Transportation of Water

The term conveyance or transportation of water refers to taking of water from source to purification plants and from treatment plant to consumers. Water supply system broadly involves transportation of water from the sources to the area of consumption, through free flow channels or conduits or pressure mains. Depending on the topography of the land, conveyance may be in free flow and/or pressure conduits. Transmission of water accounts for an appreciable part of the capital outlay and hence careful consideration for the economics is called for before deciding on the best mode of conveyance. Care should be taken so that there is no possibility of pollution from surrounding areas.

If the source is at higher level than the treatment plant, the water can flow under gravity, automatically. Similarly, after necessary purification of water, it has to be conveyed to the consumers. Therefore, for conveyance of water some sort of devices or structures is required. The arrangement may be in the form of open channels, aqueducts, tunnels or pipes.

1) Open Channels

In any water supply systems, raw water from source to treatment plants may be carried in open channels. These can be constructed by cutting in high grounds and banking low grounds. Economical sections of open channels are generally trapezoidal while rectangular sections prove economical when rock cutting is involved. The channels are to be properly lined to prevent seepage. These kind of channels need to be taken along the gradient and therefore the initial cost and maintenance cost may be high. While open channels are not recommended for conveyance of treated water, they may be adopted for conveying raw water. If these kind of channels are unlined, they have to be run with limited velocity of flow so that it does not affect scouring. As water flows only due to gravitational force a longitudinal uniform slope should be given. The velocity of water in channels should not exceed the permissible limit, otherwise scouring will start in the bed and water will get dirty. In channels there is always loss of water by seepage and evaporation.

2) Aqueducts

The term aqueduct is usually restricted to closed conduits made up of masonry. These can be used for conveyance of water from source to treatment plant or for distribution. Aqueducts normally run half to two-third full at required capacity of supply in most circumstances. In ancient times, rectangular aqueducts were most commonly used, but these days circular or horse-shoe shaped ones are more common. Masonry aqueducts unless reinforced with steel, are usually constructed in horse-shoe cross-section. This cross-section has good hydraulic properties and resists earth pressure well. It is economical and easy to build.

3) Tunnels

Tunnels are also like aqueducts. This is also a gravity conduit, in which water flows under gravitational force. But sometimes, water flows under pressure and in such cases, these are called pressure tunnels. Tunnels which are not under pressure are usually constructed in horse-shoe shape. But if they convey water under pressure, circular cross section is the best. In pressure tunnels, the depth of cover is generally such that the weight of overlying material overcomes the bursting pressure. Tunnels are used to convey water into the cities from outside sources. Tunnels should be water- tight and there should be no loss of water.

4) Pipes

Pipe is a circular closed conduit used to convey water from one point to another, under gravity or under pressure. Usually pipes follow the profile of the ground surface closely. If pipes do not run full, they are called to flowing under gravity. But flow under gravity is possible only if the pipe is given a definite longitudinal slope. Pipes running full will be said to be running under pressure. Water is under pressure always and hence the pipe material and the fixture should withstand stresses due to the internal pressure, vacuum pressure, when the pipes are empty it has to withstand water hammer, when the valves are closed it has to withstand temperature stresses. Pipes are mostly made up of materials like cast iron, wrought iron, RCC, asbestos cement, plastic, timber etc.

Requirements of Pipe Material

• It should be capable of withstanding internal and external pressure
• It should facilitate easy joints
• It should be available in all sizes, transport and erection should be easy
• It should be durable
• It should not react with water to alter its quality
• Cost of pipes should be less
• Frictional head loss should be minimum
• The damaged units should be replaced easily

a) Cast Iron Pipes

Cast iron pipes are used in majority of water conveyance mains because of centuries of satisfactory experience with it. Cast iron pipe is resistant to corrosion and accordingly long lived; its life may be over 100 years.

Advantages

• Cast iron pipes are of moderate cost
• Their jointing is easier
• They are resistant to corrosion
• They have long life

Disadvantages

• They are heavier and hence uneconomical when their diameter is more than 120 cm
• They cannot be used for pressures greater than 7 kg/cm².
• They are fragile

b) Wrought Iron Pipes

Wrought iron pipes are manufactured by rolling flat plates of the wrought iron to the proper diameter and welding the edges. Such pipes are much lighter than the cast iron pipes and can be more easily cut, threaded and worked. These pipes are stronger than cast iron pipes and it can withstand higher pressure. They look much neater, but are much costlier. They corrode quickly and hence are used principally for installation within buildings. These pipes are usually protected by coating them with a thin film of molten zinc. Such coated pipes are known as galvanized iron pipes and they are commonly jointed by screwed and socketed joints.

