Properly sizing a boiler feed pump requires more than knowing your boiler’s steam capacity. The pump must deliver sufficient feedwater while overcoming operating pressure, piping losses, and elevation changes.
A complete boiler feed pump calculation determines the required flow rate, total dynamic head, suction conditions, and horsepower. These measurements establish the preliminary operating requirements for selecting equipment.
Below, we’ll walk through the formulas and worked examples using a hypothetical industrial boiler producing 10,000 pounds of steam per hour at 150 psig.
Boiler feed pump sizing follows five steps:
Our example assumes 200ยฐF feedwater and steady-state operation. Actual sizing must also account for the feedwater-control arrangement and maximum operating conditions.
Your pump must replace the water leaving the boiler as steam and through blowdown. Calculating that total demand is the starting point for determining pump capacity.
Boiler blowdown removes water to control dissolved solids and maintain water quality. When blowdown is expressed as a percentage of total feedwater, use:
Formula:
Feedwater Demand = Steam Production รท (1 โ Blowdown Fraction)
Example Calculation:
For a boiler producing 10,000 lb/hr of steam with 5% blowdown:
10,000 รท (1 โ 0.05) = 10,526 lb/hr
What This Means:
The boiler requires approximately 10,526 pounds of feedwater per hour.
This calculation assumes blowdown is measured as a percentage of feedwater. If your facility expresses blowdown as a percentage of steam production, the calculation changes.
Condensate recovery reduces fresh makeup-water requirements but does not reduce the total feedwater the boiler must receive.
Pump capacity is typically measured in GPM. Converting feedwater mass flow into volumetric flow requires the water’s density at its operating temperature.
Formula:
Flow (GPM) = Feedwater Demand (lb/hr) รท [60 ร Density (lb/gal)]
Example Calculation:
At 200ยฐF, water density is approximately 8.02 lb/gal.
10,526 รท (60 ร 8.02) = 21.9 GPM
For illustration, applying a 10% capacity allowance:
21.9 ร 1.10 = 24.1 GPM
What This Means:
The preliminary design flow is approximately 24.1 GPM.
However, a 10% allowance is not appropriate for every boiler. Intermittent feedwater controls may require greater instantaneous pumping capacity than continuously modulating systems. Final flow requirements should reflect the actual operating arrangement and manufacturer guidance.
Flow determines how much water the pump delivers. Total dynamic head determines the energy required to move that water through the system.
Boiler operating pressure is only part of the calculation. The pump must also overcome elevation changes and hydraulic resistance from piping, valves, and feedwater equipment.
Total pump head combines pressure differential, elevation, and system losses.
Formula:
Total Head (ft) = Pressure Head (ft) + Elevation Difference (ft) + System Losses (ft)
Pressure head is calculated as:
Pressure Head (ft) = [Pressure Difference (psi) ร 2.31] รท Specific Gravity
These simplified relationships assume negligible velocity-head differences between the chosen reference points.
Example Calculation:
For our hypothetical boiler, assume:
First, calculate pressure head:
(150 ร 2.31) รท 0.961 = 360.6 ft
Then calculate total dynamic head:
360.6 + 10 + 25 = 395.6 ft
What This Means:
The pump must develop approximately 396 feet of head at the required flow.
The 25-foot loss assumption is illustrative. Actual calculations should include piping friction, fittings, control valves, economizers, and other equipment. Where applicable, account for the differential pressure needed across feedwater-control valves.
Pump performance curves commonly express pressure requirements in feet of head rather than psi.
Formula:
Pressure (psi) = [Head (ft) ร Specific Gravity] รท 2.31
Example Calculation:
Using the calculated head of 395.6 feet:
(395.6 ร 0.961) รท 2.31 = 164.5 psi
What This Means:
The pump requires approximately 164.5 psi of differential pressure under the assumed conditions.
This is not necessarily the pump’s discharge gauge pressure. Final sizing should account for actual suction pressure, maximum boiler pressure, and equipment operating requirements.
A pump can meet its required flow and discharge head while still experiencing cavitation from inadequate suction conditions.
Net positive suction head available (NPSHa) represents the pressure head available at the pump inlet above the liquid’s vapor-pressure head.
Hot feedwater requires special attention because higher temperatures raise vapor pressure.
Formula:
NPSHa = [(Pโ โ Pแตฅ) ร 2.31 รท SG] + Hโ โ H๐ป
Where:
Example Calculation:
Assume an atmospheric feedwater tank at sea level supplying 200ยฐF water:
NPSHa = [(14.7 โ 11.5) ร 2.31 รท 0.961] + 10 โ 2
NPSHa โ 15.7 ft
What This Means:
The system provides approximately 15.7 feet of NPSH available under these conditions.
Compare this value against the selected pump’s NPSH required (NPSHr) at its operating flow, with an appropriate margin. Also consider suction conditions and manufacturer performance data when evaluating cavitation risk.
Once flow and total head are established, you can estimate the mechanical power required to operate the pump.
Pump shaft horsepower depends on hydraulic duty and efficiency. This differs from boiler horsepower, which measures boiler steaming capacity.
Formula:
Pump HP = (GPM ร Head ร SG) รท (3,960 ร Pump Efficiency)
Pump efficiency must be expressed as a decimal.
Example Calculation:
Using our calculated requirements:
Pump HP = (24.1 ร 395.6 ร 0.961) รท (3,960 ร 0.65)
Pump HP โ 3.6 HP
What This Means:
The estimated pump shaft power is approximately 3.6 horsepower.
This does not establish the required motor nameplate rating. Motor selection must consider the manufacturer’s performance curve, maximum absorbed power, operating range, and applicable service requirements.
Our example establishes the following preliminary operating requirements:
These values provide a starting point for evaluating industrial boiler feed pumps.
However, a pump’s rated capacity alone does not determine whether it suits your facility.
Manufacturer performance curves should confirm that the selected pump delivers the required flow and head within an appropriate operating range. Efficiency, NPSH requirements, minimum continuous flow, and performance at reduced demand also matter.
Consider how the feedwater-control system operates. Intermittent pumping, modulating valves, variable-speed controls, and recirculation requirements can affect the appropriate equipment configuration.
Facilities with critical steam requirements may also benefit from duty/standby arrangements that allow feedwater delivery to continue during maintenance or pump failure.
Accurate boiler feed pump sizing starts with understanding your system’s actual operating requirements. Flow, pressure, suction conditions, and efficiency all influence the equipment needed for reliable feedwater delivery.
Whether you’re installing a new boiler, replacing an aging pump, or refurbishing an existing system, Industrial Boilers America can help evaluate your feedwater requirements and identify suitable equipment.
Contact our team to discuss boiler feed pump sizing and equipment options for your facility.
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