Pump Affinity Laws Calculator: Speed and Impeller Trim
- Sullair dealer since 1984
- Serving industry since 1978
- No form fill, nothing gated
What this calculator tells you
Change a pump's speed or trim its impeller and the whole curve moves. Predict the new flow, head, and horsepower before you commit, and see what the power change is worth per year.
Your Pump
Results
Annual energy savings:
$0 per year
Ratio (new / current): 0
New flow: 0 GPM
New head: 0 feet
New power: 0 BHP (was 6)
Slowing a pump 10% cuts its power draw about 27%. If a VFD or trim looks attractive here, we can confirm it against your actual system curve.
How this is calculated
- Ratio r = new / current (speed RPM or impeller diameter).
- Flow: Q2 = Q1 x r.
- Head: H2 = H1 x r².
- Power: P2 = P1 x r³.
- Annual dollar change = (P1 - P2) x 0.746 x hours x rate. This assumes the BHP you entered is at the motor shaft; motor losses would adjust the wire-side number slightly.
- The affinity laws assume the system curve stays put and efficiency holds, which is close for modest changes. Large changes need a check against the published curve.
Estimates only. Actual results depend on your equipment and operating conditions. Call 800.825.3222 for exact sizing.
Common questions
What are the pump affinity laws?
For a change in speed or impeller diameter with ratio r = new / current: flow changes in proportion to r, head changes with r squared, and power changes with r cubed. A 10 percent speed reduction cuts flow 10 percent, head about 19 percent, and power about 27 percent, which is why slowing a pump down with a VFD saves so much energy.
How much can you trim a pump impeller?
Manufacturers typically publish curves down to about 80 to 85 percent of the full impeller diameter. Trimming more than about 15 percent moves you off the published curve: efficiency falls faster than the affinity laws predict and the pump may not make the head you expect. Verify large trims with the manufacturer before cutting metal.
Is a VFD or an impeller trim better for reducing pump output?
A trim is a cheap one-time fix when the pump is permanently oversized for the duty. A VFD costs more up front but adjusts continuously, captures the cube-law savings at every operating point, and adds soft-start benefits. The right answer depends on how much the demand varies, so check the numbers against your actual system curve.
Want a tech to verify these numbers at your facility?
Request received. If you need someone now, call 800.825.3222.
Please fill in all required fields (name, company, email, phone, preferred contact method, ZIP, and city), or call 800.825.3222.
These are estimates. Your pump curve has the real answer.
Our pump team checks flow, head, and NPSH against the manufacturer curve before anything is quoted. Since 1978, across 14 Midwest locations.