Pump Motor Selection Calculation Worksheet

Pump Power Formula:

\[ Power (HP) = \frac{Flow Rate \times Head \times Specific Gravity}{3960 \times Efficiency} \]

Flow Rate:

GPM

Head:

feet

Specific Gravity:

dimensionless

Efficiency:

decimal (0-1)

Calculated Power:

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1. What is Pump Power Calculation?

The pump power calculation determines the required horsepower for a pump based on flow rate, head, fluid properties, and pump efficiency. Proper sizing ensures efficient operation and prevents motor overload.

2. How Does the Calculator Work?

The calculator uses the standard pump power formula:

\[ Power (HP) = \frac{Flow Rate \times Head \times Specific Gravity}{3960 \times Efficiency} \]

Where:

\( Flow Rate \) — Gallons per minute (GPM)
\( Head \) — Total dynamic head in feet
\( Specific Gravity \) — Density relative to water (1.0 for water)
\( Efficiency \) — Pump efficiency (typically 0.5-0.9)
3960 — Conversion factor (HP × ft-lb/min per HP)

Explanation: The formula accounts for the work needed to move fluid against gravity and system resistance, adjusted for pump efficiency.

3. Importance of Proper Pump Sizing

Details: Correct pump sizing prevents energy waste, reduces wear, and ensures reliable operation. Oversizing leads to inefficiency while undersizing causes premature failure.

4. Using the Calculator

Tips: Enter flow rate in GPM, head in feet, specific gravity (1.0 for water), and pump efficiency (0.5-0.9 typical). The calculator provides both calculated power and next standard motor size with 10% safety factor.

5. Frequently Asked Questions (FAQ)

Q1: Why add a safety factor to motor size?
A: A 10-20% safety factor accounts for system variations, aging, and ensures the motor isn't operating at maximum capacity continuously.

Q2: What are typical pump efficiencies?
A: Centrifugal pumps typically range from 50-85% efficiency. Positive displacement pumps can reach 90%. Consult manufacturer curves for specific pumps.

Q3: How do I determine total dynamic head?
A: TDH = Static Head + Friction Loss + Pressure Head. Must account for all system resistances including pipe friction, fittings, and elevation changes.

Q4: What if my fluid isn't water?
A: Adjust specific gravity (SG) - SG = fluid density/water density. For viscous fluids, additional corrections may be needed.

Q5: What are standard motor sizes?
A: Common NEMA sizes include: 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, 100, 125, 150 HP.