cve 471 - 5. water supply 2 - middle east technical university

CVE 471 Water Resources Engineering 1/58

Assist. Prof. Dr. Bertuğ Akıntuğ

Civil Engineering ProgramMiddle East Technical University

Northern Cyprus Campus

CVE 471CVE 471WATER RESOURCES ENGINEERINGWATER RESOURCES ENGINEERING

WATER SUPPLYWATER SUPPLY


55. WATER SUPPLY. WATER SUPPLY

Overview

Elements of Municipal Water Supply SystemsDistribution ReservoirsPipesPumps and Valves

Hydraulics and Operation of Gravity PipelinesDesign of Transmission Lines

Gravity LinesPumped LinesMixed Lines

Construction and Maintenance of Municipal Water Supply Systems



Elements of Municipal Water Supply System

Water is conveyed from a source or combination of sources to community in mainly closed conduits.

pressurized flow is possiblepollution control

Possible element of a water transmission system:canals and flumesgrade aqueductsgrade tunnelspipelinesvalvespumpspressure reducing chambers etc….

1915-19 Winnipeg, Manitoba, Canadahttp://www.mhs.mb.ca/docs/pageant/24/winnipegaqueduct.shtml




Transmission pipelineshttp://www.wau.boku.ac.at/fileadmin/_/H81/H811/Skripten/811356/05_Supply_Storage_distribution.pdf




Typical arrangement of a municipal water supply system

Distribution reservoirs are designed to meet the hourly variations in water demand and to store extra water for fire fighting and emergencies.




In the main transmission line Qdesign= Dmd = Dad x (P.F.)day

In a main feeder Qdesign= Dmh = Dmd x (P.F.)hour

OR

Qdesign= Dmh = Dad x (P.F.)hour x (P.F.)day




A water distribution network is normally composed of pipes, valves, hydrants, and pumps. Distribution of water is possible by

gravity (storage reservoir is required at a sufficient altitude)pump without storage (may be required during any emergency), andpump with storage (the most common way).

Types of distribution systems depends on street plantopography,location of supply works,level of service dictated.




Types of distribution systems: a) a branching pattern with dead ends,b) a gridiron pattern,c) a gridiron pattern with central feeder.

A branching pattern with dead endssuitable for strip-shaped districts where water flow in one directionduring any repair downstream of the section cannot take water

A gridiron patternpreferred for flat and wide terrains

A gridiron pattern with a central feederpreferred as the auxiliary main is in a looped pattern



Overview








Distribution ReservoirsIn large cities (population > 100,000) pumping stations and distribution reservoirs are operated in conjunction with each other.Pumping stations usually pump the average daily demand to distribution reservoirs during the minimum demand hours of the day.Distribution reservoir release the water when the demand is above the average daily demand to meet the fluctuations in use.Location: as close as the center of use as possible.Water Level: high enough to permit gravity flow at satisfactory pressure.




Distribution Reservoirs (con’t)May be located in critical points in the city in such a way that the hydraulic grade line is still above the highest storey buildings.Elevated tanks: circular cross-section (cylinder gives a max. volume).Buried reservoirs: rectangular

cross-section (easy to construct)Small: concrete or stone masonryLarge: reinforced concrete with dividing walls

Cylindrical Reservoirs: reinforces concrete or steel




Distribution Reservoirs (con’t)A fill layer of about one meter thick is placed.Lower temperature in summer (better quality and taste)

Buried distribution reservoirhttp://www.wau.boku.ac.at/fileadmin/_/H81/H811/Skripten/811356/05_Supply_Storage_distribution.pdf




Distribution Reservoirs (con’t)

Buried distribution reservoirhttp://www.wau.boku.ac.at/fileadmin/_/H81/H811/Skripten/811356/05_Supply_Storage_distribution.pdf




Distribution Reservoirs (con’t)The capacity of a distribution reservoir is determined by the summation of the following components:

Storage to meet hourly fluctuations,Required storage to put out a fire with certain duration (Cfire)Storage to meet emergencies



Overview








PipesPipes materials:

reinforced concrete (no corrosion problem),asbestos cement (limited use cancerous effect of asbestos fibers),ductile iron (corrosion problem interior is coated with cement),steel (good for large diameter and high pressure, buckling under high negative pressure)plastic (widely used, highly smooth, light, easy to install and remove)

The minimum pipe size required in a distribution network is dictated by the population.



