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close this bookWater and Sanitation Technologies: A Trainer's Manual (Peace Corps, 1985)
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close this folderSession 34 - Gravity water systems: Part II
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View the documentAttachment 34A: Design guidelines and layouts for simple gravity water systems

Attachment 34A: Design guidelines and layouts for simple gravity water systems

Simple Design System Layouts: The following are four common layouts for a simple rural system

Figure 1 depicts the general schematic for placing storage at the point of use.


Figure 1

General Comments for Figure 1:

- The inflow to each reservoir can be regulated so that each area receives a set allotment of water. If the people at that reservoir tend to waste water then they can only waste their allotment and not that of others. In a standpipe system with storage at the source, wastage would be much greater if taps were left open.

- The small reservoir acts to break pressure in the system. This means that faucets at the point of use have only the head of the reservoir itself, and will last for a longer period of time because of the reduced pressure. Reduced head at the point of use also reduces wastage.

- Storage at the point of use means that the main distribution pipe is in use at all times. It can therefore be of smaller diameter and thus reduce costs. (Influence on total cost will depend on the flow of the source compared to average daily usage as this will influence storage costs.)

- It is sometimes easier to obtain community support and cultivate feelings of ownership and consequently, improve maintenance through the construction of small scattered reservoirs as compared to one large distant reservoir and standpipes. Additionally, the construction of small reservoirs allows each segment of the community to work at its own pace during construction and a lack of community organization will be less likely to impede the project.

Figure 2 depicts the general schematic for placing storage between source and distribution. This system also has many beneficial features:


Figure 2

General Comments for Figure 2:

- The pipeline to the point of storage is small as in Figure 1.

- The tank serves as a break pressure point in the system and can be placed to regulate pressure at the distribution points.

- Only one storage tank (larger in size) need be constructed and maintained.

- Water may be treated to improve quality at one center point.

- Standpipes can be easily placed at the desired distribution points.

In Figure 3, no storage is proved, and distribution is direct to public standpipes. This type of system requires a larger diameter pipeline, which in many cases, increases the cost of the overall system substantially. This system also requires a strong source flow to provide peak demand without storage.


Figure 3

In Figure 4, storage is provided at or near the water source, then distribution is direct to public standpipes. The pipeline diameter is the same as in Figure 3 and this is the most expensive option. In certain cases, however, storage may be included with construction of the intake and prove cost-effective for small systems.


Figure 4

SUMMARY OF SUGGESTED GUIDELINES

1. Quality: The quality of the water should always meet local standards and be acceptable to the users. Optimally, there should be no fecal coliforms in any sample from the proposed source. If this is not possible, then there should be an average of less than 50 fecals/100 ml for all samples with no single sample exceeding 100 fecals/100 ml. For levels above this, simple treatment is advisable such as slow sand filtration or chlorination. Other characteristic quality problems such a turbidity or odor may be treated by aeration or settling chambers.

2. Quantity: Optimally, the source should be able easily to supply the ADF. When calculating storage capacity for a system, the rule of thumb is, use 1/2 the ADU. More specifically, storage capacity is dependent upon the ratio of source flow over ADF. For a ratio between one and two, then storage is 1/2 ADU, between two and three, then storage is 1/4 ADU, between three and four, 1/8 ADU, and if greater than four, no storage is required. In regards to per capita consumption, the figure should be between 25-80 liters/per person/per day.

3. Convenience: This criteria is dependent on the standards of each local community; however, it is desired that no more than 20% of intended users have to walk more than 100 meters to obtain water. Also, no more than 100 users per standpipe is desirable.

4. Design Flows: For lengths of pipe from source to storage, use ADF. For lengths from storage to distribution, use ADF multiplied by four.

5. Pipe Size: The smallest diameter of pipe with a calculated head loss less than the measured available head is desirable. Five percent should be added to losses to account for fittings and bends, 5% more to account for errors. Ten to fifteen meters of head should be available at any distribution point to ensure adequate pressure at the tap. The HGL should always lie above the ground profile of any system, and if possible 7 meters of residual head should be available at all points.

6. Number of Taps: This is determined by dividing peak demand by the maximum flow per tap. For a standard 3/4" tap, the maximum flow is .225 liters/sec.

APPENDIX A

GLOSSARY

AVAILABLE HEAD

The actual difference in elevation between the two points in question.

