Polyethylene for water - polymer pipes
Putting effective water distribution systems in place is essential for society and the economy. Polyethylene pipe systems may provide solutions for the global water challenge, says Christophe Salles, Application Marketing Manager, Drinking Water and Gas, at Borealis’ Pipe Business Unit, France.
In Europe, many existing water supply and treatment infrastructures need to be upgraded as a matter of urgency. Those that were installed 50 or even 100 years ago, are no longer efficient, creating mounting losses for their owners and operators, and posing a potential threat to public health and the environment.
A recent study of Italy’s water supply and treatment infrastructure showed that the country’s ageing pipe system loses around 40% of its clean drinking water. This represents annual losses for its operators of 3.9-5.2bln euros.
Before changing or selecting new pipework, local authorities need to consider economic, ecological and quality factors when deciding on the material to use. It is important to not only look at the immediate cost of installation but also the long-term costs of a network system, ongoing maintenance and repair, as well as operational costs.
Material choice is key
In the water industry a number of materials are used for pressure water mains. Historically, iron pipes were applied in large diameter mains. These were thick walled, and it is mainly due to this wall thickness that many of these pipes, some laid as early as the mid 19th century, are still operating today. However, they are heavily graphitised and therefore susceptible to brittle failure.
In the 1960s ductile iron pipes were introduced, with thinner walls where the internal graphite flakes were formed into spheres to reduce the potential for cracking.
The 1960s also saw the introduction of PVC pipes for pressure water systems, but problems with brittleness and premature failures of large diameter thick walled versions hindered their market penetration. Today modified PVC pipe systems are frequently used. These are either molecularly oriented or plasticised pipes produced in diameters up to 630mm. For water pipes with a large diameter, glass fibre reinforced plastic is sometimes used. However, following issues with joints and failures of large pipelines, a number of countries no longer use this material for pressure pipes.
The first polyethylene (PE) pipes were produced almost 50 years ago. Proven ease of installation, low maintenance and corrosion free performance are some of the reasons behind its continued use.
Following on from PE materials such as PE63 and PE80, the first high density polyethylene (HDPE) pipe materials, such as PE100 arrived on the market at the end of the 1980s. These grades have different strengths and facilitate the use of higher service pressures, or pipes with thinner walls for the same service pressures, while offering high mechanical performance. This means they are unaffected by ground movement and traffic vibration.
The development of PE100 materials with low sag properties (reduced sagging of the material during pipe production). such as Borealis’ BorSafe HE3490-LS, introduced in the late 1990s, allowed for cost-effective production of thick-walled pipes.
Alternative installation methods can be used for PE pipes, such as ‘trenchless’ laying techniques where pipes are pulled through the earth. This produces less environmental disturbance than other systems.
Moreover, plastic pipes demonstrate extraordinary durability. A study by the University of Osaka, Japan, following the 1995 Kobe earthquake, demonstrated that the PE gas distribution system was the only one able to continue operating with no failures.
In most European countries, PE pipe systems are established as the most popular material for small and medium sized water distribution. However, for large sized water pressure pipes, other materials tend to be preferred, in particular ductile iron. This selection is often based on tradition or because designers and consultant engineers are unfamiliar with the benefits of flexible and weldable plastic pipes.
Together with Thames Water and UK pipe producer GPS, Borealis, headquartered in Vienna, Austria, has developed a whole-life costing (WLC) model that allows a real cost comparison of different materials for large diameter water mains, including all cost elements of a project plus long-term service reliability and maintenance costs.
The principle of WLC analysis is to calculate all expenses associated with a project throughout its lifecycle to a common base, so that true comparisons can be made between materials. The WLC represents the sum of money to be set aside today to meet all eventual costs, after allowing for the accumulation of interest for future commitments.
Using a specific formula, the WLC model provides comparative costs for basic pipes’ installation and lifecycle for a project calculated over a 50 year lifespan.
The results for each of these elements for two pipe diameters installed in an urban environment are shown in the graphs.
In the 400mm pipe project the model shows PE to be both the lowest cost material to install and the cheapest whole lifecycle cost despite being significantly more expensive per metre of pipe.
In the 900mm pipe project, PE is again the best choice in terms of WLC, although the expense of installation is higher. In both cases, despite the apparent additional cost of butt fusion equipment, the longer pipe lengths result in lower joint costs. Narrower trenches also lead to reduced installation prices as reinstating road surface is expensive.
As water becomes scarcer, the durability and reliability of the distribution system will be the governing factor when choosing a pipe material. Having an effective water delivery system is essential for the future and this requires significant investment. Non-corrosive PE and polypropylene pipe materials can make a significant contribution to preserving drinking water.
Classification to ISO 12162
The MRS value is the Minimum Required Strength, calculated after long-term pressure testing for a minimum life-time of 50 years transporting water at 20°C.