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The properties of polyethylene make it a material that is used more and more in the world today. Discovered accidentally in 1933 by two chemists from the company ICI, the first industrial production began in early 1940. However, it was only after the 1970s that polyethylene pipes really took off. The quantities produced have continued to increase since then, reaching 16 million tonnes worldwide in 2005.
This success is due to its many excellent qualities:
• Firstly, the sustainability of networks that are completely watertight, because the tubes are delivered in long sections (coils, reels, bars) allowing for a reduction in the number of connections. On the other hand, the pipes are joined by welding (arc welding or butt welding). There will, therefore, be no leakage of water, gas or polluting fluids into the soil.
• The second property of polyethylene is the fact that it can preserve the quality of the drinking water being conveyed, because there is no interchange between the pipes and the drinking water consumed at the tap and PE has a waterproof outer layer. PE will hold back most liquids as it has a high level of chemical resistance.
• Being light and easy to handle, they can be laid out simply and rapidly in the trench in long stretches. The suppleness of the pipe allows it to adapt to any soil type and accommodate any expansion. They are flexible, easy to cut and offer a number of solutions for connections according to the site conditions.
• Last but not least, another advantage of PE is the fact that the pipes are recyclable at the end of their useful life: This is achieved through collecting, washing, sorting and shredding. An analysis of the life cycle of HDPE shows that there is very little environmental impact during its production process and throughout its whole life.
The “NF” quality label
II. These last few years, constant improvements in the quality have enabled polyethylene pipes to establish their worth and reputation, acquiring the “NF” quality label, which is widely recognised in France and throughout Europe.
They are very much in demand in the field of gas or drinking water conveyance piping, but, on the other hand, in the irrigation sector few users demand such high standards and tend to focus more on the price without attaching too much importance to the quality of the purchased product, which can lead to serious problems after a few months of use.
A more quality-conscious approach by the users that takes into account selection criteria such as having the “NF” quality label or compliance with the ISO 9001 standard, would help to reduce the risks regarding quality when buying polyethylene pipes and connections (below, an example of a pipe marked with the NF quality label is shown below).
As far as the pipes and fittings are concerned, the evolution of polyethylenes means that the previous classification - based solely on density: low, medium and high - has become obsolete.
There are various types of polyethylene (PE).
Each PE class is categorised on the basis of its minimum required strength (MRS).
In the field of irrigation, which product features are compulsory, essential or only advisable?
They depend, first and foremost, on the use of the product, but, in any case, we are going to look at the criteria relating to product life and crack propagation resistance and these will be linked to the tensile strength of the materials in order to be able to check rapidly how well the material reacts after its transformation into a pipe form. In general terms, we can say the following:
• The hose reels require very high quality PE products. A PE tube will have to “work” under difficult conditions in terms of pressure, temperature and draw. This is a product that has to comply with more rigorous safety criteria than that of a simple pipe laid on the ground with a maximum pressure of 6 Bar. The tensile strength and shrinkage in length are the most important criteria to be considered for this application. We can also mention the thermal stability as?well?as?the content and dispersal of carbon black, which has a direct influence on all of the aforesaid characteristics.
• The full cover sprinkler system in polyethylene is a different product. The general criteria outlined above must be?supplemented with other factors linked to the product’s use (lightness, rigidity, conditioning…). In view of the low pressures used (around 5 bar) and a reduced mechanical load, the PE used in full cover sprinkler systems allows for the employment of different resins that can offer the best solution at the fairest cost. Nevertheless, the effects of “water hammer” in this type of system should not be overlooked..
• Micro-irrigation should, of course, conform to the general quality criteria with a particular emphasis placed on a resistance to cracking, bearing in mind the number of “insert” connections that are used, even though, over the last few years, we have seen the development of connection technologies that are less restrictive.
• Pressurised water supply pipes have very different conditions of use, whether or not they are buried or vulnerable to “water hammer”. With these conditions of use, unknown when they were manufactured, it is possible to define and maintain the quality level of all of the applications.
• Golf courses and green space installations. The many advantages of polyethylene have made it the material of choice for irrigation pipes on golf courses. In particular, it has physical benefits such as its insensitivity to electric charges and it does not require coating or cathodic protection.
It is resistant to mechanical stress or damage, such as accidental knocks, land movement (earthquakes, subsidence, settlement and compaction) and it does not require any anchor or support blocks. It is unaffected by the chemical agents in the soil, particularly in saline environments. This is a significant asset because many people complain about the complete disappearance of the nuts and bolts of mechanical fittings in certain aggressive types of soil.
The fact that polyethylene pipes can be supplied in long sections, combined with their flexibility, means that they can be used in a similar manner to buried electric cables. This same technique means that systems can be installed on sites with several networks, resulting in improved turnaround times and minimum inconvenience for the users. With the criteria used in selection being strength and durability under a constant pressure, a polyethylene pipe with a minimal pressure of 10 bar (non food) is generally preferred by the design consultants.
The polyethylene market
The world production of polyethylene and all blended plastics reached 80 million tonnes in 2008 and 45 million tonnes for the plastic processors. The French market for PE pipes represented 108,000 tonnes in 2009. In France, 60% of pipes used in irrigation are made of polyethylene.
75% of the tonnage is used in 3 main applications: water,
electric cable casing, sewerage.