6.3 Distribution systems
The water from service reservoirs is distributed by a network of pipes of various sizes, laid beneath the streets, pavements and verges of our towns and cities. Any part of a distribution system can be isolated by valves at appropriate points. Figure 44 shows both a loop (as at A) and a spur or dead end (as at B) within a typical distribution layout. Looped or ring mains are always preferred to spurs or dead ends because when the rate of flow is restricted in a long spur, the water will remain there for a long time, and its quality may deteriorate. Bacteria can proliferate in stagnant water at dead ends.
In the event of repair work, the whole length may need to be isolated. In a ring system, water can flow through a lightly loaded section to meet demands elsewhere. There is also the advantage that one section of a ring may be isolated without cutting off the sections on either side. For example, the section ab of the ring main shown in Figure 44 may be isolated. However, water could still flow in sections ac and cb. The distribution grid is arranged so that any pipe can be taken out of service without cutting off the output to the others.
A mains pipeline can be subject to a variety of loading conditions. There is the static loading due to the internal water pressure in the pipe. Also, pipelines are often subject to water hammer. Changes in flow direction and velocity at bends, contractions, expansions and partly closed valves lead to additional loading on the pipe, as does the thermal expansion or contraction of the pipe material.
The choice of pipe material clearly depends on the magnitude and nature of these stresses. However, other factors have to be considered:
the ground conditions, in so far as these affect the possibilities of ground movement and corrosion;
the corrosive nature of the water;
the size of pipe;
the ease of jointing;
The joints between adjacent lengths of pipe must be watertight. Occasionally, joints are welded, but it is more usual to find some form of sealing ring contained in a socket at the end of one pipe or at each end of a separate collar.
Cost, as the last factor, is by no means the least important. It is usual to aim for the minimum overall cost. The materials used for transmission mains and their respective merits are listed in Table 5. Asbestos cement pipes are no longer laid in the UK. The existing asbestos cement pipes are replaced by more modern substitutes, e.g. MDPE, whenever they develop cracks, etc. All MDPE underground pipes are colour-coded to prevent accidents – water is blue, gas is yellow, electricity is black, and telephone is grey.
Table 5 Materials used for transmission mains
|Steel||High strength and ductility resulting in light weight. Jointing easy with mechanical or (in large sizes) welded joints.||Requires careful protection against external and internal corrosion.|
|Cast iron||Modern ductile iron pipes have strength and ductility approaching those of steel. Push-fit joints are easy to make. The pipes are cast in an automatic machine to obtain light but strong walls free from imperfections.||Requires special protection against external corrosion in some types of ground. Often subject to growth of incrustations or tuberculations internally unless lined with concrete.|
|Asbestos cement||Economical and resistant to decay under most soil conditions. Light and easy to handle and lay.||Vulnerable to damage during handling.|
|UPVC (unplasticised polyvinyl chloride) and MDPE (medium density polyethylene)||Light and flexible, easy to handle and install. Low cost compared with metal pipes.||Deformation and stretching due to stress over a period of time. Susceptible to aromatic hydrocarbons (e.g. petrol) penetrating the pipe wall and contaminating the water supply.|
|Prestressed concrete||Sometimes economic in large sizes.||Heavy and somewhat inflexible in use. Cannot be cut on site and joints allow only limited angular deflection.|