High safety requirements in terms of hygiene and security of supply are the most important aspects in the planning and construction of drinking water supply networks. Therefore, it is necessary to use all available resources as far as possible. Local resources should be available for the local residents inside their region.
A proper transportation of potable water by appropriate equipment is essential. During the configuration of a supply system, special attention must be paid to the efficiency of the pumps, to the inner pipe diameter and to the proper selection of pipe material. But it is just as important to consider all constructions of potable water systems. They must correspond to the rest of the supply system in terms of functional reliability, safety requirements and durability.
Prefabricated constructions out of HD PE are used in drinking water supply systems for over 50 years. They are characterized by
• Low weight to reduce the transport costs and make the installtion easy
• High strength to withstand all internal and external loads
• High flexibility to allow a wide range of variety of styles and shapes
• Short time for the installation on site
• Low maintenance based on the smooth interiors resist growth of algae and bacteria and is easy to clean
• Highest standards in hygiene
• High level of safety
• Guaranteed reliability
• Economic efficiency
They have been developed for sparsely populated areas (e.g. mountains) where central drinking water supply is not economically feasible.
But a clear separation between a central and a decentralized drinking water supply is disappearing. Drinking water is a precious good. Where drinking water can be obtained from springs or wells, it is used to supply the people. A supply system consists of pressure pipes, manholes and reservoirs. The complete system has to be tight, fast to install, easy to operate and should have a possibly long lifetime and must meet all hygienic requirements. The great advantages of the prefabricated constructions that were learned to appreciate in the mountains also help in densely built-up areas. Now, they are used regardless of the population density in the drinking water systems.
The basement for all constructions is usually a spirally wound polyethylene pipe with a double walled structure. This double wall contains an internal solid wall layer, a reinforcing profile and a final external solid wall layer. These kinds of pipes provide low weight, high ring stiffness and low costs. They can be designed in a way that they withstand all internal and external loads. Together with the characteristics of the material, it is ideal for manufacturing collecting shafts and water reservoirs.The following report shows that structural double-walled pipes are a very attractive option for the fabrication of storage tanks and collecting shafts regarding costs and safety during installation and usage.
Situation in the drinking water supply area
The following figure shows a scheme of drinking water supply area. The water from different sources is collected and transported through pipes to the collecting shafts. Here, a quality and quantity control can be made. After this, the water is forwarded to a storage tank. The storage tank is used as a buffer; if necessary, the water can be treated here additionally. At last supply to the consumer can be carried out.
The following constructions are required for a safe drinking water supply:
• Source collection chambers
• Well shafts
• Transport Pipes
• Drinking water reservoirs
• Valve Chambers
• Firefighting Tanks
Requirements for constructions
The constructions must complete the supply system of pressure water pipes. Moreover, they must fulfil the requirements of safety and reliability. They are meant to collect and store potable water. At the same time the tanks and shafts must leave room for the measuring and controlling technology such as for pumps to pass on drinking water. However, the design should enable for easy inspection and maintenance as well.
Further requirements are:
• Short installation time
• Little maintenance
• High level of safety
• Guaranteed reliability
• Economic efficiency
To fulfil all these requirements prefabricated constructions are used. The basis is always a structured double wall pipe that is made out of polyethylene.
Choice of material - Polyethylene
Polyethylene has proven itself successfully for potable water applications for a long time. For more than 50 years it has been used for water supply systems as well as for collecting systems. Its neutral properties towards taste are constantly given. It has also been tested successfully for use of food storage. It is tasteless and resistant to corrosion. Pipes and other semi-finished products made from polyethylene provide a smooth and pore-less textured surface. These characteristics reduce the maintenance work and simplify the cleaning procedure. Because of the mentioned advantages it makes sense to design the complete drinking water construction out of this material as well.The welding of all inlets and outlets create a complete plastic system, which guarantees tightness and safety.
