Quick answer: Size a boiler feed pump in three steps. First set the flow from the boiler steam rate plus a margin for blowdown and swings — usually 1.15 to 1.25 times the maximum evaporation. Second set the head from the boiler pressure (converted to metres), plus the feed control valve drop, pipe friction and static lift. Third work out the motor power from the flow, head and pump efficiency. Because feed water is hot and close to boiling, the suction side and NPSH need as much care as the numbers.
What a boiler feed pump does
A boiler feed pump pushes water from the feed tank or deaerator into a steam boiler that is already under pressure. It has to raise the water to a pressure higher than the boiler is holding, and it has to keep up with the rate at which the boiler turns that water into steam. If it cannot, the drum level falls and the boiler trips on low water — a safety shutdown you never want to see.
Feed pumps are almost always multistage centrifugal pumps, because a single impeller cannot build the head that boiler pressure demands. The three numbers that define the pump are flow, head and power, and each is sized with a deliberate margin.
Step 1 — Flow rate
The pump must replace every kilogram of water the boiler boils off, plus the water lost to blowdown, plus a margin so it can recover the drum level after a sudden demand. Feed water is close to the density of cold water, so 1 kg/h of steam needs about 1 litre/h of feed.
Feed flow (kg/h) = maximum steam rate (kg/h) × margin
margin = 1.15 to 1.25 for a modulating pump (covers blowdown + swing)
margin = up to 1.5 for a single on/off pump feeding one boiler
Feed flow (m³/h) ≈ feed flow (kg/h) ÷ 1000The larger 1.5 margin is used when a single pump switches on and off to hold the level, because it must catch up quickly during the on periods. A pump that modulates continuously against a control valve can use the smaller margin.
| Boiler steam rate | Feed flow at 1.2 margin |
|---|---|
| 500 kg/h | 0.60 m³/h |
| 1,000 kg/h | 1.20 m³/h |
| 2,000 kg/h | 2.40 m³/h |
| 5,000 kg/h | 6.00 m³/h |
| 10,000 kg/h | 12.0 m³/h |
Enter steam rate, boiler pressure and lift — get the feed flow, total head and motor power with margins applied.
Open the Boiler Feed Pump Calculator →Step 2 — Total head
The pump has to beat the boiler pressure and then some. Total head is the sum of everything working against it, converted into metres of water. A useful conversion is that 1 bar of pressure equals about 10.2 metres of water head.
Total head (m) = boiler pressure head
+ feed control valve drop
+ pipe friction
+ static lift (pump to boiler inlet)
− suction head from the feed tank- Boiler pressure head. Size against the safety-valve set pressure, not the normal working pressure, so the pump can still feed when the boiler is at its highest. Convert bar to metres with the 10.2 factor.
- Feed control valve drop. The modulating valve that trims the flow needs a pressure drop to control well — allow around 20% of the boiler pressure, or a minimum of about 1 bar.
- Pipe friction. The friction loss along the feed line at full flow, usually a few metres.
- Static lift. The height the water is physically raised from the pump to the boiler feed connection.
- Suction head. The feed tank or deaerator sits above the pump, which helps the pump and is subtracted from the required head.
Step 3 — Motor power
Once you have the flow and head, the shaft power follows from a standard water formula. The 367 constant already includes the density of water and gravity, so you only need flow in m³/h and head in metres.
Shaft power (kW) = Q (m³/h) × H (m) ÷ (367 × pump efficiency)
Motor rating (kW) = shaft power × motor margin (about 1.15)Multistage feed pumps run at an efficiency of roughly 55–70%. Use 0.6 if you have no data. The motor margin covers voltage dips, wear and the highest point on the pump curve.
| Flow | Head | Shaft power (0.6 eff.) | Motor (next size up) |
|---|---|---|---|
| 1.2 m³/h | 110 m | 0.6 kW | 0.75 kW |
| 2.4 m³/h | 120 m | 1.3 kW | 1.5 kW |
| 6.0 m³/h | 130 m | 3.5 kW | 4.0 kW |
| 12.0 m³/h | 140 m | 7.6 kW | 9.3 kW |
The part people get wrong — NPSH and hot water
Feed water leaves a deaerator at around 100–105 °C, which is close to boiling. Hot water flashes to steam very easily when the pressure drops at the pump inlet, and that flashing — cavitation — wrecks feed pumps faster than anything else. This is why the suction side deserves as much attention as the head calculation.
Every pump needs a minimum inlet pressure to stop the water boiling, called the net positive suction head required (NPSHr). The system must supply more than that, the net positive suction head available (NPSHa). The rule is simple:
NPSHa must exceed NPSHr, with a safety margin of about 0.5–1 mFor hot feed water the vapour pressure is almost the same as the operating pressure, so NPSHa comes mainly from the height of the deaerator above the pump. That is why deaerators are always mounted high on a steel structure — the elevation buys the suction head the pump needs. If a feed pump keeps failing, low NPSH is the first thing to check. The NPSH and suction lift calculator works through the numbers.
Example — a 2,000 kg/h boiler at 10 bar
A steam boiler produces 2,000 kg/h at a working pressure of 10 bar, with a safety valve set at 11 bar. The deaerator sits 6 m above the pump, and the boiler feed point is 4 m above the pump.
