centrifugal pump curve calculator​

Centrifugal Pump Curve Calculator: How to Use & Interpret Results

A pump performance curve (or H-Q curve) shows the relationship between head (H), flow rate (Q), efficiency (η), and power (P). A pump curve calculator helps select the right pump for your system by simulating performance.

1. How a Pump Curve Calculator Works

A calculator uses affinity laws and manufacturer data to predict:

• Head vs. Flow (H-Q curve)

• Efficiency vs. Flow (η-Q curve)

• Power vs. Flow (P-Q curve)

Inputs Required:

✔ Flow Rate (Q) – Required capacity (e.g., m³/h, GPM).
✔ Total Head (H) – Pressure needed (e.g., meters, feet).
✔ Fluid Properties – Density, viscosity (water = 1 cP, 1000 kg/m³).
✔ Pump Speed (RPM) – Standard (1450/2900 RPM) or variable.
✔ Impeller Diameter – If trimming is considered.

2. Online & Software-Based Pump Curve Calculators

(Most manufacturers provide their own selection tools—check KSB, Xylem, Flowserve, etc.)

3. Manual Calculation Steps (Using Affinity Laws)

If no tool is available, you can estimate a pump curve using:

A. Affinity Laws (For Speed Change)

$$\frac{{\mathrm{<span\; style="font-size:\; 18px;">Q</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">1</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">Q</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">=</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">N</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">1</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">N</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">,</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">H</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">1</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">H</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">=</span>{<span\; style="font-size:\; 18px;">(</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">N</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">1</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">N</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">)</span>}^{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}<span\; style="font-size:\; 18px;">,</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">P</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">1</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">P</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">=</span>{<span\; style="font-size:\; 18px;">(</span>\frac{{\mathrm{<span\; style="font-size:\; 18px;">N</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">1</span>}}}{{\mathrm{<span\; style="font-size:\; 18px;">N</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">2</span>}}}<span\; style="font-size:\; 18px;">)</span>}^{\mathrm{<span\; style="font-size:\; 18px;">3</span>}}$$Q2Q1=N2N1,H2H1=(N2N1)2,P2P1=(N2N1)3

Where:

• $\mathrm{<span\; style="font-size:\; 18px;">Q</span>}$Q = Flow rate

• $\mathrm{<span\; style="font-size:\; 18px;">N</span>}$N = RPM

• $\mathrm{<span\; style="font-size:\; 18px;">H</span>}$H = Head

• $\mathrm{<span\; style="font-size:\; 18px;">P</span>}$P = Power

B. System Curve Calculation

$${\mathrm{<span\; style="font-size:\; 18px;">H</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">y</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">t</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">m</span>}}<span\; style="font-size:\; 18px;">=</span>{\mathrm{<span\; style="font-size:\; 18px;">H</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">t</span>}\mathrm{<span\; style="font-size:\; 18px;">a</span>}\mathrm{<span\; style="font-size:\; 18px;">t</span>}\mathrm{<span\; style="font-size:\; 18px;">i</span>}\mathrm{<span\; style="font-size:\; 18px;">c</span>}}<span\; style="font-size:\; 18px;">+</span>{\mathrm{<span\; style="font-size:\; 18px;">H</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">f</span>}\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">i</span>}\mathrm{<span\; style="font-size:\; 18px;">c</span>}\mathrm{<span\; style="font-size:\; 18px;">t</span>}\mathrm{<span\; style="font-size:\; 18px;">i</span>}\mathrm{<span\; style="font-size:\; 18px;">o</span>}\mathrm{<span\; style="font-size:\; 18px;">n</span>}}<span\; style="font-size:\; 18px;">+</span>{\mathrm{<span\; style="font-size:\; 18px;">H</span>}}_{\mathrm{<span\; style="font-size:\; 18px;">p</span>}\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">s</span>}\mathrm{<span\; style="font-size:\; 18px;">u</span>}\mathrm{<span\; style="font-size:\; 18px;">r</span>}\mathrm{<span\; style="font-size:\; 18px;">e</span>}}$$Hsystem=Hstatic+Hfriction+Hpressure

• Static head (vertical lift).

• Friction head (pipe losses, calculated via Darcy-Weisbach).

C. Operating Point

• Intersection of pump curve and system curve = best efficiency point (BEP).

4. Example Calculation

Given:

• Required flow = 100 GPM

• Required head = 50 ft

• Pump speed = 1750 RPM

Steps:

1. Find a pump curve (e.g., from manufacturer datasheet).

2. Locate 100 GPM on the curve → Check if head ≥ 50 ft.

3. Check efficiency (e.g., 70% at BEP).

4. Calculate power:

   $$\mathrm{<span\; style="font-size:\; 18px;">P</span>}<span\; style="font-size:\; 18px;">=</span>\frac{\mathrm{<span\; style="font-size:\; 18px;">Q</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">H</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">\rho </span>}}{\mathrm{<span\; style="font-size:\; 18px;">3960</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">\eta </span>}}<span\; style="font-size:\; 18px;">(</span>\text{<span style="font-size: 18px;">in HP</span>}<span\; style="font-size:\; 18px;">)</span>$$P=3960×ηQ×H×ρ(in HP)

• $\mathrm{<span\; style="font-size:\; 18px;">P</span>}<span\; style="font-size:\; 18px;">=</span>\frac{\mathrm{<span\; style="font-size:\; 18px;">100</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">50</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">1</span>}}{\mathrm{<span\; style="font-size:\; 18px;">3960</span>}<span\; style="font-size:\; 18px;">\times </span>\mathrm{<span\; style="font-size:\; 18px;">0.7</span>}}<span\; style="font-size:\; 18px;">\approx </span>\mathrm{<span\; style="font-size:\; 18px;">1.8</span>}\text{<span style="font-size: 18px;"> HP</span>}$P=3960×0.7100×50×1≈1.8 HP

5. Key Takeaways

Match pump curve to system curve for optimal efficiency.
Avoid operating far from BEP (causes cavitation/overload).
Use VFDs to adjust speed and shift the curve.
Consider NPSH (Net Positive Suction Head) to prevent cavitation.

Free Online Calculators:

• Hydraulic Institute (HI) Pump Tools

• Engineering Toolbox Pump Calculator

• Grundfos WebCAPS

Would you like help interpreting a specific pump curve?

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