Environment i lecs 18 to 23 wbb34

ENGR. SAJJAD ALI MANGI (LECTURER)BE (Civil)-QUEST NawabshahME (Environment)-NED University Karachi

Email: [email protected]

Water Distribution SystemLecture No:18Dated: 7th Jan 2014

General Consideration for Water Distribution SystemLecture No:18 Dated: 7th Feb 2014

The water, as it leaves the treatment plant, is of desired quality. But the standard of this quality is to be maintained when the treated water if flowing through the network of distribution system.The water should also be supplied in a way useful to the consumers. For these reasons, the following general considerations of the planning of distribution system should be observed in its design:1. Circulation of water2. Construction and design3. Contamination by sewage4. Earth cushioning (laid under road)5. Economy 6. Fire demand7. Gradients8. Leakages 9. Repairs10. Safety from Pollution11. Sanitation12. Unsafe cross connection.


1. Circulation of water: The layout of distribution system should be such that there is free circulation of water and the number of dead ends should be very few. Where dead ends are unavoidable, hydrants will be provided to act as washouts.

2. Construction and design: The construction and design of water distribution system should be such that sufficient water is available at all times at desired pressure in all portions of the distribution system.

3. Contamination by sewage: The water pipe should be laid above the sewers at a vertical distance of about 2 meters and the horizontal distance between the water pipes and sewer should be at least 3meters .

4. Earth cushioning: The mains which are laid under roads should be provided with a minimum earth cushioning of 75 to 90 cm height from the top of mains.

5. Economy : The layout and Design of distribution system should be economical. The cost of distribution system forms a substantial part to the extent of about 90% of the total cost of the water supply project. Hence water distribution system should be carefully designed by taking into various factors such as pumping head, type of pipes, storage requirement, pipe diameters etc


6. Fire demand: The distribution system should be so laid that water for fire demand is available in required quantity at desired pressure at number of points along it.

7. Gradients: It is not necessary to lay main at constant gradients. But the gradients of mains should be in general follow the natural contours of ground. The gradient line should not rise above the hydraulic gradient line which means that at every point along the mains, there should be a positive pressure greater than atmospheric pressure.

8. Leakages: The distribution system should be fairly watertight and the loss of water due to leakages should be brought down to the minimum possible extent.

9. Repairs: The distribution system should be so laid as to permit easy repairs. The broken or worn out parts of the equipments for various operations should be properly replaced.

10. Safety from Pollution: The layout of Distribution system should be such that it does not contribute to the pollution of water flowing in it.


11. Sanitation: The sanitation of area through which the distribution system is passing should be good that there are no chances for water to be polluted during repair or replaced of pipe lines.

12. Unsafe cross connection: The distribution system should not have any unsafe cross connections from which there are chances for contaminated water to enter it.

Methods of Water Distribution System Lecture No:19 Dated: 10th Feb 2014

Depending upon the topography of the area /country, the following three may be adopted for distribution of water.

1) Gravity System, 2) Gravity and Pumping System combined, 3) Pumping System1. Gravity System: In this method, water is conveyed through pipes by gravity

only. The gravity system is the most reliable method of distribution. But it is useful only when the source of water supply is situated at a higher level than that of distribution area. As shown in figure A the gravity system with hydraulic gradient during max. and min. demands. In this case fire, motor pump may be used to develop high pressure for the fire fighting purpose.

2. Gravity and Pumping System combined: In this system, the treated water is pumped and stored in an elevated distribution reservoir. The excess water during low consumption remain in the elevated reservoir and it is supplied during the peak period. The pumps are usually worked at constant rate and this rate of pumping is so adjusted that the excess quantity of water stored in reservoir during low consumption is nearly equal to the extra demand of water during peak period. As shown figure B it shows the combined gravity and pumping system with hydraulic gradient during max. and min. demands.

Methods of Water Distribution System Lecture No:19 Dated: 10th Feb 2014

This method of distribution is usually applicable in most of the cases and it has the following advantages;i) in case of a fire, motor pumps can be used to develop high pressure or fire demand can directly be satisfied from pump house after closing the inlet valve for elevated reservoir.ii) In this method, the pumps are generally worked at uniform rate.iii) This method is economical iv) This method is fairly reliable in the sense that some quantity of water is available from elevated reservoir even during breakdown of pumps.

