EN 12845 Pipe Friction Loss: Hazen-Williams

The EN 12845 friction loss formula, material-based C coefficients, and the NFPA-versus-EN confusion that causes most site errors.

Opening a hydraulic report at a logistics warehouse and finding the designer had calculated mild steel pipe with C = 140, I knew the work was wrong from the start. The pump pressure came out exactly right — but the pipe should have been C = 120, and the 15–20 % difference in friction made it uncertain whether the most remote sprinklers met the minimum pressure requirement. Hazen-Williams looks like a simple formula; on site the error is rarely the formula itself, but the C coefficient and the internal diameter put into it.

The SI version of the formula

The standard requires friction loss to be calculated to no less than:

p = (6.05 × 105 × Q1.85) / (C1.85 × d4.87) × L

The units are entirely SI:

• p — pressure loss in the pipe, bar
• Q — flow through the pipe, L/min
• d — mean internal diameter (not DN), mm
• C — coefficient by pipe type and condition
• L — total straight pipe plus fitting equivalent length, m

The formula is mathematically identical to the NFPA 13 version; only the constant and unit system differ. NFPA 13 works in gpm, psi and inches; EN 12845 in L/min, bar and mm. The same number cannot appear in both systems, and mixing the two without conversion produces a wrong answer from the outset.

C coefficients

EN 12845 lists C values by pipe type. The ones most used on site:

Pipe type	C value
Mild steel (welded)	120
Galvanised steel	120
Cast iron	100
Ductile iron	110
Spun cement	130
Cement-lined cast iron	130
Stainless steel	140
Copper	140
Glass reinforced plastic (GRP)	140

The standard notes that the list is not exhaustive and a manufacturer's documented value may be used. But if a project report uses a C outside the table, the approving authority will ask for the evidence.

The wet versus dry system difference

NFPA 13 requires C = 120 for mild steel in a wet system and C = 100 for the same pipe in a dry system, because the water-air cycle accelerates internal corrosion. EN 12845 makes no such distinction. Its table gives a single value for mild steel: 120. Designing a dry system does not lower C.

The practical consequence: a designer in Turkey calculating a dry system with C = 100 on NFPA reflexes may be told the EN 12845 submission is over-conservative; conversely, revising an NFPA design to the EN C = 120 under-calculates friction if the review reverts. Which standard you are applying must be settled in writing from the start; a hybrid calculation mixing both sets of C values will not be accepted.

Worked example — DN50 steel pipe, 100 L/min, 100 m

A practical site question: in an OH2 facility, a DN50 mild steel line carries 100 L/min to the sprinklers, with a total length of 100 m including fitting equivalents. What is the friction loss?

The inputs:

• Q = 100 L/min
• C = 120 (mild steel)
• d = 54 mm (typical mean internal diameter for DN50 mild steel; confirm from the manufacturer's certificate)
• L = 100 m

Step by step:

• Q^1.85 = 100^1.85 ≈ 5012
• C^1.85 = 120^1.85 ≈ 7015
• d^4.87 = 54^4.87 ≈ 2.735 × 10⁸

Numerator: 6.05 × 10⁵ × 5012 ≈ 3.032 × 10⁹
Denominator: 7015 × 2.735 × 10⁸ ≈ 1.919 × 10¹²
Unit friction: p/L ≈ 1.58 × 10^-3 bar/m

Over 100 m: p ≈ 0.16 bar.

The same line calculated at C = 100 (NFPA dry system logic) gives 5012/7015 = 0.715, so friction is 1/0.715 = 1.40 times higher — about 0.22 bar over 100 m. That 0.06 bar difference can be enough to drop the most remote sprinklers on a block or two below the minimum pressure.

Velocity check — the line's second test

EN 12845 limits velocity to 6 m/s through valves, strainers and flow indicators, and 10 m/s elsewhere. With 100 L/min through DN50 (54 mm internal, 2290 mm² cross-section):

v = (100 / 60) L/s × 1000 cm³/L / 22.90 cm² ≈ 0.73 m/s.

