Pump Motor Capacity Calculation Worksheet

Pump Motor Capacity Formula:

\[ \text{Capacity (HP)} = \frac{\text{Flow Rate (GPM)} \times \text{Head (feet)} \times \text{Specific Gravity}}{3960} \]

Pump Motor Capacity:

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1. What is Pump Motor Capacity?

Pump motor capacity refers to the horsepower (HP) required to move a fluid at a specified flow rate against a given head, accounting for the fluid's specific gravity. It's essential for proper pump selection and system design.

2. How Does the Calculator Work?

The calculator uses the standard pump capacity formula:

\[ \text{Capacity (HP)} = \frac{\text{Flow Rate (GPM)} \times \text{Head (feet)} \times \text{Specific Gravity}}{3960} \]

Where:

Flow Rate — Gallons per minute (GPM) of fluid being pumped
Head — Total dynamic head in feet that the pump must overcome
Specific Gravity — Ratio of fluid density to water density (water = 1.0)
3960 — Conversion factor to horsepower

Explanation: The formula calculates the theoretical horsepower required, not accounting for pump efficiency which would require additional division by the pump efficiency factor.

3. Importance of Pump Capacity Calculation

Details: Proper pump sizing is critical for energy efficiency, system reliability, and preventing motor overload. Undersized pumps won't meet system demands while oversized pumps waste energy and may cause operational issues.

4. Using the Calculator

Tips: Enter flow rate in GPM, head in feet, and specific gravity (1.0 for water). For viscous fluids or systems with significant friction losses, consider adding a safety factor to the calculated capacity.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between theoretical and actual pump horsepower?
A: Theoretical HP doesn't account for pump efficiency. Actual HP = Theoretical HP / Pump Efficiency (typically 50-85% for centrifugal pumps).

Q2: How do I account for pump efficiency?
A: Divide the calculated HP by the pump's efficiency (as decimal, e.g., 0.75 for 75% efficiency) to get required motor HP.

Q3: What if my fluid has high viscosity?
A: High viscosity fluids require additional power. Consult pump performance curves or manufacturer data for viscous corrections.

Q4: Should I round up the calculated HP?
A: Yes, standard practice is to round up to the next available motor size (e.g., 5.3 HP → 7.5 HP motor).

Q5: What about NPSH requirements?
A: This calculator determines power requirements only. Net Positive Suction Head (NPSH) must be checked separately to avoid cavitation.