Boiler Feed Pump Recirculation Valve Calculator

What Does This Recirculation Calculator Do?

Boiler feed pump is a multistage centrifugal pump. When boiler load drops, feed demand drops. If pump runs at very low flow, water inside pump gets hot, starts to flash and damages impeller and seal. It also vibrates and cavitates.

A recirculation valve, also called bypass valve or automatic recirculation valve (ARV), sends a part of discharge water back to deaerator or feed tank. This keeps total flow through pump above minimum, even if boiler takes little water. This calculator finds that minimum flow and sizes the valve.

Recirculation Flow Formula

Thermal: Q_thermal = (P_heat × 3600) / (rho × Cp × ΔT_allow)
Stable: Q_stable = Q_BEP × Min Stable %
Required: Q_rec = max(Q_thermal, Q_stable)

Valve: Kv = Q_rec × sqrt(SG / ΔP) , Cv = Kv × 1.156 P_heat = 0.6 × P_BEP (shutoff power), rho 958 kg/m³ at 105°C, Cp 4.22 kJ/kgK, SG 0.958 ΔP = Discharge pressure - Deaerator pressure (bar)

Thermal flow stops overheating. Stable flow stops vibration. You must meet both, so use the larger value.

Rule of thumb: For most boiler feed pumps, minimum continuous flow is 25% to 40% of BEP. High pressure pumps with high head need 30% or more. If maker does not give value, use 30% and check thermal calc.

How to Use the Calculator

Enter BEP point. Rated flow, head and shaft power from pump curve, not motor nameplate.
Set efficiency at low flow and temp rise. Efficiency drops to 40-55% at low flow. Allow 5-8°C rise.
Set stable %. Use maker value if given, else 30%.
Enter pressures. Discharge pressure minus deaerator pressure gives valve drop. Higher drop needs smaller valve.
Click Calculate Min Flow. You get required recirculation flow, Kv and suggested size.

Understanding Results

Thermal Min Flow

Flow needed to carry away heat at low load. For small pump 22 kW, it is only 2-4 m³/h. For large 200 kW pump, it is 20 m³/h or more.

Stable Min Flow

Flow needed to avoid recirculation inside impeller and vibration. For 50 m³/h BEP at 30%, it is 15 m³/h. This is usually bigger than thermal flow and controls.

Valve Kv / Cv

Kv is metric valve size. Kv 5 means 5 m³/h flow at 1 bar drop. Cv is US version, 1.156 times Kv. Use this to pick valve trim from maker table. Do not pick valve by pipe size only.

Suggested Valve Size

Estimate from Kv table: Kv up to 6 = DN25, Kv 6-12 = DN32, Kv 12-20 = DN40, Kv 20-35 = DN50. Verify with valve maker for high pressure drop and cavitation trim.

Worked Example

Feed pump BEP 50 m³/h, 120m, 22 kW, efficiency at low flow 48%, allowed rise 7°C, stable 30%, discharge 14 bar g, deaerator 0.2 bar g.

Step	Calc	Result
Heat power	0.6 × 22	13.2 kW
Thermal flow	13.2×3600 / (958×4.22×7)	1.68 m³/h
Stable flow	50 × 30%	15.0 m³/h
Required recirc	max(1.68, 15.0)	15.0 m³/h
Valve ΔP	14 - 0.2	13.8 bar
Kv	15 × sqrt(0.958/13.8)	3.95
Cv	3.95 × 1.156	4.57

So set ARV to open at about 15 m³/h total pump flow. Valve Kv 3.95 fits DN25 trim. Add 10% margin, so select Kv 4.4 valve.

Recirculation and Feed Pump Sizing

Recirculation flow is part of pump duty. When you size feed pump, add recirculation to boiler demand. Our boiler feed pump calculation tool gives you flow, TDH, NPSH and power at full load. Use that BEP flow as input here, then add recirculation flow to check if pump still meets boiler plus bypass at low load.

ARV vs simple bypass: ARV opens automatically when main flow falls. Simple bypass with orifice is fixed and wastes energy at high load. Use ARV for variable load boiler.
Where to return: Return to deaerator or feed tank, not to pump suction directly, unless you have cooler. Hot return to suction can flash.
Deadhead protection: If boiler stop valve closes, pump runs on recirculation only. Without recirc, pump deadheads in 2-3 minutes.
VFD does not remove need: VFD slows pump at low load and cuts recirc need, but pump still needs min flow at that speed. Recirc still needed for startup and trip.

Common Mistakes

Using motor kW instead of shaft power: Motor rating is larger. Use curve power at BEP.
Sizing valve by line size: Recirc line is smaller than main line. Size by Kv, not pipe DN.
Ignoring drop: At high drop, valve cavitates and trims wear. Use multi-stage trim for drop above 10 bar.
Setting ARV too low: If ARV opens at 10% instead of 30%, pump still vibrates and heats up.

Frequently Asked Questions

What is minimum flow for boiler feed pump?

Usually 25% to 40% of BEP flow. Check pump maker curve. If not given, use 30% and verify thermal rise stays below 7°C.

What is a recirculation valve?

A valve on pump discharge that sends flow back to feed tank when boiler demand is low. It keeps flow through pump above minimum and prevents overheating and cavitation.

How to calculate recirculation flow?

Thermal flow = heat power / (density × Cp × allowed temp rise). Stable flow = BEP × stable %. Required flow is the larger of the two.

Do I need ARV if I have VFD?

Yes, still needed for startup, trip and very low speed. VFD reduces recirc flow, but does not remove need for protection.

Basis: Hydraulic Institute pump minimum flow method, KSB and Sulzer feed pump guides, and ARV maker data (Schroedahl, ClydeUnion). Thermal method uses shutoff power 60% of BEP and feedwater at 105°C. For final selection check pump maker min flow curve and valve maker Kv table for high pressure drop.