Laying & jointing :
Pipes are laid underground with minimum cover of 1 m. Care should be taken to locate other utilities in ground and avoid damage to them. All specials and valves should be available and installed with pipe without leaving gap for subsequent installation. Width of trench at bottom shall provide 200 mm clearance on both sides of pipe. Pipe line shall be laid as straight as possible with minimum horizontal and vertical bends.
The bend should not exceed 2 degree or as recommended by manufacturer.
Provide proper bends and thrust blocks and anchors at bends and dead ends.
Transportation, handling and storage should be proper and follow manufacturers recommendations.
Pipes over 300 mm dia shall be handled and lowered into the trenches with the help of Crane or chain pully block.
The socket end should face upstream when laying on flat ground and should face the up gradient when pipe runs uphill. All lumps, blisters and excess coating material shall be removed from socket and spigot end of each pipe and outside of the spigot and inside of the socket shall be wire-brushed and wiped clean and dry from oil & grease before the pipe is laid.
Testing of laid pipe line :
The field pressure to be imposed should not be less than the maximum of
1.5 times the maximum sustained operating pressure,
Sum of the maximum sustained operating pressure and the maximum surge pressure.
Losses during test shall be less than 0.1 liter per mm of pipe dia. per KM of pipeline per day for each 30 meter head of pressure applied.
Economic size of rising Main :
Appendix 6.5 of CPHEEO Manual gives method for finding economical size. Based on it a design template on axle sheet can be used to find economic size by giving input values specific to the requirement.
Appurtenances :
On line valves are provided in larger mains at 1-5 km interval to facilitate repairs. Non rising spindle Sluice valves are used for isolating or scouring and not intended for continuous throttling as erosion of the seats and body cavitation may occur. Butterfly valves are used to regulate and stop the flow. In large size butterfly valves are cheaper and occupy less space. These may involve higher friction loss than sluice valve. Also not suitable for continuous throttling. Sluice valve and butterfly valve for higher sizes require geared hand wheel or power driven actuators. Scour valves are provided in valleys/depressions to dewater pipe line.
Air valve size is one twelfth of pipe diameter when it serves purpose of only release of air and one eighth when it serves purpose of admission as well as release of air. Kinetic air valves are used to release air entrapped during running of pipe line.
Pressure relief valves, Check valves, Pressure Reducing Valves, Ball Float Valves, Shut off valves are used for specific purposes.
Peak factor:
Economic size is when it runs 24 hours. As such power availability determines pumping hours. Peak factor for Rising main is “24/Pumping hours”.
Minimum and maximum Velocity of flow:
Low velocities reduce head loss and so energy cost but pipe dia required becomes larger. High velocities increase head loss and so energy but require smaller pipe dia. As such economic pipe dia should be selected. Maximum velocity may be 3 m/sec to prevent erosion.
Water Hammer:
Water hammer pressures are computed as per clause 6.17.1. If the operating pressure plus surge pressure exceeds 1.1 times internal design pressure then protective device required. In no case maximum operating pressure plus surge should exceed field hydrostatic pressure.
Free Flow:
Open channels and gravity aqueducts and tunnels provide free flow. Open channels have restricted use in water supply in view of losses due to evaporation and seepage and possibility of pollution and misuse (theft) of water. Aqueducts and tunnels flow three quarter full. mean velocities which will not erode channels after ageing range from 0.3 to 0.6 m/s for unlined canals and 1 to 2 m/s for lined canals.
Flow under Pressure:
Pressure aqueducts and Tunnels are ordinarily circular in section
Head Loss in Pipes:
Hazen-Williams formula (CPHEEO manual clause 6.2.1 a) for pressure conduits
For circular conduits
Where : Q = Discharge in cubic meter per hour ,
d = diameter of pipe in mm
V = Velocity in mps, r = hydraulic radius in m ,
S = slope of hydraulic gradient , C = Hazen-Willams coefficient
CPHEEO manual Table 6.1 of Manual give value of Hazen William coefficient of roughness
Head Loss in Pipes:
Mannings formula (CPHEEO manual clause 6.2.1 b) for free flow conduits are generally used.
For circular conduits
Where : Q = Discharge in cubic meter per hour ,
d = diameter of pipe in mm
V = Velocity in m/s, r = hydraulic radius in m ,
S = slope of hydraulic gradient n = Mannings coefficient of roughness
and Table 6.2 give value of Mannings coefficient of roughness.
Modified Hazen william Formula, clause 6.2.4 obviates the limitations of Hazen William formula and can be used for more accuracy.
CPHEEO manual Clause 6.2.8 give design recommendations for use of Modified Hazen Williams Formula and Table 6.4 provides value of roughness coefficient in MHW formulae.
Head loss due to Specials and Appurtenances :
Pipe line transitions and appurtenances add head loss which is expressed as KV2/2g where V is velocity in m/sec and g is acceleration due to gravity in m/sec square. Value of K for different fittings are given in CPHEEO manual Table 6.5.
Minimum Velocity :
Minimum velocity be 0.6 m/s to avoid deposition & corrosion. However where it is inevitable due to minimum pipe diameter criteria, lower velocities may be adopted with adequate provision for scouring. Pipe Material Options :
Pipelines involve major investments in water supply as such judicious selection of pipe material is necessary. Selection basis should consider durability, life and cost which includes installation and maintenance cost. Pipe material options are CI,DI,MS,GI, RCC, PSCC, BWSC,AC,PVC, Polyethylene, GRP etc. Technical factors affecting decision are availability in market, internal pressure, roughness coefficient, hydraulic and operating conditions, internal and external corrosion problems, laying and jointing, type of soil, ease of transportation, special conditions etc. The Manual has stipulated check list, CPHEEO manual Table 6.7, to facilitate decision for selection of pipe material
Hydraulic Design
The design of water supply conduits depends on the resistance to flow, available pressure or head, and allowable velocities of flow. Generally, Hazen-William’s formula for pressure conduits and Manning’s formula for free flow conduits are used.
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