chapter 11 gravity flow water systems final november 16 2008

8/8/2019 Chapter 11 Gravity Flow Water Systems FINAL November 16 2008

1/40

Chapter 11 1

Chapter 11. Gravity Fed Water Supply

Systems

1

11.1 Fluid Mechanics and Pipe Networks

11.1.1 Quantity of Flow in a Pipe

As water flows in a pipe, a certain volume of water passes through per unit time. This is

referred to as theflow rate ordischarge, and is in units of volume per time (e.g., L/sec, m3/sec,

or ft3/sec). For a given flow rate, if the pipe has a small diameter, then the water has a high

velocity, and conversely, if the pipe is large, then the velocity is low. This can be quantified by

Equation 11-1, which is referred to as the continuity equation:

Q = VA (11-1)

where Q is the volumetric flow rate (length cubed/time), Vis the velocity (length/time), andA is

the cross-sectional inside area of the pipe (length squared).

11.1.2 Pressure (or Head) at Various Locations along the Pipe

A pipe will carry its maximum discharge as long as the pressure anywhere in the pipe

does not fall below atmospheric pressure. If the pressure does go below zero, then asiphon will

be established. This is a negative pressure in the pipe that can result in contamination being

drawn into the pipe if a crack or hole develops. Negative (or even low) pressures are therefore

1Contributors: Matthew A. Niskanen, Nathan Reents, John Simpson, Stephen Good


2/40

Chapter 11 2

to be avoided. For this reason, it is important to be able to determine the pressure at all locations

in the pipe network.

Assuming no change in elevation of a pipe, pressure diminishes in the pipe as water flows

downstream. This pressure loss (also called head loss)is due to friction with the pipe walls and

also to the swirling flow patterns found at the entrance to pipes, valves, pipe bends, and where

the flow exits the pipe.

Thepressure (or head) loss due to friction can be determinedas follows, assuming fully

turbulent flow:

52

2

216

DgLQfhL

= (11-2)

In Equation 11-2, hL is the frictional head loss (length),fis afriction factor,L is pipe length

(length), Q is flow rate in pipe (length cubed/time),gis the gravitational constant (length per

time squared), equals 3.1416, andD is the pipe diameter (length). Friction factors can be

determined for different relative roughness and pipe diameters as provided in Tables 11-1 and

11-2.

In addition to losing energy due to friction with the pipe walls, losses occur in any kind of

pipe fitting due to swirling flow patterns. These are called minor losses and can be quantified as

follows:

2

(min ) 2 416

2L or

Qh K

g D= (11-3)

whereKis a function of pipe geometry and type of fitting. Values ofKcan be found from Table

11-3.


3/40

Chapter 11 3

Term (1)

Term (2)

Term (3)

Term (4)

11.1.3 The Energy Equation

Using Equations 11-1 to 11-3 and Tables 11-1 to 11-3, it is possible to calculate the

amount of energy (or pressure) that is present at any location along any of the pipes in a water

distribution system. Pressure is sometimes conveniently expressed in terms ofhead, which is

the pressure divided by the specific weight of water, . The pressure at any point in a pipe can be

thought of as how far the water would rise if a vertical tube was connected to the pipe at that

point. TheEnergy Equation is used for this purpose and is written as follows:

Lp hzDg

QpHz

Dg

Qp+++=+++ 24

2

2

2

2214

1

2

2

11 88

(11-4)

In Equation 11-4,p is pressure (force/length squared), is the specific weight of water

(force/length cubed), Q is the flow rate (length cubed/time),gis the gravitational constant

(length per time squared),D is the pipe diameter (length),zis the height above some reference

elevation (length),Hp is the pressure head supplied by a pump (length), and hL is the total head

loss (length). In SI units,gis equal to 9.81 m/s2, and in English units, 32.2 ft/s

2. The specific

weight of water is a function of temperature. At 10oC (50

oF) the specific weight of water is

9.804 kN/m3

(62.41 lb/ft3).

The subscript 1 in Equation 11-4 denotes an upstream point in the pipe, and the subscript

2 refers to a downstream point. Term (1) in Equation 11-4 is called thepressure head, Term (2)

the velocity head, and Term (3) the elevation head. Term (4) is the head loss term. The Energy

Equation can be solved forD and is used to size pipes, as illustrated later in Example 11-1.

11.1.4 Use of the Hydraulic Grade Line

The hydraulic grade line (HGL) is a plot ofz+p/ (i.e, the elevation head plus the

pressure head). This is Terms (3) and (1) in the Energy Equation (Equation 11-4). The ground


4/40

Chapter 11 4

elevation is first plotted, and then a line of p/ is plotted above that. Figure 11-1 shows a HGL in

relation to the topography for the situation that will be examined in Example 11-1.

An easier and more intuitive method of plotting the HGL is to start at the upstream water

surface (i.e., at the source) and then subtract head loss as you go downstream. There are minor

losses in the inlet to the pipe at the source (term a in Figure 11-1), then friction losses along the

pipe. When pipes are joined, the head at the downstream end of the upper pipe is the head

available at the upstream end of the lower pipe. This process can be continued along the entire

pipeline. The slope of the HGL will be steeper asD is decreased orQ is increased, since both

situations will result in higher velocity and therefore higher head loss, as can be seen in Equation

11-2.

Using the Energy Equation (11-4) it is possible to plot the head at all points along the

pipe, thereby allowing any points of excessively high or low head to be detected for later

correction. (When determining whether the head is too high or too low, it is the pressure head

that is of concern. Visually, this is the elevation difference between the hydraulic grade line and

the ground surface.) As stated above, low pressure regions may not be able to provide the

needed discharge, and high pressures can burst the pipe. This is illustrated later in Example 11-

1.

