Water Manual for Refugee Situations (UNHCR, 1992, 160 p.) 10. Water distribution systems (introduction...) General Types of pipeline systems Valves and taps Other system components Considerations for pipeline designs Pipeline construction

Water Manual for Refugee Situations (UNHCR, 1992, 160 p.)

10. Water distribution systems

· Water distribution systems are needed to convey the water drawn from the source, through treatment and storage facilities, to the points where it is delivered to the users.

· Water distribution systems should be kept simple. They should, however be designed and constructed in a proper way as they represent a substantial capital investment that should always be useful and cost-effective.

· An appropriate water distribution system should ensure an even coverage of water needs among the camp population.

· Under normal circumstances, water distribution in refugee camps should be carried out through public distribution standposts. Service and administrative buildings should be provided with house connections. Staff housing should, whenever possible, be provided with private connections.

· The design, construction, operation and maintenance of the water supply system should be carried out bearing in mind the need to minimize eater wastage. This is particularly important in systems based on low yield water sources or on those requiring treatment or pumping.

General

1. Water distribution systems (or "reticulation") should be built to deliver the required quantity of water to individual users and under a satisfactory pressure. In refugee camps, water reticulations are always a major investment and as such require careful design, construction, operation and maintenance. Under normal refugee situations, distribution systems should cater for the domestic and sanitary requirements of the refugees, camp administration and service centres. Garden or livestock watering may be unavoidable cultural factors which should be covered, as far as possible, in many refugee camps (See 9.11). As the water demand in refugee camps varies considerably during the day, the pipeline network should be designed to supply the "maximum hourly demand", usually estimated to be 30% higher than the estimated average hourly demand (daily water demand divided by 24) (See 3.1-9).

2. The main system components are the pipelines; other basic components are break pressure tanks (See 10.8), valves (See 10.6), service reservoirs (See 9.5-6) and watering points (See 10.9).

Types of pipeline systems

3. From a layout point of view, there are two types of piped distribution systems (See Fig 33):

i) Branched systems are those that convey water from a distribution main to different consumption points, following a treelike pattern; all their branches finish in dead-ends. Their design is straightforward but has a main disadvantage in the fact that it causes stagnant water pockets in all dead-ends. If repairs are necessary, large areas must be cut off from service. Head losses, due to heavy local demands - or during a fire - (See 7.13) may be excessive unless the pipes are quite large.

ii) Looped network systems usually have a ring mains to which secondary pipes may be connected. Their design is much more complicated; with them the possibility of stagnant water is reduced. If part of the pipeline needs cleaning or repair, it may be isolated from the rest of the system (with appropriate valves); all watering points outside of it may continue to be supplied.

Fig. 33 Pipeline Layouts

4. Pipelines can be classified in accordance with the tasks they should perform:

i) Trunk mains convey water from the sources to other points in the distribution system over long or short distances. They may be pumping mains if the water is coming under pressure from a pumping system or gravity mains if gravity is the only force used to generate flow. Distribution mains are those to which standposts and other service connections are connected.

ii) Service pipes connect the mains to a camp's section, a standpost or a house connection.

iii) Plumbing pipes form the pipework within standposts, showers, houses, etc.

5. In refugee camps, the most commonly used pipe materials are polyvinyl chloride, known as PVC and high density polythene, known as HDPE; under special circumstances, especially when the pipeline has to withstand high pressures, galvanized iron (GI) pipes are used. The use of asbestos-cement pipes for human water supply should be avoided. The choice of pipe materials should be decided bearing in mind availability on local markets, the cost, the diameters available and their pressure ratings. Resistance to corrosion and mechanical damage, as well as transportation requirements to the project site should also be considered. Although both PVC and HDPE pipes are relatively easy to transport in view of their light weight (HDPE has the additional advantage of being provided in rolls for pipe diameters of 160 mm. or less, thus reducing the number of necessary joints), both have the disadvantage of being very easy to tap in unauthorized ways (illegal connections). This can be avoided to a large extent by laying the pipe in appropriate trenches and then covering them. This is, in any case, strictly required by PVC, which is a material that degrades when exposed to sunlight, losing part of its strength and becoming brittle; care should therefore be taken to cover PVC pipes when they are stocked in the open.

