Short note: Water supply and distribution system

Water supply in buildings brings treated water from the public main (or a well/tank) to taps and fixtures in adequate quantity and at adequate pressure.

Systems of distribution

1. Direct (up-feed) system: water comes straight from the main to the fixtures; used where mains pressure is sufficient and supply is continuous. Cheap, but there is no storage and the pressure varies.
2. Indirect (down-feed) system: water is first stored in an underground sump, pumped to an overhead tank on the roof, and distributed by gravity. Used in multistorey buildings and where supply is intermittent (usual in Nepal).
3. Combined system: a mix of both; ground-floor taps are fed directly, upper floors via the tank.
4. Hydropneumatic system: a pressure vessel and pump, with no overhead tank.

 Main -> [sump] -> pump -> [overhead tank] -> down pipes
                                   |-> floors / fixtures

Design

1. Demand: 135 L/person/day for residences (NBC/IS 1172); offices 45 L; schools 45 L; hospitals about 340 L per bed.
2. Storage: overhead tank about 1/3 of the daily demand; underground sump 1/2 to 1 day's demand.
3. Pipes: sizes chosen from demand and velocity (0.9-2.4 m/s); pressure loss computed by the Hazen-Williams equation; minimum residual head at the highest tap about 3-5 m.
4. Layout: short, straight runs; valves on branches; pipes (GI/HDPE/PVC/CPVC) in ducts, not under foundations; provision of air release and drain valves.