Boiler Feed Pump Calculation

How to use it

1. Pick a mode. Quick estimate needs steam output, boiler pressure, feedwater temperature, tank pressure and two water levels. Advanced adds blowdown, spray, recirculation, margins, running pumps, line friction and your pump’s NPSH requirement.
2. Choose metric or US units. Switching converts the numbers already in the boxes, so 10 bar g becomes 145 psi g rather than 10 psi g.
3. Enter your plant’s values. Pressures are gauge unless the label says absolute. Water levels are measured from the pump centreline.
4. Read the duty point, then the working. The cards give flow, differential pressure, head, power and NPSH. The tables show how each one was built up, and Show the working lists every formula with your numbers in it.

The method, step by step

A boiler feed pump has to deliver more water than the boiler turns into steam, at a pressure higher than the boiler’s. The calculation follows that order.

1. Flow. Start from maximum steam output. Add blowdown, since it leaves as water: feedwater = steam ÷ (1 − blowdown fraction). Add spray and any other users of the pump. If a minimum-flow recirculation line stays open, the pump carries that too. Then apply your design margin and split across the pumps that run together.
2. Discharge pressure. Boiler pressure, plus every drop between the pump and the drum: economiser, feed control valve, heaters, check valves, line friction. Then add the static head from the pump up to the drum water level.
3. Suction pressure. Tank pressure in absolute terms, plus the water level above the pump, minus suction line friction.
4. Differential pressure and head. Δp = discharge − suction. Head is Δp ÷ (ρ × g), using the density of the water at its actual temperature.
5. Power. Hydraulic power = volume flow × Δp. Shaft power = hydraulic ÷ pump efficiency. Motor input = shaft ÷ motor efficiency. The minimum motor rating adds your margin on top of shaft power.
6. NPSH available. (Tank pressure − vapour pressure) ÷ (ρ × g), plus water level, minus suction losses. Compare it with the pump’s NPSH required plus the margin your pump supplier asks for.

Water density and vapour pressure come from IAPWS-IF97, the industrial standard for water and steam properties. The engine reproduces the standard’s published verification values, and the page’s tests check them on every change.

Why feedwater temperature changes the head

A pump makes head, and the pressure that head produces depends on how dense the water is. Hot water is lighter, so every bar of pressure rise takes more metres of head. Using the cold-water figure of about 10.2 m per bar understates the head a hot feed pump has to make.

At 150 °C the same pressure rise needs 9% more head than at 20 °C. Pump curves are drawn in head, so that 9% is the difference between a pump that reaches the drum and one that doesn’t.

Power is different. It depends on flow and pressure, not on head, so this calculator works it out as Q × Δp. That way temperature can’t creep into the power figure through the density.

NPSH, and the deaerator trap

NPSH available is how far the pressure at the pump inlet sits above the point where the water would boil, expressed as head. If the pump needs more than that, the water flashes in the impeller eye and the pump cavitates.

Feedwater is hot, so vapour pressure is large and can’t be ignored. At 90 °C it’s 0.70 bar. Take a tank open to the atmosphere with 5 m of water above the pump. NPSH available is 8.29 m before suction losses, and nearly all of it comes from the water level.

A deaerator takes this to the limit. It holds the water at saturation, so tank pressure and vapour pressure are equal and cancel out. NPSH available is just the water level above the pump, minus suction losses. Raising the deaerator pressure doesn’t help, because the water temperature rises with it. That’s why deaerators sit high above their feed pumps.

If you enter feedwater hotter than the boiling point at the tank pressure, the calculator refuses. Water at 150 °C in a tank at atmospheric pressure would flash, and that has no valid NPSH figure.

Boiler horsepower and the 0.069 rule

One boiler horsepower is 34.5 lb/h of steam from and at 212 °F, or 15.65 kg/h. So 100 boiler hp is 1,564.9 kg/h of evaporation. Choose boiler hp as the steam output unit and the calculator converts it for you.

US pump makers turn that into a rule of thumb: 0.069 US gpm per boiler horsepower, since 34.5 lb/h of water is about 0.069 gpm. They then multiply for control type:

Quick mode applies these same multipliers to mass flow. It then converts to volume at the real feedwater density instead of the 8.34 lb/gal the rule assumes, so its gpm comes out a little higher for hot water.

Shutoff head and the safety valve

The pump has to be able to feed the boiler even when the boiler is at its safety valve setting. A common practice is for pump pressure at zero flow (shutoff) to be at least 3% above the lowest safety valve set pressure. That lets the pump still feed the boiler while the valve is lifting.

Advanced mode works out the minimum shutoff head from that rule, the drum water level and the suction conditions. Compare it with the shutoff head on the pump curve. Your boiler code may set a different rule, and where it does, the code applies.

Worked example: two pumps feeding a 50 t/h boiler

These are the Advanced mode’s example values, so you can load them with Reset and follow along.

• Steam: 50,000 kg/h, 2% continuous blowdown, 1,500 kg/h attemperation spray
• Pumps: 2 running together, 10% recirculation always open, 10% flow margin
• Boiler: 45 bar g operating, safety valve 50 bar g, drum water level 25 m above the pumps
• Discharge drops: economiser 1.5 bar, control valve 3 bar, other 0.5 bar, plus 120 m of 102.3 mm bore pipe with fittings totalling K = 12
• Suction: deaerator at 0.3 bar g, saturated, water level 12 m above the pumps, 20 m of 154.1 mm bore pipe with K = 4
• Pump and motor: 72% and 95% efficient, 10% motor margin, 5% head margin, NPSHr 4 m, 1 m NPSH margin wanted

The deaerator is saturated at 107.4 °C, where water weighs 952.9 kg/m³.

Friction comes to 6.82 m in the discharge line at 2.05 m/s and just 0.08 m in the suction line at 0.50 m/s. The pumps need 52.97 bar g at discharge and see 2.43 bar a at suction.

NPSH available is 11.92 m, and 12 m of it is water level. That’s the deaerator case: the 0.3 bar g of tank pressure contributes nothing, because the water is already at its boiling point.

Preliminary estimate vs final pump selection

This calculator gives you a duty point. That’s where a pump enquiry starts, not where it finishes. A pump supplier or engineer still needs to check:

• The pump curve. The duty point should land near best efficiency, and shutoff head has to clear the safety valve rule.
• NPSH required at your flow. It changes along the curve, and the margin you need depends on the pump and the application. ANSI/HI 9.6.1 gives guidance.
• Minimum continuous flow. This sets the recirculation line’s size and whether it stays open.
• Materials and temperature limits for your feedwater chemistry and temperature.
• Motor selection from real frame sizes, plus the starting method and supply. The calculator only gives a minimum rating.
• Codes. Boiler and pressure-equipment codes set requirements for feed capacity, pressure and redundancy that differ between jurisdictions.

What this does not cover

• Pump curves and parallel-running interaction. Two pumps running together share the flow evenly here. On a real system curve they don’t always.
• Transients. Sudden load swings, deaerator pressure decay and pump trips all eat into NPSH faster than steady-state figures show.
• Velocity head at the pump nozzles. It’s small at normal feed velocities and depends on nozzle sizes the calculator doesn’t know.
• Standard motor sizes. Frame lists vary by standard and maker, so the page gives a minimum rating, not a size to order.

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