Each type of pipe can withstand a different amount of pressure (or head). This maximum

allowable pressure for a certain type of pipe can be found from the pipe manufacturer as is

provided in Table 11-4. If you cannot find this information, then an approximate value to use is

150 ft of head for PVC pipe and 600 ft for galvanized iron pipe. These are low estimates.

Actual values depend on the specific material, the wall thickness, and operating temperature.

11.2 Components of a Gravity Fed Water Supply System

A gravity fed water system consists of many components as described in detail in this

section. The hydraulic design process is demonstrated later in Example 11-1.


5/40

Chapter 11 5

Box 14-1. Eight items to consider when determining whether a gravity flow water

project is feasible.

1. Water at the source should not appear turbid, have an odor, carry a lot of sediment, or becontaminated with pathogens or agricultural chemicals.

2. The owner of the land where the source is located should be trustworthy, educated onhow their activities could influence the water source, and have provided written

permission.

3. For gravity systems, the community houses should be located at an elevation below thesource.

4. The minimum flow of the source (during the dry season) should cover the needs of thecommunity else it must be supplemented with other sources of water.

5. If the source is located below the community, a pump system will be required and thus,the community has to have access to power (human, renewable, or fossil fuel) to lift the

water. The availability of parts to repair a pumping device must also be considered.

6. There needs to be a suitable place for a storage tank located above the community.7. There must be funds available from nearby organizations, government, and community

members to construct the system initially.

8. The members of the community should show interest and be willing to provide labor andthe funds to construct and maintain the system.

11.2.1 The Source

Gravity fed water supply systems typically begin by identifying an adequate and

permanent source of safe water in an area of higher elevation than the community. This water

could be surface water collected behind a dam or groundwater collected by means of a

springbox. Water at the source should not appear turbid, have an odor, carry a lot of sediment, or

be contaminated. If the flow of the source does not cover the needs of the community during the

dry season, it will need to be supplemented with other sources of water. The owner of the land

where the source is located should be trustworthy, understand how his or her activities could

influence the water source, and provide written permission for use of the source.


6/40

Chapter 11 6

Dams are used when the available water source is a stream or river. The advantage of

constructing a dam is that during the rainy season, the water will easily pass over the structure

and not damage it. The main disadvantage is that this water source is more susceptible to

contamination, and generally carries more sediment than the groundwater of a natural spring.

Dams can be built out of many materials, depending on availability in the region.

Sedimentation tanks (Figure 11-2) are often needed when using dammed surface water,

due particularly to the high turbidity of water during the rainy season. The tank is built in order

to slow the flow of the water, causing particles to settle out by gravity.

For the tank to work properly, a plumber must periodically open the clean out valve to

release sediment located in the bottom of the tank. Sediment in the conduction line can cause

obstructions and unnecessary wear along the inner walls of the pipe. A sedimentation tank also

serves as a break pressure tank (discussed later), because the pressure is returned to atmospheric

pressure. However, they are typically placed near the source in order to remove sediments as

early as possible so as to minimize damage to pipes caused by suspended particles

11.2.2 The Conduction Line

A conduction line transmits water from the elevated source to a storage tank. The tank

then stores the water accumulated during periods of lower demand for use during periods of

larger demand. Pipe material is typically PVC or galvanized iron. Table 11-5 compares the

advantages and disadvantage of each, and an explanation of where they are typically used. As

can be seen from this table, the choice of pipe material affects the roughness of the pipe, and

therefore the head loss due to friction.

Water velocity through pipe is typically maintained between 0.5 to 3 m/s. If the velocity

is lower than 0.5 m/s, suspended solids in the water may settle and collect in the pipe. This can

increase head loss and lead to clogging in the pipe. Sediment clean out valves (wash outs) that


7/40

Chapter 11 7

are strategically placed at low points allow removal of accumulated sediment; however, a lapse

in maintenance will lead to problems. If a low velocity cannot be avoided, frequent line flushing

will have to take place, or a sedimentation tank may have to be installed at the beginning of the

pipeline or the springbox can be sized to provide for sedimentation. If the velocity is greater

than 3 m/s, the interior of the pipe can be seriously eroded leading to increased frictional losses

and a reduction in the design life. A larger pipe size will reduce the velocity and also lead to

lower frictional losses. Table 11-6 provides velocity limits for different pipe sizes.

Pipe sizing is not only based on survey data, but also peak flow from the water source. A

conduction line is typically designed to carry the maximum daily flow from the source to the

storage tank. The tank then acts to store the water carried during the periods of lower demand

for use during periods of larger demand. Pipe diameters can change going from one pipe

segment to another. Pipe sizes usually decrease as a pipeline progresses downstream from one

pipe segment when there is decreasing discharge required. The head loss in the contraction

fitting from a large to smaller diameter pipe should be included in calculations. Table 11-3

provided the coefficient to be used for contractions.

Pipe size can be calculated by hand if necessary (Reents 2002; Simpson 2003). Software

programs such as GoodWater(Good 2008) are also available to assist the design of gravity fed

water systems. GoodWateris further unique because it also incorporates principles of

sustainability. A copy can be obtained by contacting this books lead author. Commonly used

pipe size diameters are: , 1, 1.5, 2, 3, and 4 inches. Larger, more expensive pipes are rare in

rural water projects. GI pipe is usually sold in 6 m (20 ft) sections and PVC pipe is usually solid

in 19 ft sections. The actual pipe inner diameter is slightly different than the nominal size.

Box 14-2. Community understanding of pipe size.

You may observe that some individuals do not understand why the diameter of the pipe changes

as the distribution system gets closer to the community. These individuals may suggest for


8/40

Chapter 11 8

example to use 2-inch pipe on the entire system instead of a smaller size pipe. They may

initially look in disbelief when you explain the same or a larger pipe size will not be used for the

whole transmission line.