Overview








PumpsPumps: mechanical energy potential energyTypes of pumps are named according to the direction of the movement of water from the rotating element of the pump (impeller).The specific speed:

where Q: discharge (m3/s)N: rotative speed of impeller (rpm)Hp: Head (m)

2/3p

sHQN

n =




Pumps (con’t)Centrifugal Pumps:

For the requirement of high heads (low ns).Impeller in series having radial flow characteristics are adopted.

http://en.wikipedia.org/wiki/File:CetriFugal_Pump.jpg http://www.thomasnet.com/articles/pumps-valves-accessories/centrifugal-principles




Pumps (con’t)Axial-flow Pumps:

For greater flow rates under low heads (high ns).Having axial flow characteristics with respect to the pump axis.

http://en.wikipedia.org/wiki/File:Axial_2.png http://www.directindustry.com/prod/weir-minerals/axial-flow-pump-23306-376724.html




Pumps (con’t)Mixed-flow Pumps:

For medium head and discharge (medium ns).

http://www.shakthipumps.com/download/mixed-flow-pumps.pdfhttp://www.pattersonpumps.com/axial.html#




Pumps (con’t)Power of the pump:

where Pp: the power (kW).γ: specific weight of water (kN/m3)Q: discharge (m3/s)Hp: head of the system by the pump (m)ηp: the efficiency of the pump

p

pp

QHP

ηγ

=




Pumps (con’t)Multiple pump operations are often used to increase the discharge or head.

Pumps in Series: Increase the head

Pumps in Parallel: Increase in discharge




Valves Sluice or gate valves are used to isolate the flow especially during repair works.

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




Valves Butterfly valves are widely used at the exits of distribution reservoirs for regulation or isolation purposes.

http://en.wikipedia.org/wiki/Butterfly_valvehttp://www.scanco.no/produkter_ventiler.php




Valves Check valves are used to stop flow automatically in the reverse direction.

http://www.checkall.com/valvestyles/un3/un3enlargedphoto.htm

http://www.spiraxsarco.com/resources/steam-engineering-tutorials/pipeline-ancillaries/check-valves.asp

http://www.checkvalves.co.uk/




Valves Hydrants are those appurtenances used for the withdrawal of pressurized flow from the network for fire fighting.

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



Overview







Hydraulics and Operation of Gravity Pipelines



Hydraulics and Operation of Gravity Pipelines

when (inflow > Qmax) Q - Qmax spills over the reservoirwhen (Q0 < inflow < Qmax) 0 < reservoir water level < Hmax

when (inflow < Q0) free surface flow in the pipe (empty reservoir).

To prevent free surface flow in the pipe use control valve at the pipe exitThe valve dissipates the excess potential energy, Hv.

Then flow is pressurized in the pipe and the water level in the reservoir is controlled.



Overview







Design of Transmission Lines

The pipe size should be determined to meet future requirements.The system should confirm to the required hydraulic performance:

velocity criteriapressure criteria etc…..

In this section design principles ofgravity lines,pumped lines, andmixed lined

are introduced.




Design of Gravity LinesFor a gravity pipeline, neglecting operating cost, the optimum diameter minimizes the capital cost of the pipeline.

The optimum diameter minimum diameter which can convey the specified design discharge with the available gravity head.

In gravity lines there are several design criterions:Velocity: 0.5 – 2.0 m/sPressure head: 3-5 m – 80 m




Design of Gravity Lines (con’t)

Step 1:Determine the control points (C,E,F) and their topographic elevations zc, zE, zF.Add the minimum required pressure head Pmin/γ to these elevations.Determine the energy grade line slopes (S1, S2, S3), between the reservoir and the control points and select minimum slope.

( )AC

cALPzHS )/( min

minγ+−

=





Step 2:Compute the pipe diameter for line A-C. Using Darcy-Weisbach equation,

and select the nearest larger commercial available diameter, D.

5/1

min2

28

=

SgfQDcom

π





Step 3:Compute velocity

If umin < u < umax, the selected diameter, D, is used in the projectIf u < umin, a booster pump may be installed at the reservoir site to increase the velocity to umin.

4/2DQu

π=





Step 3: (con’t)The additional head supplied by the booster pump, Hp, is compute using

If u > umax, reduce the velocity to umax, by increasing the pipe diameter from Since the velocity, and hence headlosses

are reduced, install a pressure reduction valve or allow increased pressures along the pipeline if (P/γ) < (Pmax/γ)





Step 4:After determining the diameter for pipe segment A-B-C and computing the piezometric level at point C (HC), repeat the above procedure for the remaining segments C-D-E-F.Note that for the above example point E now becomes the control point.Therefore, first the diameter for segment C-D-E, and then the diameter of segment EF are determined.





