ISO 9906 Pump Performance Calculator — H-Q Curve, NPSH & Affinity Laws
ISO 9906 — Rotodynamic pumps — Hydraulic performance acceptance tests — Grades 1, 2 and 3
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ISO 9906 is the international standard for hydraulic performance acceptance tests of rotodynamic pumps — centrifugal, mixed-flow and axial-flow types. It defines the measurement conventions, grade tolerances (Grades 1, 2 and 3) and the agreed formula for available net positive suction head (NPSHa) used to assess cavitation risk at the pump inlet. ISO 9906:2012 supersedes the 1999 edition and consolidates the former Grades 1 and 2 with an additional Grade 3 for less demanding industrial applications.
MechanixCalc does NOT run ISO 9906 acceptance tests — no tolerance grade is implemented, and nothing here compares an as-built pump against a guarantee. What the Pump & Fan Selection calculator does is the design-side job: enter the pump H-Q curve, system geometry and fluid properties, and it builds a Darcy-Weisbach system curve, locates the operating point where the two cross, runs an NPSH energy balance for cavitation, scales the duty point across speed and impeller trim with the affinity laws, and analyses series and parallel configurations against the same system curve — with a branded PDF report showing the method. Use it to choose a pump; use an ISO 9906 test to accept the one that arrives.
Calculators that implement ISO 9906
What ISO 9906 covers
- H-Q pump curve operating point — bisection-solved intersection of the pump characteristic and the system resistance curve
- Cavitation assessment — available net positive suction head from the suction-side energy balance, compared against the pump's required NPSHr
- Pump efficiency, hydraulic power and shaft power at the duty point; motor sizing to the next IEC standard frame
- Affinity laws for speed change and impeller trim (Q ∝ N·D, H ∝ N²·D², P ∝ N³·D³) with variable-speed drive energy-saving analysis
- Specific speed nq classification — radial/centrifugal, mixed-flow or axial — with best-efficiency-point (BEP) efficiency estimate
- Series and parallel multi-pump configuration analysis with combined H-Q curve and heterogeneous pump support
Governing formulas
P_hyd = ρ · g · Q · H and P_shaft = P_hyd / η_pwhere P_hyd = hydraulic power (W); ρ = fluid density (kg/m³); g = 9.81 m/s²; Q = volumetric flow at the operating point (m³/s); H = total head at the operating point (m); P_shaft = shaft power (W); η_p = pump efficiency at the operating point (dimensionless, 0–1)
NPSHa = (P_atm − P_v) / (ρ · g) − H_s − h_fwhere NPSHa = available net positive suction head (m); P_atm = absolute pressure at the pump suction (Pa); P_v = fluid vapour pressure at operating temperature (Pa); ρ = fluid density (kg/m³); g = 9.81 m/s²; H_s = suction lift — positive when the pump is above the source (m); h_f = friction loss in the suction line (m). Cavitation risk flagged when NPSHa < 1.1 × NPSHr
Q₂/Q₁ = (N₂/N₁)·(D₂/D₁) H₂/H₁ = (N₂/N₁)²·(D₂/D₁)² P₂/P₁ = (N₂/N₁)³·(D₂/D₁)³where Q = volumetric flow (m³/h); H = total head (m); P = shaft power (kW); N₁, N₂ = rotational speeds (rpm); D₁, D₂ = impeller diameters (m or mm, ratio only). Laws are exact for dynamically similar operating points; they provide a close approximation across the normal VSD speed range
Frequently asked questions
What is ISO 9906 used for?
ISO 9906 is the international standard for hydraulic performance acceptance tests of rotodynamic pumps (centrifugal, mixed-flow and axial-flow). It defines how to measure the pump's H-Q characteristic, efficiency and net positive suction head, and the permissible tolerances (Grades 1, 2 and 3) between the measured performance and the manufacturer's guarantee. Engineers also use the ISO 9906 conventions — particularly the NPSHa formula and the agreed definition of total head — when sizing pumps and checking cavitation in new system designs.
What is the difference between ISO 9906 Grades 1, 2 and 3?
The three grades define how closely the as-tested pump performance must match the manufacturer's guaranteed curve. Grade 1 has the tightest tolerances (±2.5 % on flow, ±2 % on head at the guaranteed duty point) and is used for critical applications or where precise energy accounting is required. Grade 2 (±4 % flow, ±3 % head) is the standard industrial acceptance level. Grade 3, introduced in ISO 9906:2012, carries wider tolerances and is intended for general-service pumps where close hydraulic matching is less critical.
How does the ISO 9906 NPSH formula work?
Available net positive suction head is the suction-side energy balance NPSHa = (P_atm − P_v)/(ρ·g) − H_s − h_f, where P_atm is the absolute pressure at the suction free surface, P_v is the fluid vapour pressure, H_s is the suction lift and h_f is the suction-line friction loss. It is written without a separate velocity-head term because the manufacturer's NPSHr is measured at the suction flange under the same convention — including one on the system side would double-count it. The MechanixCalc pump calculator computes NPSHa this way and reports a cavitation risk below a 1.1× ratio, and a marginal duty below a 1 m margin or a 1.3× ratio. Those acceptance thresholds are common design practice, not clauses of ISO 9906.
How do the affinity laws help with variable-speed drives?
The affinity laws state that flow scales linearly with speed, head scales with speed squared, and power scales with speed cubed. Halving the speed therefore reduces shaft power to one eighth — an 87.5 % saving under ideal conditions. The MechanixCalc pump calculator computes the actual part-speed operating point by intersecting the speed-scaled H-Q curve with the real system curve (including static head), giving an honest power saving rather than the overstated result produced by directly scaling the full-speed duty point.
Is the ISO 9906 pump calculator free?
You can use the full pump calculator during a free 30-minute preview with no sign-up required, and a free 14-day account trial unlocks every MechanixCalc tool with no credit card. The branded PDF engineering report and saved calculations are included in the free 14-day trial and in every paid plan.
Related standards
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