Quick Answer
A boiler feed pump calculator converts your boiler's steam capacity (TPH), operating pressure and feedwater temperature into required flow (m³/h), total dynamic head (m), hydraulic power, shaft power and the next IEC standard motor rating (kW / HP). Feedwater density is temperature-corrected automatically and a 10% design margin is applied by default.
How to Use This Calculator
The calculator answers two separate questions about the same pump. The left column sizes the motor; the NPSH block underneath tells you whether the pump will cavitate. You can use either half on its own — the NPSH check runs even if you never enter a steam flow.
- Enter the boiler capacity. This is the rated steam output. Pick TPH, kg/h or lb/h from the dropdown beside the box — 1 TPH is 1,000 kg/h, and 1 lb/h is 0.453592 kg/h.
- Enter the boiler operating pressure. Bar, PSI, kg/cm² or MPa. The tool converts internally: 100 PSI becomes 6.8948 bar, 10 kg/cm² becomes 9.8067 bar, and 1 MPa is exactly 10 bar.
- Enter the feedwater temperature in °C. Density is corrected automatically, so a 40 °C reading and a 150 °C reading do not produce the same pump.
- Choose the feedwater source. Deaerator means a closed vessel where the water sits at its own boiling pressure. Vented tank means the surface is open to atmosphere. This single button changes the NPSH formula.
- Enter the water level above the pump centreline in metres or feet — the vertical drop from the tank’s lowest working level down to the pump shaft.
- Enter the pump’s NPSH required if you have the manufacturer’s curve. Leave it blank and you still get NPSH available; fill it in and you also get the margin and a verdict.
- Open Advanced to change pump efficiency (default 75%), motor efficiency (default 92%), design margin (default 10%) or suction pipe friction loss (default 0.5 m).
The Formulas
Motor sizing runs as a chain. Each step feeds the next, and the tool shows every intermediate value so you can check it against your own working:
| Step | Formula | Output |
|---|---|---|
| Design flow | Q = capacity × (1 + margin) ÷ ρ | m³/h |
| Total dynamic head | H = P ÷ (ρ × g) × 1.15 | m |
| Hydraulic power | Ph = Q × H × ρ × g ÷ 3,600,000 | kW |
| Shaft power | Ps = Ph ÷ ηpump | kW |
| Motor input power | Pm = Ps ÷ ηmotor | kW |
| Motor rating | smallest IEC frame ≥ Pm | kW |
The 1.15 factor on head is a 15% allowance for pipework and suction losses between the pump and the boiler drum. The IEC ladder the tool selects from runs 0.37, 0.55, 0.75, 1.1, 1.5, 2.2, 3, 4, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90 kW and on up to 1,000 kW in 31 standard sizes.
NPSH available uses a different formula, and it is the one that decides whether the pump survives:
NPSHa = (Psurface − Pvapour) ÷ (ρ × g) + hstatic − hfriction
Vapour pressure here is not read from a table. Water’s saturation curve is steeply convex, so interpolating between 10 °C steps misses by over 1% near boiling, and on a vented tank that error lands directly on your NPSH figure in metres. The calculator uses the closed-form Region 4 equation from the IAPWS-IF97 industrial formulation, which is exact across the whole range. You can cross-check any value it prints against the phase-change data in the NIST Chemistry WebBook entry for water.
Why a Deaerator’s Pressure Never Helps
This is the result most worth taking away, and it surprises people who have specified deaerators for years.
A closed deaerator holds its water at saturation — the liquid is already boiling at whatever pressure sits above it. So Psurface and Pvapour are the same number, and the first term of the NPSH equation is not small. It is exactly zero. Switch the tool to Deaerator and it prints a pressure term of 0.00 m at every temperature you can type.
| Deaerator temperature | Vapour pressure | Pressure term | NPSHa on an 8 m platform |
|---|---|---|---|
| 100 °C | 1.014 bar a | 0.00 m | 7.50 m |
| 120 °C | 1.987 bar a | 0.00 m | 7.50 m |
| 150 °C | 4.761 bar a | 0.00 m | 7.50 m |
| 180 °C | 10.026 bar a | 0.00 m | 7.50 m |
The vapour pressure climbs almost tenfold across that range and the NPSH available does not move at all. Every metre of it comes from the 8 m platform, minus the 0.5 m of suction line loss. That is precisely why deaerators are mounted on tall steel structures rather than beside the pump: the height is not a convenience, it is the entire NPSH budget.
