TPi April 2008

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

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.

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

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

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April 2008 Tube Products International