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AquaShareware

Dick Bouman, April 2014

Hydraulic design for gravity based water schemes

Aqua for All Koningskade 40 2596 AA Den Haag P.O.Box 93218 2509 AE Den Haag +31 (0) 70 351 725 www.aquaforall.nl info@aquaforall.nl

KvK 27248417 IBAN NL81 RABO 038384584

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Aqua for All is a Dutch organization, aiming to mobilize expertise and resources from the Dutch private sector to contribute to reaching the 7th Millennium Development Goal on water and sanitation.

AquaShareware products are open source support materials for the water and sanitation sector and may be used free of charge, but with reference to Aqua for All. Any suggestion for correction, addition or remark is welcome at info@aquaforall.nl.

The author of this AquaShareware product, Mr. Dick Bouman, is head of the program desk and experienced in water resources and water supply. Mr. Dick Bouman started his career in 1981 with a water resource mapping in the Morogoro Region for DHV Consultants (now Royal Haskoning DHV), with emphasis on low flow analysis of small streams.

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Contents

1. Introduction.........................................................................................................................4 2. Design ................................................................................................................................5

2.1 Preliminary work ............................................................................................................5 2.2 Design criteria ...............................................................................................................6 2.3 Hydraulics .....................................................................................................................8 2.4 Pipes ...........................................................................................................................14 2.5 Storage tanks ..............................................................................................................15 2.6 Other Devices ............................................................................................................16 2.7 Valves .........................................................................................................................17

3. Specific Issues .................................................................................................................18

3.1 Too high pressures......................................................................................................18 3.2 Air locks ......................................................................................................................19 3.3 Water hammer ...........................................................................................................19 3.4 Preferential flow in TEES.............................................................................................19

4. Final Remarks ..................................................................................................................20

References ....................................................................................................................21

Appendix A - Example of a hydraulic calculation ...............................................................22 Appendix B - Explanation of terms in design of water schemes .........................................25

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1. Introduction

Gravity schemes are water distribution schemes without pumping; only the power of the earth gravity force is used to bring the water from a high entry point to the lower outlets. Compared to pumped schemes, gravity schemes require less operational cost for power and pump operation and less maintenance cost for pumps.

It looks simple: just let gravity do its work. But many gravity schemes do not function properly and fail to distribute the water evenly among the water points. In many cases, taps at the higher end and at the lower end get insufficient or irregular water.

This hand out starts with an introduction on design, pipe hydraulics and pipe materials, required for the further calculations, followed by several critical aspects.

The theory of hydraulics can also be applied to other than purely gravity schemes:

Distribution from a raised tower is very similar to a gravity scheme The pumping of water through a pipe line follows the same hydraulics logics, but

starts with a high pressure instead of zero/atmospheric pressure

Appendix B gives a summary of the most important terminology.

Figure 1 Example of a gravity scheme with branched distribution. A=source, B=division box, C and B21 are Storage Tanks

A

B

B2

B11

D

C

B3

E

B12B21B1

D1

C1B22

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2. Design

2.1 Preliminary work Most schemes exist of a central transmission main between an intake and the main Storage Tank. From the Storage Tank the water is distributed to water points (public or private). Most systems are so called closed systems (the water flow can be stopped by closing valves and taps). In case of abundant yield at the source and high pressure differences, an open system is recommended, where the flow in the pipelines is not interrupted by valves and taps. In this general instruction, the closed system is used as a reference, except when the exception of an open system is mentioned.

Appendix A provides a module to exercise the theory provided in this manual.

1. Before starting a hydraulic calculation, one should start with the analysis of water sources and the calculation of the minimum flow at the available sources. Preferably, the once in 20 years minimum flow of the source is taken as the maximum design flow. This flow can be determined from at least 3 discharge measurements during the dry season in 3 consecutive months. Most commonly, the decline of the discharge follows an inverse logarithmic formulae. In that case, the minimum flow can be calculated for a period that lasts as long as the once in 20 years longest drought. This period can be obtained from meteorological statistics.

Figure 2 Example of an excel file to determine once in 20 years minimum (at 347 days)

The analysis of the potential of a natural spring is elaborated in a separate Aqua for All Shareware product: Dick Bouman (January 2013) Determination of the potential of natural springs for water schemes. This analytical tool holds for stable situations, but the situation might worsen with changing land use patterns and climate change (more extreme events: higher intensities and longer droughts).

For small streams the theory is rather comparable, but the abstraction devices are different and need a precaution for high floods, sediment load and contamination.

One should also analyse the water quality, the risk of contamination and the risk of damage by erosive forces.

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2. One should also check whether it is likely that the water can be brought down by gravity. This can be done from topo-maps, from Google Earth (look at altitude indication), or by using GPS and altitude measurements in the field. If height differences are too high, the designer has to find solutions to deal with the high pressure or to break the pressure. If height differences are too little, GPS and Google may not be accurate enough for design purposes and land surveys are required.

3. Thereafter one should make a sketch map of the area and the concentrations of present and future population. Make a first draft plan of the lay out map of the scheme (source, mains, distributions, siting of stand posts and siting of tanks and other devices).

4. Then one should analyse the future demand at the end of the life time of the scheme (population, other users and the use per consumer); calculate the required stand posts per area and define the required flow per stand post.

5. Finally, one can make per sub-section an analysis of the required design flow, working from the end to the top and using design criteria such as a risk factor for the mains and a peak factor for the distribution lines. One should check whether the required flow is below the minimum flow of the source. If not, one should look after additional or alternative sources, or one could adopt the preliminary design (smaller distribution, restriction on additional use or elimination of branches that require peak demand from the main line).

From the resulting preliminary sketch one can start with the hydraulic calculations.

2.2 Design criteria Examples of design criteria for closed and open systems are summarized in table 1.

CLOSED SYSTEM OPEN SYSTEM

OBSERVATION

Domestic Water Use 20-50 l/capita/day

20-50 l/cap/day Mean between public standposts (67%) and house connections (33%)

Peak Factor in Main 1.15 3 To compensate for losses

Peak Factor in distribution

4 3 This factor is used to determine peak design flow per Public Standpost: Q = nr of people per PS* use (l/d) * peak factor / 86,400. (l/sec)

Storage capacity of Tank

0.5 * daily water demand in distribution

0 For open system not required; only if there are sub-systems with many house connections

Maximum pressure in pipes

(50 m) (50 m) PN6 pipes have 60 meter; but most taps can only have less than 50; other wise a pressure break is required

Preferred velocity in pipe at design flow

0.5 2.0 m/s 0.5-2.0 m/s

Min. and Max velocity at design flow

0.2 3.0 m

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