Large diameter HDPE pipes with structured walls
The polyethylene pipes with structured walls have been originally designed for gravity lines. They provide low weight, high ring stiffness and low costs. The wall structure is a combination of a solid liner and a reinforcing profile. The pipes are produced and controlled according to the international standards DIN 16961, EN 13476 and ISO 9969. The pipes are helically wound on a steel-mandrel. The mandrel ensures the inner diameter of the pipe. All pipes can be delivered from ID 300 to ID 3500. The winding technology allows an individual and customized design of the pipes for each project to fulfil all requirements regarding internal and external loads like inner pressure, soil- and traffic loads. Thus, any ring stiffness can be achieved and pipes for high loaded drain systems and manholes, even in large diameters, can be manufactured. All pipes that are used for drinking water supply have a blue inner surface. For pipes, tanks and shafts which are buried and which have to withstand heavy soil or traffic load, the ring stiffness is one of the most important design factors. The ring stiffness SR [kN/m²] is defined as the capability of the pipe to withstand the external pressure or forces. It depends on the moment of inertia of the wall, on the mean radius and the modulus of elasticity.
Ec [Nmm²] = Modulus of elasticity
l [mm4/mm] = Moment of inertia of the wall
rm [mm] = Mean radius
One of the major advantages of these pipes is the high ring stiffness at low weight. A comparison to solid wall pipes with same stiffness makes it obvious. A typical pipe that is used as a storage tank with a diameter of 3000 mm is a combination of a solid inner liner with a reinforcing profile and a final external layer. This pipe has a long time ring stiffness of S 50years = 3.1 kN/m². To achieve the same ring stiffness with a solid wall pipe a wall thickness of s = 89 mm is necessary. Whereas the structural double-walled pipe has a weight of 359 kg/m, a solid wall pipe has a weight of more than 828 kg/m.
The total weight of a complete prefabricated collecting shaft ID 3000 mm is about 1200 kg. A comparison with rigid concrete material makes the advantage in terms of weight even more obvious: a concrete construction would have a weight of about 18.000 kg (18 tons).
Factory control and assurance of quality
The whole production process is monitored by a quality management system. Which includes the internal quality control and the external (third-party) quality control.
The internal quality control is divided into three phases.
Phase one - intake control
When purchasing the raw material, the manufacturer’s test report must routinely be requested. On receipt, an intake control regarding melting index and density is carried out. After the material has passed the intake control, a test-pipe is produced to verify its technical properties. Melting index and density are once more tested at this stage. Furthermore, the durability of the material under short term tension is tested, a bending test is carried out and the heat resistance is examined. After this intensive procedure, the material is preliminary approved for production.
Phase two – during production control
During the production the complete production process including the production parameters and all working steps and must be continuously supervised and documented. So, corrections can be made immediately. The most important dimensions are measured continuosly.
Phase three – finished product control
After the production, the final product (pipe, fitting, manhole) is tested and compared with the requirements. This is the final control of all products where all parameters like surface, dimensions, material, colour and marking are checked. The results of the internal control testing shall be recorded, and, as far as possible, statistically evaluated. Records are kept for at least five years, and submitted to the inspection body on request. If the product passes all tests a quality certificate according to DIN EN 10204 can be made.The external quality control should take place twice a year. Of course The frequency of all tests must be agreed in accordance with the external quality control party – which can be “TÜV Rheinland GmbH”.All checked samples shall be considered representative for the whole production. Raw material tests should be carried out before adding the product to the production cycle. Undamaged / non affected samples can be sold afterwards. Those tests made in Production Plants are exceeding the requirements of the international standards like DIN16961, DIN EN 13476, ISO 9969, DVGW W300, etc.
The collecting shaft serves as a control station where quality and quantity of the water can be checked before it flows into the reservoir or continues to the transport lines. Every collecting shaft has to be divided in a dry chamber and one or several wet chambers. The entrance is always located in the dry chamber, which serves as a base for maintenance and service work. The different water chambers are arranged in such a way, that each spring is directed into a separate intake chamber which is also divided into an intake area and a settlement area which are separated by a scum board. Thereby it is possible to treat them individually. At the same time some particles like sand can settle down.The standard production has an inside diameter of ID 1200 up to ID 3500 mm. All pipe constructions consist of the structural double wall. They have a side entrance ore a shaft lid and can be delivered with a conical entrance. Whereas constructions with DN 1200 include only one intake, constructions with DN 3500 can include up to six intakes. A special drainage facilitates the cleaning. The water flow of each spring can be measured by installing a rectangular or a triangular-notch thin plate wire.
Drinking water reservoirs
Drinking water reservoirs are used as buffer for fresh water. Thus, variations in consumption can be well- balanced. The size of the reservoir has to be chosen in relation to the amount of fresh water supply, so that a sufficient supply of drinking water can be guaranteed at all times. The water rervoirs consist of an operating room and two or more storage chambers. This chambers are accessible from the operating room via a stainless-steel safety door which is located in the seperating wall. The operating room includes all necessary valves and measuring equipment. Through sight glasses, the internally illuminated storage tank can be visually inspected at any time. Each water reservoir has also an intake, an overflow, an extraction, and an outlet.