- Flow: 2,000 × 1.2 = 2,400 kg/h = 2.4 m³/h.
- Boiler pressure head: 11 bar × 10.2 = 112 m.
- Control valve drop: ~20% of 10 bar = 2 bar = 20 m.
- Friction + static lift: say 5 m friction + 4 m lift = 9 m.
- Total head: 112 + 20 + 9 = 141 m (the elevated deaerator handles the suction side).
- Shaft power: 2.4 × 141 ÷ (367 × 0.6) = 1.5 kW, so a 2.2 kW motor gives comfortable margin.
The result is a multistage pump of about 2.4 m³/h at 141 m head on a 2.2 kW motor, fed from a deaerator raised at least 6 m to protect against cavitation.
Boiler pressure to head — quick reference
Since pump head is worked in metres, convert the boiler pressure first. One bar of pressure is about 10.2 metres of water. Always use the safety-valve setting, which sits above the working pressure.
| Boiler pressure | Pressure head | Approx. total head with losses |
|---|---|---|
| 4 bar | 41 m | 60–70 m |
| 7 bar | 71 m | 95–105 m |
| 10 bar | 102 m | 130–145 m |
| 14 bar | 143 m | 180–200 m |
| 17 bar | 173 m | 215–235 m |
The right-hand column adds a typical allowance for the control valve drop, friction and lift. It shows why feed pumps build such high heads — and why they are almost always multistage.
Feed water temperature and NPSH
The hotter the feed water, the higher its vapour pressure, and the more suction head the pump needs to stop it flashing. This is the single reason deaerators are mounted high.
| Feed water temperature | Vapour pressure | Suction condition |
|---|---|---|
| 60 °C | 0.20 bar | Comfortable |
| 80 °C | 0.47 bar | Watch NPSH |
| 95 °C | 0.85 bar | Needs elevation |
| 105 °C (deaerator) | 1.2 bar | Deaerator must be raised several metres |
As the water nears boiling, almost all of the local pressure is vapour pressure, so there is very little margin left at the pump inlet. The height of the deaerator above the pump is what makes up the difference. Run the numbers with the NPSH and suction lift calculator.
Types of feed pump and control
- Multistage centrifugal — the standard choice. Several impellers in series build the high head boiler pressure demands, at steady flow.
- Positive-displacement — used on very small or high-pressure boilers where a centrifugal pump cannot reach the head; delivers a fixed volume per stroke.
How the pump matches the boiler’s changing demand is a separate choice:
- On/off level control — the pump runs flat out to refill, then stops. Simple, but it needs the larger flow margin and cycles the motor.
- Modulating feed valve — the pump runs continuously and a control valve trims the flow to hold a steady drum level. Smoother, and easier on the boiler.
- Variable-speed drive — the pump speed follows demand, saving energy and removing the need to throttle a valve. It also lowers the head at part load, so check the pump still meets boiler pressure at minimum speed.
Minimum flow and recirculation
A multistage pump must never run for long against a closed or nearly closed valve. With almost no flow, the energy it puts into the water turns to heat, the water in the casing gets hot, and the pump can be damaged in minutes. When the boiler turns down below the pump’s minimum continuous flow, fit a recirculation line back to the feed tank so a safe minimum always passes through. Variable-speed pumps still need this check at their lowest speed.
Practical sizing notes
- Two pumps, not one. Boiler codes generally require a standby feed pump so the boiler is never left without feed if one pump fails. Each pump is sized for the full duty.
- Size against safety-valve pressure. Using the normal working pressure leaves no head when the boiler is riding high, and the pump stalls just when it is needed.
- Do not oversize the head. A pump with far too much head runs hard against a throttled control valve, wastes energy and can overheat at low flow. Match the head to the real system.
- Watch the minimum flow. Multistage pumps need a minimum continuous flow to avoid overheating; fit a recirculation line if the boiler can turn down below it.
- Hot water, cool head. Keep the suction line short, straight and full-bore, and never throttle the pump inlet.
Key points to remember
- Flow = steam rate × 1.15 to 1.25 (up to 1.5 for on/off pumps).
- Head = boiler pressure (at safety-valve setting) + valve drop + friction + lift.
- 1 bar of pressure is about 10.2 m of head.
- Motor power (kW) = flow × head ÷ (367 × efficiency), plus a margin.
- Hot feed water makes NPSH the make-or-break check — keep the deaerator high.
For the suction side see the NPSH calculator, and for pump head in general read how to calculate pump head.
Frequently asked questions
How do you size a boiler feed pump?
Set the flow from the steam rate times 1.15 to 1.25, the head from the boiler pressure plus valve drop, friction and lift, and the motor power from the flow, head and pump efficiency.
What flow should a boiler feed pump have?
Between 1.15 and 1.25 times the maximum steam rate for a modulating pump, and up to 1.5 times for a single on and off pump.
What head does a boiler feed pump need?
Enough to beat the safety valve pressure (bar x 10.2 gives metres) plus the feed control valve drop, pipe friction and static lift.
Why do boiler feed pumps cavitate?
Feed water near 100 C flashes to steam at the pump inlet when the available NPSH is too low. Mounting the deaerator high provides the suction head that prevents it.
How many boiler feed pumps are needed?
At least two, each sized for the full duty, so the boiler is never left without feed water if one pump fails.