Pump Affinity Calculator Formula
Understand the math behind the pump affinity calculator. Each variable explained with a worked example.
Formulas Used
New Flow Rate (Q2)
flow2 = flow1 * ratioNew Head (H2)
head2 = head1 * pow(ratio, 2)New Power (P2)
power2 = power1 * pow(ratio, 3)Variables
| Variable | Description | Default |
|---|---|---|
flow1 | Original Flow Rate (Q1)(m³/h) | 100 |
head1 | Original Head (H1)(m) | 30 |
power1 | Original Power (P1)(kW) | 15 |
speed1 | Original Speed (N1)(RPM) | 1450 |
speed2 | New Speed (N2)(RPM) | 1750 |
ratio | Derived value= speed2 / speed1 | calculated |
How It Works
Pump Affinity Laws
The affinity laws predict how centrifugal pump performance changes with rotational speed or impeller diameter. They are essential for variable-speed pump sizing.
Speed Change Laws
Q2/Q1 = N2/N1 (flow scales linearly)
H2/H1 = (N2/N1)² (head scales with speed squared)
P2/P1 = (N2/N1)³ (power scales with speed cubed)
The same relationships apply for impeller diameter changes (replacing N with D). The cubic power relationship makes VFDs very effective for energy savings.
Worked Example
A pump at 1450 RPM delivers 100 m³/h at 30 m head, consuming 15 kW. Speed increased to 1750 RPM.
flow1 = 100head1 = 30power1 = 15speed1 = 1450speed2 = 1750
- 01Speed ratio = 1750 / 1450 = 1.2069
- 02Q2 = 100 × 1.2069 = 120.7 m³/h
- 03H2 = 30 × 1.2069² = 30 × 1.4566 = 43.7 m
- 04P2 = 15 × 1.2069³ = 15 × 1.7581 = 26.37 kW
Ready to run the numbers?
Open Pump Affinity Calculator