📚 What's in This Guide
Most online boiler feed pump calculators reduce sizing to four numbers — boiler capacity, pressure, a rough safety factor, and an efficiency guess — then multiply. That gets you in the right neighborhood, but it skips the two things that actually cause boiler feed pump failures in the field: an NPSH check that accounts for how hot the water really is, and a motor sized for what one pump actually does, not the whole system's flow.
This calculator works through the full duty point — required flow (with blowdown and design margin), Total Dynamic Head broken into its real components, an NPSH available calculation that uses the actual vapor pressure of water at your feedwater temperature, and per-pump power and motor sizing across common duty/standby arrangements.
1. Calculating Required Feedwater Flow
The starting point is the boiler's maximum continuous steam rating. But the pump has to deliver more than that number, because water leaves the system two ways the steam meter doesn't count:
Continuous blowdown (typically 2–5% of feedwater flow — not of steam output) removes dissolved solids that concentrate in the boiler as it evaporates pure steam; because blowdown is drawn off as a share of the water entering the boiler, dividing by (1 − blowdown%) is the correct relationship, not simply adding blowdown% on top of the steam rate. Design margin (typically 10–20%) covers control-system response time, load swings, and future capacity headroom, and is applied on top. If you already know your required feedwater flow, you can enter it directly and skip this derivation entirely — see the "Known Feedwater Flow" mode in the calculator above.
That mass flow (kg/hr or lb/hr) then has to become a volumetric flow (m³/hr or gpm) for pump selection — and that conversion depends on density, which changes meaningfully with temperature. This calculator looks up feedwater density at your actual entered temperature (from a saturated-liquid-water reference table) instead of assuming cold-water density, which typically overstates the required volumetric flow by several percent for hot feedwater.
2. Total Dynamic Head (TDH)
TDH is everything the pump has to push against, converted to a single head figure in meters or feet:
where Suction Credit = Suction Vessel Pressure Head + Static Suction Head − Suction Friction Loss.
- Pressure head — the boiler's operating pressure, converted to a head using the feedwater's actual density (not the textbook "1 bar ≈ 10.2 m" shortcut, which assumes cold water).
- Feedwater control valve drop — a major, frequently-omitted term in modulating-control systems; the pump has to develop enough head to still push water through a partially-closed valve at load.
- Static discharge head & friction — elevation and piping losses between the pump and the boiler drum/economizer inlet.
- Suction credit — if the pump draws from a pressurized or elevated deaerator/feed tank, that head reduces the pump's job. Ignoring it (as several calculators do) oversizes the pump.
3. NPSH: Why Feedwater Temperature Is the Real Driver
Boiler feedwater is frequently drawn from a deaerator running close to its boiling point — that's the point of deaeration. As temperature rises, water's vapor pressure rises sharply and non-linearly, eating directly into the margin between available suction pressure and the point where the pump starts flashing water to vapor at the impeller eye (cavitation).
This calculator computes the vapor pressure term from your entered feedwater temperature using the IAPWS-IF97 saturation-pressure equation — the same industry-standard correlation behind modern steam tables, valid continuously from 0°C to the critical point (~374°C) with no accuracy gaps at any single temperature — rather than a rough approximation. A small NPSH miscalculation is exactly what causes an otherwise correctly-flow-and-head-sized pump to cavitate in service.
The result is compared against your pump's NPSH required (from the manufacturer's curve) with a margin check. Commonly cited guidance — consistent with the Hydraulic Institute's ANSI/HI 9.6.1 NPSH margin guideline — calls for a minimum margin of roughly 1 m (3.3 ft) or a margin ratio (NPSHa ÷ NPSHr) of about 1.1, whichever is larger, with higher-energy or critical services often warranting more.
4. Hydraulic Power, Shaft Power & Motor Sizing
Hydraulic power is the useful work done lifting and pressurizing the fluid; shaft power is what the motor actually has to deliver once pump inefficiency is factored in:
Shaft Power = Hydraulic Power ÷ Pump Efficiency
Motor Rating = Shaft Power × (1 + Motor Margin%), then rounded up to the next rating on the common IEC 60034-1 preferred power series (kW). This is a preliminary selection aid, not a guarantee of availability — actual motor frame size (a separate IEC 60072 mounting/dimension standard) and which exact ratings a given manufacturer stocks still depend on speed, poles, and the specific vendor's range.
