Pipe Sizing Principles
Undersized pipes create high velocity (noise, erosion), pressure drop, and pump strain. Oversized pipes waste money. Target velocity is the starting point.
Required Diameter from Flow Rate
Q = A × v → A = Q / v
d = √(4A / π) = √(4Q / πv)
Target velocities:
Water supply: 1.0-3.0 m/s
Hot water return: 0.5-1.0 m/s
Gas pipe: 5-15 m/s (low pressure)
Steam: 25-50 m/s
Example: Q=0.005 m³/s, v=2 m/s:
d = √(4×0.005 / π×2) = 0.0564 m → use DN65 (64.9mm bore)Hazen-Williams Pressure Drop (Water)
h_f = 10.67 × L × Q^1.852 / (C^1.852 × d^4.87)
C = roughness coefficient (PVC=150, Cast iron=100, Steel=120)
L = length (m), d = diameter (m)
Short form: allow 1-5 Pa/m for design runsNominal Pipe Size vs Bore
- DN15 (½") internal bore: ~15.8mm
- DN25 (1") bore: ~26.6mm
- DN50 (2") bore: ~52.5mm
- DN100 (4") bore: ~102mm
Calculate pipe sizing: Free Pipe Sizing Calculator
Pipe Sizing for Flow Rate
Flow velocity: V = Q/A, where Q = flow rate (m³/s), A = pipe cross-section (m²) = π r². Target velocities: hot water heating pipes 0.5–1.0 m/s; cold water mains 1.0–2.0 m/s; compressed air mains 6–10 m/s; steam (low pressure) 20–40 m/s. Higher velocities increase friction losses and noise; lower velocities in hot water risk Legionella growth (dead legs). Recommended approach: choose target velocity, calculate required area = Q/V, find pipe diameter D = 2√(A/π), round up to next standard size.
Friction Loss Calculation
- Darcy-Weisbach: ΔP = f × (L/D) × (ρV²/2). f = Darcy friction factor (0.01–0.05 for turbulent flow in typical pipes).
- Hazen-Williams (water only): V = 0.8492 × C × R^0.63 × S^0.54. C = roughness coefficient (copper: 130, steel: 100, PVC: 140, cast iron old: 80–100).
- Equivalent length method: Fittings (elbows, tees, valves) add resistance equivalent to straight pipe length. 90° elbow ≈ 30 × D equivalent length. Globe valve ≈ 340 × D.
Frequently Asked Questions
What nominal bore (NB) or DN sizes are standard?
Standard pipe sizes follow DN (diamètre nominal) series: DN15 (½"), DN20, DN25 (1"), DN32, DN40, DN50 (2"), DN65, DN80, DN100 (4"), DN150 (6"), DN200 (8"), DN250 (10"), DN300 (12"). The DN number approximates internal bore in mm (actual dimensions vary by pipe schedule/series). Always specify both DN and pipe schedule (thickness): Schedule 40 (standard wall) or Schedule 80 (heavy wall) are common.
What is the difference between ID and OD of a pipe?
OD (outside diameter) is fixed for a given pipe size — fittings, flanges, and clamps are designed to OD. ID (inside diameter) varies with wall thickness (schedule/series). A DN100 Schedule 40 pipe has OD = 114.3 mm and ID = 102.3 mm; Schedule 80 has the same OD = 114.3 mm but ID = 97.2 mm (thicker wall). Flow calculations use ID; structural calculations use OD and wall thickness.
How do I account for fittings in pressure drop calculations?
Two main methods: (1) Equivalent length method — convert each fitting to an equivalent length of straight pipe and add to actual pipe length before applying friction formula. (2) K-factor (velocity head loss) method — ΔP_fitting = K × ρV²/2, where K is a fitting-specific loss coefficient. The K-factor method is more accurate for complex fittings; equivalent length is simpler for estimating purposes. Allow 20–50% extra for fittings as a rough design allowance.