Water Supply Systems and Storage
This domain tests system layout, tank sizing, pumping and the valves that keep a supply reliable.
Core rules and concepts
- Layouts: direct supply from the main, or indirect supply through an underground sump, pump and overhead tank. Indirect supply needs an air gap at the tank inlet and a covered, insect-proof tank.
- Typical design demand is 135 litres per capita per day for domestic use; storage is usually sized for one to two days.
- Valves: gate or sluice valve for isolation, ball valve at fixtures, check (non-return) valve after a pump, float valve in the tank, air release valve at high points, water meter at the connection.
- Suction line needs a foot valve and strainer, and should be short and rising towards the pump.
- Pressure at a tap comes from height: about 0.1 bar per metre of head.
Worked example
Size an overhead tank for five people at 135 lpcd with 1.5 days of storage. Demand = 5 × 135 = 675 L/day. Storage = 675 × 1.5 = 1012.5 L, so specify a 1000 L tank. Then check pressure: if the tank water level is 8 m above the tap, pressure ≈ 0.78 bar, which is adequate. Reasoning: multiply per-capita demand by users, multiply by storage days, then round up to a standard tank size.
Traps that cost marks
- Sizing storage for less than one day, leaving no reserve.
- Confusing pump delivery head with suction lift; suction lift is limited to about 7 m in practice.
- Ignoring the float valve, which then overflows the tank.
- Omitting the foot valve and losing prime.