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centrifugal pump calculation examples

Centrifugal Pump Calculation Examples

Here are step-by-step worked examples covering key centrifugal pump calculations, including:

  1. Pump Power (P)

  2. Head (H)

  3. Flow Rate (Q)

  4. Specific Speed (Nₛ)

  5. NPSH (Net Positive Suction Head)


1. Pump Power Calculation

Problem:
Calculate the power required for a pump delivering 50 m³/h of water at 30 m head, assuming 80% efficiency.

Solution:
Use the pump power formula:

P=ρgQHηP=ηρ⋅g⋅Q⋅H

Where:

  • ρρ (water density) = 1000 kg/m³

  • gg (gravity) = 9.81 m/s²

  • QQ (flow rate) = 50 m³/h = 0.0139 m³/s

  • HH (head) = 30 m

  • ηη (efficiency) = 0.8

Calculation:

P=10009.810.0139300.8=5116 W=5.12kWP=0.81000⋅9.81⋅0.0139⋅30=5116 W=∗∗5.12kW∗∗

Conclusion: The pump requires ~5.1 kW of power.


2. Total Head Calculation

Problem:
A pump draws water from a 3 m deep well and discharges it to an open tank 25 m above. Friction losses are 2 m. Calculate the total dynamic head (TDH).

Solution:

TDH=Hstatic+Hfriction+HvelocityTDH=Hstatic+Hfriction+Hvelocity

  • Hstatic=25 m (discharge)+3 m (suction)=28 mHstatic=25 m (discharge)+3 m (suction)=28 m

  • Hfriction=2 mHfriction=2 m

  • HvelocityHvelocity (negligible for most cases)

TDH=28+2=30 mTDH=28+2=∗∗30 m∗∗

Conclusion: The pump must overcome 30 m total head.


3. Flow Rate from Pump Curve

Problem:
A pump curve shows:

  • At 2500 RPM, H = 40 m when Q = 0.02 m³/s.

  • If speed reduces to 2000 RPM, what’s the new Q and H?

Solution:
Use Affinity Laws:

Q1Q2=N1N2,H1H2=(N1N2)2Q2Q1=N2N1,H2H1=(N2N1)2

  • New speed ratio: 20002500=0.825002000=0.8

  • New flow: Q2=0.02×0.8=0.016m3/sQ2=0.02×0.8=∗∗0.016m3/s∗∗

  • New head: H2=40×(0.8)2=25.6mH2=40×(0.8)2=∗∗25.6m∗∗

Conclusion: At 2000 RPM, Q = 16 L/s, H = 25.6 m.


4. Specific Speed (Nₛ) Calculation

Problem:
A pump runs at 2900 RPM, delivers 100 m³/h at 50 m head. Find its specific speed (Nₛ).

Solution:

Ns=NQH3/4Ns=H3/4NQ

  • N=2900 RPMN=2900 RPM

  • Q=100 m³/h=0.0278 m³/sQ=100 m³/h=0.0278 m³/s

  • H=50 mH=50 m

Ns=2900×0.0278503/4=2900×0.16718.8=25.7Ns=503/42900×0.0278=18.82900×0.167=∗∗25.7∗∗

Conclusion:

  • Ns=25.7Ns=25.7 → Radial-flow impeller (typical for high-head pumps).


5. NPSH Available (NPSHₐ) Calculation

Problem:
A pump takes water from a 2 m deep tank (atmospheric pressure = 101.3 kPa). Pipe friction loss is 0.5 m, and vapor pressure is 2.34 kPa (at 20°C). Find NPSHₐ.

Solution:

NPSHa=PatmPvapρg+HstaticHfrictionNPSHa=ρgPatm−Pvap+Hstatic−Hfriction

  • Patm=101.3 kPa=101300 PaPatm=101.3 kPa=101300 Pa

  • Pvap=2.34 kPa=2340 PaPvap=2.34 kPa=2340 Pa

  • Hstatic=2 mHstatic=2 m

  • Hfriction=0.5 mHfriction=0.5 m

NPSHa=10130023401000×9.81+20.5=10.1+20.5=11.6mNPSHa=1000×9.81101300−2340+2−0.5=10.1+2−0.5=∗∗11.6m∗∗

Conclusion: The system provides 11.6 m NPSHₐ.


Summary of Key Formulas

ParameterFormula
Pump Power (P)P=ρgQHηP=ηρgQH
Total Head (TDH)TDH=Hstatic+HfrictionTDH=Hstatic+Hfriction
Affinity LawsQ1Q2=N1N2,H1H2=(N1N2)2Q2Q1=N2N1,H2H1=(N2N1)2
Specific Speed (Nₛ)Ns=NQH3/4Ns=H3/4NQ
NPSH AvailableNPSHa=PatmPvapρg+HsHfNPSHa=ρgPatm−Pvap+Hs−Hf

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