What Does This Friction Loss Calculator Do?
Feedwater pipe looks short, but friction adds up. Every meter of pipe, every elbow and every valve eats a bit of head. If you ignore it, pump head looks enough on paper but not in real plant and boiler level hunts.
This calculator finds friction loss in boiler feedwater pipe using Darcy-Weisbach. It uses real water density and viscosity at your feedwater temperature, roughness of pipe material and K factor for fittings. You get velocity, Reynolds number, friction factor, head loss in meters and pressure drop in bar and psi, plus loss from elbows and valves.
Friction Loss Formula
Velocity: v = Q / A , A = π × D² /4Reynolds: Re = rho × v × D / muColebrook: 1/√f = -2 log( e/3.7D + 2.51/(Re√f) )Head loss pipe: hf = f × (L/D) × v²/(2g)Fittings: hf_fit = K × v²/(2g) , K = sum of elbows and valvesPressure drop: ΔP = rho × g × hf_total (Pa) , ΔP bar = ΔP /100000
e is roughness in mm, D is internal diameter, L is straight length, g 9.81 m/s². For laminar Re less than 2300, f = 64/Re. For turbulent, Colebrook is solved by iteration.
How to Use the Calculator
- Enter feedwater flow. Use max flow including boiler steam plus blowdown rate plus recirculation if ARV open.
- Enter pipe ID and length. ID is actual bore, not nominal. Length is straight pipe only, not including fittings.
- Pick material and temperature. Commercial steel 0.045 mm for new pipe, 0.2 mm for old rusty pipe. Temperature 105°C for deaerator outlet, 80°C for feed tank.
- Enter fittings. Count 90° elbows. For valves, enter total K. Gate full open 0.15, swing check 2.5, globe 10. Use 3-5 K for typical feed line with 6 elbows and 2 checks.
- Click Calculate. You get head loss and pressure drop.
Understanding Results
Velocity
Feedwater velocity should be 1.0-2.0 m/s on pump suction to keep NPSH and 2.0-3.5 m/s on discharge. Above 3.5 m/s, friction rises fast with square of velocity and noise increases.
Reynolds Number
Above 4000 flow is turbulent and Colebrook applies. Below 2300 laminar, f = 64/Re. Most feedwater lines are turbulent with Re 100k-500k.
Friction Factor f
Depends on roughness and Reynolds. New steel at Re 200k gives f about 0.019-0.022. Old rusty pipe gives 0.025-0.03. Lower f means lower loss.
Head Loss and Pressure Drop
Head loss in meters adds directly to pump TDH. Pressure drop in bar is head loss × rho × g. For 5m loss at 105°C, drop is about 0.46 bar.
Worked Example
Feedwater 50 m³/h, ID 52.5 mm (DN50), length 80 m, 6 elbows K 0.3 each =1.8, valves K total 3, roughness 0.045 mm, temp 105°C.
| Step | Calc | Result |
|---|---|---|
| Area | π×0.0525²/4 | 0.002165 m² |
| Velocity | (50/3600)/0.002165 | 6.41 m/s? Wait - correct: 50 m³/h =0.01389 m³/s /0.002165 =6.41? Actually 52.5mm small for 50 m³/h - velocity high. Use DN80 77.9mm gives 2.56 m/s |
| Use DN80 ID 77.9mm | Area 0.004766 | v =2.91 m/s for 50 m³/h? Let's calc: 0.01389/0.004766=2.91 m/s |
| Re | 954×2.91×0.0779/0.000272 | 795k turbulent |
| f Colebrook | e/D 0.045/77.9=0.000578 | 0.0185 |
| hf pipe | 0.0185×(80/0.0779)×2.91²/(2×9.81) | 8.21 m |
| hf fittings | (1.8+3)×2.91²/(2×9.81) | 2.07 m |
| Total hf | 8.21+2.07 | 10.28 m |
| ΔP | 954×9.81×10.28/100000 | 0.96 bar |
So 80m DN80 line with 6 elbows loses about 10.3 m head, 0.96 bar. If your pump head is 120m, this is 8.5% loss, acceptable. If ID was 52.5mm, velocity would be 6.4 m/s and loss would be over 50m, too high.
Friction and Whole Boiler System
Friction loss is part of your total dynamic head. If you miss it, pump cannot reach boiler pressure at full flow.
For correct base flow, use our complete boiler feed pump calculation hub to get flow, TDH and NPSH. That flow already includes steam demand plus blowdown. Then add friction from this calculator to suction and discharge lines to get true TDH. Also check boiler efficiency at your feed temperature because hotter feedwater changes density and friction slightly and saves fuel.
- Suction line: Keep friction below 0.5m to protect NPSH. Use larger pipe and fewer elbows on suction.
- Discharge line: 2-5% of pump head as friction is normal. Above 10% means pipe too small.
- Old pipes: Roughness grows with age. 0.2 mm old steel has 30% more loss than new 0.045 mm pipe.
Common Mistakes
- Using nominal diameter as ID: DN50 nominal is not 50mm ID. DN50 Sch40 ID is 52.5mm, Sch80 is 49.2mm. Use real ID.
- Ignoring fittings: 6 elbows and 2 checks can add 20-30% extra loss on top of straight pipe.
- Using cold water properties: Feedwater at 105°C is thinner. Using 20°C viscosity overestimates friction by 10%.
- Velocity too high: Above 4 m/s, friction grows as square and water hammer risk rises on valve closure.
Frequently Asked Questions
What is normal feedwater velocity?
1.0-2.0 m/s on suction, 2.0-3.5 m/s on discharge. Lower on suction to keep NPSH, higher on discharge okay but not above 4 m/s.
What is Darcy-Weisbach formula?
Head loss = f × (L/D) × v²/(2g). f from Colebrook using roughness and Reynolds number.
How to reduce friction loss?
Increase pipe ID, reduce elbows, use long radius elbows, keep pipe new and smooth, keep velocity below 3 m/s.
Is Hazen-Williams same?
Hazen-Williams is simpler C factor method for water at 20°C. Darcy-Weisbach with Colebrook is more accurate for hot feedwater with temperature correction.
Basis: Darcy-Weisbach with Colebrook-White per Crane Technical Paper 410, Hydraulic Institute and CIBSE. Water density and viscosity from IAPWS steam tables interpolated 20-120°C. Roughness values per Moody chart: commercial steel 0.045 mm, stainless 0.015 mm. K factors: 90° flanged elbow 0.3, gate full open 0.15, swing check 2.5 per Crane. For final pipe sizing keep total friction including fittings below 10% of pump TDH.