AVERAGE DAILY USE

The average volume of water which flows through the water system during a 24-hour period. It is based on the total population served and the projected per capita usage and is usually expressed in cubic meters (m3). The Average Daily Use is a hypothetical quantity. In reality, on some days water usage is greater and on some days it is less; it is only rarely the same. Moreover, this figure is the basis used for determining other design parameters such as storage volume and design flows. The Average Daily Use for a system supplying 1,000 persons with 100 liters per day is:
1,000 x 100 liters or 100 m3.

AVERAGE DAILY FLOW

The flow of water necessary to supply the Average Daily Use if the water were flowing continuously. It is used as the basis for selecting design flows for pipes. The Average Daily Use in m3 divided by 86.4 gives the Average Daily Flow in 1/s that is necessary to supply that amount.

DESIGN FLOWS

The flow used in the calculation of head losses to determine the pipe diameter. It is chosen by the designer based on the number of users, level of service, and type of storage to be provided and is thus related to the Average Daily Flow.

ESTIMATED MINIMUM FLOW

The best estimate of the low flow from the water source that can be made with the available data.
If long-term estimates are not able to be made, flow measurements should be taken during the dry season in the area. It is best to be conservative in estimating the minimum flow to be used in designing the system.

HEAD

The pressure or force per unit area that is avail able or must be overcome in order to transport water. Head may be supplied by gravity or by mechanical means such as a pump. Although it is a pressure, it is generally referred to in meters or feet of elevation, which is the equivalent pressure that would be exerted by a standing column of water of that height.

HEAD LOSS

A loss of pressure (or head) in a closed pipeline due to friction between the pipe and the flowing water. The head loss is affected by flow of water, the distance it is carried, the diameter of the pipe, all fittings and valves in the system, and the inside surface of the pipe. Calculated head losses are compared with the available head to determine if the desired flow of water will be obtained using selected pipe diameters.

PEAK DEMAND

The highest flow of water expected to occur on any given day. This flow usually lasts for only a very short time period and does not necessarily occur every day. For designing standpipe systems, it is taken as four times the Average Daily Flow.

STATIC HEAD

The various pressures that would be obtained in the water system if it were full of water and the water was not flowing. It is different for each point in the system and depends on the elevation relative to the highest point in the system.

DYNAMIC HEAD LEVEL

The pressure head levels of a flowing system caused by the loss of head through friction. These points vary throughout the system and plot the HGL.

HYDRAULIC GRADIENT LINE (HGL)

An imaginary line that plots the head loss at any given point in the pipeline. It is determined by friction loss factors and always slopes downward along the direction of flow.

RESIDUAL HEAD

The difference in elevation between any point on the pipeline and that point's dynamic head level.

STATIC HEAD LEVEL

Highest point in the system.

STEPS IN SURVEY AND DESIGN

I. The Field Survey

1. Become familiar with the water source and the village; consult with the community with regard to the acceptability of the water source and proposed system and their commitment to participation.

2. From community survey results, decide on the feasibility of constructing the water system.

3. For systems considered feasible, a detailed survey is made including: source flow measurement, pipeline distances, and ground level profile.
Someone from the village should assist in this survey. At this time, tentative locations for distribution points are decided upon in consultation with the villagers, taking into account their own wishes, population distribution, etc.

II. The Design Process

The survey data is used to design the system generally as follows:

1. Decide on general system design, type of distribution facilities, etc.

2. Calculate the project life and growth rate to find the total population to be served. Decide on per capita usage and calculate the average daily use (ADU).

3. From the ADU, calculate the average daily flow (ADF) and by comparison to the minimum source flow, determine the desired storage and its placement. The placement of the distribution points should be set at this time as well.

4. From the average daily flow and the type of system, the design flows for the pipe are selected, taking into account peak demand.
The number of taps required is calculated.

5. The pipeline profile is drawn and an approximate hydraulic gradient line (HGL) plotted by making points of desired head.

6. From the design flows and elevations, the exact head losses for various diameter pipes are calculated for each unbroken section of the pipeline.

7. The most appropriate pipe is chosen for all sections of the pipeline based on the calculated head losses. The true hydraulic gradients are then plotted on the profile.

8. From the elevation profile and general scheme of the system, the need and placement of break pressure tanks, air valves, wash-outs, etc. are determined and recorded on the general sketch.

9. Detailed drawings, specifications, materials lists and budgets can now be prepared.

REFERENCE: Practical Design Notes for Simple Rural Water Systems; A. Scott Faiia. CARE, Indonesia, 1982.

RIGID PVC FRlCTIONAL HEADLOSS FACTORS

These are the approximate headless factors, in m/100m (%), for new rigid PVC pipe. Flows are in liters/second.