In this situation, the individual believes that the smaller 1.5-inch pipe was too small to

provide water for the entire community and the system would run dry. They may visually show

the difference in size of the diameters of the two pipes with their thumb and finger, and explain

how the larger 2-inch pipe will carry more water. In this case, the individual is thinking about

the amount of water the pipe could carry in a 2-D fashion, instead of thinking about the volume

each diameter carries. The importance of careful planning and integration into a community will

allow you to earn the trust and respect of community members, so you can then explain to them

the fundamentals of fluid mechanics and the design in their terms.

Example 11-1. Using the Energy Equation to solve for pipe size.

Suppose the discharge from a pipe by a group of users is 0.7 ft3/s. We want there to also

be at least a small amount of head available everywhere in the pipe, 10 ft, for example. This

allows for sufficient flow and takes care of possible errors (e.g., in the land survey). PVC is

readily available in this instance, so it is specified as the pipe material. Water enters the pipefrom an inlet tank in which the water surface is 5 ft above the ground level. (Because we are

designing the pipe that comes after the storage tank, we are designing the distribution line. The

same process would also be used for the conduction line.)

The task here is to choose a suitable pipe diameter and then plot the head along the pipe

to make sure that all points have sufficient pressure to allow the design flow rate (0.7 ft3/s) to

pass, while no points have a pressure high enough to cause the pipe to burst. The ground levels

are shown in Figure 11-1. There is a high point in the ground 200 ft downstream of the intake

(Point A), and the end of the pipe (Point B) is another 100 ft from the high point. There will be a

small amount of head loss at the entrance (indicated as a near the storage tank in Figure 11-1),

but that will be considered later.


9/40

Chapter 11 9

The allowable head loss due to friction in the pipe until Point A is calculated by taking

the water surface elevation at the source (120 ft above mean sea level) and subtracting the

ground elevation at Point A (90 ft) but then adding in the 10 ft of head assumed to be available as

a safety factor. This results in a head loss (hL) due to friction of 20 ft. Equation 11-2 is then

solved for D, using hL = 20 ft.

2 3 2

552 2 2

(200 )(0.7 / )16 16(0.008) 0.25 3.0

2 2(32.2 / ) (20 )L

LQ ft ft sD f ft in

g h ft s ft = = = =

The idea here is to find a value of pipe diameter,D, that satisfies both Equation 11-3 and

Table 11-2. The value off= 0.008 was guessed since we do not knowD, which is required for

the ks/D ratio listed in Table 11-2. We will now check the value offby using the value of D

(0.25 ft) that we got when using Equation 11-3.

6sk 0.0001 1.3 10(308.5 / )(0.25 )

mmx

D mm ft ft

= =

This value ofks/D is less than the lowest value (0.00001) in Table 11-2. In this case, the

value offcould be lowered, but we dont know how much since it is below the range of dataused to make Table 11-2. Maintain the value off= 0.008 to be conservative. A high value off

results in more head loss, so if the water will flow with a higher head loss, then it will surely

flow if the real head loss is less than we calculated.

If this value ofks/D hadnt matched, then we would have needed to use the value ofD

obtained to get another value of the friction factor, f, from Table 11-2, calculateD again and

check ifks/D matches that of Table 11-2. This is continued until Table 11-2 and Equation 11-3

are both satisfied. It may take 3 or 4 iterations of this procedure to converge on an answer.

If the resulting diameter is available as commercial pipe, we can stop here. If it is not

commercially available, we should round up to the next largest commercially available pipe size.


10/40

Chapter 11 10

Lets assume that 3-in. pipe is not available, and that the next largest size is 4 in. This

would lead to a head loss of:

2 3 2

2 5 2 2 5

(200 )(0.7 / )16 16(0.008) 5.0

2 2(32.2 / )3.14 (0.33 )L

LQ ft ft sh f ft

g D ft s ft = = =

Rounding up to a 4-in diameter reduced the head loss due to friction from about 12 ft

down to 5 ft.

Now we can consider the head loss at the inlet calculated from Equation 11-3, with a

value of 0.5 forKfrom Table 11-3, assuming a square-edged entrance with r = 0.

( )( )

232

) 2 4 42 2

0.7 /16 16(0.5) 0.52

2 2(32.2 / )3.14 0.33L ml

ft sQh K ft

g D ft s ft = = =

The minor head loss is thus 0.52 ft, which corresponds to a in Figure 11-2. The total

head loss is the summation of the frictional head loss and the minor head loss:

HL-total = 5.0 ft + 0.52 ft = 5.52 ft

Using the Energy Equation (11-4) with the water surface in the tank as the upstream

point, and Point A as the downstream point, we can simply subtract the total head loss, 5.52 ft,

from the water surface elevation. This yields a value of 120 ft 5.52 ft = 114.48 ft. This value

is above the ground level at that point by a value of 114.48 ft 90 ft = 24.48 ft, which is above

our minimum specified value of 10 ft, so the correct discharge can go through this pipe.

If this value were not above the minimum value, then we would have had to increase the

pipe diameter by one pipe size and check again. We would continue to increase the pipe size

until we found the smallest commercially available pipe that would leave at least 10 ft of head at

all points along the pipeline.


11/40

Chapter 11 11

To design the pipe size for the downstream section of pipe, we repeat the process, but

using a head of 114.48 ft as the upstream value. If 4-in diameter pipe is maintained all the way

until the downstream end of the pipe, the hydraulic grade line at the end of the pipe can be

calculated by subtracting the frictional head loss from the value of head at Point A. The

frictional head loss can be calculated as follows:

2 3 2

2 5 2 2 5

(100 )(0.7 / )16 16(0.008) 2.5

2 2(32.2 / )3.14 (0.33 )L

LQ ft ft sh f ft

g D ft s ft = = =

Therefore the value of the hydraulic grade line at the end of the pipe is 114.48 ft 2.5 ft =

112 ft. Since the ground elevation here is 65 ft, that means the head is 112 ft 65 ft = 47 ft.

