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April 2008 Tube Products International
59
less than traditional types of installation), with minimised
disturbance to the neighbourhood being particularly
important in urban areas. They offer new possibilities
for the relining or replacing of old pipelines. PE, due
to its properties, is the reference material for these
installation methods. However, these techniques can be
more aggressive and demanding to pipe materials, which
can be in contact with hard or sharp objects.
There is a clear trend, across different European countries,
for classic installation in trenches, to move from the
traditional sandbed laying to sandless bedding, and to use
as-dug material as backfill material to fill the trench after
installation of the pipe system. Depending of the type of
soil in the specific area of installation, this also creates a
new challenge for pipe materials.
These new demanding conditions have pushed for
the development of PE materials with improved
properties, in particular slow crack growth
resistance, and for new pipe systems to use the
best of such materials, sometimes in the form of
multilayer pipes. These multilayer pipe systems
can provide solutions to practical problems, while
offering maximum security in installation and
jointing at minimum additional cost. Combining the
best materials to meet specific loading conditions in
the structure, they provide protection and long-term
durability.
Slow crack growth initiation in
pipe installation
There are two main situations that can initiate the
slow crack growth phenomenon in a pressure
PE pipe. The first cause is scratches or notches
created at the outside surface of the pipe before
installation by improper handling or storage, or
during installation when the pipe can be accidentally
damaged. The common rules of practice mention that
pipes showing an external scratch of more than 10%
of the wall-thickness should not be installed, for safety
reasons; in practice, we cannot rule-out the possibility that
some damages on the construction site are not detected
during quality control procedures before installation of the
pipe.
The second main cause is ‘point-load’: an external local
stress is created at the surface of the pipe, for example by
a rock pressing directly against the pipe wall. Over time,
this rock will create a stress concentration at the opposite
inner surface of the pipe which, combined with the internal
pressure, will eventually propagate through the pipe wall,
from the inside to the outside.
▼
Rauprotect II gas pipe without sand embedding
1st
Generation
HDPE
2nd
Generation
HDPE
3rd
Generation
HDPE
3rd
Generation
HDPE
Pipe installed since
1965
1975
1990
1990
Classification
PE63
PE80
PE80
PE100
Design stress
5.0 MPa
6.3 MPa
6.3 MPa
8.0 MPa
Stage II knee point 80°C
100-300 H
1000-3000
>10 000
>10 000
Notch test, 4,6 Mpa/80°C
10
100-200
>1,000
>1,000
Rapid crack propagation/
Pc110SDR11
<3 bar
<3 bar
>10 bar
>10 bar
s
Table 1
:
Evolution of pipe properties