What This Boiler Feed Pump Calculator Gives You
A boiler feed pump delivers treated water from the feed tank or deaerator into the boiler against boiler pressure, so it is a high-head, moderate-flow duty served by multistage centrifugal pumps. This calculator turns the boiler nameplate data and the feed tank arrangement into a complete pump duty: design flow in m³/hr, LPM and US GPM, total head, NPSH available with a check against the pump's NPSHr, shaft power and the next standard IEC motor. For the full method explained step by step, read the boiler feed pump calculation guide.
How to Use the Calculator
- Boiler evaporation rate from the nameplate or datasheet, in kg/hr, TPH or lb/hr, with a 15–25% flow margin for blowdown and feed control.
- Boiler pressure: enter the safety-valve set pressure. The 3% pressure margin then covers accumulation. If you only know the working pressure, raise the margin by the gap to the safety-valve setting.
- System losses: economizer, feed control and check valve drop in bar, the static height from tank water level to the boiler feed inlet, and total pipe friction.
- Feed tank: its pressure (0 for vented), the water level above the pump, suction loss and site altitude. Add the pump's NPSHr when you have a candidate pump.
- Pump efficiency and motor margin, then calculate.
Formulas Used
Q (m³/hr) = Evaporation (kg/hr) × (1 + Margin%) ÷ ρ
Pdis = Pboiler × (1 + Pressure margin%) + ΔPeconomizer+valves
H (m) = (Pdis − Ptank) × 10⁵ ÷ (ρ × 9.81) + Static head + Friction
NPSHa (m) = (Patm + Ptank − Pvapour) ÷ (ρ × g) + Hs − hf,suction
Pshaft (kW) = ρ × g × Q × H ÷ (3.6 × 10⁶ × η) → motor = next IEC size ≥ Pshaft × (1 + margin)
Water density follows the feedwater temperature, so hot water needs about 10.5–10.7 m of head per bar instead of 10.2 m. Vapour pressure comes from the IAPWS-IF97 saturation equation, and atmospheric pressure is corrected for altitude.
Example: 10 TPH Boiler with a Deaerator
A 10,000 kg/hr boiler with the safety valve set at 17 bar g, fed from a deaerator at 0.2 bar g and 105 °C. Flow margin 20%, economizer and valves 1.5 bar, deaerator water level 7 m above the pump, boiler feed inlet 2 m above that level, total friction 6 m of which 0.6 m on the suction side, pump efficiency 55% and a candidate pump with NPSHr 3.5 m.
- Flow = 10,000 × 1.20 = 12,000 kg/hr ÷ 954.7 kg/m³ = 12.57 m³/hr
- Discharge pressure = 17 × 1.03 + 1.5 = 19.01 bar g
- Head = (19.01 − 0.2) × 10⁵ ÷ (954.7 × 9.81) = 200.9 m, + 2 m static + 6 m friction = 208.9 m. The deaerator's 0.2 bar saves about 2 m of head.
- NPSH available: 101.3 + 20 = 121.3 kPa on the surface against 120.9 kPa vapour pressure leaves only 0.05 m from pressure, so NPSHa = 0.05 + 7 − 0.6 = 6.45 m, a 2.95 m margin over the 3.5 m NPSHr
- Power: hydraulic 6.83 kW → shaft 12.4 kW → × 1.15 = 14.3 kW → 15 kW motor
The same boiler fed from a vented tank at 85 °C would gain about 4.6 m of NPSH from atmospheric pressure, which is why small plants without a deaerator rarely have suction problems, while deaerator plants depend entirely on how high the deaerator is mounted. Check other suction layouts with the Pump NPSH Calculator.
Design Tips for Boiler Feed Pump Selection
- NPSH is the #1 failure point. Use the lowest working level in the tank and the hottest feedwater it will see. Keep suction lines short, one size larger than the pump suction nozzle, and watch strainers on the suction side.
- Provide standby capacity. Boiler codes generally require two independent means of feeding — typically 2 × 100% pumps (one working, one standby).
- Design at the safety-valve condition. With 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, orifice or automatic recirculation valve 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 a preliminary duty for concept and tender stage; final selection must match a manufacturer's tested curve, including NPSHr at the duty point.
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 safety valve set pressure plus accumulation, plus economizer and feed check valve drops, minus any pressure already in the feed tank or deaerator, converted to metres of water (≈10.2–10.7 m per bar depending on temperature), plus static height 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 efficiency at small feed duties is 35–65%. Small boilers sometimes use regenerative turbine pumps.
How is NPSH available calculated for a feed pump?
NPSHa = (absolute pressure on the tank water surface − vapour pressure at feedwater temperature) ÷ (ρ × g) + water level above the pump − suction loss. In a deaerator at saturation the pressure term is zero, so NPSHa comes only from elevation. Keep NPSHa at least 0.5–1 m above the pump's NPSHr.
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).