Page 55 - Reliability and Maintainability of In service Pipelines

44 Reliability and Maintainability of In-Service Pipelines

1.6.4 DETERIORATION OF OTHER TYPES OF PIPES

1.6.4.1 Seamless Pipes

Seamless pipes are derived from metal, however, they are also available with a
plastic coating inside the steel pipe. In the case of seamless pipes the presence of
plastic linings inside these pipes can allow pipes to have a relatively higher
strength than most plastic pipes, and at the same time maintain the resistance to
corrosion. There is also potential for corrosion of metal if the plastic cracks, caus-
ing the exposure of metal to fluid flow.
Seamless pipes undergo extreme forces during the manufacturing process;
therefore, a thick high-pressure lubricant is applied to the surface of the pipe to
prevent cracking, which can result in a defective pipe. Following this process, the
pipe undergoes heat treatment in which the high pressure lubricant must be
removed. The aggressive chemicals used to dissolve the solvent on the surface of
the pipe can still remain as a residue long after the manufacturing process is com-
pleted and can result in corrosion. This is especially problematic in seamless
pipes with thin diameter walls.
As tensile and yield strengths are related to the thickness of the pipe wall
thickness, it is important that this thickness is not depleted in the slightest due to
corrosion, otherwise it can result in pipe failure.
Wall thickness is also important in terms of heat transfer, as these types of
pipes can be prone to temperature fluctuations, as well as high heats; this can be
particularly problematic if manufacturing processes are not properly followed.

1.6.4.2 Glass Reinforced Epoxy Resin Composite (GRE)

GRE used in pipelines are known to be corrosion resistant and the cause of failure
is often the result of poor design methods. Although using this material for pipe
production is effective and capable of withstanding most waters and alkali envir-
onments, the issue lies within exposure to strong acids. This exposure alone how-
ever may not be enough to cause deterioration, but rather becomes possible with
a combined effect of poor design methods and applied loads.
With GRE, the choice of glass fiber is critical. Given an unsuitable glass fiber
type is used, exposure to corrosive chemicals can cause the fiber to break down
and break the resin, altering the structural composition of the composite. This can
result in a structurally inadequate material and if used in pipes, can alter their
strength and ability to resist corrosion.
The deterioration rate of the glass fibers exposed to corrosive environments
depends on the level of diffusion and osmosis of corrosive chemicals into the
fibers. Various factors, including thermal gradients and pressure gradients, can