Piping, Plumbing, and Fire Protection Systems
Piping questions combine hydraulic sizing with code minimums: pipe diameter from a flow or fixture-unit load, drainage slope, and sprinkler density. Mabhas 16 governs sanitary plumbing and Mabhas 3 governs fire protection.
Core rules
- Hazen–Williams (SI): h_f = 10.67 L Q^1.852/(C^1.852 D^4.87), with Q in m³/s and D in m; C ≈ 120–140 for new steel, copper and cement-lined pipe.
- Fixture units: the water-supply demand is not the arithmetic sum of fixture flows; use the probability curve so a building with many fixtures gets a diversified demand.
- Drainage: gravity lines run at 1–2% slope; a vent is required to protect trap seals.
- Sprinklers: design by density/area - for light hazard typically 0.1 gpm/ft² over the hydraulically most remote 1500 ft², with a minimum residual pressure at the base of the riser.
- Pumps and tanks: fire pumps must not be bypassed by domestic demand; verify the available supply curve against the system curve.
Worked example
Water at Q = 0.010 m³/s through D = 0.10 m, L = 100 m, C = 120. Q^1.852 = 1.98×10⁻⁴; C^1.852 = 7.09×10³; D^4.87 = 1.353×10⁻⁵. h_f = 10.67 × 100 × 1.98×10⁻⁴/(7.09×10³ × 1.353×10⁻⁵) = 0.211/0.0959 = 2.20 m.
Traps
- Summing fixture units as if they were simultaneous flow.
- Using the internal diameter of a nominal pipe size as if it were exact; schedules differ.
- Applying Hazen–Williams with Q in L/s or D in mm without conversion.
- Forgetting that a dry standpipe and a wet system have different acceptance test criteria.