Chemical Engineering March 2021 - 41

Delamination
New at 12 o'clock
New at 8 o'clock
FIGURE 9. Here are two additional transverse cross-sections of as-fabricated fiberglass in HDPE-lined
fiberglass pipe
FIGURE 10. Shown here are the inside (left) and outside (right) surfaces of a joint between FRP
pipe sections
to be kept below 20% of the pipe
manufacturer's pressure rating.
Otherwise, the design pressure can
be reduced by up to 50%. Lined
FRP and FRP requires cyclic loading
to be considered during design.
Failure 6. A fiberglass (FRP) pipe
used in saltwater service was lined
with HDPE along its bottom (6
o'clock) side. The failed section, a
non-failed section downstream of
the failure, and a third piece (representing
an as-fabricated piece)
were examined. Specifically, transverse
cross-sections of the failed
area were compared to those of
an as-fabricated pipe of the same
size (see Figures 8 and 9). Note the
extensive intralaminar cracking in
the failed cross-section in contrast
to its absence in the as-fabricated
pipe. Both the new and failed pipe
had delaminations.
Delaminations
are common in FRP with high glass
content; the high glass content confers
higher strengths. The pipe had
been subjected to severe pressure
swings (greater than 20%), and it
failed due to cyclic loading.
FRP installation errors
During site installation, smaller sections
of pipe are joined - these
joints are critical. Typically, two pipe
sections are butted against each
other, and the gap between the pipe
is filled with a " putty. " Then, the joint
is wrapped with wide multiple plies
of FRP reinforcement saturated with
resin. The joint must have adequate
reinforcement coverage along the
outside surfaces.
Nonmetallic materials, such as
the liner and FRP, are viscoelastic.
While the characteristic can be
challenging to explain, its evidence
is common to find - often, damage
is introduced during installation,
but the leak does not develop
immediately. " Viscoelasticity is the
property of materials that exhibit
both viscous and elastic characteristics
when undergoing deformation.
Viscous materials, like
honey, resist shear flow and strain
linearly with time when a stress is
applied. Elastic materials (such as
steel) strain instantaneously when
stretched and just as quickly return
to their original state once
the stress is removed. Viscoelastic
materials have elements of both
of these properties and, as such,
exhibit
time-dependent strain.
Whereas elasticity is usually the result
of bond stretching along crysCHEMICAL
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MARCH 2021
tallographic planes in an ordered
solid, viscosity is the result of the
diffusion of atoms or molecules inside
an amorphous material " [4].
FRP and plastic parts require exceptional
care during installation
and handling. Otherwise, they can
crack, and the damage may not be
evident until much after a hydrotest.
Most lined FRP failures develop
because of installation damage [5].
Hydrostatic tests are essential but
may not detect small damage that
can grow in service.
Failure 7. Figure 10 shows a joint
between two FRP pipe sections.
Figure 11 shows a cross-section
of the joint. The external reinforcement
and coverage along the pipe
were insufficient, and the pipe
cracked during handling. Guidelines
for the reinforcements of joints
are given in DIN 16966, CSA Z662,
and ASME NM.2.
Slow crack growth in HDPE pipe
HDPE piping is light-weight, corroPipe
Thickness
Reinforcement
Cracks
FIGURE 11. A cross section of a joint between FRP
pipe sections is shown here
FIGURE 12. In this photo, one can see the intersection
from a tee branch to the main run, which
developed the crack pointed at with a red arrow
FIGURE 13. This cutaway shows the fracture surface
of the tee branch to tee main run
41

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Chemical Engineering March 2021

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