Step 5:Install a control valve at point F, and determine the necessary headloss at the valve, to maintain pressurized flow at segment E-F.



Overview








Design of Pumped LinesFor a pumped line, the economical diameter is the one which minimizes the total cost.

investment cost of pipes (placement and installation costs),cost of pump,energy cost of operation (operation and maintenance costs).

A small diameter reduces initial cost but increases pumping and energy costs.

For the most economical diameter:




Design of Pumped Lines (con’t)Step 1:

Compute the annual energy cost, CE, to overcome friction and static head:

where Pp: power of the pump.γ: the specific weight of water (kN/m3).Q: design discharge (m3/s).E: energy cost ($/KWh).t: annual operating hours.η: the pump efficiency.Hp: the required pump head (m). Hp=Hs+hfHs: the static head between the lower and upper reservoir (m)hf: the friction headloss along the pipe connecting these reservoirs.




Design of Pumped Lines (con’t)Step 1: (con’t)

Using Darcy-Weisbach equation, the annual energy cost, CE, can be expressed in terms of diameter D:





Pipe cost, CD, per unit length for various diameters is obtained from manufacturers, and converted to annual cost, CP, :

where i : interest rate.N: economic life of the project in years.





Initial pump cost, Cpi, is normally expressed in terms of monetary units per kW of installed power. The power of the pump in kW units is

where γ : the specific weight of water (kN/m3).Q: the discharge (m3/s)

The annual pump cost:





Ignoring annual operation and maintenance costs, the economic diameter is obtained as the value corresponding to the minimum total annual cost.

Cp : annual cost of pipeCE: annual energy costCpump: pump costCT: total cost



Overview








Design of Mixed LinesA pipeline having both

gravitational and pumped flow rates.

Topographic elevation of point C is higher than the reservoir level.In such a case it is convenient to build a small reservoir at C.The pump may be

located at reservoir site A orlocated as a booster pump along the line.

Topographic elevation of point C is higher than the reservoir level.




Design of Mixed Lines (con’t)If the pump is located at reservoir site A, the economic pipe diameter can be computed considering line ABC as a pump discharge lineThe required pump head:

The diameter of line CD is then determined as a gravity pipeline.




Design of Mixed Lines (con’t)If a booster pump is selected, then its location, X, along BC can be determined as follows:

Flow will be transmitted until the booster pump by gravity.Use any available diameters, that keep velocity within the limits (umin < u < umax).The location of booster pump(LBX, zX)

Then the economic pipe diameter for section X-C can be determined as that of a pumped discharge line.

Required Pump Head:




Design of Mixed Lines (con’t)If a small diameter is used until the buster pump,

the initial cost will be lessthe energy cost will be high (the pump will be in at a lower elevation)

Note that for the booster pump applications the followings must also be considered:

cost of energy line transportation, andcost of employment of guards, etc…



Overview








The selection of suitable route for a pipeline has an important bearing on the capital cost and operation cost.

A pipeline route is selected fromaerial photostopographic and cadastral planson-site inspections, andother data available on the terrain, obstacle, and local services.

For suitable route Jointly consider Cost and Practicality.

The general level of pipeline route should be as close to hydraulic grade line as possible to minimize pressures and hence pipe costs.




If a pipeline is laid parallel to the natural ground, then it will have many peaks and depressions as the natural ground surface.Air released from water and trapped at peaks

reduces the opening of the waterway,increases energy loss,may interrupt flow.

In this case, air valves are required.In order to minimize the number of air valves the pipeline can be laid

on a more straight profile deeper in the ground,close to the ground surface with a nearly straight profile and covered with earth-fill in depression along the route.




The selection of profile is achieved by cost evaluations.In setting up the alternative profiles:

In order to minimize air entrainment problems: (pipe slope) > 0.005On very straight ground surfaces the minimum slope can be droped to 0.002.No upper limit for the maximum slope.

In long supply lines frequent changes in direction should be avoided in order to conserve head and pressure.sharp bends should also be avoided to reduce the dynamic impact of the water thrust.

The bends should be blocked with concrete to take the dynamic action.




Laying of pipes:excavation of trenches,transportation and handling of pipes and appurtenances,backfilling and repaving.

Pipes should be laid on firm beds in order to prevent the problems due to settlement.The dept of burial of pipes may be governed by

the depth of frost linethe external loads acting on the pipe

cve 471 - 5. water supply 2 - middle east technical university

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