2.1. POLITICAL, SOCIAL, HUMAN AND ECOLOGICAL ASPECTS
Since prehistory, mankind has used artificial pipes and channels to transport water to settlements and fields. In the Middle Ages wooden tubes were used. Some are still in service in some districts of Paris and London. Water distribution systems must be constantly expanded to satisfy the needs of the population. In many countries, population is more and more sensitive to the fact that water problem is also a sanitary problem. Some disease have appears by introduction of bacteria into drinkable water through the orifice of the leak.
2.2. ELEMENTS OF A WATER GRIDS AND WATER NETWORK DENSITY
Pipelines are complicated three-dimensional structures that include straight pipes, nozzles, pipe-bends and different welded joints. In Western Europe, the distribution of drinking water has been practically completed over the last 20 years and now the problem of renewing the grids is considered as an urgent matter in terms of money and time. Average networks loose about 30% by leaks or ruptures. In some Mediterranean countries, more than 80% is lost by leaks, breaks and illegal withdrawing. Studies made in North America indicate that each year 10.4 billion litres of drinking water never reaches the tap. The most important part is lost by leak. The annual cost of the lost resources is estimated to US$3.6 billion in the USA and CAN$625 million in Canada. By reducing leak and rupture of the water pipes which cause about two thirds of the losses considerable economy can be made. In these countries, it has been pointed out that water distribution companies or city water offices needs new methods and technologies in order to reduce rapidly, efficiently and economically the water losses.
The materials used for water piping are grey cast iron, ductile iron, PVC, cement-based materials and sometimes steel. Grey cast iron has been the main standard material for the past 150 years: the majority of distribution piping installed in North America and Western Europe, beginning in the late 1800s up until the late 1960s, was manufactured from this material.
Grey cast iron is among the most common material used in the water distribution pipes of developed countries: about half of the networks in North America and more than a third in France. Grey cast iron is a problematical material because it is relatively brittle and susceptible to corrosion. Because of this and their age, grey cast iron pipes have the highest number of failures per kilometre per year, and they must be replaced as soon as possible. Ductile cast iron pipes were introduced in the late 1960s in Western Europe and in the late 1970s pipe in North America to replace grey cast iron pipe. Ductile cast iron piping is stronger and more resistant to corrosion than grey cast iron. Its lifespan is estimated at 100 years. Steel pipes have been used since the 1930s in Western Europe. The older ones are joined with lead and have to be replacing as soon as possible. The lifespan is estimated at 100 years for recent pipes. Polyvinyl chloride (PVC) pipes have been used in Western Europe and Australia since the early 1960s and in North America since the late 1970s.
Cement-based materials include reinforced or pre-stressed concrete, cement-mortar linings and asbestos-cement. Two general components of cement-based materials include the aggregates and the binder. Asbestos cement water mains were installed in North America, Europe, and Australia from the late 1920s to the early 1980s to replace the failed cast iron pipes and still form a significant component of water distribution networks of many cities.
The operating conditions of the piping can be quite severe, with internal pressure, bending, and thermal and cyclic loading combined with the influence of internal and external corrosive environments. The potential synergy of such parameters can lead to an increase in the risk of damage and unexpected fracture of these structures. Corrosion reduces the thickness of metal pipes and thus their ability to resist failure. The primary stress is due to internal pressure which produces a uniform circumferential tension stress across the wall. Pipes can also be subjected to secondary loadings, due to the soil, frost, bending loading due to the vehicular loading, and thermal stress. Bending produces an axial compression and tension stress while soil pressure and the changes in temperature induce axial and circumferential stresses.
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