This 47 ft of head is now the inlet value for any pipe(s) attached here that continue downstream

(not shown). Additional pipes can be calculated in the same manner, always using the outlet

value from the pipe upstream as the inlet value of the pipe being calculated.

The maximum pressure occurs at the location where the difference between the hydraulic

grade line and the ground elevations is the largest. This occurs at point B in Figure 11-2. 47 ft is

not more than the maximum allowable pressure for PVC (150 ft), so the pipe will not burst.

However, if the pressure is too high at any point in the pipe, you can decrease the pipediameter upstream to increase the amount of head loss, or you can put galvanized iron pipe in the

spots with excessively high pressure. An additional option is a break pressure tank, discussed in

Section 11.2.3. If a smaller diameter pipe or a break pressure tank is used, make sure you have

enough head to get the water back up any hill downstream of this point. If the head is greater

than 600 ft, the maximum allowable pressure for galvanized iron pipe, then you must decrease

the pipe diameter(s) upstream. If a large amount of pressure is needed to cross a valley and then

carry the water back up the other side, and no pipe is strong enough to withstand the pressure, the

pipe can be suspended across the valley.


12/40

Chapter 11 12

11.2.3 Break Pressure Tanks

Static head is the difference in elevation from the source to any point on the ground

profile. Static head must always be considered because excessive pressure in the pipe from

elevation head can rupture the pipe or pipe connections. Piping is rated by its ability to withstand

an internal pressure without breaking. This is a function of the type of pipe material and the

thickness of the pipe wall. Previously, Table 11-4 provided information on the pressure limits

of particular piping material.

Break pressure tanks are strategically placed along the pipeline to eliminate excessive

pressure that will rupture the pipe or cause failures at the joints. The function of a break pressure

tank is to allow the flow to discharge into the atmosphere, thereby reducing its hydrostaticpressure to zero, and establishing a new static level. Figure 11-3 depicts a break pressure tank

and its components. In Figure 11-3, as the entry pipe turns vertically downward, there are holes

placed in its sides to help dissipate the pressure and protect the inside of the box from damage.

A storage tank will also function like a break pressure tank. A general rule of thumb is that static

head at any given point within the conduction line (with a good factor of safety) should not

exceed 100 meters of head. This means that a break pressure tank should be installed

approximately every 100 m change in elevation from the source yet still leave enough pressure to

flow to the storage tank.

There is no minimum required capacity for a break-pressure tank, as long as water is able

to drain from it as quickly as it is discharged. The dimensions of the tank are influenced more by

the size of the fittings (such as control valves, float valves) which must fit inside of it, and the

size of the pipe wrenches which must be able to be rotated inside as well (Jordan 2000). When

adding a break pressure tank, similarly to when reducing pipe size, it is important to check that

the head that will be lost is not needed farther downstream to bring the water back up to a

localized high point.

Figure 11-4 shows what happens to the HGL of a system when a break pressure tank is

added to the system and the frictional pipe loss plotted over the ground profile. Proper location


13/40

Chapter 11 13

of break-pressure tanks is an important part of a successful design. The ideal placement is a

location where the exiting flow will have an immediate drop in elevation to regain energy. In

general, the top of a hill is a good location for a break-pressure tank as long as the next high

point along the conduction line is at a lower elevation.

There is sometimes a float valve (similar to western-style toilet) located inside the break

pressure tank that helps regulate flow inside the tank. In this case, the float valve shuts off when

the break pressure tank is full (water is not being used downstream). However if the float value

does not shut off, water is lost in the overflow tube. Most float values have much lower pressure

head limits (around 60 m of head) then standard tubing materials, and if break pressure tanks are

to be installed, the final pressure at the tank entrance should be double checked. The other

option is that the break pressure tank will overflow when the water demand is not great which is

allowable when the water source provides more water than necessary. Placing rocks or

something hard where the water falls will disperse energy and avoid erosion at this spot. Float

valves break fairly often, therefore, it is important to minimize the number of break pressure

tanks and make sure the water committee has replacements and knows how to install them.

11.2.4 Clean Out Valves and Air Release Valves

Collection of sediment and air in the pipe are the two most common obstructions found in

water systems that result in partial and/or complete blockage of the pipeline. Figure 11-5 shows

a typical profile of a water supply system and proper placement of air release and sediment clean

out valves. Air relief valves are best installed at high spots along the conduction line marked as

B and D. Not all high points will require air relief valves however, and once the system is

completed, air valves can be installed starting from the source, working downward until air

blockages are not a problem. Sediment cleanout valves should be installed at any low spots

along the pipeline which are marked as A, C, and E. During construction it is important to

always ensure that sediment clean out valves are installed at the low points in the terrain and the

air release valves are installed in the low points of the terrain. This may require that a section of

pipe is cut.


14/40

Chapter 11 14

The minimum size for sediment clean-out valves is typically a 1-in gate valve. For

larger mainlines a relationship of D/3, where D is the inner diameter of the mainline, can be used

to calculate the size of the required valve.

The purpose of an air valve is to release this trapped air. Because air is less dense than

water, it sometimes is retained in the higher areas of the pipeline, and thus blocks the movement

of water at the design flow. The location of air valves is especially important where the hydraulic

grade line is close to the level of the terrain. Commercially-available air-release valves are

available that work by a spring attached to a ball that allows air to escape if it builds up to

enough pressure to push the ball away from the opening and seal itself again when the pressure

reduces. Blockages due to air typically occur after pipes have been emptied for cleaning or

service. Standard, non-automated valves can also be installed if the cost of automatic air release

valves is too high, or they are not available in the area. In this case the person or persons

servicing the pipeline opens all the air valves while walking up the pipeline to the area to be

cleaned. After the work is completed, these persons can return the same way, closing the air

valves manually once water has arrived at each location and all air has been forced out. These

valves are typically enclosed in a concrete box to provide easy access and prevent damage by

humans and animals.