Raising a deaerator’s operating pressure does nothing for NPSH. The water heats to the new saturation temperature and the pressure term stays at zero. If the pump is cavitating, the fixes are height, a shorter or wider suction line, or a pump with a lower NPSH requirement — not more deaerator pressure.
What a Vented Feed Tank Loses as It Heats
A tank open to atmosphere behaves in the opposite way. Here Psurface is fixed at 101,325 Pa while Pvapour climbs with temperature, so the pressure term is real — and it drains away as the tank gets hotter.
| Tank temperature | Vapour pressure | Pressure term |
|---|---|---|
| 40 °C | 7.38 kPa | 9.65 m |
| 60 °C | 19.95 kPa | 8.44 m |
| 70 °C | 31.20 kPa | 7.31 m |
| 80 °C | 47.41 kPa | 5.66 m |
| 90 °C | 70.18 kPa | 3.29 m |
| 95 °C | 84.61 kPa | 1.77 m |
Heating a vented feed tank from 60 °C to 95 °C throws away 6.67 m of NPSH available — 79% of it, without anyone touching the pump or the pipework. The term reaches zero at 99.97 °C, the point where water’s vapour pressure equals one atmosphere. Above that a vented tank cannot hold liquid water at all, and the calculator says so rather than printing a number: enter 110 °C on the vented setting and the pressure term goes to −4.51 m with a note that the duty needs a closed, pressurised source.
Put the two sources side by side on the same 8 m of steel and the result inverts the usual assumption. A vented tank at 90 °C gives 10.79 m of NPSH available. A deaerator at 105 °C on the identical platform gives 7.50 m. The vented tank wins by 3.29 m, because its water is 31.1 kPa below its own boiling point while the deaerator’s water is exactly at it.
Worked Example
A 10 TPH boiler at 10.5 bar, feedwater at 105 °C, deaerator 8 m above the pump centreline, 0.5 m of suction friction, and the default 75% pump / 92% motor efficiencies with a 10% design margin:
| Quantity | Value |
|---|---|
| Feedwater density at 105 °C | 954.65 kg/m³ |
| Design flow (10% margin) | 11.52 m³/h (50.7 GPM) |
| Total dynamic head | 128.9 m (423.0 ft) |
| Hydraulic power | 3.86 kW |
| Shaft power | 5.15 kW |
| Motor input power | 5.60 kW |
| Recommended motor | 7.5 kW (10.1 HP) |
| Overall efficiency | 69.0% |
| Vapour pressure at 105 °C | 1.209 bar a |
| NPSH available | 7.50 m (24.6 ft) |
Note the gap between 5.60 kW of motor input and the 7.5 kW frame the tool recommends. That is not padding the calculator adds — it is the nearest standard size above the computed duty, and the next one down, 5.5 kW, would be under it.
The design margin is quietly expensive. Dropping it from 10% to 0% moves motor input from 5.60 kW to 5.09 kW — a 10% change, because head here is set by boiler pressure and does not rise with flow. But it also moves the recommended frame from 7.5 kW down to 5.5 kW. A 10% margin bought a 36% larger installed motor, because the IEC ladder has no rung in between.
Reading the Margin Verdict
Enter the pump’s NPSH required and the tool compares it against NPSH available two ways at once — an absolute margin in metres and a ratio — then labels the result. A ratio on its own is not enough, because 1.5× of a very small NPSHr is still a very small margin, so an absolute floor rides alongside it.
| Verdict | Condition | Example at NPSHr = 5.0 m |
|---|---|---|
| Safe margin | ratio ≥ 1.5 and margin ≥ 1.0 m | 8.0 m platform → NPSHa 7.50 m, ratio 1.50 |
| Marginal | ratio ≥ 1.1 | 6.5 m platform → NPSHa 6.00 m, ratio 1.20 |
| Cavitation risk | anything below | 5.0 m platform → NPSHa 4.50 m, ratio 0.90 |
Those thresholds are the calculator’s own, chosen to be conservative for boiler feed duty, where the consequence of getting it wrong is impeller damage rather than a noisy pump. Treat them as a screening test, not as a substitute for the pump maker’s own margin recommendation for the specific model.