FLOW

1/2"

3/4"

1

1¼"

1½"

2"

2½"

3"

4"

0.1

4.2

1.0

0.25

0.08






0.15

8.8

2.2

0.53

0.17

0.07





0.2

15.0

3.7

0.9

0.28

0.12





0.25

22.0

5.5

1.35

0.44

0.18





0.3

31.0

7.8

1.9

0.6

0.25





0.35

41.0

10.0

2.45

0.8

0.34





0.4

53.0

13.0

3.1

1.0

0.43





0.45

66.0

16.3

4.0

1.25

0.54

0.13




0.5


19.0

4.8

1.5

0.65

0.16




0.55


23.5

5.6

1.8

0.78

0.19




0.6


27.5

6.6

2.1

0.9

0.22




0.65


32.0

7.8

2.4

1.04

0.25




0.7


36.0

8.7

2.7

1.19

0.28




0.75


41.0

9.9

3.1

1.32

0.33

0.1



0.8


45.0

11.0

3.5

1.5

0.37

0.12



0.85


52.0

12.5

4.0

1.7

0.41

0.14



0.9


57.0

14.0

4.5

1.9

0.45

0.15



0.95


63.0

15.0

4.9

2.1

0.5

0.17



1.0



16.5

5.4

2.25

0.55

0.18

0.08


1.05



18.0

5.8

2.5

0.6

0.20

0.09


1.1



19.5

6.3

2.7

0.67

0.22

0.1


1.15



21.5

6.9

2.95

0.71

0.24

0.11


1.2



23.0

7.3

3.2

0.78

0.26

0.12


1.3



26.5

8.6

3.75

0.9

0.29

0.13


1.4



30.0

10.0

4.25

1.0

0.34

0.15


1.5



35.0

11.2

4.9

1.15

0.39

0.17


1.6



39.0

12.5

5.5

1.3

0.43

0.19


1.7



44.0

14.2

6.05

1.45

0.49

0.21


1.8



49.0

15.9

6.9

1.6

0.54

0.24


1.9



55.0

17.4

7.5

1.8

0.6

0.26


2.0



60.0

19.0

8.0

2.0

0.66

0.28


2.2




22.5

9.7

2.35

0.79

0.34


2.4




26.8

11.5

2.75

0.9

0.4


2.6




31.0

13.3

3.2

1.05

0.45


2.8




35.1

15.2

3.7

1.2

0.52


3.0




40.0

17.0

4.2

1.36

0.6


3.2




45.0

19.3

4.7

1.52

0.68


3.4




50.0

21.9

5.25

1.7

0.75


3.6




56.0

24.0

5.8

1.9

0.84

0.2

3.8




62.0

26.0

6.3

2.1

0.9

0.22

4.0




69.0

29.0

7.0

2.3

1.0

0.24

4.5





36.0

8.8

2.8

1.2

0.3

5.0





44.0

10.5

3.5

1.5

0.37

5.5





62.0

12.5

4.2

1.75

0.44

6.0






14.7

4.9

2.1

0.52

6.5






17.0

5.6

2.4

0.6

7.0






19.5

6.5

2.8

0.7

GI FRICTIONAL HEADLOSS FACTORS

These are approximate headloss factors, in m/100m (%), for new GI pipe. Flows are in liters/second.

FLOW

1/2"

3/4"

1"

1¼"

1½"

2"

2½"

3"

4"