Installing too many clean out and air release valves will not only increase the cost, but

also make the system more susceptible to leaks and vandalism during the operation of the

system. Financially, the engineer must review the cost impact of adding cleanouts and air relief

valves as they can become a very expensive portion of the system cost. The maintenance and

upkeep of these types of valves are very important to prolonging the operating life of the system.

11.2.5 The Storage Tank

Storage tank design is covered in a separate chapter. The tank should be located to allow

for at least 10 m of head at all points in the distribution network (some suggest this value can be

5-10 m of head). It is also preferable that it be located close to the community for easy

maintenance. Because storage tanks act as break pressure tanks, they should be placed at a


15/40

Chapter 11 15

location where the exiting flow will have an immediate drop in elevation in order to regain

energy.

11.2.6 Distribution Lines

Distribution networks consist of the pipe and accessories required to connect the storage

tank to the users. Similarly to the design of the conduction line, distribution pipe design should

follow the process described previously in Example 11.1 or use of software such as GoodWater.

Branched Networks

A branched system is one in which there are no loops in the network. In other words,

water can reach each tap stand by only one path. Many water systems in the developing world

are branched systems, except perhaps in large cities.

As previously mentioned, the flows required for any pipe in a branched network (not a

looped one) are determined by the user demand at the end of that pipe. The flow of any pipe

coming into a junction is the sum of all the flows going out of that junction. Figure 11-6

illustrates this point.

As discussed above, the hydraulic grade line of each pipe can be determined using the

value at the downstream end of the pipe just upstream of the pipe in question as the new

upstream head value. HGL calculations proceed from the source to the end of each branch line

once the pipe diameters have been selected.

Looped Systems

If there is a tap stand in a system that water can reach by more than one path, then the

system has one or more pipe loops. This provides the advantage of adding redundancy to the

design. If there is a blockage or break in a pipe, water can still go to the downstream taps

through an alternate path. It adds cost to the design, however, due to the need for more piping.


16/40

Chapter 11 16

It also makes it harder to calculate the flow and pressure at points throughout the system. For

analyzing looped networks, there are computer programs as well as the classical Hardy-Cross

Method that can be done by hand (although it takes a while) or spreadsheet (Crowe et al. 2001).

11.2.7 Tap Stands

Tap Stands

Water is distributed from the distribution line to a user via a tap stand. Tap stand design

is based on community needs, especially the type of container commonly used for collection.

Consideration should also be given to what activities (e.g., washing dishes, laundry) will take

place near the tap stand in addition to water collection.

Typically, PVC piping is used to connect each tap stand to the distribution line.

Piping within the community is not buried as deep as in the main conduction line. It is usually

60 cm deep for normal terrain and 80 cm deep to cross roads. In areas of erosion, GI pipe may

be used to cross roads.

A water distribution system can be based on community tap stands, or separate tap stands

can be provided for each participating household. The collection and sanitary disposal of

greywater is a design consideration as well. Figure 11-7 shows the components of a typical tap

stand. Figure 11-8 depicts various types of tap stands, and Figure 11-9 shows the integration of

greater collection, drainage, and shutoff valves. Concrete skirts not only inhibit erosion at the tap

stand, but can allow a user to direct excess water to nearby gardens.

A larger PVC pipe (approximately 10 cm diameter) that is cut in half length wise and

held together with hose clamps or bailing wire can work as a mold for the taps shown in Figure

11-7 and the top illustration of Figure 11-8. For this particular design, the piping and fittings

that exit the top and bottom of the tap stand are placed inside the mold, which is filled with

concrete and then cured. Encasing the tap stand pipe in concrete protects it from large animals,

children, and carts.


17/40

Chapter 11 17

A minimum head of 10 m pressure is recommended at the tap to ensure the delivery of

sufficient flow from the tap. When determining the minimum head required at the tap, it is

important to consider the frictional losses at the elbows and gate valves shown in Figure 11-7.

Standard taps purchased at a hardware store can generally withstand approximately 60 m of head

before they are damaged. If the static head is over 60 m prior to the placement of community tap

stands, a break-pressure tank with a float valve or a break pressure valve can be installed (Reents

2003).

Valves

Valves are typically placed outside of each house, enabling the plumber to control access

to the system. The valves should be enclosed in a valve box which may be locked so that only

the plumber can adjust them. If the water fee is not paid, water supply can be terminated.

Lockable faucets can also be purchased. Valve boxes should be located on public property close

to the road for ease of access. Additional valves should be installed to isolate sections of the

water system for repairs. With strategically located valves, access to the system can be given

for part of the day to each section of the town. Valves should be closed slowly to prevent

damage from water hammer.

11.2.8 Other System Components

The components discussed in previous sections form the basic parts of a water system.

Other non-traditional items may also be included. In order to improve the quality of the water in

the system, other components should be considered. These components can add significant cost

and maintenance constraints to a system and their additions should be carefully considered.

Aeration units can be installed in the line to improve the taste and smell of water. These are

simple tanks where water enters from above and falls through the air to an outlet at the bottom of

the tank.


18/40

Chapter 11 18

An aeration unit will not remove pathogens. In this case, different filters may be installed

along the pipeline. A simple rock or sand filter can be installed at the intake if water is of poor

quality. These will require maintenance though and cannot be guaranteed to remove pathogens.

Slow sand filtration can also be used to improve water quality. These filters can require large

amounts of space and can also be complicated to install. Water may also be treated at the home

with point-of-use filtration systems.