Valves and taps

6. All pipeline systems require the use of valves to control flows and pressures as well as for closing or opening a pipeline or a section of it (Fig 34). As the pipeline must always follow the terrain's topography, some valves are used for the release of air that may be trapped at high points (air valves) and to facilitate emptying and scouring the pipeline to flush out sediments that may have been deposited at low points (wash out valves). Sluice valves are fitted to the pump outlets in the case of pumped supplies, but are also installed to isolate pipeline sections during operation or maintenance activities; these valves are also known as gate valves. Non-return valves consist of a flat disk set pivoted within the pipe in such a way that it may be forced open by water flowing in one direction but also forced shut, thus impeding the flow, if water tends to flow in the opposite direction. Float valves function with the same principle; the driving force is given to the mechanism by the upwards movement of a floater or buoy, thus allowing the automatic closure of inlet pipes before tanks overflow. Other valves, such as the butterfly valves, screw plug valves or ball valves are also used for flow control tasks and are built on the principle of a plug, diaphragm or jumper which is forced into the pipe's opening, thus reducing or shutting off completely the flow; as their sealing device (gasket) wears down rather quickly, they require constant attention and periodical renewal; this may become an important maintenance problem if the valves have a frequent use; an additional disadvantage of these valves is that, due to their design, they cause considerable pressure-head losses, even when completely opened. Stopcocks, also known as water taps or faucets, used at water distribution outlets at public standposts or house connections, are normally designed in accordance with the same principles of screw plug valves. They therefore suffer from the same shortcomings related to the short working life of gaskets, thus creating a major maintenance problem, especially when distribution is carried out through public distribution standposts; these taps may be opened and closed hundreds of times during a single day; as their malfunctioning is one of the main causes of water wastage, this should be given close attention during the planning and implementation of a preventive maintenance programme. Recently, very sturdy, easy to repair and maintain self-closing taps (known as water saving taps) have been developed specifically to address these problems; their introduction in public distribution standposts at refugee camps has proven successful in minimizing water wastage and camp maintenance costs.

Fig. 34a Type of Valves: gate valve

Fig. 34b Type of Valves: plug valves

Fig. 34c Type of Valves: water-saving tap

Other system components

7. Valve boxes should always be built to protect control valves from undesirable tampering, which may upset the hydraulic behaviour of an entire water supply system or some of its components; valve boxes are to protect control valves - and the whole supply system - from this type of disturbance; they may be attached to other structures (e.g. storage tanks) or placed independently along the pipeline. They may be made from many materials, depending on local availability, but they should always be provided with a secure cover, adequate drainage, and a size large enough to allow easy operation and maintenance.

8. Whenever it becomes necessary to reduce hydrostatic pressures in gravity pipelines, break-pressure tanks are used. These tanks permit the flow to discharge into the atmosphere, thus reducing pressures to zero; a new static level is, therefore, established. Strategic placing of break-pressure tanks minimizes capital costs, as the need to use GI pipes or more expensive, higher grade plastic pipes is reduced (See 10.5). Cement masonry, concrete or any other suitable material may be used for their construction.

9. The most common water distribution facility used in refugee camps is the public distribution standpost or tapstand. These structures should be designed and built bearing in mind that no other component in the water supply system will suffer more abuse and that they should always be adapted to social and cultural needs of the beneficiary refugee population. This is particularly important in view of the fact that standposts are more than a physical structure; they will normally become a social gathering point where several day-to-day activities (water collection, clothes washing, bathing) will take place (See 6.29). This means that, as part of their design, enough attention should be paid to their location and to the additional facilities necessary to make them sanitary and attractive. The control of water wastage at standposts should also be given importance. Users should never fail to turn off the taps and constant maintenance should be ensured to avoid leaky or broken taps; self-closing, water saving taps have proven effective in this context and their installation in tap-stands should be encouraged (See 10.6). The use of prefabricated distribution standposts may be considered during emergency situations, especially if other system components, such as pumping sets, storage or filtration tanks, etc., are also being brought in as prefabricated packages or kits; these should, however, be of sturdy construction and should allow the use of water saving taps. No single standpost location is likely to meet all ideal requirements; selecting the most appropriate ones will always be a process of compromise. Standposts should be located in places where distances to water users are minimal; as a guideline, 200 metre distances are advisable for most refugee camps, while in less congested situations, such as in rural refugee settlements, a minimum distance of 500 metres may be acceptable. The need to drain away all waste water should also be given consideration; the costs for this drainage system may be substantially reduced by locating other service components, such as laundry or bath/shower facilities, in the vicinity of the standpost or by using some of this waste water (free of soap or detergents, please!) in fruit or vegetable garden irrigation. Water pressure at standposts should not be too high, never higher than 4 bars (40 metres); very low pressures should also be avoided (absolute minimum: 0.70 bars or 7 metres). While it would be desirable that a single tap would not be used by more than 20 beneficiaries on average, this figure could be as high as 100, depending on the characteristics of each particular refugee situation; to provide an appropriate coverage, multiple tap standposts may be constructed; common designs allow for the installation of 5 to 10 taps in each post.

