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close this book Water purification, distribution and sewage disposal for Peace Corps volunteers
close this folder Section 4: Characteristics of an adequate system
View the document Overview:
View the document Essential components of an adequate system
View the document General requirements for an adequate system
View the document Relationship of distribution system to community
View the document Headloss and distribution systems
View the document Lesson plans

General requirements for an adequate system

An adequate system is one which will deliver the required amount of potable, palatable water to all outlets at a prescribed or satisfactory pressure.

MAPPING THE PROPOSED SYSTEM

A drawing or map and a profile of the distribution system should be made showing the location of each component in relation to others (see Section l on drawing topographic maps). Below are a number of ways that the distribution system layout can be illustrated,


Layout of a combination water system

SYSTEM CAPACITY

Once these factors have been determined, the storage capacity can be determined. The following guidelines should be considered in arriving at adequate storage capacity:

1. As a rule-of-thumb, the storage required should be approximately equal to a days consumption of water.

2. Minimum capacity should be large enough to handle morning and afternoon peaks. In no case should it be less than half a day's supply.

3. If it is not possible to store a day's requirement, long periods of pumping should be adopted with no interruptions.

4. Where possible, ground storage is preferable.

PIPES

In selecting pipes, the following suggestions should be followed:

1. The velocity of flow of water should not be more than 6 ft. per second in main pipes; and 3 ft. per second in feeder pipes.

2. Pipe sizes for mains must be at least 2 inches in diameter.

3. Using the nomograph attached, determine the pipe size which will deliver water at a desired rate.

Pipe Flow Calculation

This chart helps determine the flow of water from several sizes of pipe when you know the height of the water source.

The nomograph applies to steel pipe. Fig. 34 should be used to find the equivalent pipe length. The length of pipe run can be paced off. One can crudely sight with their eyes and attempt measuring the reservoir height. To make this measurement with any accuracy requires some type of surveyor's instrument.

To use the nomograph, first find the number of pipe diameters there are in the pipe lengths. This is accomplished by dividing the pipe diameter in inches into 12 x pipe lengths in feet; or divide the pipe diameter in centimeters into 100 x the pipe length in meters.

Then with the straight edge connect the pipe diameter on the d scale in inches (l loch = 2.54 cm) with the reservoir height in feet on the h scale. Mark on the index scale where the straight edge crosses. Now connect this point on the index with the number of pipe diameters in the pipe length as calculated and observe the reading on the Q scale. This will be the discharge in gallons per minute.


Pipe Flow Calculation

Pump Size and Horsepower Requirement

For preliminary sizing of a pump used to lift liquid to a known height through simple piping, follow these steps:

1. Determine the quantity of flow desired in gallons per minute.

8.33 pounds = 1 gallon.

2. Measure the height of the lift required (from the point where the water enters the pump suction piping to where it discharges.)

Example:

Assume a reservoir height of 30 feet, a pipe size of 1 1/2" diameter and 100 feet in pipe length, what will the discharge rate be?

First divide the length by the diameter each in inches

1200/1.5 = 800

now convert 1.5 on the d scale with 30 feet on the h scale and make a mark on the index scale. Connect this mark with 800 on the L/D scale and read the flow as 60 gallons per minute on the Q scale.

Headloss Calculation

How to compensate for the headlosses due to pipe fittings: Express in terms of the equivalent to the length and size of pipe which would produce an equivalent loss if, instead of adding fittings, additional pipe was added. Note in table below that headless due to pipe fittings can be neglected for relatively long pipes.

TABLE 7: ALLOWANCE IN EQUIVALENT LENGTH OF PIPE FOR FRICTION LOSS IN VALVES AND THREADED FITTINGS

Diameter of fitting

90° std. ell

45° std. ell

90° side tee

Coupling or straight

Gate valve

Globe valve

Angle valve

Inches

Feet

Feet

Feet

Fee

Fee

Fee

Fee

3/2

1

0.6

1.5

0.3

0.2

8

4

1/2

2

1.2

3

0.6

0.4

15

8

3/4

2.5

1.5

4

0.8

0.5

20

12

1

3

1.8

5

0.9

0.6

25

15

11/4

4

2.4

6

1.2

0.8

35

18

11/2

5

3

7

1.5

1.0

45

22

2

7

4

10

2

1.3

55

28

21/2

8

5

12

2.5

1.6

65

34

3

10

6

15

3

2

80

40

31/2

12

7

18

3.6

2.4

100

50

4

14

8

21

4

2.7

125

55

5

17

10

25

5

3.3

140

70

6

20

12

30

6

4

165

80