To manage water in the network splitter-boxes can be installed. These boxes can be

much more cost effective then installation of valves and more precisely control the distribution

of water. They consist of a simple box where water enters, flows over a partition, and exits into

different tubes. These boxes are particularly important where water must be divided into specific

fractions, for example, when one community must receive exactly half or a quarter of the water

from a source. The partition divides the box into two halves, and the half where the water exits

may be sub-divided into as many sections as the designers requires. The placement of these sub-

sections corresponds exactly to the ratio of water which that subsection is to receive. For

instance if the user wants the water distributed into three pipes, one with 50% of the water, one

with 25% of the water, and one with 25% of the water, a Splitter-box with three sub-sections

should be installed (Figure 10-10). The partitions of the sub-sections are arranged so that water

flows over the main partition in the correct rations.

In addition to the standard tap stand, there are various other types of extraction points

which can be included in a water system. Locations such as schools, churches, and community

centers are locations where special needs may need to be addressed. Public bathing or showering

areas may also be a priority in the community. A specially designed area for washing clothes

that has multiple faucets can also be considered in communities that have a traditional washing

area.


19/40

Chapter 11 19

11.3 Construction Tips

Typically, the community digs the trench and installs the pipeline. It should be buried

about one meter below ground, because it may cross agricultural fields which get plowed on a

regular basis or may be burned to clear brush. The erosion of soil due on hilly terrain can also

expose piping which has not been buried deeply enough. In addition, burying the conduction

line reduces illegal tapping for household or agricultural use. One-meter sticks can be cut and

provided for workers to measure consistent depth along the trench. Small marks can also be

notched on picks and shovels to identify trench depth.

PVC pipe is connected as described in Chapter 7. As discussed in that chapter, PVC

pipes are glued so that the bell end (i.e., the female end) is positioned against the flow of water to

prevent future leakage. Be careful not to accidentally introduce sediment into the pipes during

joining. Extreme care must be taken when connecting pipes of different materials, especially

PVC to galvanized iron. These areas are especially problematic, as the connection sleeve is

often very short. A longer section of PVC can be glued the night before into the adaptor so that

the connection is much stronger.

Experience suggests that 20-30 pieces of 6-m PVC pipe can be laid through rocky soilduring a normal work day with a work brigade of 15 persons. The count can increase to

approximately 40 pieces of pipe if the soil is sandy and absent of rocks. When installing sections

of galvanized iron piping it is important to install universal unions every 8 to 10 pieces of pipe.

Having an ax available to cut through roots and narrow shovels as well can reduce the time

required to dig a wide trench.

Box 11-3. Typical Water Project Construction Timeline.

Water supply projects are large endeavors requiring the coordination of laborers,

materials, weather, and other factors. The order in which a project is constructed can


20/40

Chapter 11 20

significantly affect its progress. Below is a suggested project plan and justification for its

ordering.

1. First 1/3rd of Main Pipeline Starting with the conduction line is easier thenstaring with a complicated intake structure, which

often necessitates hauling cement and other heavy

materials. Use this time to organize labor brigades

and work out any problems. Connect pipes to bring

water where people are working

2. Intake Structure(s) Interest in the project will be highest near thebeginning, take advantage of this time to work on

sections farthest from the community.

3. Finish Main Pipeline Once the conduction line is finished water can bebrought to the tank location. This will excite

community members and provide further

motivation.

4. Storage Tank Working on the storage tank once the conductionline has been completed allows water to be

available water to mix concrete and cement without

hauling water long distances.

5. Entire Distribution Network The distribution network will proceed rapidly ascommunity members are working near their homes

and can see daily progress.

6. All Tapstands It is important that all tap stands are connected atthe same time. Workers are liable to stop working

on the project once water arrives near their homes.To be fair to all community members ensure that

all tap stands are installed over a short timeframe.

11.4 Operation, Maintenance, and Security

The operation and maintenance of a water supply system is vital to its long term

sustainability. The most important asset of a well-operated water supply system, aside from its

design, is the formation and management of a community water committee (see Chapter 3). This

group of elected individuals has the task of assuring the water system will last as long as


21/40

Chapter 11 21

possible. They collect fees to maintain the system, determine what to do if users do not pay their

fees, and arrange necessary system repairs and maintenance.

The second most important asset of operating and maintaining a gravity fed water supply

system is to have several trained plumbers who reside in the community. Training local

plumbers starts during the projects conceptual design, and begins in earnest on the first day of

construction. During the duration of the construction, these individuals should learn how to

install and replace every element of the system. Additionally, they need to be trained: 1) to walk

the conduction line; 2) inspect the line for leaks; 3) release air from the system, if present; 4)

clean sediment valves; 4) inspect and clean an emptied storage tank once a year; 5) maintain

bleach in a system chlorinator, if required; and 6) understand how to purchase parts that need

replacement and/or repair. Compensation for these individuals is a decision made by the

community water committee, and can be monetary or in kind (e.g., rice, beans).

A final aspect of water supply system design issecurity. While designing a system,

provide secure boxes/hatches to access the valves, tanks, and other places where a system has

potential for tampering. The hatches on valve boxes and tanks, whether concrete or steel, can be

designed with a padlock for security. This also includes protecting vulnerable areas such as the

springbox or intake dam from animals.

Insert Table 11-7

Further Reading

American Society of Civil Engineers (ASCE) (1969).Design and construction of sanitary and

storm sewers, ASCE Press, New York, New York.

Annis, J. (2006). Assessing progress of community managed gravity flow water supply systems

using rapid rural appraisal in the Ikongo District, Madagascar. Masters Report.


22/40

Chapter 11 22

Michigan Technological University, Houghton, Michigan.

(January 14, 2008).

Crowe, C.T., Roberson, J.A., and Elger, D.F. (2001).Engineering fluid mechanics, 7th

Ed. John

Wiley and Sons, Inc., New York, New York.

Good, S. (2008).Development of a decision support system for sustainable implementation of

rural gravity flow water systems, M.S. Thesis, Civil & Environmental Engineering,

Michigan Technological University.