10. The need to include appropriate washing/laundry facilities as a standard infrastructure component of a refugee camp is often overlooked. Washing cooking dishes and clothes is a basic need and, as such, should be appropriately covered by the camp infrastructure. If not, more wasteful, and perhaps less sanitary alternatives, will be developed by the refugees themselves. It is not possible to give general rules or guidelines for the design or construction of appropriate laundry or bathing facilities, as they should respond to the individual needs, as well as to cultural and religious practices of the refugee users. Therefore, their design should be entrusted to qualified engineers who should take into account cultural habits, sanitation requirements as well as the need to minimize water wastage.

11. In some circumstances, there will be a need to provide appropriate watering points for cattle (See 10.1) or for the filling of animal driven carts or water tankers (See 3.5). Adequate designs for these facilities are available in the literature. Their location (normally outside camp boundaries) should, as a rule, be away from refugee water supply standposts. These facilities should always be provided with appropriate access and efficient drainage facilities.

Considerations for pipeline designs

12. Water moving through a piping system is subject to friction with the inner surface of the pipes and therefore continuously loses pressure in the direction of flow; this loss is proportional to the length of pipes, to the roughness of their interior and to the square of the velocity. These friction losses may be calculated by using formulas; different graphs may also be used for this purpose (See Fig. 35). This means that in a pipeline system with flow under dynamic equilibrium, pressure drops in the direction of flow in accordance to what is known as the hydraulic gradient, which also represents the energy levels at each point along the pipeline.

Fig. 35a Graphic Determination of Friction Head Losses in Pipes: HDPE pipes*

Fig. 35b Graphic Determination of Friction Head Losses in Pipes: PVC pipes*

Fig. 35c Graphic Determination of Friction Head Losses in Pipes: galvanized steel pipes*

* All pipe diameter are in millimetres

13. The amount of energy remaining in the pipeline system by the time the desired flow has reached the distribution points is what is called residual head, and may be either positive or negative. While positive heads indicate the presence of energy in excess and that there is enough energy to move an even greater flow through the pipeline, negative heads would indicate that, within the pipeline, there is not enough energy to move the desired quantity of water. If a pipeline with a positive residual head is allowed to discharge into the atmosphere, the flow will increase until the residual head is reduced to zero; this flow, which for the given conditions of each pipeline is always maximum, is called the natural flow of the system. In a gravity fed pipeline, the natural flow should always be smaller than the safe yield of the water source (See 6.20 and 6.38), otherwise, the pipe would drain faster than it can be filled and the result will be that the pipe will not flow fully and any standpost located in this section would not function normally.

14. As already mentioned, high velocity flows within a pipeline increase friction losses. At the same time, with high velocities, suspended particles can also cause excessive erosion of the pipes; if the velocity is too low, these suspended particles may settle and collect at low points within the pipeline, which may even clog if provisions have not been made for sedimentation (See 8.14-16) of the water or for the provision of appropriate wash-out points for the pipeline (See 9.6).

15. Air blocks are bubbles of air that remain trapped, particularly at high points of a pipeline; their size may be such that they could interfere with the normal flow of water through this section. They may become very important (and problematic) in the case of pipelines which are subject to periodical drainage and refilling and provisions should be made to install air valves (See 9.6) at all high points of the pipeline.

16. The bases for the design of any pipeline is the graphic plotting of the topographic survey along the pipeline's route in the form of an "altimetric profile" showing the variation in soil elevations from the source to storage, treatment and distribution points. This survey should have been previously carried out as part of the basic studies to assess the beneficiaries' needs and to produce the conceptual design and budgets required for project approval and funding (See 5.1; 12.8). The hydraulic design comes next; the possibility of using gravity as the only driving force for the water to flow is assessed (See 10.12) and, if insufficient, the calculations for pumping requirements are made; all system components (including treatment facilities, storage, pumping and gravity mains, distribution lines and taps for which it may be possible to use standard models) are also designed (hydraulic and structural designs) and the final checking for hydraulic soundness and efficiency is done, bearing in mind the ultimate goal of providing a cost effective and reliable supply of safe drinking water to the refugees. The final drawings, showing all technical details of the system, are then finished and will accompany the topographical profile (showing also the pipeline's hydraulic gradients) and the planimetric map showing the exact location of all system components. Once this is done, the documents are ready for "blue-printing". Detailed estimates of materials, labour and money required for the construction are then calculated.

