Boiler Feed Pump Calculator

A boiler feed pump takes water from the deaerator or feed tank, at low pressure, and pushes it into a boiler drum that may be at hundreds of psi. A boiler feed pump calculation answers two questions: how much head the pump must develop, and how much power that takes at the shaft and at the motor terminals. The calculator does both with the standard pump equations and shows each step, in US or metric units.

How to use the calculator

1. Flow. Choose the boiler steaming rate (lb/h, klb/h, kg/h, t/h or boiler horsepower) plus a flow margin, or enter the feedwater flow directly in gpm, m³/h, L/s or L/min.
2. Pressures. Enter the boiler drum pressure the pump must feed into and the deaerator (suction) pressure, both as gauge pressures in psi, bar or kPa.
3. Static head. The height of the drum water level above the deaerator water level. It is often negative, because deaerators are mounted high to give the pump suction head.
4. Friction losses. The total for suction and discharge, including the feed control valve, economizer and heaters, in ft or m.
5. Water. Pick the feedwater temperature (the density is looked up) or type a density.
6. Efficiencies. Pump efficiency from the curve at the duty point, and motor efficiency from the nameplate.

The result is the shaft (brake) power, with total dynamic head, motor input, hydraulic power, flow, density, pump differential pressure and wire-to-water efficiency beneath it.

Boiler feed pump head calculation by hand

Take a 400 boiler horsepower firetube boiler at 150 psig, a deaerator at 5 psig, feedwater at 220 °F, 20 ft of static lift, 35 ft of friction losses, a 60% efficient pump and a 92% efficient motor.

Flow (capacity): 400 BHP × 34.5 lb/h = 13,800 lb/h of steam. With a 15% margin, the pump must deliver 15,870 lb/h. Water at 220 °F has a density of about 955.1 kg/m³ (59.62 lb/ft³, or 7.97 lb/gal), so the flow is 15,870 ÷ 7.97 ÷ 60 ≈ 33.2 gpm.

Head: the pressure difference is 150 − 5 = 145 psi. At this density one psi is 144 ÷ 59.62 = 2.415 ft of water, so the pressure head is about 350.2 ft. Adding the static and friction head:

TDH = 350.2 + 20 + 35 = 405.2 ft (123.5 m)

Power: water horsepower = gpm × ft × SG ÷ 3,960 = 33.2 × 405.2 × 0.955 ÷ 3,960 ≈ 3.2 hp (2.42 kW). Dividing by the pump efficiency gives 2.42 ÷ 0.60 = 4.04 kW, or 5.4 hp at the shaft. Dividing again by the motor efficiency gives 4.04 ÷ 0.92 = 4.39 kW of electrical input.

The metric route gives the same answer: pressure head = ΔP ÷ (ρ·g), and hydraulic power = ρ·g·Q·H in watts.

Head per psi at feedwater temperatures

The familiar “2.31 ft per psi” is for cold water. Deaerated feedwater is hot and lighter, so each psi is worth more feet of head:

Densities are saturated liquid water from the NIST Chemistry WebBook (IAPWS-95). In a boiler feed pump calculation, using 2.31 for 300 °F water would understate the pressure head by about 8%.

Boiler feed pump sizing calculation in practice

• Capacity margin. Feed pumps are sized above the maximum steaming rate to cover continuous blowdown, spray attemperation and control-valve authority. The margin varies by plant and by code; use your design basis.
• Intermittent vs continuous feed. On/off level control on small boilers needs a pump that can refill faster than the boiler evaporates, so the margin is larger than for modulating feed valves.
• Pressure margin. Many specifications size discharge pressure above the safety-valve set point, not just the operating pressure, so the pump can still feed while the valves lift.
• NPSH. Hot, saturated water in the deaerator flashes easily. The static height of the deaerator above the pump provides the NPSH; check it against the pump’s NPSH required separately.
• Running cost. Multiply the motor input by operating hours to get energy use, then use the kWh calculator or the electricity cost calculator to price it.

Common mistakes

• Mixing absolute and gauge pressure. Use gauge on both sides, or absolute on both; the difference is what counts.
• Cold-water head factor. Using 2.31 ft/psi for 230 °F feedwater undersizes the head.
• Forgetting the feed valve. The control valve’s pressure drop at full flow is often one of the largest friction items.
• Applying the margin twice. If the flow already includes blowdown, set the margin to cover only what is left.
• Confusing brake horsepower and motor input. Size the motor on shaft power with a service margin; pay the utility for motor input.

Glossary

• TDH (total dynamic head): total energy per unit weight the pump adds, expressed as a height of liquid.
• Deaerator: heated tank that strips oxygen from feedwater and serves as the pump’s suction source.
• Boiler horsepower: 34.5 lb/h of water evaporated from and at 212 °F.
• Wire-to-water efficiency: pump efficiency × motor efficiency; the share of electrical input that reaches the water.

To work out a fluid’s density from mass and volume, use the density calculator.