Boiler Feed Pump Calculator

Boiler Feed Pump Calculation: Capacity, Head & Power Explained

In brief, the boiler feed pump calculation can be reduced to 3 questions. How much water needs to be moved? What resistance is the pump working against? How much power does it take to do that? You can run your own boiler numbers with the calculator above for instant results. Enter steam rate, drum pressure, feedwater temperature, pipe losses and elevation. The calculator will provide you with required flow, total dynamic head, an NPSH cavitation check and motor power all in one.

This guide walks you through each boiler feed pump calculation by hand, in both SI and imperial units, so you can verify the output of the tool, or do a sizing problem yourself from scratch. Below the tool You will see the capacity formula with unit conversions, the full TDH formula with a pressure to head conversion box, an NPSH section on risk of cavitation in the deaerator, a power formula in kW and BHP, and a full worked example to bring it all together.

What a Boiler Feed Pump Calculation Involves

Each boiler feed pump calculation sizing exercise answers three related questions. First, capacity. How much water does the pump have to deliver taking into account the steam being generated, the water lost to blowdown and a safety margin for upset conditions? Second, head. What is the resistance (drum pressure, elevation, friction) the pump must overcome to be able to get that water into the boiler? Third, power: how much motor power does the pump really need for that flow and that head?

These three pieces are in order, capacity feeds into the head calculation and both feed into the power calculation. So it is worth working through them in sequence and not jumping straight to a horsepower number. Each is discussed below with its formula, a plain-English explanation, and a worked example.

Step 1: Feed Pump Capacity Calculation

The starting point for any boiler feed water pump sizing exercise is figuring out how much water the pump actually needs to move. That’s more than just the steam generation rate, since water is also lost to blowdown, and a safety margin is standard practice to cover upset conditions.

Feedwater Flow = Steam Generation Rate + Blowdown + Margin

Blowdown is water purposely drained from the boiler drum to control dissolved solids. This is generally 2% to 5% of the steam generation rate depending on water treatment and boiler design. Also, a margin of 15% to 25% is common practice to deal with load swings, control lag as well as future capacity, rather than sizing a pump with zero headroom.

Feed Pump Capacity Calculation: Unit Conversions

Steam rates are generally quoted in kg/h or t/h (SI) or lb/h (imperial). Pumps are rated in volumetric flow, m³/h, or US GPM. To convert between mass flow and volumetric flow, you need the density of water at the actual feedwater temperature, not cold-water density, because hot feedwater is much less dense than water at room temperature.

Volumetric Flow (m³/h) = Mass Flow (kg/h) ÷ Density at Feedwater Temperature (kg/m³). US GPM = m³/h × 4.403

Mini example: Boiler produces 10,000 kg/h steam, 3% blowdown, 20% margin.

Blowdown = 10,000 x 3% = 300 kg/h Base flow = 10,000 + 300 = 10,300 kg/h Required capacity = 10,300 kg/h × 1.20 = 12,360 kg/h

With feedwater at about 105°C, which is about 955 kg/m³ density for water (compared to about 998 kg/m³ at room temperature):

Volumetric flow = 12,360/955 ≈ 12.94 m³/h In US GPM: 12.94 x 4.403 ≈ 57 GPM

Step 2: Total Dynamic Head (TDH) Calculation

Once the flow is known, the next step is to determine the pump head with a boiler feed pump calculation, which is the total resistance that the pump must overcome, usually referred to as Total Dynamic Head (TDH). This is the total of the pressure the pump has to overcome, any change in elevation and friction losses in the piping, less any help with the boiler feed water pump sizing the pump gets from suction side pressure.

TDH = Discharge Pressure Head + Static Elevation Head + Friction Losses − Suction Pressure Head

The discharge pressure head is derived from the pressure in the boiler drum, plus a safety-valve margin. The pump has to operate against a little more than the normal operating pressure in the drum. The suction pressure head is the deaerator pressure and any static height the deaerator is above the pump. Both of these help the pump, not hinder.

