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

Apr 10, 2008

Since the introduction of the first PE pipe more than 40 years ago, PE raw material producers and pipe producers have continuously joined their efforts to propose better and better pressure pipe solutions to the end-users and installers. The development of modern installation techniques, in particular trenchless technologies, has created a need for improved systems, with enhanced insensitivity to installation conditions. 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 paper 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 RAUPROTECTII solution.

1. INTRODUCTION
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.
  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 > 1000 > 1000
Rapid crack propagation/Pc 110SDR11 < 3 bar < 3 bar > 10 bar > 10 bar
Figure 1: evolution of pipe properties (Source: Kania, et al.)
Pipes made from these modern materials are considered as perfectly adapted to classic installation conditions: open trench with sand-bedding of the pipe, and refill of the trench with carefully chosen reinstatement materials.
However, the last 10 years have seen new challenges for pipes, with the development of new installation techniques.
2. NEW DEMANDING INSTALLATION CONDITIONS
2.1 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.
2.2 Sandless bedding
There is also a clear trend, over different European countries, in the case of installation in open trenches, to move from the traditional sand-bed laying to sandless bedding and to re-use 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, higher stresses can be created to the pipes.
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 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.
3.SLOW CRACK GROWTH RESISTANCE OF PE PIPES:
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. 
4. DEVELOPMENT OF A NEW PE MATERIAL WITH OUSTANDING RESISTANCE TO SLOW CRACK GROWTH
4.1 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, comonomer 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 optimized for a manufacturing process or end use application.
The bimodal process consists of two polymerization reactors in series. Figure 1, 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 polymerization of the ethylene monomer and suitable amounts of comonomer, continuing in a series mode into the second reactor.
The principal advantages of the process are:
  • Independent control is applied to the reactors to steer comonomer distribution and to tailor molecular weight distribution (MWD).
  • Flashing between the reactors guarantees independent reaction mixture compositions. A wide density range, from LLDPE to HDPE can be produced.
  • Different comonomers can be incorporated according to needs, e.g. butene or hexene.
  • The MFR2 of the different reactors can be varied across a very broad range, from <<0.1 to more than 1000 g/10min.
  • The process offers great flexibility with regard to the type of comonomer which can be incorporated into the correct regions of the polymer. For example, the use of the comonomer hexene in the bimodal Borstar process results in polymers having extremely high resistance to Slow Crack Growth.
4.2 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.
Test Standard Test conditions Result Requirement
NPT (a) ISO 13479 80°C / 9,2 bar > 18000 h 165 h1
500 h2
FNCT (b) ISO DIS 16770-3 80°C / 4 MPa / Arkopal ca. 6000 h 3500 h3
ACT (c )   90°C / NM-5 ca. 4000 h 330 hrs4
PENT (d) ASTM F-1473 80°C / 4,4 MPa >10000 hours 50 hrs
a) Notched Pipe Test b) Full Notch Creep Test c) Accelerated Creep Test d) Pennsylvania University Notch Test

1 = EN 1555 and EN 12201 requirement
2 = PE100+ association requirement
3 = DVGW requirement for sandless bedding
4 = Provisional DVGW requirement for sandless bedding

Figure 7: Summary of test results for Borsafe™HE3490-LS-H (Source: Kania et al.)
5. THE RAUPROTECT II RAUSISTO PIPE SYSTEM
5.1 The challenge
The increasing demand for heavy-duty gas, potable water and sewer pipes suitable for the modern and cost-effective trenchless technologies, as described earlier, pushed the pipe manufacturer REHAU to develop a new pipe system solution, able to provide a high safety level to the pipe installer and to the end-user even in the most demanding conditions :The RAUPROTECT II RAUSISTO system.
The challenge given to Rehau’s development engineers to design this completely new system solution was based on:
  • the demonstrated technical fact that higher stress concentration is located at the opposite inner surface of the pipe while external point loads and internal pressure are combined during the whole lifetime of the pipe system.
  • The requirement for this new pipe to keep the traditional key benefits of PE pipes, e.g. flexibility, ease of installation and weldability .
5.2 The solution
To achieve an outstanding crack resistance meeting new market expectations and needs, REHAU have then chosen to use Borsafe™HE3490-LS-H , the best available PE100 VRC-Quality (VRC= Very Resistant to Crack) available on the market for the co-extrusion of a thick inside layer of a smooth pressure pipe dimensioned according to the 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-effective, technical superior pipe system quality to the market that fulfills all the market expectations. The well-know testing institute “Dr. Hessel Ingenieurtechnik” in Roetgen-Germany confirmed that the RAUPROTECT II RAUSISTO system solution based on Borsafe™HE3490-LS-H showed best results against point loads and can therefore be used without risks for trenchless technologies and in trenches without sand embedding. Another advantage worth mentioning for the installer and end-user is that RAUPROTECT II RAUSISTO pipes, can be installed as standard PE pipe: in particular, they can be butt-fused or jointed with standard-fittings and welding parameters. No expensive special tooling for the removing of outside protection layers is necessary.
5.3 Innovative traceability:
Together with long life-time expectation , traceability and easy identification of the pipe after installation are becoming to be perceived as highly important. To improve the traceability of that heavy-duty pipe system, REHAU has decided to marks all RAUPROTECT II RAUSISTO pipes with durable, laser-marked 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.
5.4 Field test:
Gas pipe installation in Holzthaleben / Germany One of the first important projects using the new developed RAUPROTECT pipe system was in Holzthaleben in Germany.
The gas distribution company E.ON has chosen this system for a new gas distribution system, installed in very demanding conditions: the pipes had to be installed in very tough rocky soil , using horizontal drilling and ploughing-in installation techniques. 2,4 km of pipes diameter 63 SDR11 and 2,3 km of diameter 110 SDR 11 have been successfully installed for that project.
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 testings have been successfully fulfilled.
6.CONCLUSION
PE pressure pipes systems are designed for a longlife time, and are showing today the best leakage statistics compared with.
They have proven, since several decades to offer a safe solution for gas and drinking water distribution, contributing to protect these precious fluids. They are in particular showing the best leakage statistics among all pipe materials. One of the key reasons for this success is the innovation potential shown by both PE raw material producers and pipe producers; recently, a new market need for tougher materials for installation in demanding conditions has pushed for the development of a new generation of pipes.
The new RAUPROTECT II RAUSISTO system, based on the combination of BorSafe™HE3490-LS-H, a new PE100 material with outstanding resistance to slow crack growth and modern state-of-the-art pipe-design will continue to secure the safe transport and distribution of potable water and gas for generations to come!

Contact

Guido Kania (REHAU Senior Manager R&D, Civil and Underground Engineering)

91058 Erlangen, Germany

Phone:

+49 9131 92-5546

Fax:

+49 9131 92-51-5546

E-Mail:

Guido.Kania@REHAU.com

Internet:

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