(January 14, 2008).

Jordan, Jr. T.D. (1980).A handbook of gravity-flow water systems, Intermediate Technology

Publications, London.

Menon, Shashi. 2005.Piping Calculations Manual. New York, NY: McGraw-Hill.

Niskanen, M.A. (2003). The design, construction, and maintenance of a gravity-fed water system

in the Dominican Republic, M.S. Report, Civil & Environmental Engineering, Michigan

Technological University. (January 14,

2008).

Purcell, Patrick. 2003.Design of Water Resource Systems. London, UK: Thomas Telford

Reents, N. (2002). Designing water supply systems in rural Honduras, M.S. Report, Civil &

Environmental Engineering, Michigan Technological University.

(January 14, 2008).

Servicio Autnomo Nacional de Acueductos y Alcantarillados (SANAA) (1999).Normas de

Diseo para Acueductos Rurales V.1.0, Tegucigulpa, Honduras.


23/40

Chapter 11 23

Simpson, J.D. (2003). Improvement of existing gravity-fed rural drinking water systems in

Honduras, M.S. Report, Civil & Environmental Engineering, Michigan Technological

University. (January 14, 2008).


24/40

Chapter 11 24

Figure Captions

Figure 11-1. Topography and hydraulic grade line (HGL) from a water storage tank (at 120 ft

elevation) to a water user (at 65 feet). The drop in the HGL represented by the letter a after the

water tank is the minor loss caused by the pipe inlet.

Figure 11-2. Profile view cross section of horizontal settling tank and components.

Figure 11-3. Profile view cross section of break pressure tank and components. Float valves can be

installed to prevents the overflow of water.

Figure 11-4. Pressure head plotted over ground profile (determined by the topographic survey) to

create the HGL. The top solid line shows the frictional head loss, which is determined by the source

flow and the size of pipe. Here the difference in head from the source to the storage tank is

approximately 260 m, therefore 1-2 break pressure tanks might have been recommended. In the

real scenario depicted here only one break pressure tank was recommended for the midpoint of the

elevation change. This was because of issues of land use and to project costs. Note that the static

head at the midpoint is 130 m, which is greater than the 100-m rule of thumb mentioned in the text.

However, if PVC RD17 pipe was used, the allowable pressure would be 176 m (Table 11-4) so the

system should not fail from pipe rupture (Niskanen 2003).


25/40

Chapter 11 25

Figure 11-5. Locations of sediment clean out valves (points A, C, and E) and air release valves

(points B and D) (adapted from Jordan 1980).

Figure 11-6. Determination of flow rate in each pipe of a branched system. Neighborhood needs

are determined by adding individual user needs. In this system, the flow rates would be calculating

by starting at the right and working backwards to the left.

Figure 11-7. Components of a typical tap stand design.

Figure 11-8. Tap stands with concrete post and concrete erosion protection (top), concrete post and

underground pipe to minimize erosion (middle), and detached wooden post and large stones to

provide erosion protection (bottom).

Figure 11-9. Tap stand design integrated with greywater evacuation, drainage, and optional

shutoff valves and meters.

Figure 11-10. Splitter-box that divides water into 50%, 25%, and 25% pipes.


26/40

Chapter 11 26

Figure 11-1. Example how topography and hydraulic grade line (HGL) change from a water

storage tank (at 120 ft elevation) to a water user (at 65 feet). The drop in the HGL represented by

the letter a after the water tank is the minor loss caused by the pipe inlet.

Figure 11-2. Profile view cross section of horizontal settling tank and components.


27/40

Chapter 11 27

Figure 11-3. Profile view cross section of break pressure tank and components. Float valves can be

installed to prevents the overflow of water.


28/40

Chapter 11 28

Ground Profile - Frictional Losses/ Spring to Tank - Los Arroyos/Los Botados

700

750

800

850

900

950

1000

0 200 400 600 800 1000 1200 1400 1600

Distance (m)

Elevation(m)

--2" diam. friction loss

1-1/2" diam. friction loss--

--pressure returns to zero

--2" diam. friction loss

1-1/2" diam. friction loss--

ground profile

Figure 11-4. Pressure head plotted over ground profile (determined by the

topographic survey) to create the HGL. The top solid line shows the frictional head loss,

which is determined by the source flow and the size of pipe. Here the difference in head

from the source to the storage tank is approximately 260 m, therefore 1-2 break pressure

tanks might have been recommended. In the real scenario depicted here only one break

pressure tank was recommended for the midpoint of the elevation change. This was

because of issues of land use and to project costs. Note that the static head at the midpoint

is 130 m, which is greater than the 100-m rule of thumb mentioned in the text. However, if

PVC RD17 pipe was used, the allowable pressure would be 176 m (Table 11-4) so the

system should not fail from pipe rupture (Niskanen 2003).

Break-pressure

Tank


29/40

Chapter 11 29

Figure 11-5. Locations of sediment clean out valves (points A, C, and E) and air release valves

(points B and D) (adapted from Jordan 2000).


30/40

Chapter 11 30

Figure 11-6. Determination of flow rate in each pipe of a branched system. Neighborhood needs

are determined by adding individual user needs. In this system, the flow rates would be calculating

by starting at the right and working backwards to the left.

Neighborhoodneeding

Q=0.3 ft3/s

Neighborhood

needingQ=0.1 ft

3/s

Neighborhoodneeding

Q=0.2 ft3/s

NeighborhoodneedingQ=0.4 ft

3/s

Water

Source

Q=0.1 ft3/s

+ 0.3 ft3/s

= 0.4 ft3/s

Q=0.4 ft3/s+ 0.2 ft

3/s

= 0.6 ft3/s

Q=0.6 ft3/s

+ 0.4 ft3/s

= 1.0 ft3/s


31/40

Chapter 11 31

Figure 11-7. Components of a typical tap stand design.