Pressure-to-Head Conversion

Since drum pressure, deaerator pressure and friction losses are normally expressed in pressure units rather than head, conversion between the two requires the specific gravity of the water at its actual temperature (hot water has a specific gravity less than 1, unlike the SG of 1 normally assumed for cold water):

Head (m) = Pressure (bar) × 10.2 ÷ Specific Gravity Head (ft) = Pressure (psi) × 2.31 ÷ Specific Gravity

Mini example: Converting 10 bar(g) drum pressure to head at a feedwater specific gravity of 0.955 (about 105°C water).

Head = 10 x 10.2/0.955 ≈ 106.8 m

If the cold-water specific gravity (1.0) were used here instead of the actual hot-water value, the head would be understated by about 5%. This is a small-looking error, but it adds up once it is combined with every other term in the TDH calculation.

Step 3: NPSH, Avoiding Cavitation

NPSH (Net Positive Suction Head) is a different check than TDH and it’s arguably the one most likely to get skipped in a quick sizing exercise, even if you get it wrong, cavitation causes potentially serious damage to a pump over time.

NPSHa (available) = Absolute Suction Pressure Head + Static Suction Head − Friction Losses − Vapor Pressure Head

NPSH calculation is the actual amount of suction side pressure margin available at the pump inlet. This is compared to NPSHr (required) which is a value given by the pump manufacturer on the specific pump curve and not calculated from a general formula. The general rule of thumb is this:

NPSHa ≥ NPSHr + 0.5 to 1 m margin

The classic case of cavitation risk is deaerator-fed boiler feed pump calculation, and it is worth understanding why. The deaerator operates at the saturation temperature and pressure of the water so that the vapour pressure of the water is very close to the actual pressure at the pump suction. Because NPSHa subtracts vapour pressure head from available pressure head, these two terms are almost equal and opposite to one another, and the main thing that actually protects the pump from cavitation is the static height of the deaerator above the pump. That is why, also, deaerator installations are normally located well above the pump, on an elevated structure instead of at grade.

Step 4: Pump Power Calculation (kW)

The last step is to calculate with pump power calculation kW the amount of power the pump actually needs once we have flow and head. Hydraulic power is the theoretical minimum power required to move that flow against that head, and shaft power includes the pump’s own inefficiency.

Hydraulic Power (kW) = Q (m³/h) × H (m) × ρ (kg/m³) × 9.81 ÷ (3.6 × 10⁶) Shaft Power (kW) = Hydraulic Power ÷ Pump Efficiency

In imperial units, brake horsepower uses a similar structure:

BHP = GPM × Head (ft) × Specific Gravity ÷ (3,960 × Pump Efficiency)

Once the shaft power is known the motor is generally sized with an added 10% to 15% margin to the calculated shaft power and then rounded up to the next standard motor frame size available from the manufacturer, rather than specifying an oddly specific kW rating that does not match a real, off-the-shelf motor.

Complete Worked Example (Full Boiler Feed Pump Calculation)

These are the different boiler feed pump calculation applied to one realistic example. A 20 t/h boiler at 10 bar(g) drum pressure is fed from a deaerator at 0.2 bar(g) and 105°C. The static elevation between the pump and drum is 12 m, and the total friction losses at the discharge side of the pump is 1.5 bar. Some supporting values not stated in the base scenario (e.g., suction side friction, static suction height and pump efficiency) are assumed to be realistic, typical figures and are clearly labeled. These are needed from the real site layout, for real projects, and for the selected pump curve.

You can see that the absolute pressure head (12.95 m) and the vapor pressure head (12.90 m) are quite close together. This is due to the deaerator being at saturation. They nearly cancel each other out and the 5 m static suction height is the main factor that keeps the NPSHa positive. The manufacturer’s curve for this size pump indicates NPSHr ≈ 3.0 m, so the margin is about 1.52 m, well above the minimum recommended of 0.5 to 1 m, and this configuration is acceptable.

