Introduction
In most European countries today polyethylene (PE) piping systems are well established as the most popular material for small and medium sized water distribution mains. However for large sized water pressure pipes other materials tend to be preferred, particularly at sizes above 355mm (14 inch) where most new large diameter water mains are still laid in ductile iron.
Often, material selection for a large diameter project is commonly-based on the cost of the pipe rather than a full analysis of installation costs, including the cost of trenching and the bedding/surround, and future operational and maintenance costs. However, when all these factors are taken into account in the initial project cost the most economic selection may indeed change.
Material overview
To focus in on the historical use of pipes for large diameter water mains, grey cast iron pipes manufactured by vertical (“pit”) - or more recently centrifugal casting- created very thick walled pipes. Mainly due to this large wall thickness many of these pipes laid in the mid 19th century are still operating today although they are heavily graphitized and therefore very susceptible to brittle failure.
In the 1960’s ductile iron (DI) pipes were first produced where the internal graphite flakes were formed into spheres to reduce the potential for cracking. As cracking was believed to be less of a problem the wall thickness was also reduced.
PVC pipes were introduced for pressure water systems in the 1960s but problems with brittleness and premature failures of large diameter thick walled pipes hindered their market penetration. Today in the UK only modified PVC pipe systems are used. These are either molecularly oriented (MOPVC) or plasticized (MPVC) pipes produced in diameters up to 630mm (24 inch).
High performance filament wound glass fibre reinforced pipes (GFRP) are too expensive for water pipes and therefore the lower cost composite pipes of the “Hobas” sandwich construction are selected. Joint tightness problems and premature failures of large strategic pipelines have lead to the material no longer being used in the UK for pressure pipes.
PE pipes have been used for small diameter house connection since the 1960s but the first full PE systems were first introduced in the water industry in1984. Above this size the PE pipes tended to be more expensive. In the 1990s, the thinner walls of the PE100 pipes, for a given pressure rating, made the pipes more competitive as well as lighter and easier to handle on site, which extended the penetration of PE into larger diameters.
The development of PE100 materials with Low Sag properties such as BorSafe HE3490-LS, introduced in the late 1990s was another breakthrough for large diameter PE pipes, allowing cost-effective production of thick-wall pipes.
Whole life costing model
To help installers and system operators obtain a clearer perspective on these issues, Borealis, together with Thames Water and UK pipe producer GPS, has developed a whole-life costing model that allows the cost comparison of different materials for large diameter water mains. The model encompasses all cost elements of a project plus the long-term service reliability and the maintenance costs.
The principle of whole-life cost (WLC) analysis is to calculate all costs associated with a project throughout its life to a common base, so that true comparisons can be made between options. Thus, the WLC represents the sum of money to be set aside today to meet all the eventual costs, both present and future, after allowing for the accumulation of interest on that part of it intended for future commitments.
The WLC is estimated by discounting all the anticipated operation and maintenance costs, calculated at present day prices, by a factor which takes account of time from the start of the project to when the expenditure would be incurred by using the equation, saved in this version as one of the images. For information:
N Analysis period (years)
r Discount rate (%)
t Year of cost/benefit
ct Cost (initial cost, operation & maintenance cost)
The computer model has been set up to compare pipe materials, (in that case: ductile iron (DI) and polyethylene (PE) in two different pipe nominal bore sizes 400mm and 900mm. It is of course possible to adjust the model to cover other size ranges. To overcome the difference in the dimensioning system of PE pipes the two closest outside diameter series from the UK standards were chosen, namely 450mm and 1000mm.
The pipe prices used in the following examples are based on the UK water industry price at the time June 2006. Installation costs are based upon costs obtained from UK installers for the following components:
1. Number of joints completed per day;
2. Cost of jointing labour per hour;
3. Cost of plant hire for jointing per day;
4. Purchase cost of pipe per meter;
5. Labour cost of testing and commissioning per day.
Maintenance costs were calculated using the average cost of repairing a leak and the probability of failure for each type of system as derived from the UKWIR database. This may be revised in the future as more information becomes available particularly relating to the age of the pipe.
Finally, to provide a practical basis for the whole costing an example of a typical model large diameter project has been set up, comprising 5km of pipe containing two off takes, four sluice valves and two air valves. The discount rate used in the calculation is 5.1% as this is the current return on capital for the UK water industry.
This model provides comparative costs for the following elements:
1. Basic pipe cost (€ per meter);
2. Total installation cost (€ per meter);
3. Total installation cost for model project (in 1000 €);
4. Whole life cost for model project over 50 years (in 10,000 €).
Clear comparison
The results for each of these elements for the two different pipe diameters installed in an urban environment are shown in Figures 1 and 2.
In the 400mm project in an urban environment PE proves to be both the lowest cost to install and the lowest whole life cost despite being significantly more expensive per meter pipe.
In the 900mm project in urban environment, PE is again the best choice in terms of whole life costing although the installation cost 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 lower installation costs as the cost of the reinstatement of the road surface is expensive.
PE future choice
The WLC model highlights, that in virtually most cases the unique properties of PE work to the benefit of the installer and system operator. The calculation demonstrates that when all project costs for a large diameter water mains scheme - from material choice and installation to on-going repair and maintenance - are considered from the outset, the most economic selection may indeed place PE as the best option.
As water becomes scarcer, the durability and reliability of the system will be the governing factor within the material selection process. Having an effective water distribution system is essential for the future and this requires significant investments. Non-corrosive polyethylene and polypropylene pipe materials can make a significant contribution towards preserving drinking water.
Contact:
Christopher Salles
Application Marketing Manager Pipes, Borealis
Tel: +33 146148284
Email: christophe.salles@borealisgroup.com
www.borealisgroup.com