FieldEngineersKitEngineering Reference & ASME Code Basis
1. Core Formula & Variable Definitions
Classical pump affinity laws (rotodynamic similarity) · Hydraulic Institute / OEM impeller trim guidance (screening) · Typical TA practice: confirm VFD setpoints and shop cuts on the manufacturer curve
Q₂/Q₁ = (N₂/N₁)·(D₂/D₁)
H₂/H₁ = (N₂/N₁)²·(D₂/D₁)² · P₂/P₁ = (N₂/N₁)³·(D₂/D₁)³
Affinity screening · VFD speed change · impeller trim limits (OEM / HI guidance)
Classical affinity assumes geometric similarity and roughly constant efficiency. Use speed mode for VFD / RPM changes, diameter mode for shop impeller cuts during TA / shutdown, or combined when both change. Soft trim warning when D₂/D₁ < 0.80; hard warning below ~0.70. Confirm final duty on the OEM curve and recheck NPSH after large speed-up.
- N (Rotational speed) — Pump shaft speed (RPM). VFD / pulley / gear changes N₂.
- D (Impeller diameter) — Outside impeller diameter. Shop trim sets D₂ < D₁.
- Q (Capacity) — Volumetric flow at the compared duty point (m³/h or GPM).
- H (Head) — Total head at that capacity (m or ft).
- P (Brake power) — Shaft / brake power at that duty (kW or HP).
- D₂/D₁ (Trim ratio) — Diameter fraction after cut. Soft screen ~0.80; hard screen ~0.70.
2. Allowances, Tolerances & Standards
Affinity is a first-pass TA tool. Efficiency, NPSHr, and vibration limits are OEM-specific.
About 20% diameter cut — common OEM ceiling for many centrifugals.
Beyond typical screening envelope — do not machine without OEM approval.
Check motor power, max RPM, and NPSH after aggressive VFD moves.
Real η often drops slightly after trim or off-design speed — correct on the curve.
Quick Reference Lookup Table
| Case | Ratio | Focus |
|---|---|---|
| Speed 1480→1780 RPM · 50 m³/h · 25 m | N₂/N₁ ≈ 1.20 | Default VFD up-speed |
| Trim 250→230 mm · 50 m³/h · 25 m | D₂/D₁ = 0.92 | Typical shop cut |
| Trim 280→210 mm | D₂/D₁ = 0.75 | Soft trim warning band |
| Speed 1750→1450 RPM · 200 GPM | N₂/N₁ ≈ 0.83 | Imperial VFD turndown |
| Combined VFD + trim | (ND) product | TA both-change case |
3. Material & Code Limitations
Rotodynamic (centrifugal / mixed-flow) affinity screening. Not for PD pumps or multiphase duties.
| Material Group | Temperature Range | Allowable Stress / Limit | Engineering Notes |
|---|---|---|---|
| Centrifugal water / process pumps | N/A — kinematic similarity | Motor / shaft limits apply | Best when specific speed and geometry stay similar. |
| Deep impeller trims | N/A | OEM trim chart | Affinity over-predicts if the cut destroys similarity. |
| VFD above nameplate RPM | N/A | Max continuous RPM | Power rises with N³ — check driver and coupling ratings. |
Code Applicability & Safety Boundaries
- Field screening only — not an HI performance acceptance document.
- Efficiency assumed constant; correct power on the OEM curve.
- NPSHr roughly scales with speed² — recheck suction margin after speed-up.
- Export / PDF is a TA worksheet — not a stamped pump datasheet.
4. Step-by-Step Worked Example
Field VerificationShowHide
Step-by-Step Worked Example: VFD 1480 → 1780 RPM
During a TA review, raise a water pump from 1480 to 1780 RPM with the impeller diameter unchanged. Baseline duty: 50 m³/h, 25 m head, 5.5 kW brake power.
Speed ratio
Capacity & head
Brake power
Open /speed-1480-to-1780-rpm-50m3h-25m for the live default. Pair with TDH & NPSH before locking the VFD setpoint.
5. Frequently Asked Questions & Technical References
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For similar operation: Q ∝ N·D, H ∝ (N·D)², and P ∝ (N·D)³. Holding diameter fixed isolates VFD speed effects; holding speed fixed isolates impeller trim.
Many OEMs screen near 20% maximum diameter cut (D₂/D₁ ≈ 0.80). Toward 30% (≈ 0.70) typically needs manufacturer approval. This app soft-warns below 0.80 and hard-warns below 0.70.
Yes as a first pass. Confirm the predicted H₂ against the system curve, motor current, and NPSH. Pair with the TDH & Pump Power and NPSH calculators.
Because P ∝ (N·D)³ while Q ∝ N·D. A modest speed-up can demand a much larger driver margin than the flow increase suggests.