The calculator will not guess NPSH required for you, and no honest tool can. NPSHr is a measured property of one specific impeller at one specific flow, read off the manufacturer’s curve. Leave the field blank and you still get NPSH available, which is the half that depends on your plant rather than on the pump.
Why Hot Feedwater Costs More Motor
Feed the same 10 TPH boiler at 10.5 bar from water at three different temperatures and something clean falls out:
| Temperature | Density | Flow | Head | Motor input | Motor |
|---|---|---|---|---|---|
| 40 °C | 992.2 kg/m³ | 11.09 m³/h | 124.1 m | 5.39 kW | 5.5 kW |
| 105 °C | 954.7 kg/m³ | 11.52 m³/h | 128.9 m | 5.60 kW | 7.5 kW |
| 180 °C | 886.9 kg/m³ | 12.40 m³/h | 138.8 m | 6.03 kW | 7.5 kW |
From 40 °C to 180 °C, volumetric flow rises 11.9%, head rises 11.9%, and motor input rises 11.9%. The three move by exactly the same percentage because they have exactly one cause: density fell 10.6%, and all three quantities scale as 1 ÷ ρ. Substituting Q = ṁ ÷ ρ and H = 1.15 P ÷ (ρ g) into the power equation cancels one ρ and leaves hydraulic power proportional to 1 ÷ ρ and to nothing else.
The practical version: hot feedwater is good for boiler efficiency and good for deaeration, but it costs pump power in direct proportion to the density drop, and it can push you across an IEC frame boundary — 40 °C lands on a 5.5 kW motor here while 105 °C lands on 7.5 kW.
If you are converting a steam flow given in pounds per hour into a volumetric pump duty, the lb/hr to GPM converter does that step for any fluid density. And once you have a flow and a suction pipe size, the Reynolds number calculator will tell you whether that suction line is laminar or turbulent, which is what sets the friction loss you feed back into the NPSH box here.
Frequently Asked Questions
Because a deaerator holds its water at saturation. The pressure pushing down on the surface and the vapour pressure trying to boil the liquid are the same number, so they cancel completely. The subtraction (Psurface − Pvapour) gives exactly zero at 100 °C, at 180 °C, and at every point between. This is a property of the vessel, not an approximation in the calculator.
No. Raising the pressure raises the saturation temperature, the water heats to match, and the two terms cancel again at the new condition. NPSH available is unchanged. The levers that do work are lifting the deaerator, shortening or enlarging the suction pipe to cut friction loss, or selecting a pump with a lower NPSH requirement.
No. NPSH available depends only on temperature, source type, static height and friction loss, so the NPSH card fills in as soon as you enter a height — the capacity and pressure boxes can stay empty. The motor figures will read zero until you fill them, which is correct rather than a fault.
This tool flags a safe margin at a ratio of 1.5 or better combined with at least 1.0 m of absolute headroom, and treats anything below a 1.1 ratio as a cavitation risk. Those are deliberately conservative screening thresholds for boiler feed service. The pump manufacturer’s recommendation for the specific model and duty point always takes precedence.
It picks the smallest standard IEC frame at or above the computed motor input, and those sizes are coarse. A duty of 5.60 kW has to go to 7.5 kW because 5.5 kW would be undersized. The gap is not a safety factor added by the tool — it is the spacing of real motors, and it is why a small change in design margin can move you a whole frame.
It is an allowance for pipework and suction losses between the pump discharge and the boiler drum, applied on top of the head needed to overcome boiler pressure alone. It is a planning-stage figure. If you have a real pipe schedule with fitting losses calculated, that number should replace this allowance rather than sit on top of it.
Because pump power here scales as 1 ÷ density. Hot water is less dense, so the same mass of steam demands more volume per hour and more metres of head to reach the same pressure. Between 40 °C and 180 °C, flow, head and motor input all rise by 11.9% — one figure, because they share one cause.
The pressure term goes negative, because the water’s vapour pressure now exceeds atmospheric. At 110 °C it reads −4.51 m. That is the calculator telling you the arrangement is impossible: an open tank cannot hold water above 99.97 °C as liquid. Switch to a closed, pressurised source for that duty.
