Pump Affinity Laws Calculator - Speed, Impeller & Source Gaps
Calculate flow, head, and power changes from pump speed or impeller diameter adjustments
Use local pump affinity-law arithmetic to screen how a speed ratio or impeller-diameter ratio changes entered flow, head, and shaft horsepower. The screen also keeps the source boundaries visible: actual operation depends on the pump curve, system curve, static head, efficiency, minimum flow, NPSH margin, fluid properties, manufacturer trim curves, motor/VFD limits, utility tariff, and qualified review. The energy line is a simple local prompt from entered HP, hours, and $/kWh; it is not a payback model, energy audit, or VFD approval.
Verify pipe pressure drop at new flow rate
Pipe Pressure Drop Calculator →Size belt drive for speed change without VFD
Belt Drive Calculator →Check motor efficiency at reduced speed
Motor Efficiency Calculator →Review pump affinity source boundaries
Pump Affinity Source Guide →How It Works
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Enter Current Operating Conditions
Input the pump's current flow rate (GPM), total head (feet), shaft horsepower, and operating speed. These values should come from a pump curve, field measurement, or documented local review basis.
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Choose Speed or Impeller Ratio
Select whether the local prompt uses a speed ratio (N2/N1) or impeller diameter ratio (D2/D1). The screen does not replace manufacturer VFD limits, sheave review, or impeller trim curves.
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Review Local Affinity Outputs
Review the local flow, head, and shaft-power prompts. Flow uses ratio to the first power, head uses ratio squared, and shaft power uses ratio cubed.
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Treat Energy as a Prompt
Enter electricity cost and annual hours to display a simple local energy-cost prompt from shaft horsepower. Actual savings still require motor/VFD efficiency, tariffs, load profile, controls, installed cost, and metered verification.
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Document Source Gaps
Verify the pump curve, system curve, static head, efficiency, operating region, NPSH margin, minimum flow, OEM data, electrical safe-work controls, and qualified review before acting.
Built For
- Plant teams documenting a first-pass source gap before VFD or sheave review
- Pump technicians screening an impeller-trim prompt before checking manufacturer trim curves
- HVAC teams comparing local speed prompts against chilled-water or condenser-water pump curves
- Municipal water operators gathering pump/system curve and minimum-flow review items
- Reliability teams checking whether a proposed speed change needs NPSH or operating-region review
- Energy auditors separating local cube-law prompts from measured duty-cycle and tariff analysis
- Process teams reviewing speed control, throttling, bypass, and control-valve questions before engineering review
Features & Capabilities
Affinity Law Equations
Applies local Q2/Q1 = ratio, H2/H1 = ratio^2, and P2/P1 = ratio^3 prompts for speed-ratio or impeller-ratio review.
Energy-Cost Prompt
Displays a simple entered-HP, hours, and $/kWh energy prompt while warning that motor/VFD efficiency, tariffs, load profile, demand charges, installed cost, and verification are outside the screen.
Source-Boundary Warnings
Calls out pump curve, system curve, static head, NPSH margin, minimum flow, operating region, VFD/motor, OEM, OSHA, NFPA 70E, and qualified-review gaps.
Impeller Trim Boundary
Treats impeller diameter changes as local prompts and warns that manufacturer trim curves and pump geometry determine actual performance.
Large-Ratio Review Prompts
Flags low-speed, speed-increase, large-ratio, and large-trim conditions as review triggers rather than pass/fail results.
Comparison
| Review Item | What the Local Screen Shows | What Still Needs Source Review | Typical Decision Owner |
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| Speed ratio | Flow/head/power prompts from N2/N1 | System curve, static head, NPSH, minimum flow, motor/VFD limits | Pump/electrical review |
| Impeller ratio | Flow/head/power prompts from D2/D1 | Manufacturer trim curve, geometry, efficiency, radial load, balance | Pump OEM or qualified shop |
| Energy-cost prompt | Entered HP-to-kW, hours, and $/kWh arithmetic | Measured duty cycle, tariffs, demand charges, VFD/motor losses, cost | Energy auditor or engineer |
| Safe work | Warning prompts only | LOTO, electrical work practices, guards, permits, OEM instructions | Employer and qualified safety/electrical staff |
Assumptions
- Entered values represent a local review basis for the same centrifugal pump design.
- The screen shifts local flow, head, and shaft power by the selected speed or impeller ratio only.
- Energy-cost output uses entered shaft HP, 0.746 HP-to-kW, entered hours, and entered $/kWh.
- Pump/system curves, static head, efficiency, NPSH, minimum flow, trim curves, and VFD/motor losses remain outside the arithmetic.
- Safe work requires current OEM, employer, OSHA, NFPA 70E, and qualified review.
Limitations
- Does not apply to positive displacement pumps (gear, piston, diaphragm, progressive cavity).
- Does not calculate a pump/system curve intersection or static-head dominated behavior.
- Does not approve impeller trimming, VFD operation, sheave changes, controls, or motor loading.
- Does not determine minimum flow, suction recirculation, cavitation, NPSH margin, vibration, pressure class, or relief limits.
- Does not model motor efficiency, VFD efficiency, harmonics, bearing currents, cable effects, tariffs, demand charges, installed cost, incentives, or payback.
References
- DOE Continuous Energy Improvement in Motor Driven Systems - affinity-law equations and system-curve caveats.
- DOE Variable Speed Pumping: A Guide to Successful Applications - static-head and variable-speed application boundaries.
- DOE/HI Improving Pumping System Performance sourcebook - pump-system assessment context.
- Hydraulic Institute standards and resources - current authorized pump standard review.
- OSHA 1910.147, OSHA 1910.333, and NFPA 70E - hazardous energy and electrical safe-work context.