Page 463 - Pipeline Pigging Technology

Pipeline Pigging Technology

It is difficult to predict how long a coating material will last on the inside
of a pipeline. UCISCO has been coating lines since 1977, and these coatings
are still in place. The expected method of failure is flaking or chipping of the
coating. The lines can then be recleaned (to remove the old coating) and
recoated.

COATING APPLICATION

Coating is applied to in-place pipelines by placing the coating material
between two pigs and propelling the pig train through the line. Several types
of pigs, including multiple-cup-and-disc, bi-directional disc, and spherical, are
commonly used. UCISCO prefers inflatable spheres because they are revers-
ible, non-collapsible, can negotiate tight bends without leaving gaps, and will
conform to internal pipe irregularities. Spherical pigs also produces thicker
coating layers, usually 4-6mils (dry film thickness), as opposed to 1-3mils for
other types of pig, which means that a line needs only one to two coats if done
with spherical pigs.
The coating thickness is controlled by the size of the spheres (shear
pressure on the coating) and the speed of the pig train. The speed is controlled
by the differential pressure across the pig train, which is determined by the
pressure differential upstream and downstream. Nitrogen is used as both the
driving force and back pressure, because its flowrate and velocity can be
easily controlled by the same pumping equipment used to clean the line, and
because its inertness prevents any possibility of flashing of the solvent
material (usually MEK) in the line. Typically, two coats are applied, one in
each direction, to ensure thorough coating of welds, joints, and plugged
laterals.
The "double-plug extrusion" process has several limitations. The coating
serves as a barrier for future corrosion or product contamination, but it will
not repair or cover leaks, or add structural strength to the line. All leaks must
be repaired before coating, including those that can be uncovered during
cleaning. While this method can clean and coat much longer lengths than
most alternative methods, it cannot coat through diameter changes, and lines
must be broken at these points.

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