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русскийFor most water and HVAC-type flows in stainless steel pipe, a practical Darcy friction factor is typically f ≈ 0.018–0.022 (fully turbulent, “smooth-to-mildly-rough” range). For higher Reynolds numbers (very fast flow), f often trends toward ~0.015–0.018; for lower turbulent Reynolds numbers (near 5,000–20,000), f may be ~0.03–0.04.
To be accurate, compute f from Reynolds number (Re) and stainless steel roughness (ε) using an explicit correlation (e.g., Swamee–Jain or Haaland) or the Colebrook equation.
Friction factor for stainless steel pipe: what value to use
Use the Darcy friction factor (also called Darcy–Weisbach friction factor) unless your chart or software explicitly says “Fanning.” The Darcy factor is 4× the Fanning factor.
A fast, defensible estimate when you don’t yet know the exact flow is:
- Water in typical stainless piping (Re ~ 50,000–300,000): f ≈ 0.018–0.022
- Very high Re (~1,000,000): f often approaches ~0.015–0.018
- Lower turbulent Re (~5,000–20,000): f commonly ~0.03–0.04
Then refine with the calculation steps below once you know diameter, flow rate, and fluid viscosity.
Stainless steel roughness: the input that drives the result
In turbulent flow, friction factor depends strongly on relative roughness (ε/D). Stainless steel is generally “smooth,” but the assumed ε still matters.
| Surface / assumption | Absolute roughness, ε (mm) | Absolute roughness, ε (m) | When to use |
|---|---|---|---|
| Clean stainless (common design assumption) | 0.015 | 1.5×10⁻⁵ | New/clean pipe, conservative-but-smooth baseline |
| Slightly aged/film build-up (rule-of-thumb) | 0.03 | 3.0×10⁻⁵ | If you expect deposits or less-controlled service |
| Unknown condition (design margin) | 0.045 | 4.5×10⁻⁵ | When you need extra conservatism |
Compute relative roughness as ε/D using the internal diameter (not nominal size). Even small changes in D or ε/D can noticeably change f in the fully turbulent region.
Step-by-step calculation (Re → f) you can trust
1) Compute Reynolds number
For a full circular pipe:
Re = (V·D)/ν
- V = average velocity (m/s)
- D = internal diameter (m)
- ν = kinematic viscosity (m²/s)
2) Choose the right flow regime rule
- Laminar (Re < 2300): f = 64/Re
- Transitional (2300–4000): avoid “precision”; confirm with test data or use conservative margins
- Turbulent (Re > 4000): use ε/D with an explicit correlation
3) Turbulent flow: practical explicit formulas
Two widely used explicit options (Darcy f):
- Swamee–Jain: f = 0.25 / [log10( (ε/(3.7D)) + (5.74/Re^0.9) )]^2
- Haaland: 1/√f = -1.8·log10( [ (ε/(3.7D))^1.11 ] + [ 6.9/Re ] )
If you’re iterating in software, the classic reference is Colebrook (implicit):
1/√f = -2·log10( (ε/(3.7D)) + (2.51/(Re·√f)) )
Worked example: stainless pipe friction factor and pressure drop
Assume water near 20°C, clean stainless roughness ε = 0.015 mm (1.5×10⁻⁵ m), and a pipe internal diameter D = 0.0525 m (about a 2-inch Schedule 40 ID). Flow rate Q = 50 gpm (0.003154 m³/s).
Calculate velocity and Reynolds number
- Area A = πD²/4 = 0.002165 m²
- Velocity V = Q/A = 1.46 m/s
- Kinematic viscosity ν ≈ 1.0×10⁻⁶ m²/s
- Re = (V·D)/ν ≈ 7.6×10⁴
- Relative roughness ε/D ≈ 2.86×10⁻⁴
Compute friction factor (Swamee–Jain)
Darcy friction factor f ≈ 0.0203
Translate f into pressure loss (Darcy–Weisbach)
For length L = 100 m, density ρ ≈ 998 kg/m³:
ΔP = f·(L/D)·(ρV²/2) ≈ 41 kPa per 100 m (about 4.2 m of water head per 100 m).
Quick reference table: stainless steel friction factor vs Reynolds number
The values below assume ε = 0.015 mm and D = 0.0525 m (ε/D = 2.86×10⁻⁴), using the Swamee–Jain correlation. Use this to sanity-check your results.
| Reynolds number (Re) | Darcy friction factor (f) | Typical interpretation |
|---|---|---|
| 5,000 | 0.038 | Low turbulent; f still relatively high |
| 10,000 | 0.031 | Early turbulent; sensitive to Re |
| 50,000 | 0.0219 | Common design region for pumped water |
| 100,000 | 0.0194 | Mid turbulent; f stabilizes |
| 1,000,000 | 0.0156 | Very turbulent; approaches roughness-controlled behavior |
Common pitfalls that cause wrong friction factors
- Using nominal pipe size instead of internal diameter: f depends on ε/D and pressure loss depends on L/D, so ID matters twice.
- Mixing Darcy and Fanning friction factors: if your result seems 4× off, this is the usual reason.
- Ignoring fluid temperature: viscosity changes Re; colder water increases ν and can increase f.
- Assuming stainless is always “perfectly smooth”: welds, scaling, or product buildup can justify using higher ε than new, clean pipe.
- Expecting high precision in transitional flow: treat 2300–4000 as uncertain and design with margin.
Bottom line: stainless steel pipe often yields f around 0.02 in common turbulent water services, but the most reliable number comes from Re and ε/D using a standard correlation.
