Liquid Coating Praised
Line’s Polyurethane Rehab
Corrosion Free After 10 Years
It has been 10 years since Natural Gas Pipeline Company embarked on a rehabilitation
project of a 26-inch underground gas transmission line, installed in 1953 on the system
stretching from South Texas to near Chicago. The first section that was rehabilitated was
a 24.2-mile segment, starting at Refugio and extending to about 20 miles from Victoria,
TX. The original coal tar enamel used to coat the pipe had not held up to the climatic
conditions that prevail in the Gulf Coast region of Texas or the severe mechanical and
chemical stresses brought about by the acidic soil.
This rehabilitation project was innovative for several reasons. At the time of the
project, technology and equipment were available to handle the majority of objectives that
had to be met, but not for a line-travel, thin-film coating machine. There was no
commercial equipment available at the time to apply any type of thin film coating over the
ditch. Another unusual element of the project was the use of a urethane-based coating on a
gas line, notorious for unusually high temperatures.
NGPL had decided to use a plural component urethane coating for corrosion protection,
developed by TIB Corrosion Protection Division, based in Germany. The coating material,
Protegol, was selected as a one-layer system for its:
- Adhesion capacity,
- Abrasion resistant properties, and
- Chemical resistance.
Also, Protegol’s extremely short curing time, allowing a pipe to be inspected and
buried within minutes after application offered diverse advantages.
CRC-Evans Pipeline International was asked to design and manufacture a new machine to
apply the quick set Protegol material to a 20-mil to 24-mil thickness at a rate of a mile
per day, on the line. The innovative idea of designing a spray ring was born. The eventual
line-travel machine was equipped for each step of the application procedure (removal and
cleaning, blasting, coating) and was ready for line coating in mid 1989. The new equipment
made it possible for the first time to refurbish a pipeline without having to dismantle
it.
A high pressure water jet system was used to remove the old coal tar enamel coating,
followed by a line travel blast machine that used a combination of shot and grit to clean
the pipe surface to SA 2.5/ near white metal blast and with a 2 to 4 mil anchor pattern.
Many leaders in the field of coating application agree that correct surface preparation is
the single most important factor in the success or failure of a protective coating
application. Even the best coatings may fail when applied over a poorly prepared surface.
The over-the-ditch line travel system developed by CRC-Evans consists of two major units:
a self-propelled, variable-speed line travel coater and a track-mounted equipment skid. A
side boom cradles the pipe for the coater and tows the equipment skid. The line travel
coater has eight stationary airless spray guns for uniform 360-degree coverage. A mixing
manifold accurately distributes the two-component coating material to the guns. The
onboard material holding tanks have sufficient capacity to coat up to one mile of 30-inch
pipe.
There have been many improvements made to the original design of the line-coating
equipment used on the NGPL project, particularly to the blasting and coating systems. The
original spray apparatus has been upgraded from the eight stationary guns to three
oscillating guns, which rotate to 120 degrees. This provides a very uniform coating
thickness. Another performance-enhancing improvement has been a computerized metering
system, self-contained within a skid that moves with the equipment. Operators can control
all application ratios from inside the skid, and the system even provides for automatic
shutdown before any mis-metered material is applied to the pipe.
One of the most significant advancements in the line travel coating equipment was the
containment of the old coating and other debris that is blasted from the pipe, prior to
application of the new coating. This was not designed into the original system and the
improvement provides for an environmentally safe blasting process.
Sid Taylor, vice president engineering and marketing for CRC Evans said, "Prior to
the NGPL project, liquid-applied, plural-component coatings were considered only for
rehabilitation of short sections of pipe. Today, liquid coatings are efficiently applied
to long sections of large-diameter pipe, and are generally accepted as having excellent
mechanical, chemical and electrical properties."
TIB said many tests have been conducted that support the corrosion-resistance superiority
of urethane coatings. Says Taylor, "There is now a wealth of information available on
the reduced production time and other performance efficiencies of the line travel coating
application process, much of which is available from CRC-Evans." The design of the
first spray ring technology was the trigger for many application companies. The
line-coating equipment is being used worldwide today, with spray rings designed for two
guns on some current projects.
"What has not been made available, until recently, is data that would determine the
long-term success or failure of the NGPL rehabilitation, in terms of continued corrosion
protection. Corrosion-free lines, over many years, would be the true measure of long-term
success. We decided to investigate, on the 10-year anniversary of the groundbreaking NGPL
project, how well the Protegol coating, applied with the first ever line-travel coating
equipment was holding up, in comparison to the previous coating."
Phil Dusek, currently manager of risk engineering for KN Energy (formerly NGPL), was the
chief engineer of corrosion control in 1989 when the rehabilitation project took place.
Says Dusek, "The condition of the rehabilitated line section is excellent. Current
density tests from field engineers have consistently been 0.015 milliamps per square foot,
or lower, with the last test conducted recently this year." The current density data
represents a measure of how much bare pipe is exposed. The smaller the number, the better
the measurement. Dusek said the current density test results are some of the best that
they’ve achieved, with any of the various corrosion protection methods used on NGPL
lines. P&GJ
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Pipeline
& Gas Journal