The power calculation uses the same temperature-corrected density as the flow calculation — hot feedwater is measurably less dense than cold water, which affects both mass flow conversion and power. Critically, this calculator sizes the motor against one pump's individual duty flow, not the total system flow — a common shortcut in simpler tools that quietly oversizes every motor in a multi-pump arrangement.
5. Duty/Standby Pump Arrangements
| Arrangement | Each Pump Sized For | Typical Use |
|---|---|---|
| 1 × 100% | Full required flow | Small/simple systems where downtime for pump maintenance is acceptable |
| 2 × 100% | Full required flow (each) | Full standby redundancy — one pump runs, one is spare |
| 3 × 50% | Half the required flow (each) | Two pumps run together to meet 100% duty, third is spare — common on larger industrial boilers |
Select an arrangement above and the calculator shows both the per-pump duty flow and the per-pump motor rating, since that's what actually gets ordered from a pump vendor — not the system total.
6. Worked Example: Sizing a 20 t/hr Boiler Feed Pump
Here's the full calculation, start to finish, for a boiler running at 20,000 kg/hr steam capacity — the default values loaded in the calculator above, so you can follow along and cross-check every number.
Inputs: Boiler capacity 20,000 kg/hr · Blowdown 3% (of feedwater) · Design margin 15% · Feedwater temperature 105°C · Boiler pressure 12 barg · Control valve differential 1.5 bar · Static discharge head 2 m · Discharge friction 3 m · Suction vessel 0.3 barg · Static suction head +4 m · Suction friction 0.4 m · NPSHr 3 m · Pump efficiency 72% · Motor margin 15%.
Step 1 — Flow: Feedwater before margin = 20,000 ÷ (1 − 0.03) ≈ 20,619 kg/hr. Add 15% design margin: 20,619 × 1.15 ≈ 23,711 kg/hr. At 105°C, feedwater density is about 954.6 kg/m³ (not 1,000), giving a volumetric flow of 23,711 ÷ 954.6 ≈ 24.84 m³/hr.
Step 2 — Head: 12 barg converts to roughly 128.1 m of head at this density; the 1.5 bar control valve differential adds about 16.0 m; static discharge and friction add 2 m and 3 m. On the suction side, the 0.3 barg vessel plus 4 m static head minus 0.4 m friction gives a suction credit of about 6.8 m. TDH = 128.1 + 16.0 + 2 + 3 − 6.8 ≈ 142.4 m.
Step 3 — NPSH: At 105°C, water's vapor pressure is about 1.209 bar absolute (computed via IAPWS-IF97, not assumed). Working through the suction-side pressures gives NPSHa ≈ 4.71 m. Against the entered NPSHr of 3 m, that's a margin of 1.71 m and a ratio of 1.57× — comfortably above the ~1 m / 1.1× minimum, so the calculator shows a "good" status.
Step 4 — Power & motor, single pump (1×100%): Hydraulic power = 24.84 m³/hr × 142.4 m × 954.6 kg/m³ × 9.81 ÷ 3,600,000 ≈ 9.2 kW. Shaft power = 9.2 ÷ 0.72 ≈ 12.8 kW. With a 15% motor margin: 12.8 × 1.15 ≈ 14.7 kW, rounded up to the next standard rating: 15 kW.
Step 5 — Same job, split 3×50%: Switching the arrangement toggle to "3 × 50%" doesn't change the flow or TDH totals — it changes how they're divided. Each duty pump now handles half the flow (≈12.42 m³/hr) at the same 142.4 m TDH, dropping shaft power to roughly 6.4 kW per pump and the motor selection to 7.5 kW each — with a third pump on standby.
That's the complete duty point: 24.84 m³/hr @ 142.4 m TDH for a single pump, or 12.42 m³/hr @ 142.4 m TDH per pump for a 3×50% arrangement — the exact numbers you'd hand to a pump vendor or check against a manufacturer's curve.
