Size a boiler feed water pump in seconds — calculate design flow rate, total dynamic head, and recommended motor power (kW / HP) from boiler evaporation rate and working pressure. Based on IBR, EN 12953 and standard steam engineering practice.
What is a Boiler Feed Pump?
A boiler feed pump (also called a feed water pump or BFW pump) delivers treated water from the feed tank or deaerator into the steam boiler against the boiler's operating pressure. Because it must overcome the full boiler pressure plus system losses, it is a high-head, comparatively low-flow duty — which is why multistage centrifugal pumps are the industry standard for this application. Correct sizing is critical: an undersized pump cannot maintain drum level (a serious safety risk under IBR / ASME rules), while an oversized pump wastes energy and suffers from throttled, unstable operation.
How to Use This Calculator (Step by Step)
- Enter boiler evaporation rate — the maximum steam output in kg/hr, TPH or lb/hr from the boiler nameplate or datasheet.
- Set the flow safety margin — 15–25% is standard practice to cover blowdown losses, on/off or modulating feed control, and safety-valve lift conditions. Boiler codes such as IBR and EN 12953-6 require feed capacity above peak evaporation.
- Enter boiler working pressure and feedwater temperature — the calculator automatically corrects water density for temperature (hot water is lighter, so more head is needed per bar).
- Add system losses — pressure margin, economizer and feed check valve pressure drop, static lift from feed tank to boiler drum, and pipe friction.
- Enter pump efficiency and motor margin — then click Calculate to get the complete pump duty: flow, head, shaft power and the next standard motor rating.
Boiler Feed Pump Sizing Formulas
Step 1 — Feedwater Flow Rate
Q (kg/hr) = Boiler Evaporation × (1 + Margin%)
Q (m³/hr) = Q (kg/hr) ÷ ρ [ρ = water density at feedwater temp, kg/m³]
Step 2 — Discharge Pressure & Total Head
Pdis = Pboiler × (1 + Pressure margin%) + ΔPeconomizer+valves
H (m) = Pdis × 10⁵ ÷ (ρ × 9.81) + Static Head + Friction Losses
Quick rule of thumb: 1 bar ≈ 10.2 m of water head (cold water). At 85–105 °C feedwater the density drop makes it ~10.6–10.7 m per bar — this calculator applies the correction automatically.
Which pressure to design for: the pump must still feed when the boiler is at its highest pressure, which is the safety valve set pressure plus accumulation. If the safety valve is set above the normal working pressure (often 5–10% above), include that difference in the pressure margin, not just the 3% accumulation. The calculation also assumes a vented feed tank; for a pressurised deaerator, subtract its pressure from the discharge pressure.
Step 3 — Pump Power & Motor Selection
Phydraulic (kW) = ρ × g × Q × H ÷ (3.6 × 10⁶)
Pshaft = Phydraulic ÷ ηpump
Motor kW = next standard rating ≥ Pshaft × (1 + service factor)
Worked Example: 5 TPH Boiler at 10.5 bar g
A 5,000 kg/hr boiler operating at 10.5 bar g with 85 °C feedwater, 25% flow margin, 3% pressure margin, 1 bar economizer/valve drop, 5 m static head and 5 m friction:
- Feedwater flow = 5,000 × 1.25 = 6,250 kg/hr ≈ 6.5 m³/hr (ρ ≈ 968.5 kg/m³)
- Discharge pressure = 10.5 × 1.03 + 1.0 = 11.8 bar g → ≈ 124 m + 5 + 5 = ~134 m total head
- Hydraulic power ≈ 2.3 kW → shaft power at 45% efficiency ≈ 5.1 kW → ×1.15 = 5.85 kW → select 7.5 kW (10 HP) motor
For the full step-by-step method with pipe sizing and NPSH, read the Boiler Feed Pump Calculation guide. Check suction conditions with the Pump NPSH Calculator.
Design Tips for Boiler Feed Pump Selection
- NPSH is the #1 failure point. Feedwater at 85–105 °C is near its vapor pressure. Elevate the feed tank/deaerator (commonly 3–8 m above pump centerline), keep suction lines short and one size larger than the pump suction nozzle, and ensure NPSHa exceeds NPSHr by at least 0.5–1 m.
- Provide standby capacity. Boiler codes generally require two independent means of feeding — typically 2 × 100% pumps (one working, one standby).
- Check the safety-valve condition. For on/off feed control the pump must still deliver water when the boiler is at safety-valve set pressure.
- Minimum flow protection. Multistage pumps overheat at low flow — provide a recirculation line or orifice back to the feed tank.
- Check motor power at run-out. The motor margin here is over the duty point; on a flat pump curve, power at maximum flow can be higher, so confirm the maximum power on the vendor curve.
- Verify against the vendor curve. This tool gives preliminary duty for concept/tender stage; final selection must match a manufacturer's tested curve (Grundfos CR, KSB Movitec/HGM, Wilo MVI, Kirloskar etc.).
Frequently Asked Questions
How do you calculate boiler feed pump capacity?
Multiply the boiler's maximum evaporation rate by 1.15–1.25. The margin covers blowdown, feed-control cycling and safety-valve lift. Divide kg/hr by the water density at feedwater temperature to get m³/hr.
How much head does a boiler feed pump need?
Enough to deliver against the highest boiler pressure it will meet, usually the safety valve set pressure plus accumulation, plus economizer and feed check valve drops, converted to metres of water (≈10.2–10.7 m per bar depending on temperature), plus static lift and pipe friction.
Why is the feed pump sized at 1.25 × evaporation?
The pump must refill the boiler faster than steam leaves it, replace blowdown, and hold drum level even during safety-valve lift. Boiler codes such as IBR and EN 12953-6 require feed capacity above peak evaporation.
Which pump type is used for boiler feed water?
Vertical or horizontal multistage centrifugal pumps (high head, moderate flow). Typical feed-duty efficiency is 35–60%. Small boilers sometimes use regenerative turbine pumps.
Why is NPSH so critical for feed pumps?
Hot feedwater is close to boiling; any suction pressure dip flashes it to vapor and cavitates the pump. Elevate the tank, shorten the suction line, and keep NPSHa > NPSHr + 0.5–1 m margin.
Disclaimer: Results are for preliminary engineering estimation only. Final pump selection must be verified against manufacturer performance curves and applicable codes (IBR / ASME Section I / EN 12953).