Conception et fonctionnement des adductions par canalisations sous pression à forts débits

Design and operation of high-discharge water supply duct systems under pressure

Ingénieur à la Société du Canal de Provence et d'Aménagement de la Région provençale, Le Tholonet

Résumé

Introduction Water supply systems above a certain size contain the following sub-systems, from the river intake to the final user : 1. Upstream conveying system. 2. The supply system proper. 3. The distribution system. The upstream conveying system consists of high-discharge canals or tunnels (depending on topography). The distribution system is a ramified pressure pipe network ending at the users' offtakes. Between these two sub-systems, there is almost invariably a third, which can be considered to constitute the supply system proper. This comprises appreciable lenghts of large-diameter pipe frequently operating under high pressure and conveying flows of a few cubic metres per second. System features depend on local topography ; if the upstream conveying system is at a sufficient height, the pipe flow is under gravity, and if not, pumping is resorted to. The S.C.P. owns a number of sub-sistems of this type, with a total pipe length of approximately 65,000 m. Others planned include "Berre-Ouest", "Berre-Nord", and extensions to the " Aix-Nord", "ToUilon-Est" and other existing systems (see Fig. 1). Specific design problems for all these projects are discussed in this report. Pipe runs Pipe runs must be as straight as possible, because of the high cost per lineal metre pipe length. Design discharge When demand varies considerably during a peak day it often costs less to install a compensation reservoir and operate the water supply system continously. Per kilometre pipe length and compensating storage costs can be calculated rapidly from Figs. 3 and 4. Available head. Requirement for booster station Since pipe cost deoreases with available head loss (Fig. 3), addition of a booster station to increase this head may result in a more economical system. This is unlikely for a system in continuous operation, but where a booster station is required to get the water over a high point, the economic optimum generally consists in increasing the strict minimum geometric head required. The cost of a booster station and corresponding power expenditure can be evaluated from Fig. 5. Stagewise implementation. Duplication of pipes Although implementation of a water supply system in two stages (duplication of pipes) is very difficult in practice, it undoubtedly is an attractive economic proposition for systems expected to supply a steadily increasing demand. The time at which duplication is most economical can be calculated from Fig. 6. Materials used Steel and prestressed concrete. Operation The water supply systems described operate "on demand", under downstream control. The delivery stations limit the rate of flow called for to the proportions subscribed for by the users ; some control arrangements maintain the piezometric gradient below the guaranteed contract value (sec Figs. 7 and 8). All installations, are protected against transient effects (e.g. hammerblow). Fig. 10 shows results obtained with a large S.C.P.-owned system.