7. What to Look for in a Boiler Feed Pump Calculator
The underlying hydraulic formulas are standard engineering — the differences between tools come down to completeness and whether real-world details are modeled or assumed away:
| Feature | Why it matters | This calculator |
|---|---|---|
| Blowdown & design margin as separate inputs | A single arbitrary "safety factor" hides how much margin is actually being applied | Included |
| Temperature-corrected density & vapor pressure | Hot feedwater has meaningfully different density and much higher vapor pressure than cold water | Included |
| NPSH available vs. required, with a margin check | The most common real-world cause of feed pump cavitation and premature failure | Included |
| Suction credit from a pressurized/elevated tank | Ignoring available suction pressure oversizes the pump | Included |
| Feedwater control valve pressure drop as a TDH term | Often the single largest head component in modulating systems, frequently omitted | Included |
| Motor sized per-pump for the selected arrangement | A motor sized on total system flow is wrong for every multi-pump layout | Included |
| Single duty-point summary (flow @ TDH) | The one line a pump vendor actually needs to select or verify a curve against | Included |
| Option to enter a known feedwater flow directly | Skips the blowdown/margin derivation when you already have a fixed design flow | Included |
| Runs entirely in your browser, no lead-gen forms | Your numbers stay private | Included |
8. Methodology & Limitations
This calculator uses standard centrifugal pump sizing formulas (flow, TDH, hydraulic/shaft power) with feedwater density and vapor pressure both computed from your entered temperature rather than assumed constant. A few things worth knowing:
- Feedwater density is read from a saturated-liquid-water reference table (0–300°C) with linear interpolation between points — accurate to a fraction of a percent for sizing purposes, not a full IAPWS-95 steam-table implementation.
- Vapor pressure uses the IAPWS-IF97 Region 4 saturation-pressure equation (the industry-standard steam-table correlation), accurate to within about 0.1% of reference steam tables continuously from 0°C up to the critical point (~374°C) — suitable for engineering NPSH screening, though final verification should still use your pump vendor's own data for marginal cases.
- TDH and NPSH models use simplified, single-value head terms for friction and control valve losses that you supply — this tool does not perform pipe-by-pipe Darcy-Weisbach hydraulic calculations, so friction inputs should come from your own piping takeoff or a separate line-loss calculation.
- Pump and motor efficiency are user-supplied estimates. Final selection should always be checked against the actual manufacturer pump curve and motor datasheet — this tool is a preliminary sizing and cross-check aid, not a replacement for vendor selection software.
- NPSH required (NPSHr) must come from a specific pump's published curve at your duty flow — it is not something this calculator can determine on its own, and NPSHr changes with flow rate.
- This tool is for preliminary engineering estimation and education only and is not a substitute for a qualified engineer's review, detailed hydraulic study, or manufacturer pump/motor selection data.
9. Frequently Asked Questions
Required feedwater flow equals the boiler's maximum continuous steam rating, increased for continuous blowdown (typically 2-5%) and a design margin (typically 10-20%), then converted to volumetric flow using the feedwater density at its actual temperature.
TDH is the total resistance the pump must overcome: the boiler operating pressure converted to head, plus any feedwater control valve pressure drop, plus static discharge elevation and discharge piping friction, minus any credit from a pressurized or elevated suction source.
Boiler feedwater is often near its boiling point (for example, leaving a deaerator). As temperature rises, the water's vapor pressure rises sharply, which reduces NPSH available and increases the risk of pump cavitation if the margin against NPSH required is too small.
A commonly cited minimum is an NPSH margin of about 1 meter (3.3 ft) or a margin ratio of 1.1, whichever is greater, broadly consistent with the Hydraulic Institute's ANSI/HI 9.6.1 guideline. High-energy or critical services often warrant a larger margin — always confirm against your specific pump's published curve.
No. Every calculation runs locally in your browser. Nothing you type is sent to a server, there is no account, and there are no ads or lead-generation forms.
This calculator is for preliminary engineering estimation only and is not a substitute for a qualified engineer's review or manufacturer pump/motor selection data. See Section 7 for methodology details and references.