Design of Water Distribution System

Water distribution system needed for a conventional residential building depends on the following aspects:

1. Estimation of water requirement
2. Determination of pipe sizes
3. Pipe layout principles

1. Estimation of Water Requirement

As per Indian code recommendation (IS: 1172–1983 and SP 57–1993) for residences, 135 l of water per head per day to be used for the design of water supply system. The details of the daily consumption is given Table 27.2.

Table 27.2 Domestic consumption of water

Source: IS: 1172–1983.

The daily consumption will vary depending on the activities in a building. For example, in a hospital with beds, the consumption may be of the order of 340 l per bed per day and for a day school, the consumption will be as low as 45 l per head per day.

2. Determination of Pipe Sizes

In general for a middle class house the average number of pipes needed shall be about 8 and pipes of minimum nominal size of 20 mm diameter are enough. For WCs which are directly connected to overhead tanks, a 25 mm diameter is adequate to get sufficient flow. Minimum recommended size of pipes for different purposes in a building is shown in Table 27.3.

Table 27.3 Recommended sizes of water supply pipes

A thumb rule to assess the size of pipes which can supply two or more branch pipes is as follows (Varghese, 2012):

1. Up to 3 numbers of 10 mm branches can be supplied by a 12 mm pipe.
2. Up to 3 numbers of 12 mm branches can be supplied by a 20 mm pipe.
3. Up to 3 numbers of 20 mm branches can be supplied by a 25 mm pipe.

In order to control the output from overhead tanks, a stop valve or gate valve must be fixed in an accessible place. Apart from control this will help to stop the water supply during repairs.

For multi-storey buildings with many flats, a more complex arrangement of water distribution system should be planned.

3. Pipe Layout Principles

Following rules may be adopted in laying of water supply lines (Varghese, 2012):

1. There should not be any cross connection and backflow. Backflow can be eliminated by ensuring the under mentioned conditions.
   1. Adequate air gap is to be provided between the inlet and final flood level in all places particularly washbasins.
   2. While connecting cistern to water closet, the inlet and float valve should be suitably fixed.
   3. Positive pressure at the outlet pipes should always be assured.
2. Drainage pipes and water supply pipes should not be laid very close to each other. Especially drainage stoneware pipe which are liable to leak if damaged should not be laid above the water supply line. Further, water supply pipes should not be laid alongside of the pipes carrying foul water.
3. Planning of the distribution line should be such that it is accessible for inspection, replacement and repairs. GI pipes should not be buried as they tend to corrode in contact with soils, but they can be laid on walls.
4. All pipes including PVC pipes laid inside the building as concealed pipes should be tested for leakage.