0.1

5.9

1.58

0.38

0.12






0.15

12.25

3.4

0.82

0.26






0.2

21.45

5.65

1.4

0.44

0.19





0.25

31.65

8.5

2.1

0.68

0.28





0.3

44.91

11.9

2.9

0.92

0.4





0.35

58.2

15.8

3.8

1.2

0.52





0.4

75.5

19.9

4.8

1.55

0.67





0.45

91.9

25.0

6.0

1.93

0.84





0.5


30.0

7.3

2.35

1.0

0.25




0.55


36.0

8.7

2.75

1.2

0.3




0.6


42.0

10.2

3.25

1.4

0.35




0.65


48.0

11.9

3.8

1.63

0.4




0.7


55.0

13.6

4.35

1.82

0.46




0.75


63.0

15.4

4.9

2.15

0.52

0.17



0.8



17.4

5.55

2.4

0.59

0.19



0.85



19.4

6.15

2.65

0.68

0.21



0.9



21.8

6.9

2.9

0.74

0.23



0.95



24.0

7.5

3.25

0.82

0.26



1.0



26.2

8.2

3.6

0.88

0.28

0.12


1.05



28.5

9.0

3.9

0.97

0.31

0.13


1.1



31.0

9.8

4.2

1.05

0.34

0.15


1.15



34.6

10.6

4.6

1.15

0.37

0.16


1.2



36.0

11.5

5.0

1.25

0.39

0.17


1.3



42.5

13.3

5.7

1.45

0.45

0.2


1.4



48.0

15.3

6.6

1.65

0.52

0.23


1.5



55.0

17.5

7.65

1.9

0.59

0.26


1.6



62.0

19.5

8.45

2.1

0.67

0.29


1.7



69.0

22.0

9.5

2.35

0.75

0.33


1.8




24.2

10.5

2.6

0.82

0.36


1.9




26.5

11.7

2.85

0.9

0.4


2.0




29.5

12.8

3.2

1.0

0.44


2.2




35.0

15.3

3.8

1.2

0.52


2.4




42.0

17.9

4.45

1.4

0.61


2.6




48.5

20.5

5.15

1.6

0.71

0.17

2.8




55.0

24.0

5.95

1.85

0.82

0.2

3.0




62.5

26.7

6.7

2.1

0.92

0.22

3.2





30.0

7.6

2.35

1.02

0.25

3.4





34.0

8.4

2.65

1.15

0.28

3.6





38.0

9.4

2.95

1.28

0.32

3.8





41.0

10.3

3.25

1.42

0.35

4.0





45.0

11.2

3.55

1.55

0.38

4.5





56.0

14.0

4.45

1.95

0.46

5.0






17.0

5.45

2.25

0.56

5.5






20.0

6.5

2.8

0.68

6.0






24.0

7.5

3.35

0.8

6.5






28.0

8.65

3.9

0.92

7.0






32.0

10.0

4.45

1.05

SAMPLE PROBLEMS

Problem I:

Design for system with central storage at some point along the pipeline, serving one village


Central storage

Given:

Population = 1000
Growth Rate = 1.0%
Project Life = 10 yes.
Per capita consumption = 50 liters
Minimum Source Flow = 1.0 1/sec.
Pipe = PVC
Tap size = 3/4"


Pipeline Profile - Problem I

Step 1. Determine design population figure_________________________
Step 2. Determine average daily use_________________________
Step 3. Determine average daily flow_________________________
Step 4. Determine storage size_________________________
Step 5. Determine peak demand_________________________
Step 6. Determine number of required taps_________________________
Step 7. Determine storage placement_________________________
Step 8. Determine pipe size for all reaches along the pipeline_________________________
Step 9. Determine exact head losses and residual head at points B. C, D and E. _____________
Step 10. Plot HGL.

Problem II:

Design for system with storage at point of use, serving two villages


Storage at point of use

Given:

Population = 1500 divided evenly between the two villages
Growth Rate = 2.0%
Project Life = 15 yes.
Per capita consumption = 40 liters
Minimum Source Flow = 1.5 1/sec.
Pipe = PVC
Tap Size = 3/4"


Pipeline Profile - Problem II

Step 1. Determine design population figure_________________________
Step 2. Determine average daily use_________________________
Step 3. Determine average daily flow_________________________
Step 4. Determine storage size_________________________
Step 5. Determine peak demand_________________________
Step 6. Determine number of required taps_________________________
Step 7. Determine storage placement_________________________
Step 8. Determine pipe size for all reaches along the pipeline_________________________
Step 9. Determine exact head losses and residual head at points B. C, D and E._____________
Step 10. Plot HGL.

Problem I: Answer Sheet


Figure I

Step 1. 1100
Step 2. 55,000 liters = 50m3
Step 3. .6 liters/second
Step 4. 25m3
Step 5. 2.4 liters/sec.
Step 6. 10
Step 7. Point C
Step 8. See Figure 1

Step 9.


pt. B

pt. C

pt. D

pt. E

Head Loss

4.2

20

8

23

Res. Head

10.8

31

12

14

Step 10. See Figure I

Problem II: Answer Sheet


Figure II

Step 1. 2025
Step 2. 81,000 liters = 80m3
Step 3. .9 liters/second
Step 4. 40m3
Step 5. 3.6 liters/sec.
Step 6. 16
Step 7. Point D 20m3
Point E 20m3
Step 8. See Figure II
Step 9.


pt. B

pt. C

pt. D

pt. E

Head Loss

7.6

13.5

42

20

Res. Head

12.4

29

12

17

Step 10. See Figure II