Step 1: Power

Assuming a pump efficiency of 65%, typical for a multistage pump at this duty:

Hydraulic power = 25.89 × 138.4 × 955 × 9.81 ÷ (3.6 × 10⁶) ≈ 9.3 kW Shaft power = 9.3 ÷ 0.65 ≈ 14.3 kW Motor rating with 15% margin = 14.3 × 1.15 ≈ 16.5 kW, typically rounded up to the next standard motor size (for example, 18.5 kW)

Common Mistakes in Boiler Feed Pump Sizing

There are some common mistakes made in boiler feedwater pump sizing that happen more often than they should. Using cold-water density instead of actual feedwater temperature underestimates required flow and head because hot water is less dense than the room-temperature values often pulled from a generic reference table. If the safety-valve margin on the discharge pressure is ignored, the pump will be unable to keep up exactly when it is most needed by the boiler during a pressure excursion. If you ignore economiser and other in-line equipment losses you are understating the total friction as these items add a significant amount of resistance over and above straight pipe runs. Undersizing the NPSH margin presents a risk of cavitation and premature pump wear, especially in deaerator-fed systems where the available margin is already much reduced. Finally, “just to be safe” oversizing the pump isn’t free. An oversized pump is often running to the left of its best efficiency point, which can increase vibration, wear and energy costs over the life of the equipment, rather than actually adding a useful safety margin.

Conclusion

Sizing a boiler feed pump calculation is really three calculations in one: how much water it has to move, how much resistance it has to overcome, and how much power that takes, with an NPSH check to make sure cavitation isn’t a risk along the way. This guide includes all of the formulas in both SI and imperial units, along with a full worked example that you can check by hand or re-create in a spreadsheet. All numbers in this document are educational estimates always check final pump selection with actual manufacturer’s pump curve, your specific piping layout and the codes and standards applicable to your installation. Boiler feed pump calculator on top Run your own numbers.

FAQs

Q1. How do you calculate boiler feed pump capacity? 

Add the steam generation rate, blowdown (usually 2% to 5% of steam rate) and a margin (usually 15% to 25%) to obtain the required mass flow. Convert to volumetric flow using the density of water at the actual feedwater temperature. For example, a 10,000 kg/h boiler with 3% blowdown and 20% margin requires about 12,360 kg/h feedwater capacity.

Q2. What head should a boiler feed pump have? 

Total dynamic head is the sum of the drum pressure (converted to head plus a safety-valve margin), the static elevation between the pump and the drum, and the friction losses in the piping, less the suction-side pressure head from the deaerator pressure and elevation. Each pump head calculation has to be converted from pressure to head, using the actual specific gravity of the water, not a generic cold water value.

Q3. How do I convert boiler pressure to pump head? 

Head (m) = Pressure (bar) × 10.2 / Specific Gravity (SI units) In imperial units: Head (ft) = Pressure (psi) * 2.31 / Specific Gravity. Always use the specific gravity of the water at the actual operating temperature, for hot feedwater has a specific gravity appreciably below 1.

Q4. What is NPSH and why does it matter for boiler feed pumps? 

NPSH (Net Positive Suction Head) is a measure of the pressure margin that is available on the suction side before the water starts to vaporize inside the pump. This can cause cavitation which can damage the pump over time. This is especially important for deaerator-fed systems as deaerator water is at or very close to saturation temperature with a very narrow margin that is very sensitive to the amount of elevation between the deaerator and the pump.

Q5. How much motor power does a boiler feed pump need? 

Calculate hydraulic power from flow, head and water density, then divide by pump efficiency to get shaft power, then add 10% to 15% margin and round up to next standard motor size. Rough Si formula = Hydraulic Power (kW) = Q (m³/h) x H (m) x ρ (kg/m³) x 9.81 / (3.6 x 10⁶) Shaft Power = Hydraulic Power / Efficiency

Q6. What safety margin should I add when sizing a boiler feed pump? 

Normal practice is to have a flow margin of 15-25% and an NPSH margin of 0.5-1 m over and above the pump’s required NPSH. Too little margin and the pump may not perform as expected in the field. Too much margin and you may be operating the pump away from its best efficiency point, which will lead to increased wear and energy consumption without a true safety benefit.