3. Pumping System: In this system, water is directly pumped into the mains leading to the consumers. The number of pumps required in this system will depend on the demand of water. As figure shows C the pumping system with hydraulic gradients during max. and min. demands.

Service Reservoir Lecture No:20 Dated: 11th Feb 2014

The service or distribution reservoir are generally provided in the distribution system to store clear treated water before it is dispatched to the consumers.These may be constructed of brick masonry, Stone masonry, Reinforced cement concrete (RCC), etc

Purposes:Following are the purposes served by the service or distribution reservoir:1. If pumps are used, the provision of these reservoir make it possible to run

pumps at uniform rate.2. In case of gravity system of supply, the provision of these reservoirs will result

in main of smaller diameters.3. They furnish the facility of storage of water for meeting fluctuating hourly

demand of water.4. They maintain constant pressure in the mains. The pressure in mains, without

service reservoirs, will fall as the demand of water will increase.5. They make the design and construction of treatment units and distribution

system economical.6. They serve as storage for emergencies such as breakdown of pumps, bursting

of mains, heavy fire demand, interruption in power supply, temporary floods, etc.

Classification of Service Reservoir Lecture No:21 Dated: 12th Feb 2014

According to situation with respect to ground, the service reservoirs are classified in the following three types,

(1) Surface Reservoir,(2) Elevated Reservoir,(3) Stand Pipes,

Surface Reservoir: These are also known as ground or non-elevated reservoir. They are constructed at ground level and are mainly used to store water. It is usual practice to construct a surface reservoir in two compartments so that one can be used while the other is being cleaned or repaired. The two compartments are connected with each other by control valves.Due to storage of water, silting will take place to some extent and it can be removed by occasional cleaning through the washout pipes which are placed at the bottom of the reservoir.The surface reservoir are usually located at high points in the distribution system. In the gravity system of supply, water can directly be sent to the distribution system from the surface reservoir. In the combined gravity and pumping system, filtered water is first stored in the surface reservoir and it is then pumped to the elevated reservoir.

Classification of Service Reservoir Lecture No:21 Dated: 12th Feb 2014

Elevated Reservoir: These are also known as overhead tanks and they are constructed at an elevation from ground level. They may be of any shape rectangular, circular or elliptical. With the development of science of structural analysis, it is possible to construct the elevated reservoir in any shape to suit the architectural requirement of the structure.The elevated reservoirs are constructed in distributing areas which are not controlled by gravity system of distribution. Stand Piped: A stand pipe is a vertical cylindrical tank resting just above the ground to develop the necessary pressure. The diameter of stand pipe varies from 10m to 15m and its height varies from 15m to 30m. The stand pipes are made of RCC, Steel etc.The stand pipes are usually located on a hill or high ground so as to successfully utilize its entire capacity.The stand pipes, like elevated reservoir, are to be provided with inlet pipe, outlet pipe, overflow pipe, washout pipe and various other accessories for their efficient working, inspection and maintenance.

System of Supply of water Lecture No:21 Dated: 12th Feb 2014

based on the duration of supply, the following are the two systems of supply of water.

(1)Continues System, (2) Intermittent System(1) Continues System: In this system of supply, the water is supplied to the

consumers for 24 hours of the day. This is most ideal system of supply and it should be adopted as for as possible. The only disadvantage of this system is that considerable wastage of water occurs if consumers do not possess civic sense regarding the importance of treated water.

(2) Intermittent System: In this system of supply, the water is supplied during certain fixed hours of the day only. The usual period is about one to four hours in the morning and about the same period in the afternoon. The timings of supply of water may be changed according to the seasons of the year. This system of supply of water proves to be useful for the following two conditions:(i) the available pressure is poor and (ii) the quantity of water available is not sufficient to meet with the various demands of water. The working of this system is very simple. The distribution area is divided into several zones and the timings of each zone are so adjusted that good working pressures are maintained in each zone.

