaned per setup, reducing excavation costs and time. In
general, the maximum length that can be cleaned per setup is a function of
the inside diameter of the pipe. The ID (in inches) divided by three will give
the length in miles that can be cleaned. For example, the method can clean
up to four miles of 12-in pipeline per setup.

COATING MATERIALS

A wide variety of coatings have been used to internally coat in-place
pipelines. The "double-pig extrusion process" requires specific physical
properties, including that it be thixotropic, or lose viscosity under shear
pressure. This enables the coating to be spread onto the pipe wall with pig
pressure and then thicken immediately thereafter, to prevent the coating
from running or sagging. Also, the coating must be at least 60% solids.
The most commonly-used coating is a two-part polyamide-cured epoxy. It
is moderately chemical- and abrasion-resistant, and will withstand tempera-
tures of up to 150°F under immersion service (220°F, atmospheric service)
and pressures up to 500psig. The polyamide coating is recommended for lines
carrying potable, fresh, and saltwater, crude oils, transportation fuels, natural
gas, and some solvents. It is not recommended for lines containing strong
aromatics, strong organic acids, or high levels of sulphur dioxide or hydrogen
sulphide. The minimum cure time for this coating is seven days at 70°F,
although it may be force-cured much quicker if the line can be heated.
Many other coatings, such as polyamines and polyurethanes, have been
used, depending on the operating conditions of the line. At this time, there is
no clear choice of coatings for "hostile" environments (high-pressure and/or
high-temperature). Much testing is currently being done in this area. Also of
interest are coatings appropriate for service-water systems in njuclear power
plants.

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