1. Hazen-Williams Friction Loss Equation
The friction loss per foot of pipe is calculated according to NFPA 13 standards:
p = 4.52 × Q^1.85 / (C^1.85 × d^4.87)
- p = Friction loss per foot of pipe (psi/ft)
- Q = Cumulative flow in the pipe section (gpm)
- C = Hazen-Williams roughness coefficient (e.g. 120 for steel, 150 for CPVC/Copper)
- d = Exact internal pipe diameter (inches), looked up from nominal pipe schedule tables
2. Equivalent Length Adjustment for Valves & Fittings
Baseline equivalent lengths are determined for standard Schedule 40 steel pipe with \(C = 120\). For other pipe materials and roughness values, the equivalent length is adjusted:
Equivalent Length (adjusted) = Base Length × Material Multiplier × (C / 120)^1.85
3. Water Supply Curve at System Flow
Available pressure from the flow test at system demand flow \(Q_{sys}\):
P_supply = ((P_residual - P_static) × (Q_sys / Q_test)^1.85) + P_static
4. Elevation Head Loss
Elevation loss is calculated at \(0.433 \text{ psi/ft}\) of vertical rise (\(\Delta h \times 0.433\)).