acf_gravity_fed_system_2_sizing_en.pdf

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CONTENTS
CONTENTS .........................................................................................................................1
I.
II.
II.1.
INTRODUCTION ......................................................................................................2
PRINCIPLE OF A GRAVITY FED SYSTEM.............................................................3
WHAT ARE THE MAIN ELEMENTS OF A GRAVITY-FED SYSTEM AND THEIR
FUNCTION? .......................................................................................................................... 3
II.2. HOW DOES A GRAVITY FED SYSTEM WORK? ......................................................... 4
II.2.1.
Gravity....................................................................................................................... 4
II.2.2.
Pressure .................................................................................................................... 5
II.2.2.
How to calculate the head losses (∆P)? ........................................................... 11
III.
SIZING AND DESIGN OF GRAVITY FED SYSTEM ..............................................13
III.1. INFRASTRUCTURES POSITIONING............................................................................ 13
III.1.1.
Header tank ............................................................................................................13
III.1.2.
Storage Tank ..........................................................................................................13
III.1.3.
Break pressure tank..............................................................................................13
III.2. SIZING OF THE HEADER TANK................................................................................... 14
III.3. SIZING OF THE STORAGE TANK ................................................................................ 15
III.4. CHOSING AND SIZING THE PIPES .............................................................................. 18
III.5. PLANNING AND DESIGNING THE VALVE NETWORK.......................................... 29
III.5.1.
Regulation valves ................................................................................................... 29
III.5.2. The open/close valves type ................................................................................ 29
III.6. HOW TO AVOID THE USUAL PROBLEMS PRESENTED BY GRAVITY FED
SYSTEM? ............................................................................................................................. 29
III.6.1.
How to avoid the sometimes violent shocks in pipes?.................................. 29
III.6.2. How to avoid having air in pipes? ...................................................................... 30
III.6.3. How to avoid the deposits in pipes? ................................................................. 33
III.6.4. How to avoid water hammers?........................................................................... 34
III.7. CALCULATION EXERCISES.......................................................................................... 35
IV.
APPENDIXES.........................................................................................................42
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I.
INTRODUCTION
In order to size a gravity fed system, it is important and necessary that a technician knows
and understands how gravity fed systems work.
In this module, a first section (chapter II) is devoted to the functioning of gravity fed
system, giving definitions of concept such as gravity, pressure and head loss. A brief
description of the various infrastructures and their use is also given in this section.
A second section (chapter III) is devoted to the sizing and designs, i.e. how to size and
where to place the various infrastructures of a gravity fed system.
Calculation exercises are given at the end of the module (paragraph III.7) in order to put
into practice the theory developed in this module (exercise corrections are given in
appendix).
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II.
II.1.
PRINCIPLE OF A GRAVITY FED SYSTEM
WHAT ARE THE MAIN ELEMENTS OF A GRAVITY-FED
SYSTEM AND THEIR FUNCTION?
A simple spring catchment includes the following elements:
A
B
C
D
A. Catchment box to protect and to collect water from the spring.
B. Header tank which has several functions:
o
avoid an accidental increase of pressure “inside” the spring in case of network
blockage. (Indeed, if a spring is put under pressure, there is a risk that it
disappears).
o
allows the decantation of the suspended substances if present (sand.…) before
they enter in the pipe,
o
stabilize the flow coming from the spring.
C. Main pipeline which brings water from the spring to the users.
D. One or more water points, without tap, where the users can come to fetch water and
where water run continuously.
This type of system is called “opened” because nothing closes the network and water runs out
continuously.
But what should be done, if the spring flow is not sufficient to cover the population water
needs?
It is then necessary to close the system using taps installed on the tapstands and to store
the unused water in a tank. The diagram of such a system, called “closed” system, is as
follows:
A
B
C
D
E
F
A. Catchments box
Header tank
B. Main pipeline
C. A storage tank to store water during periods when the populations demand is low in
order to provide a more important flow when the demand increases.
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D. Distribution line to supply water to the consumers.
E. Tapstands equipped with taps where the users can come to take water.
When the difference of height between the spring and the tank or the tap is too important,
it is necessary to put small tanks, called break pressure tanks, between the two
infrastructures in order to prevent the damage of the pipes because of the effect of high
pressure.
II.2. HOW DOES A GRAVITY FED SYSTEM WORK?
II.2.1.
Gravity
A gravity fed system function thanks to the gravity. Gravity is a force which attracts all
objects on the earth surface, due to the attraction exerted by the planet’s mass. It is this
force which makes that all bodies or things always fall at the lowest point (for example, a
mango which falls from a tree).
It is thus by gravity that the water stored in tank goes down by its own weight inside the
pipes and run out from the taps. But this system works only if the pipes and taps are at a
lower level than the water level at the starting point.
To illustrate this fact, let’s consider the example of the drawing of picture 1:
-
Tap 1:
water does not run out because the tap is on a higher level than the water level
in the tank.
-
Tap 2:
water runs out from the tap but with low pressure (i.e. low power) because the
tap is close to water level in tank.
-
Tap 3:
water does not run out because part of the pipes is found in the top of water
level in the tank.
-
Tap 4:
water runs out from the tap with a good pressure.
Fig.1: Illustration of the gravity force
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