Well below the limit. Had the same line carried 360 L/min for OH3, v ≈ 2.6 m/s — still safe. The limit is approached when DN25 and DN32 lines are pushed to carry high flows; friction explodes there and the velocity limit is breached too.

Fitting losses

Fitting losses are given as equivalent straight pipe length, referenced to C = 120, with a correction for other C values. For DN50:

• 90° screwed elbow: 1.5 m
• 90° welded elbow (r/d = 1.5): 0.69 m
• Standard tee, flow through the branch: 2.9 m
• Fully open gate valve: 0.38 m
• Alarm or swing check valve: 2.4 m
• Butterfly valve: 2.2 m

A DN50 line with four elbows, one tee and one alarm valve has an equivalent length of 4 × 1.5 + 2.9 + 2.4 = 11.3 m. That is added to the straight pipe: L = 100 + 11.3 = 111.3 m, raising the 0.16 bar figure above to about 0.18 bar.

Common errors

• Carrying the NFPA C table into an EN calculation. Copper is C = 150 under NFPA and 140 under EN; a small gap that is still a 5 % friction difference.
• Using nominal instead of internal diameter. Writing DN50 and entering d = 50 produces roughly a 20 % error; substituting 50 mm for 54 mm raises d^4.87 by a factor of 1.55 and drops friction proportionally.
• Not balancing the calculation. Around a loop, the pressure loss sum must close to within (0 ± 1) mbar and flow at a junction to within (0 ± 0.1) L/min. Software output that does not check this is rejected.
• Ignoring the required precision. The standard fixes unit precision: 0.01 m for length, 1.0 L/min for flow, 1 mbar for pressure. Showing five decimal places in a spreadsheet looks scientific but the standard wants it cut at 1 mbar.
• Forgetting the velocity limit. Using DN50 for 600 L/min because friction looks acceptable means 4.4 m/s; even within the 6 m/s valve limit, flow stability suffers.

Comparison with NFPA 13

NFPA 13 writes Hazen-Williams in gpm and psi, with a constant of 4.52. Its C table is close to EN for most materials but differs on several: mild steel in a dry system is C = 100 (120 in EN), copper is 150 (140 in EN), and galvanised steel is 120 in both. Converting a design from NFPA to EN requires checking C material by material; a blanket assumption that everything is 120 produces errors.

Turkish context

BYKHY gives no explicit formula for sprinkler hydraulics; it refers to the provisions of the relevant standard. In practice, Turkish projects are drawn to either NFPA 13 or EN 12845, and European clients — particularly in FMCG, automotive, pharmaceuticals and logistics serving the EU market — generally require EN 12845. A contractor working on NFPA reflexes and calculating a dry system at C = 100 will be told the pipe is conservatively oversized under EN review; the reverse mistake surfaces late, and if under-calculated friction is found at an insurer's site audit, the fix is not a bigger pump but larger pipe — which is fabrication cost.

Frequently asked questions

Does the C coefficient differ between wet and dry systems under EN 12845?

No. EN 12845 gives a single C per pipe type with no wet/dry distinction. Mild steel is 120 in all cases.

Are the NFPA 13 and EN 12845 C values the same?

Close for most materials, different for some. NFPA uses C = 100 for mild steel in dry systems where EN uses 120 throughout. Copper is NFPA 150 and EN 140; ductile iron is NFPA 140 and EN 110.

Is d the internal or external diameter?

The mean internal diameter in mm — not the nominal size. DN50 mild steel runs 52–54 mm internally; take it from the manufacturer's certificate.

What is the velocity limit and how does it relate to friction?

6 m/s through valves, strainers and flow meters, and 10 m/s elsewhere. Even where friction is acceptable, a design exceeding the velocity limit is rejected.

How are fitting losses added?

Each fitting has an equivalent straight pipe length at C = 120. That equivalent length is added to the straight pipe run, and L in the formula is the total.

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