Development of a new high performance PE pressure pipe system, from design phase to reality

REHAU and Borealis, have gathered their respective long expertise and knowledge in PE pipes and raw materials to develop a new pipe system with outstanding crack resistance to meet new market expectations and needs. This article will describe the different steps the two companies have passed together, from analysis of the market needs to the first actual projects of the newly created RAUPROTECT II solution.

Among many applications, Polyethylene (PE) pipes are used worldwide for water supply and gas distribution, contributing significantly to improve the reliability and the quality of the networks. Since the first pipe productions almost 50 years ago, PE has developed an impressive successful track record, and is today the preferred choice for these applications. Ease of installation, low maintenance and corrosion free are some of the well-known reasons for this success story. Another key reason is the constant efforts of both raw materials producers and pipe producers for improved quality and reliability. After the development of HDPE pipe materials like PE63 and then PE80, the first PE100 materials arrived on the market at the end of the 1980s. These materials were a real milestone in the development of HDPE pipe materials because they facilitated the use of higher service pressures or pipes having thinner wall at the same service pressures and at the same time they also offered the end-user other advantages such as high resistance to Slow Crack Growth and high resistance to Rapid Crack Propagation. Pipes made from these modern materials are considered as very well-adapted to classic installation conditions.

The last 10 or 15 years have anyway seen new challenges for pipes, with the development of new installation techniques.

Trenchless installation technologies
Lot of modern techniques (pipe bursting, directional drilling, relining, etc…) have been developed combining faster and cheaper installation (up to 60% less than traditional type of installation), with also minimize disturbance to the neighbourhood, the latest benefit being specially important in urban areas. They offer new possibilities for relining or replacing of old pipelines. PE, due to its properties, is the reference material for these installation methods.

But these techniques can happen to be more aggressive and demanding to pipe materials, which can be in contact with hard or sharp objects.

Sandless bedding
There is also a clear trend, over different European countries, for classic installation in trenches, to move from the traditional sandbed laying to sandless bedding and to use or as-dug material as backfill material to fill-up the trench after installation of the pipe system. Depending of the type of soil in the specific area of installation, this is also creating a new challenge for pipe materials. 

These new demanding conditions have pushed for the development of PE materials with improved properties, in particular their Slow Crack Growth Resistance, and subsequently of new pipe systems to take the best of these better materials, sometimes in the form of multilayer pipes; these multilayer pipe systems can provide solutions to practical problems and 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

In a practical way, two main situations can initiate slow crack growth phenomenon in a pressure PE pipe: The first one is scratches or notches created at the outside surface of the pipe before installation with 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. With 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.

Optimizing the raw material: the polymerization process

In the pipe business the flexibility of the bimodal (or multimodal) process for producing polyethylene materials has provided the greatest scope for producing "tailor-made" materials. The choice of catalyst, co monomer type-, content-, and selective distribution thereof in the polymer chains, and the selection of process parameters in each reactor all effect the development of the polymer structure and the properties of the end product. Changing these variables enable the properties to be optimised for a manufacturing process or end use application.

The bimodal process consists of two polymerisation reactors in series. Figure 5 shows a simplified view of the basic principles of the bimodal process. The Borstar^® low pressure slurry loop and gas phase reactor process is illustrated. The catalyst is fed into the first reactor, where polymer is formed as powder particles through polymerisation of the ethylene monomer and suitable amounts of co monomer, continuing in a series mode into the second reactor.

The process offers great flexibility with regard to the type of co monomer which can be incorporated into the correct regions of the polymer. For example, the use of the co monomer hexene in the bimodal Borstar process results in polymers having extremely high resistance to Slow Crack Growth.

Borsafe™HE3490-LS-H

With the help of the Borstar process and according to the description in the previous section, it has been possible to create a material which enables the pipe producer to make any dimension from 20mm to up to more than 2000mm and wall thicknesses of >100mm. Not only does this material provide the possibility to make any pipe dimension, it also possesses an excellent resistance to impact failures e.g. Rapid Crack Propagation, and excellent resistance to internal pressure manifested by the PE100 designation according to ISO 9080 standard extrapolation method.

Resistance to slow crack growth (SCG)
We have seen before that SCG is a key parameter when in comes to lifetime of the pipes; we are now able to demonstrate that this new material has taken yet another step of at least one decade in the resistance to slow crack growth in the notched pipe test. However we can show not only that but also an unprecedented resistance to SCG in all of the tests to which the material has been subjected until now.

RAUPROTECT II RAUSISTO

The increasing demand for heavy-duty gas, potable water and sewer pipes suitable for the modern and cost-effective trenchless technologies forced the pipe manufacturer REHAU to develop a new system solution based on the latest state-of-the-art raw material Borsafe™HE3490-LS-H: The RAUPROTECT II RAUSISTO generation was born.

The design of the RAUPROTECT II RAUSISTO system solution is based on the known stress concentration at the opposite inner surface of the pipe while external point loads and internal pressure are combined for the whole lifetime of the pipe system.

To archive an outstanding crack resistance to meet new market expectations and needs, REHAU used the best available PE100 VRC-Quality Borsafe™HE3490-LS-H available on the market for the coextrusion of a thick inside layer of a smooth wall pressure pipe with overall measures of the established European standards EN 12201 / EN 1555 / EN 13244. This material combination of best PE100+association-grades for the outside layer with Borsafe™HE3490-LS-H on the inside layer brings a cost-effektive, technical superior pipe system quality to the market that fulfils the all the market expectations and can be installed with all standard pipe laying techniques, e.g. but-fuse welding or jointing with standard-fittings. No expensive special tooling for the removing of outside protection layers is necessary. And the well known testing institute "Dr. Hessel Ingenieurtechnik" in Roetgen / Germany approved, that the RAUPROTECT II RAUSISTO system solution based on Borsafe™HE3490-LS-H showed best results against point loads therefore and can be used without risks for trenchless technologies and in trenches without sand embedding.

To improve the traceability of that heavy-duty pipe system, REHAU marks all RAUPROTECT II RAUSISTO pipes with durable, lasermarked traceability-Barcode according ISO 12176-4 (Code 128, type C) and ISO 13950. Every produced meter of RAUPROTECT II RAUSISTO can now be easily tracked and welded with modern welding equipment to make pipe laying in the trench more easy and safe than ever before.

Field test: Gas pipe installation in Holzthaleben / Germany
The new developed RAUPROTECT pipe system was used in the dimension d63 SDR11 for more than 2.4 km and 2,252m d110-pipes under hardest construction side conditions in 2006 in Germany. In hard rock conditions, the pipe has been installed by horizontal drilling and by using a plow-in technology to build a new gas distribution system for the E.ON company.

One year after the installation, a routine TV-inspection on the performance of the installed RAUPROTECT II Pipe System took place. The gas pipeline showed no signs of wear, tear, deflection and point loads. All internal pressure tests have been successfully fulfilled.

The combination of best Borsafe™HE3490-LS-H PE100-Quality and modern state-of-the art pipe-design will secure the safe transport and distribution of potable water and gas for generations to come.