Methods of Layout of distribution pipes Lecture No:22 Dated: 13th Feb 2014

following are the four main methods of laying distribution pipes.(1) Dead-end Method,(2) Grid Iron Method,(3) Circular Method,(4) Radial Method,

Hydraulics of Closed Conduit Flow

• Some time also known as;- closed conduit flow- pipe flow- pressurized flow

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Lec: 23, dated: 14th Feb. 2014

Closed Conduit / Pipe HydraulicsEngr. Sajjad Ali Mangi(Lecturer)Department of Civil EngineeringMUET, SZAB Campus, Khairpur Mir’s

Closed Conduit VS Open Channel

Closed Conduit Open ChannelNo free surface of water Free water surface

Gravity or pump driven Gravity driven / flow

Cavitation & other sub-atmospheric-pressure problems

Waves & other surface disturbances

Reynolds Number Re Froude Number Fr

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Concepts – Reynolds Number

• Reynolds number is used to check whether the flow is laminar or turbulent.

• Reynolds Number is the ratio of inertia force to viscous force.

V = velocity (ft/sec)D = pipe diameter (ft) = density of fluid (lbs/ft3) = dynamic viscosity of fluid (Pa s, N s/m2)or(lb.ft.sec/ft2 ) = kinematic viscosity (m2/s) or (ft2/sec)

VDVD

Re

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Osborne Reynolds (1842–1912),

http://en.wikipedia.org/wiki/Osborne_Reynolds

Concepts – Froude Number

• Froud Number is the ratio of inertia force to gravitational force.

• V = velocityg = gravitational accelerationh = depth of water

ghVFr

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Laminar & turbulent flow

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If Reynolds Number Re < 2000 the flow is called Laminar flow.

If Reynolds Number RE > 4000 the flow is called Turbulent.

uniform & steady flow• Uniform flow

The constant characteristics with respect to space• Steady flow

The constant characteristics with respect to time. Often adopted when establishing pipe system design parameters (pressure & flow at certain locations).

• Consider unsteady (transient) phenomena to refine design (pipe pressure class and thickness)

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Conservation of Energy• In pipeline design, most often consider steady

state – flow not varying with time - first• Steady state (SS) Bernoulli Equation along a

streamline:h

gg zVpzVp

2

2

221

2

11

22

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Where as;• Pressure head p/• Elevation head z• Velocity head V2/2g• Piezometric head p/ + z

(hydraulic grade line)• Total head p/ + z + V2/2g

(energy grade line)• Head Loss h

Head Losses • There are two types of losses:1. Friction Loss

Resulting from friction between the fluid and pipe wall2. Minor Loss

Resulting from pipe entrance, transition, exit, valve and other in-line structures

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Friction Loss • Most useful head loss equation for closed-conduit flow –

Darcy-Weisbach equation

gDLf Vh f 2

2

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Dimensionless Friction coefficient

Pipe diameter

Pipe length

Pipe velocity

Friction head loss

Gravitational acceleration


Darcy-Weisbach • Laminar flow (Re<2000)

Turbulent flow in smooth pipes (Re>4000) 8.0Relog21

ff

Re64

f

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• Turbulent flow in rough pipes

• Transition between turbulent smooth and rough pipes

De

flog214.11

fDe

f Re51.2

7.3log21

Friction Loss

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Darcy-Weisbach

• Most recent development of Darcy Weisbach coefficient - Explicit equation [Swamee and Jain, 1976] applicable to entire turbulent flow regime (smooth, transition and rough pipes):

2

9.0Re74.5

7.3log

25.0

De

f

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Minor Loss• Use minor loss coefficient (k) in this form

gkVhl 2

2

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Nomograph for Hazen-Williams formula in which C= 100

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Empirical Formula of Hazen-Williams formula

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Where as;Hf = Head loss due to friction in ftsL = Length of pipe in ftsC = Frictional coefficientQ = Discharge in gpm (US gallons) 1US gallon = 3.785 Liters

Environment i lecs 18 to 23 wbb34

Engineering

Transcript of Environment i lecs 18 to 23 wbb34