c) Galvanised Iron (GI) Pipes

Gl pipes are highly suitable for distribution system. They are available in light (yellow colour code), medium (blue colour code) and heavy grades (red colour code) depending on the thickness of pipe used. Normally, medium grade pipes (wall thickness 2.6-4.8 mm) are used for water supply system. It is cheap in cost, light in weight and easy to join. It is usually affected by acidic or alkaline water. Gl pipes can be used in non-corrosive water with pH value greater than 6.5. Gl pipes are normally joined with lead putty on threaded end.

Fig. 1 GI Pipes

d) Mild Steel Pipes

Mild steel pipes are durable and can resist high internal water pressure and highly suitable for long distance high pressure piping. It is flexible to lay in certain curves. The number of joints are less as they are available in longer length. It is light in weight, easy to transport and the damage in transportation is minimum. These pipes are prone to rust and require higher maintenance. It requires more time for repairs and not very suitable for distribution piping. These are available in diameter of 150-250 mm for water supply and cut lengths of 4 - 7 m (2.6-4.5 mm wall thickness).

Fig. 2 Mild Steel Pipes

e) Cement Concrete Pipes

Cement concrete pipes may be either plain or reinforced and are best made by the spinning process. They may be either precast or may be cast-in-situ. Transportation costs are much reduced if pipes are cast in situ. Concrete pipes have low maintenance in resistant to corrosion and particularly suitable to soft and acidic waters. They however can withstand high pressure if reinforced. The plain cement concrete pipes are used for heads up to 7 m while reinforced cement concrete pipes are normally used for head upto 60 m.

Advantages

• They are more suitable to resist the external loads and loads due to backfilling.
• The maintenance cost is low.
• The inside surface of pipes can be made smooth, thus reducing the frictional losses.
• The problem of corrosion is not there.
• Pipes can be cast at site and hence the transportation problems are reduced.
• Due to their heavy weight, the problem of floatation is not there when they are empty.

Disadvantages

• Unreinforced pipes are liable to tensile cracks and they cannot withstand high pressure.
• The tendency of leakage is not ruled out as a result of its porosity and shrinkage cracks.
• It is very difficult to repair them.
• Precast pipes are very heavy and it is difficult to transport them.

f) Poly Vinyl Chloride (PVC Un-plasticised) Pipes

These are cheap in cost and light in weight. It is economical in laying and jointing and are rigid pipes. It is highly durable and suitable for distribution network. These pipes are free from corrosion, tough against chemical attack and good electric insulation. It is highly suitable for distribution piping and branch pipes. The disadvantage is that it is less resistance to heat and direct exposure to sun. Hence, not very suitable for piping above the ground. PVC pipes weigh only 1/5^th of steel pipes of same diameter.

Fig. 3 PVC Pipes

g) HDPE (High Density Polyethylene) Pipes

These are light in weight and flexible than PVC pipes. HDPE pipes are black in colour. These are suitable for underground piping and can withstand movement of heavy traffic. It allows free flowing of water and highly durable and suitable for distribution network. These are free from corrosion and has good electric insulation. It is useful for water conveyance as they do not constitute toxic hazard and does not support microbial growth.

Fig. 4 HDPE Pipes

h) Ductile Iron Pipes

Ductile Iron pipes are better version of cast iron pipes with better tensile strength. These pipes are prepared using centrifugal cast process. DI pipes have high impact resistance, high wear and tear resistance, high tensile strength, ductility and good internal and external corrosion resistance. These pipes are provided with cement mortar lining on inside surface which provides smooth surface and is suitable for providing chemical and physical barriers to water. Such pipes reduce water contamination. The outer coating of such pipes is done with bituminous or Zinc paint. DI pressure pipes are available in range from 80-1000 mm diameter in lengths from 5.5-6 m. They are about 30 percent lighter than conventional cast iron pipes. DI pipes have lower pumping cost due to lower frictional resistance.

Fig. 5 DI Pipes

i) Asbestos Cement Pipe

These pipes are composed of asbestos fibre and Portland cement combined under pressure into dense homogenous structure. These are available in large variation from 50 to 600mm.

j) Wood Pipe

These pipes are built of staves of wood held together by steel bands. Wood is less durable for pipe material and pipe must be constantly full of water to prevent crackdown due to alternate wet and dry conditions.