32/40

Chapter 11 32

Figure 11-8. Tap stands with concrete post and concrete erosion protection (top), concrete post and

underground pipe to minimize erosion (middle), and detached wooden post and large stones to

provide erosion protection (bottom).


33/40

Chapter 11 33

Figure 11-9. Tap stand design integrated with greywater evacuation, drainage, and optional

shutoff valves and meters.


34/40

Chapter 11 34

Figure 11-10. Splitter-box that divides water into 50%, 25%, and 25% pipes.


35/40

Chapter 11 35

Table 11-1. Roughness (ks) of various pipe materials (Crowe et al. 2001).

Pipe Material ks (ft)

PVC 3.3x10-7

Copper, Brass 4.9x10-6

Steel 1.5x10-4

Galvanized Iron 4.9x10-4

Cast Iron 8.5x10-4

Concrete 9.8x10-4 to 9.8x10-3

Riveted Steel 3.0x10-3 to 3.0x10-2

Table 11-2. Friction factor for use in Equation 11-1 for various relative roughness/pipe diameter

ratios (ks/D), assuming fully turbulent flow (Crowe et al. 2001).

ks /D f

0.00001 0.008

0.00005 0.0110.0001 0.012

0.0002 0.014

0.0004 0.016

0.0006 0.018

0.0008 0.0190.001 0.02

0.002 0.024

0.004 0.0290.006 0.032

0.008 0.036

0.01 0.0380.015 0.044

0.02 0.050.03 0.056

0.04 0.065

0.05 0.07


36/40

Chapter 11 36

d

r

D1D2

Table 11-3. Minor loss coefficient (K) values for various fittings (Crowe et al. 2001)

Pipe Entrance r/d0.0

0.1

>0.2

K=0.50

K=0.12

K=0.03

Contraction D1 D2

D2/D1

0.0

0.20

0.400.60

0.80

0.90

K for=60o

0.08

0.08

0.070.06

0.06

0.06

K for=180o

0.50

0.49

0.420.27

0.20

0.10

Expansion D1/D20.0

0.20

0.400.60

0.80

K for=20o

0.30

0.250.15

0.10

K for=180o

1.00

0.87

0.700.41

0.15

90o Sharp Miter

Bend

K=1.1

Pipe Fittings Globe valve wide open

Angle valve wide open

Gate valve wide open

Gate valve half openReturn bend

Tee

straight through flowside-outlet flow

90o elbow

45o elbow

K=10.0

K=5.0

K=0.2

K=5.6K=2.2

K=0.4K=1.8

K=0.9

K=0.4


37/40

Chapter 11 37

Table 11-4. Standard values for typical pipe materials (from Menon 2005; Purcell 2003) (1 bar equals 10.197

m of water).

Material Hazen-Williams C Pressure Limit (Bars) Pressure Limit

(m of head)PCV (Class B) 150 6 61

PCV (Class C) 150 9 91

PCV (Class D) 150 12 122

PCV (Class E) 150 15 125

PE (medium density) 150 12.5 127

PE (high density) 150 16 163

Iron Tubing 120 40 407

Prestressed Concrete 120 20 204


38/40

Chapter 11 38

Table 11-5. Typical Pipe Material Sizes, Advantages, and Disadvantages.

Pipe Material Lengths and Types* Advantages Disadvantages

PVC Pipe Typically 10' or 20' lengths Low cost

Limited allowable pressure

pressure a

(Polyvinyl Chloride) 1/4" - 4" diameter

Flexible, easy to place in a

trench

Possible damage

due to blunt force

Easier to vandalize

Becomes brittle

when exposed to

sunlight for an

extended period of

time

Areas not

vulnerabl

Also available in long spiral

sections, which will requirefewer joints

Modification by

cutting or burning

Smoother, therefore causes

less head loss and allows a

smaller diameter to be used

GI Pipe Typically 20' lengths Very durable Higher cost High pres

(Galvanized Iron) 1/4" - 4" diameter

Allowable pressure

is much higher

Added labor to

bend or modify

Stream orcrossings

locationspipe is exAnother o

scenario i

larger-dia

pipe surrosmaller-d

that actua

water, in

Additional tools

required to install

Requires pipethreading

Can experience

corrosion andblockage due to

calcium deposits

*Subject to specific availability in each country

.


39/40

Chapter 11 39

Table 11-6. Pipeline Flow Rate Design Limits (L/s) for Different Pipe Sizes

Pipe Diameter, mm

25 32 40 50 80 100

Minimum 0.35 0.60 0.90 1.4 3.5 6.0

Maximum 1.4 2.0 3.5 5.0 n/a n/a


40/40

Chapter 11 40

Table 11-7. Common Water Myths.

Myth Fact

Decreasing pipe size increases pressure at the tap. The Energy Equation (11-4) shows that if youdecrease the pipe size, there is more head loss,

meaning that more pressure is burned off due to

friction by the time the water reaches the end of the

pipe.

If there is not enough pressure at the tap, then the

control valve just has to be opened more.

Opening the valve may help, but if there are other

causes of head loss (too small a pipe, blockage by

sediment, etc.) then you still wont get enough flow.

Water does not flow uphill. Water can flow uphill if there is enough pressure to

push it up the hill. This can be seen in the EnergyEquation (11-4) by solving for all of the pressure

terms on one side of the equation. All thats left on

the other side of the equation is head loss and

elevation difference. If there is enough pressure

difference to overcome the head loss and elevationdifference, then water can indeed flow uphill. The

needed pressure can be supplied by a pump or for

gravity systems, by a sufficient difference inelevation.

Having a sudden drop in the pipeline is not good. Having a sudden drop is OK if the pressure stays

below the pressure that would cause the pipe torupture.

chapter 11 gravity flow water systems final november 16 2008

Documents