Owing to refinements made to the thermoplastic polyethylene and the pipes and fittings manufactured from this material, the fields of application of these products have increased.

Nowadays, pipes, fittings and customised components can be used in water supply and disposal networks with pipelaying methods that were inconceivable only 20 years ago.

Development of PE pipes for industry as well as water supply and disposal systems

In the past, the properties of PE for industrial applications as well as water supply and disposal systems were developed with regard to internal pressure resistance. This is reflected by the basic standard DIN 8074/75 – Polyethylene (PE) Pipes – PE 63, PE 80, PE 100, PE-HD.

In this context the material designation and the load classification for pipes refer to internal pressure creep rupture strength (MRS = Minimum Required Strength) for 20°C, 50 years and the test medium water.

The underlying MRS figures are 6.3 N/mm² for PE 63, 8.0 N/mm² for PE 80 and 10.0 N/mm² for PE 100. Owing to the soft pipe surfaces, PE pipes were embedded in sand, in accordance with the laying recommendations, in order to prevent damage, and hence a reduction in service life. With the further development of pipelaying methods, when installing new pipes and rehabilitating buried water supply and disposal pipelines it was necessary to improve the material in such a way that a high level of resistance to external mechanical damage would be achieved.

This goal was achieved thanks to the development of bimodal PE 100 materials with high resistance to slow and rapid crack propagation. Classification of these materials is set out in PAS 1075 – Pipes made from Polyethylene for Alternative Installation Techniques: Dimensions, Technical Requirements and Testing. In PAS 1075 (PAS = Publicly Available Specification), which is a supplement to existing standards and guidelines, there are three different types of RC pipe (RC = Resistance to Crack).

• Type 1: Single-layer solid wall pipes made of PE 100-RC
• Type 2: Pipes with dimensionally integrated protective layers of PE 100-RC
• Type 3: Pipes with dimensions conforming to DIN 8074/ISO 4065 with an outer protective casing. Inner pipe made of PE 100-RC.

In order to meet the high demands made by PAS, for the "Material" approval test the pipe must pass an FNCT (Full Notch Creep Test at 80°C and a test stress of 4.0 N/mm² in a 2% solution of Arkopal N 100 for more than 8,760 hours) and a point load test (under the same test conditions). The approval test for the pipe must take the form of a 2NCT (2 Notch Creep Test at 80°C and a test stress of 4.0 N/mm² in a 2% solution of Arkopal N 100 for more than 3,300 hours) and a point load test (under the same test conditions but for more than 8,760 hours). Based on this requirement profile it is possible for the user and planner to select a pipe that is technically and commercially optimised for his particular application.

Conclusion

On the basis of material specifications and proofs it is thus possible to recommend using pipes conforming to PAS classification types 1 and 2 for sand bed-free laying of PE pipes, e.g. in prepared, compactable excavated trench soil. If extreme loads are to be expected, as with the alternative pipe laying methods of pipe bursting and wash-boring, type 3 should be used. This largely corresponds to the recommendations in DVGW code of practice GW 321 (Controllable horizontal wash-boring methods for gas and water pipelines; requirements, quality assurance and testing), and GW 323 (Trenchless renewal of gas and water supply pipelines by bursting; requirements, quality assurance and testing).