Boiler Feed Pump Calculation

Step 1 — required capacity

A feed pump is never sized at exactly the boiler's maximum continuous rating. Continuous blowdown removes 2–5% of the feed water, load swings need headroom, the drum level control valve throttles some of the flow away, and the pump loses performance as it wears. Standard practice is a 15–25% margin.

Mass flow (kg/h) = Steam flow × (1 + margin)
Volume flow (m³/h) = Mass flow ÷ density at feed water temperature

Density matters more than people expect. Water at 105°C is 954 kg/m³, not 1,000 — that alone adds nearly 5% to the volumetric duty compared with a cold-water assumption.

Step 2 — total dynamic head

Total dynamic head is the sum of three physically distinct components:

• Pressure head — the differential between drum pressure and deaerator pressure, converted to metres of the hot feed water. This is almost always the dominant term.
• Static head — the vertical lift from the pump centreline to the drum feed inlet.
• Friction head — losses through the discharge piping, isolation and non-return valves, the feed control valve and the economiser. On an industrial boiler this is often 10–20 m.

H_pressure (m) = Δp (bar) × 10⁵ ÷ (ρ × 9.81)
TDH = H_pressure + H_static + H_friction

Step 3 — power

Hydraulic kW = (ρ × 9.81 × Q × H) ÷ 3,600,000
Shaft kW = Hydraulic kW ÷ pump efficiency
Motor input kW = Shaft kW ÷ motor efficiency

Multistage boiler feed pumps typically run at 55–75% efficiency depending on size and specific speed; small industrial units sit at the bottom of that band. Motors are then selected one standard frame size above shaft power, giving roughly a 15% service margin.

Step 4 — the NPSH check that catches people out

NPSH_a = (P_{suction abs} − P_vapour) × 10⁵ ÷ (ρ × 9.81) + static suction height − suction friction loss

Here is the important part: deaerator water is at saturation temperature by design — that is the whole point of a deaerator. So the pressure term and the vapour pressure term cancel almost exactly, and NPSH available collapses to little more than the height of the deaerator above the pump minus suction losses. That is why deaerators are mounted 7–10 m up on a steel structure rather than sitting at ground level next to the pump.

Worked example

A 10 t/h boiler at 10 bar g, feed water at 105°C from a deaerator at 0.2 bar g:

• Mass flow with 20% margin = 12,000 kg/h
• Density at 105°C = 954.7 kg/m³, so volume flow = 12.57 m³/h
• Pressure head = (10 − 0.2) × 10⁵ ÷ (954.7 × 9.81) = 104.6 m
• Plus 12 m static and 15 m friction → TDH ≈ 131.6 m
• Hydraulic power = 954.7 × 9.81 × 12.57 × 131.6 ÷ 3.6×10⁶ = 4.30 kW
• At 62% pump efficiency, shaft power = 6.94 kW → select an 11 kW motor

Typical causes of feed pump problems