Water Supply Systems and Pipe Sizing
Water Supply Systems and Pipe Sizing tests demand calculation, storage, pressure and pipe selection. Expect questions on per-capita demand, fixture units, velocity limits, residual pressure and tank capacity.
Sizing logic
- Demand: domestic supply commonly 135 lpcd; commercial and institutional demand differs. Add fire storage where required.
- Storage: size for at least one day's demand plus fire reserve; provide inlet, outlet, overflow, washout and access cover.
- Pipe sizing: use fixture units and a sizing table; keep velocity around 1.5–2.0 m/s in small branches to limit noise and erosion.
- Pressure: check residual pressure at the farthest fixture; use a booster set if the overhead tank cannot deliver.
- Layout: avoid dead legs, provide isolation valves, and slope exposed pipe to drain points.
Worked example
A branch carries 0.5 L/s through a 20 mm internal-diameter pipe. Area = π(0.010)² = 3.14 × 10⁻⁴ m². Velocity = Q/A = 0.0005 ÷ 3.14 × 10⁻⁴ = 1.59 m/s. This is acceptable for a small branch. If velocity exceeds about 2 m/s, choose the next size.
Traps
Sizing from the tank outlet rather than the farthest fixture, ignoring peak demand, or mixing lpcd with fixture units loses marks. Also remember that a larger pipe is not always better: it increases stagnation and cost.