The standard constructions are produced with an inside diameter from ID 2000 up to ID 3600 mm. They are constructed with structural double wall pipes. The front and end walls are made of reinforced PE plates. The entrance is exclusively located in the dry chamber. Exceptions to this rule are storage tanks for fire-fighting purposes. They are constructed without a dry chamber. The storage volume can be freely chosen. Usually the storage tanks have a volume of 5 m³ to 800 m³.
Valve chambers are required to separate pipes, ventilate the supply lines, reduce pressure or measure the water quantities. So that a safe drinking water supply is guaranteed.
Design and installation
Flexible plastic pipes provide a higher durability than conventional rigid pipes. In the case of a buried rigid pipe, it is the pipe alone that constitutes the structure and is subjected to the full load, whereas in the case of a flexible pipe, it is the interaction procedure between the pipe and the surrounding soil that constitutes the structure . The pipe always carries the loads together with the surrounding soil. A correct design of plastics pipes and shafts requires consideration of the actual situation at site regarding the product itself, the soil conditions, the installation procedure and the different types of loading.
The pipe design method considers the following values:
• Information about construction site, pipeline zone and the main filling. It can havedifferent modulus of deformation and Proctor density of compaction.
• Information about external loads
• Installation including the trench excavation
• Height of ground water level
• Working conditions, such as outside temperature or operating pressure
The installation of the pipes is as important as the accurate calculation beforehand. Pipes and shafts perform only as well as they are installed. A proper soil investigation and a definition of the backfill parameters are necessary for the pipe design. It is commonly known that the deflection of all flexible pipes and their capability to withstand the external loads are strongly influenced by the installation conditions. Because of the interaction between the pipe and the surrounding soil, the installation should be done properly. The embedment is the most important part of the soil- pipe structure. Regarding the compaction around the pipes, tanks or shafts, it is necessary, that the backfilling material is easily and homogenously compactable. The backfilling material should be noncohesive soil and free of big stones. The compaction of the soil should be homogeneously and evenly distributed with the result that deflection is limited.
The installation time of the constructions should be as short as possible and include:
• Supply of all materials
• the excavation
• If necessary; wrecking of existing constructions
• Preparing the bedding
• Installation of the construction
• Connecting the service pipes
• Surface reinstatement
Our Experience has shown, that depending on the constructions, the installation time can vary between one and nine days. Such a short installation time can be realized due to the high degree of prefabrication in the factory. The constructions are supplied ready for operation. Only the connection of the service pipes must be done on site.
The following pictures show the installation of a collecting shaft. The transport is done by a small truck. After preparation of the excavation and the foundation, the shaft is positioned and the service pipes are connected. A minimum working space of 0.5 m shall be provided for correct backfilling. No grain size should come below 32 mm. The material is to be filled equally in layers of 20 - 40 cm thickness and must be carefully compacted. If necessary, an extra inlet or outlet can be made at any time during the installation. After this the old shaft can be wrecked. The last step of the installation is the reinstatement of the surface after the competition of the backfill.
The following pictures show a water reservoir during transport, the installation and after the reinstatement of the surface. After the trench excavation, the bedding must be prepared. The bedding must ensure an even pressure distribution under the pipe in the surrounding area. According to EN 1610 the bedding type 1 should be suitable. The thickness of the lower bedding shall not be less then 150 mm. After that the compaction of side fill and main backfill can start. The initial backfill directly above the tank should be compacted by hand or light compaction equipment. The side fill and the main backfill are usually compacted with a medium weight vibration stamper. All remarks about the material, layer thickness and working space are valid for the storage tanks as well. Because of their low weight, easy handling and the fact that all constructions were prefabricated, both introduced constructions have been completely installed an extremely short time and in accordance to all hygienic requirements.
ConclusionThe structural HDPE pipe is the most important component of constructions like collecting shafts, water reservoirs aund valve chambers. It has been shown, that a fast and cost-effective installation of an absolute tight, safe and reliable drinking water supply system can be realised. As the pipes are designed individually for each project, a highly loaded system even in large diameters can be produced.
Author Andreas Wittner, Hawle Kunststoff GmbH