17. As mentioned before, the task of designing any water supply system should be entrusted to a qualified and well-experienced engineer. It will be the responsibility of this engineer to provide a complete record of his investigations, surveys, calculations and designs; this data will prove useful in the project approval and funding exercise, in the negotiations for project implementation (identification of implementing partners, tender procedures, contractual negotiations) and for supervision, operation and maintenance purposes (See 12.8-17). Such data should contain, at least, the following:

i) Pipelines: All relevant data on the different sections of the pipeline (pumping mains, gravity mains, branches, tap connections, etc.) (See 10.4), including pipe material, lengths and diameters. A planimetric map, at an appropriate scale, of the layout of each section of the pipeline, giving clear indication of the length and diameters of each pipeline component, the position of related structures (intakes, valve boxes, reservoirs, etc.)

ii) Surface water catchments, boreholes or wells: Description of the catchment, well or borehole as a water yielding structure (See 6.37; 6.54); results of test pumping and productivity assessments (See 6.38; 6.55); water quality characteristics (See 3.13).

iii) Intake sections: Sketches (using convenient scales) of the location of sources and the future structures to tap them; design drawings of these structures; calculations of construction needs (volumes of excavation, construction materials, etc.) and labour.

iv) Treatment facilities: details and scaled sketches of pre-treatment and treatment structures required (sedimentation, filtration, chlorination, etc.), including specific details of all piping and valves, construction requirements in terms of material, labour, special tools, etc. (See 8.8).

v) Break pressure (See 10.8) and reservoir tanks (See 9.2): Careful drawings of the designs are required, depicting all necessary construction details on the structure, pipe and valve arrangements; construction requirements in terms of material, labour, tools, etc.

vi) Distribution points: Drawings of each water outlet (individual connections to service, administration or staff accommodation buildings, public distribution standposts, animal troughs, etc.); construction requirements in terms of materials, labour, tools, etc. (See 10.9-11).

vii) Other system components: Drawings and other relevant details (location, construction characteristics, piping and valve arrangements, etc.) of special components such as valve boxes, river crossings, etc.

viii) Total estimates: Two lists, one for locally procured material and another one requiring purchase and transportation into the country or project area. Unit prices and total costs should accompany these lists. All tool requirements should also be presented, as well as other logistical details on transport of material and related costs (See 12.17).

Pipeline construction

18. Beneficiaries, not taking into account strangers, heavy animals or vehicles, may cause considerable damage to exposed pumping equipment, pipes or fittings with frustrating results. These problems should be prevented by taking practical and tailor-made steps for each project. In this context, efforts should be undertaken to make beneficiaries understand the difficulties of repairing damaged systems and the negative impact that such repairs have on their own welfare; their cooperation in protecting the system should, therefore, be fostered and encouraged.

19. The design and construction of a water supply system should be guided by the need to avoid these problems and to provide maximum protection to the whole system against adverse weather and other environmental conditions. If pipelines are not constructed properly the first time, remedial actions are difficult, time consuming and discouraging tasks, especially if they have to be undertaken as a result of carelessness or sloppy construction techniques or practices.

20. Pipes should normally be laid within trenches to protect them from damage from traffic or weather conditions. In the tropics, the proper depth of trenches should be at least 0.80 metres; deeper trenches are necessary to avoid freezing and other cold weather effects in higher latitude countries; local experience should therefore be taken into account in choosing the right depth of trenches, always bearing in mind the increased costs deeper trenches represent. Although there are no special requirements for the width of trenches, cost factors determine that this width should be kept to the minimum necessary (mainly determined by the width of the trenching equipment). The trench should be dug in sections equal to the length of the pipe to be buried in it each day and should be free of sharp rocks or bends that may interfere with the pipe; when the entire section is dug, it should be inspected before the pipe is laid.

21. Once the pipe is laid within the trench, and all connections inspected, backfilling may be carried out. The material to be used should be soft and granular; large stones should be avoided. An initial backfilling, to cover the pipes with a minimum of 20 cm. of soil, should be carried out as soon as possible after the pipe has been laid into the trench to provide protection to the pipe. Final backfilling may be carried out after the entire pipeline section has been tested.

22. Although the pipeline should, ideally, follow the route that was originally surveyed and used in the pipeline design and related calculations, it may be necessary during construction activities to introduce some detours or other changes to avoid impassable areas (rocky terrain, landslides, deep gullies) not identified by the original survey. In this case, these detours must be re-surveyed to determine how will they affect the overall hydraulic behaviour of the pipeline system and to calculate additional requirements (pipes, construction materials, other structures, etc.).

23. It is always worthwhile remembering that, within relatively short periods, visible traces of buried pipelines may disappear, making it difficult, and sometimes costly, to find a pipe trace. Permanent markers, at strategically located reference points should be used for future reference. Concrete pegs are the most commonly used markers. They should be located at all branch points, reducers, changes in pipeline direction and at regular intervals in open terrain or bush. A record of each marker, containing at least information on pipeline materials, diameters and direction of pipes should be kept at hand.

24. Leaks or damages to the pipeline should be identified before the final backfilling of the trenches is undertaken. Test pressures should be the maximum pressures possible if the system is gravity fed, or at least 20% higher than the working pressure of pumping mains. The test should be carried out continuously for at least 15 minutes for each 100 metres of pipeline; the air at all high points must be released during the filling of the pipeline, before the testing.