Drinking water distribution systems are sometimes spread over thousands of kilometres, with a network of structures that are:
- more or less interconnected,
- of varying age.
The complexity of distribution structures can make them difficult to control and in some cases may lead to heavy losses. It has in fact been estimated that between 10 and 50% of the water supplied never actually reaches the tap, resulting in a considerable financial loss to the community.
To reduce losses and improve the functioning of the distribution channel, a detailed study is required. This will lead to a greater awareness of the hydraulic behaviour of the network and pinpoint unused sections and leaks.
Before the quality management of distribution networks can be optimised, the way they function needs to be determined and described. A diagnostic study is required. This study involves a detailed “auscultation”.
[Encart :
ÉTUDES DIAGNOSTIQUE
FAIRE UN ÉTAT DES LIEUX
← ANALYSE LE FONCTIONNEMENT
→ ANALYSE LES CONSOMMATIONS → DONNÉES DE BASE POUR LES ÉTUDES ULTÉRIEURES (Schéma directeur, etc.)
↓ ANALYSE DE L'ÉTAT PHYSIQUE DES OUVRAGES ET APPAREILS DU RÉSEAU
↓ ANALYSE DES PERTES DU RÉSEAU
→ CONNAISSANCE DU RÉSEAU → ÉLABORATION DES PLANS
↓ AMÉLIORATION DU FONCTIONNEMENT ÉCONOMIQUE ET RENFORCEMENT DE LA SÉCURITÉ D’ALIMENTATION EN EAU (Quantitatif - Qualitatif)
]
Aim of the diagnostic study
For over fifteen years, Edacére has been carrying out diagnostic studies of drinking water networks. The technical solutions that are proposed are established in accordance with local constraints and allow a procedure to be defined which can become an effective tool for preventive maintenance.
[Photo : Water-well drillers have always listened to the sound of the water. And today the tradition lives on.]
of the network, aimed at bringing any malfunction to light.
This is carried out by service companies, which perform a highly detailed study of the hydraulic behaviour of the network and the differential between “water produced” and “water billed”, so as to bring to light any anomalies.
A precise analysis of the situation
Recently constructed networks already have integrated instrumentation. Information on the state of valves and rates of flow is collected and analysed. Nothing escapes the attention of the network supervisor, who can provide a continuous diagnosis of the system.
In the case of older works or those lacking the necessary equipment, a special procedure must be undertaken. A precise analysis of the situation begins with a thorough survey to measure rates of flow. The network consists of a grid, and a flowmeter is placed at each junction to measure the movement of water. This operation is carried out both during the day and at night. The state of the valves is also recorded. The resulting data are stored. The entire process may take days, or even weeks if the network is complex.
This operation allows movements of water to be measured at different points in the network. It pinpoints branches with an insufficient rate of flow, a factor that can have repercussions on water quality. It also detects any abnormal consumption, often signifying a leak.
Whenever an anomaly is found, a more precise exploration can be initiated. This investigation is based on listening for leaks.
Listening for leaks
Water-well drillers have always listened for the sound of water. And the tradition lives on! Pressurised water makes a noise as it leaks through the wall of the pipe. This phenomenon is used by the majority of the leak detection instruments on the market. They detect the sound of leaks. CMR-SMR, Agrippa Fast, Dantec, Metravib Fluide, and Sewerin all propose devices based on this principle.
A preliminary, approximate localisation can be carried out using a listening device in the zone where a leak is suspected. A ground microphone with protection from the wind can precisely localise a leak under a roadway or loose ground. This prelocalisation of the problem can be refined by using zonal acoustic monitoring devices. Metrascan, for example, is marketed by Metravib Fluide. It records the sound of the pipe second by second. The different sound levels obtained are recorded in the form of a histogram. A histogram is generated every five minutes over a period of two hours.
Agrippa-Fast, on the other hand, has opted for the SAZ system. As the firm's manager Philippe Ritter explains: “The SAZ zonal monitoring system replaces tedious and
[Encart : texte : The problem of lead
The forthcoming European directive on water is bound to raise new problems. By imposing a lead content of less than 10 µg/l it is likely to prove a very real problem. Water authority managers will have to distinguish between risks relating to the sections under public supervision, which have to be resolved by the public authority, and those relating to the sections under private responsibility. Needless to say, detailed inventories will be required.
What strategy should be used to reduce lead in drinking water? Existing water treatment cannot guarantee a concentration of less than 25 µg/l, which is thus insufficient to meet the 10 µg/l limit provided for in the directive. The replacement of lead pipes is for the time being the only solution for achieving a permanent reduction, but it is very costly and will be difficult to carry out in a short time.
In France, it has been estimated that the cost of achieving compliance in the drinking water network would be in the region of 20 000 million ECUs, or roughly 130 billion francs. This sum only takes into account the cost of replacing pipes. To this must be added the cost of replacing lead welding and apparatus containing lead, galvanised steel, PVC stabilised with lead, etc.
Lining pipes is another possible solution but has yet to be explored.]
[Photo : Water leaking through the pipe wall is under pressure and produces an audible sound. Most of the detectors on the market work by picking up the sound of leaks.]
[Photo : Figure 4 - Analysis of network losses]
Volume utile pour la DEFENSE INCENDIE
= lutte contre le feu
Volumes utiles pour le SERVICE DES EAUX - Réservoirs
- Pressions d'essais
RESEAU DE DISTRIBUTION
- Pertes d’eau
- Débits estimés
- Arrêtement des chaussées
- Infiltrations etc...
Erreurs de comptage
Ecoulements permanents
Consommations non compatibles
Pertes de recettes pour le SERVICE DES EAUX
Chasse d’égout
Non rentabilité des bassins
Dérangements privés
Arrosage
Fuites des canalisations
Ecoles
Equipement
PROGRAMME DE RENOUVELLEMENT DES COMPTEURS ANCIENS
PROGRAMME DE REDUCTION DES PERTES D’EXPLOITATION EN PERIODE ESTIVALE
PROGRAMME D’INSTALLATION DE COMPTEURS
PROGRAMME DE REDUCTION DES PERTES EN REMPLACEMENT DES VANNES ET APPUIS
PROGRAMME CONTRE LE GASPILLAGE
PROGRAMME D’AMELIORATION DU SUIVI DE CONSOMMATION - INDUSTRIELS
NOUVELLES RECETTES POUR LE SERVICE DES EAUX
with measurements in three dimensions.
The Vector part is a waterproof self-propelled robot, suitable for pipes with a diameter of between 150 and 800 mm.
The inspection is usually carried out in 1,000-metre sections, using a 7-mm diameter reinforced cable link. The chassis carries a Radiax colour camera with integrated zoom and laser. It is equipped with a computerised sensor data acquisition system. At the other end of the cable link, a PC and joystick allow the operator to pilot the unit.
When all the field data have been acquired, they have to be analysed. This is the diagnostic stage, carried out by the engineering and design department.
Making a precise diagnosis
An analysis of the volumes produced and the volumes billed provides a means of determining the level of efficiency of the network. Ideally, the highest possible figure should be obtained. Water balance is carried out using:
- • measurements by zone,
- • detection of leaks by acoustic correlation,
- • counter-measures introduced after repairs.
This provides valuable information on the functioning of the system. The information is then compared to the network plans and altimetric diagrams.
This process helps to detect unused sections and zones with insufficient flow.
Based on the results of these analyses, the engineering and design department can make proposals for improving the management of the network, for example by introducing a system of remote processing or the replacement of certain sections. ■
[Publicité : Editions Johanet – Lexique de l’Eau]
