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6.1.Water Distribution Lines
Distribution system is a network of pipelines that distribute water to the consumers (Picture 6.1).
- They are designed to adequately satisfy the water requirement for a combination of:
- Firefighting purposes
A good distribution system should satisfy the followings:
- Adequate water pressure at the consumer's taps for a specific rate of flow (i.e., pressures should be great enough to adequately meet consumer needs).
- Pressures should be great enough to adequately meet firefighting needs.
- At the same time, pressures should not be excessive because development of the pressure head brings important cost consideration and as pressure increases leakages increases too.
- Purity of distributed water should be maintained. This requires distribution system to be completely water-tight.
- Maintenance of the distribution system should be easy and economical.
- Water should remain available during breakdown periods of pipeline. System of distribution should not such that if one pipe bursts, it puts a large area without water. If a particular pipe length is under repair and has been shut down, the water to the population living in the down-stream side of this pipeline should be available from other pipeline.
- During repairs, it should not cause any obstruction to traffic. In other words, the pipelines should not be laid under highways, carriage ways but below foot paths.
A. Branching Pattern with Dead End
- Similar to the branching of a tree- it consists of:
- Main (trunk) line
- Main line is the main source of water supply. There is no water distribution to consumers from trunk line.
- Sub-mains are connected to the main line and they are along the main roads.
- Branches are connected to the sub-mains and they are along the streets.
- Lastly service connections are given to the consumers from branches.
Picture 6.1. An example for a water network
- It is a very simple method of water distribution. Calculations are easy and simple to do.
- The required dimensions of the pipes are economical.
- This method requires comparatively less number of cut-off valves.
- The area receiving water from a pipe under repair is without water until the work is completed.
- In this system, there are large number of dead ends where water does not circulate but remains static. Sediments accumulate due to stagnation of the dead end and bacterial growth may occur at these points. To overcome this problem drain valves are provided at dead ends and stagnant water is drained out by periodically opening these valves but a large amount of water is wasted.
- It is difficult to maintain chlorine residual at the dead ends of the pipe.
- Water available for fire-fighting will be limited since it is being supplied by only one water main.
- The pressure at the end of the line may become undesirably low as additional areas are connected to the water supply system. This problem is common in many less-developed countries.
B. Grid Pattern In grid pattern, all the pipes are interconnected with no dead-ends. In such a system, water can reach any point from more than one direction (Picture 6.2).
- Since water in the supply system is free to flow in more than one direction, stagnation does not occur as readily as in the branching pattern.
- In case of repair or a break down in a pipe, the area connected to that pipe will continue to receive water, as water will flow to that area from the other side.
- Water reaches all points with minimum head loss.
During fires, by manipulating the cut-off valves, much of the water supply may be diverted and concentrated for fire-fighting.
- Cost of pipe laying is more because relatively more length of pipes is required.
- More number of valves are required.
- The calculation of pipe sizes are more complicated.
C. Grid Pattern with Loops
- Loops are provided in a grid pattern (similar to the above diagram) to improve water pressure in parts of a city (industrial, business and commercial areas).
- Loops should be strategically placed so as a city continues to develop the water pressure can be continuous.
- The advantages and disadvantages of this pattern are the same as those listed under the grid pattern section.
Hydraulic Analysis of Distribution Systems
Most commonly methods used are:
- Dead-end Method
- Determine the locations of "dead-ends" providing that water will be distributed in the shortest way. At the dead-end points there will be no flow distribution.
- To apply the dead-end method for loop systems, convert it to branch system. To do this, a dead-end point is identified for each loop. The location of a dead-end point is based on the distance travelled to reach dead-end point from 2 different directions will almost equal to each other.
- Hardy-Cross Method
- This method is applicable to closed-loop pipe networks.
- The outflows from the system are assumed to occur at the nodes (NODE: end of each pipe section). This assumption results in uniform flow in the pipelines.
- The Hardy-Cross analysis is based on the principles that:
- At each junction, the total inflow must be equal to total outflow. (flow continuity criterion)
- Head balance criterion: algebraic sum of the head losses around any closed- loop is zero.
- For a given pipe system, with known junction outflows, the Hardy-Cross method is an iterative procedure based on initially estimated flows in pipes. Estimated pipe flows are corrected with iteration until head losses in the clockwise direction and in the counter clockwise direction are equal within each loop.
- Equivalent Pipe Method
- Equivalent pipe is a method of reducing a combination of pipes into a simple pipe system for easier analysis of a pipe network, such as a water distribution system. An equivalent pipe is an imaginary pipe in which the head loss and discharge are equivalent to the head loss and discharge for the real pipe system. There are three main properties of a pipe: diameter, length, and roughness. As the coefficient of roughness, C, decreases the roughness of the pipe decreases. For example, a new smooth pipe has a roughness factor of C = 140, while a rough pipe is usually at C = 100. To determine an equivalent pipe, you must assume any of the above two properties. Therefore, for a system of pipes with different diameters, lengths and roughness factors, you could assume a specific roughness factor (most commonly C =100) and diameter (most commonly D = 8"). The most common formula for computing equivalent pipe is the Hazen-Williams formula.