How to Size a Boiler Feed Pump: Flow Rate, Head, and Power Calculation

Boiler Feed Pump Sizing Overview

A boiler feed pump delivers feedwater to a steam boiler at sufficient pressure and flow to meet boiler steam generation demands. Proper sizing ensures the pump can supply flow against system backpressure without cavitation.

Step 1: Required Flow Rate

Q_min = Boiler Steam Output (lb/hr) × Specific Volume of Feedwater (ft³/lb)

Or in engineering units: Q (GPM) = Steam Output (lb/hr) / (500 × ρ_water)

For water at 212°F: ρ ≈ 60 lb/ft³, so: Q (GPM) ≈ Steam Output (lb/hr) / 500

Sizing factor: Apply a 10–25% safety margin above calculated minimum: Q_design = Q_min × 1.15 to 1.25

Example: Boiler generates 5,000 lb/hr of steam: - Q_min = 5,000 / 500 = 10 GPM - Q_design = 10 × 1.20 = 12 GPM

Step 2: Total Dynamic Head (TDH)

TDH = sum of all pressure increases the pump must provide:

TDH = Boiler Pressure Head + Static Head + Friction Head Losses − Suction Pressure

In simplified form: TDH (feet) = (Boiler Pressure − Feedwater Tank Pressure) × 2.31 / SG + Static Height + Friction

Where: - 2.31 = conversion from PSI to feet of water (at SG = 1.0) - SG = specific gravity of feedwater (≈ 1.0 for hot water near atmospheric) - Static Height = height difference between pump and boiler inlet (feet) - Friction = pressure drop in piping, valves, feedwater heaters (typically 10–15% of pressure head)

Example: Boiler at 150 PSI, feedwater deaerator at 5 PSI, static height = 20 ft, friction = 30 ft: TDH = (150 − 5) × 2.31 + 20 + 30 = 335 + 20 + 30 = 385 feet

Step 3: Pump Power (BHP)

BHP = (Q × TDH) / (3,960 × Pump Efficiency)

Where: - Q = flow in GPM - TDH = total dynamic head in feet - 3,960 = conversion constant for GPM, feet, HP - Pump efficiency = typically 0.60–0.75 for small centrifugal pumps, 0.75–0.85 for larger

Example: 12 GPM, 385 feet TDH, 65% pump efficiency: BHP = (12 × 385) / (3,960 × 0.65) = 4,620 / 2,574 = 1.79 HP

Select a motor rated for at least 2.5 HP (next standard size up, with service factor).

Net Positive Suction Head (NPSH) Check

NPSH_Required (from pump curve) must be less than NPSH_Available from the system:

NPSH_A = (Absolute suction pressure − Vapor pressure) × 2.31 / SG + Static suction head − Friction losses

If NPSH_A < NPSH_R by more than 10%, cavitation will occur — raising the suction tank pressure or lowering the pump elevation resolves this.

Feed Pump Selection Guidelines