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Fire protection pipe sizing and friction loss
Enter flow and length to compare steel pipe sizes by velocity and Hazen-Williams friction loss and get the smallest size that meets your limit.
Quick answer
960 L/min over 80 m of C=120 pipe: use DN65 (v = 4.29 m/s), friction loss about 3.16 bar including 25% for fittings.
Pipe size comparison
| Size | OD × wall | ID | v (m/s) | Δp (bar/m) | Total Δp (bar) | |
|---|---|---|---|---|---|---|
| DN25 1" | 33.7 × 3.20 | 27.3 | 27.33 | 2.8732 | 287.32 | Too fast |
| DN32 1-1/4" | 42.4 × 3.20 | 36.0 | 15.72 | 0.7469 | 74.69 | Too fast |
| DN40 1-1/2" | 48.3 × 3.20 | 41.9 | 11.60 | 0.3567 | 35.67 | Too fast |
| DN50 2" | 60.3 × 3.60 | 53.1 | 7.23 | 0.1125 | 11.25 | Too fast |
| DN65 2-1/2" | 76.1 × 3.60 | 68.9 | 4.29 | 0.0316 | 3.16 | Recommended |
| DN80 3" | 88.9 × 4.00 | 80.9 | 3.11 | 0.0145 | 1.45 | |
| DN100 4" | 114.3 × 4.50 | 105.3 | 1.84 | 0.0040 | 0.40 | |
| DN125 5" | 139.7 × 5.00 | 129.7 | 1.21 | 0.0015 | 0.15 | |
| DN150 6" | 165.1 × 5.00 | 155.1 | 0.85 | 0.0006 | 0.06 |
| Step | Result | Basis |
|---|---|---|
| Equivalent lengthLe = L·(1 + 25%) = 80 × 1.25 | 100.0 m | Le = L + ΣLtđ |
| Smallest size with v ≤ 5.0 m/sv = Q/(π·d²/4), d = 68.9 mm | DN65 · 4.29 m/s | v ≤ vmax |
| Pipe friction lossΔP = Δp·Le = 0.0316 × 100.0 | 3.16 bar | Hazen-Williams |
Sources and assumptions. Metric Hazen-Williams: Δp = 6.05·10⁵·Q^1.85 / (C^1.85·d^4.87) bar/m (Q L/min, d mm). Pipe dimensions from the series standard ⚠ check the mill catalogue for the batch. Elevation not included. ⚠ = code table value to be checked against the original standard. Preliminary results for material selection and budgeting; not a substitute for an approved fire protection design.
Suggested materials
Galvanized steel pipe DN65, BS 1387 / EN 10255 Medium80 m straight pipe14 6 m lengths
DN65 fittings (elbows, tees, grooved couplings)Threaded, welded or groovedper drawing
Frequently asked questions
What is the friction loss formula for fire protection pipe?
Metric Hazen-Williams: Δp = 6.05×10⁵ × Q^1.85 / (C^1.85 × d^4.87) bar/m, with Q in L/min, d the internal diameter in mm, and C = 120 for steel pipe.
How much flow can a DN65 fire pipe carry?
A DN65 Medium pipe (68.9 mm ID) at 5 m/s carries about 1,100 L/min (18.6 L/s).
What C-factor is used for galvanized steel pipe?
Designs usually take C = 120 for black and galvanized steel in wet systems and C = 100 for dry systems.