A water performance curve is a starting reference, not the expected operating curve for a more viscous liquid. Before using it to approve a pump or motor, establish the liquid properties at the actual temperatures, apply a documented viscosity-correction method to the matching pump curve, and check the resulting duty against the system and the pump manufacturer’s limits.
Why a Water Curve Can Mislead on a Viscous Liquid
A pump curve tested on water does not, by itself, predict the flow, head, efficiency or shaft power on oil, resin or another more viscous liquid. Internal hydraulic losses change with viscosity and pump geometry. The connected system also changes the operating point through its own pressure losses.
The result must be evaluated as a corrected pump curve against the system duty. A single correction applied to a nameplate flow, or a percentage derating applied to every pump, cannot represent every size, speed, impeller or fluid.
Define the fluid
Viscosity, density and temperature at normal, limiting and startup conditions.
Identify the curve
Exact pump size, speed, impeller and water performance data.
Apply corrections
Use the applicable HI procedure or pump-manufacturer correction.
Check the duty
Review the corrected curve, system intersection, power, NPSH and startup.
For the underlying curve-reading steps, see the centrifugal pump curve guide.
What ANSI/HI 9.6.7 Covers
The current Hydraulic Institute listing identifies ANSI/HI 9.6.7-2021 as the current edition. Its scope describes prediction of rotodynamic-pump performance for Newtonian liquids more viscous than water, for conventional radial-flow pump types within their allowable operating range. Check the standard and the pump supplier’s guidance for the exact application before using a correction.
A Newtonian-liquid method should not be treated as a complete model for shear-thinning or shear-thickening fluids, significant solids loading, entrained gas, or a liquid whose properties change during processing. Those applications need data and review appropriate to the actual fluid and pump.
Do not use a single cSt cutoff as a pump-selection rule
There is no one viscosity value that automatically makes every centrifugal pump unsuitable or requires a positive-displacement pump. Compare the corrected curve, duty range, startup behavior, fluid sensitivity and supplier limits for the specific service.
See the Hydraulic Institute’s ANSI/HI 9.6.7-2021 reference for the current standard edition and its scope.
Data Needed Before Correcting a Pump Curve
Record the properties and curve data that belong to the same operating case. If viscosity varies during a batch or after shutdown, treat each material condition that can govern the duty as a separate case.
| Input | What to record | Why it matters |
|---|---|---|
| Fluid rheology | Fluid composition; Newtonian or non-Newtonian basis; solids or gas if present. | Determines whether the selected correction method represents the liquid. |
| Viscosity | Kinematic or dynamic viscosity, units, test/source method, and the temperature for each value. | Viscosity is temperature-dependent; a value without its temperature may not describe the pump condition. |
| Density | Density or specific gravity at the service temperature, with the reference basis stated. | Required for liquid power and motor-load calculations. |
| Pump identity | Manufacturer, exact model and size, speed, impeller diameter/configuration and installed condition. | Correction depends on the applicable pump geometry and curve. |
| Water performance | Current manufacturer curve with flow, head, efficiency and absorbed-power information over the required range. | A single duty-point value may not show the corrected operating range or maximum power. |
| System duty | Minimum, normal and maximum flow/head; system curve or pressure-loss basis; control and bypass conditions. | The operating point is set by the pump and connected system together. |
| Startup and suction | Minimum startup temperature, expected viscosity after shutdown, suction pressure/level, vapor pressure and available NPSH data. | Startup torque and suction margin can govern even when normal operation appears acceptable. |
| Driver and seal | Motor nameplate, voltage/frequency, starter or VFD details, seal arrangement and support/flush requirements. | Hydraulic suitability alone does not establish driver or sealing suitability. |
When viscosity is reported in dynamic units, retain the density and temperature basis needed to convert it to kinematic viscosity. The Hydraulic Institute’s fluid-viscosity reference explains the distinction and common unit conversions.
How the Viscosity Correction Factors Are Used
The HI method uses correction factors to relate a water-performance reference to estimated performance on the viscous liquid. In the notation used here, CQ applies to flow, CH to head and Cη to efficiency. Obtain factor values using the applicable procedure and pump data; do not use generic values copied from a different pump or duty.
Apply the procedure across the curve range needed for the service, then compare the corrected curve with the system duty. If the supplier has tested or calculated performance for the actual liquid, that configuration-specific evidence should take precedence over an unsupported generic estimate.
Calculate Shaft Power and Review the Motor
For a defined operating point, shaft power follows from liquid flow, pump head, liquid density and pump efficiency. Use the actual viscous-liquid duty values and efficiency at that same point. In the US customary equation, use specific gravity on a water basis and efficiency as a decimal.
If the required process duty is specified first, locate that duty on the corrected pump/system basis rather than mixing a required flow with an unrelated corrected curve point. Review absorbed power over the full expected operating range, including credible startup and upset conditions. The final motor decision also depends on the selected pump curve, motor rating and electrical conditions, starter/VFD behavior and project requirements.
Once the corrected flow, head, density and efficiency are established, the site’s Pump Power & Motor Sizing Calculator can organize the shaft-power calculation. It does not perform ANSI/HI viscosity correction or select a pump model.
What a Verified Calculation Example Should Include
For a project example, show the exact pump size, speed, impeller, source water curve, liquid viscosity and density at the stated temperature, the documented correction method, corrected curve point and power calculation. If any inputs are illustrative, label the entire example as hypothetical and do not associate it with an OEM model or published curve.
Review Startup, NPSH and Sealing Separately
A normal-temperature duty point may not govern the application. Check the liquid’s viscosity and density at the lowest credible startup temperature, including a shutdown condition in which the liquid cools or becomes harder to move. Confirm acceleration and overload behavior against the motor, starter or VFD documentation and the pump supplier’s limits. Heating or insulation may be considered where the process design supports it; do not prescribe a universal VFD torque setting.
Establish system-side NPSH available for the relevant liquid level, pressure, temperature and suction losses. Compare it with manufacturer-supplied pump NPSH data at the corresponding flow, speed and impeller configuration, using the required project margin. For the system-side calculation, use the NPSH Available Calculator; it does not replace pump NPSH-required data.
Evaluate the mechanical seal, elastomers, chamber and flush or barrier system for the actual chemistry, temperature, solids, crystallization risk and pressure. Select any API piping plan from the complete seal-service requirements and supplier/project review, not from viscosity alone.
When to Reconsider the Pumping Technology
A centrifugal pump remains a candidate when its corrected curve meets the required duty across the operating range and its power, suction, startup, seal and material conditions are acceptable. Compare another pump size, speed or technology when the corrected duty cannot be met with suitable margin, when startup is not repeatable, or when the fluid’s rheology and shear requirements fall outside the correction method.
Use the ANSI pump technical resource library to find applicable curves and engineering references. If reviewing a Goulds 3196 or Durco Mark III installation, treat model replacement as a later verification step: confirm the exact configuration and hydraulic duty rather than inferring compatibility from the family name.
What to Include in a Viscous-Liquid Pump Review
Send the information that lets an engineer identify the curve and the governing liquid condition:
For a first-pass data package, the Centrifugal Pump Sizing Calculator collects viscosity, temperature, density and duty inputs for engineering review. It is a duty-intake tool, not an automatic viscosity-correction calculation or model recommendation.
Related engineering references: specific-gravity effects on absorbed power.
Frequently Asked Questions
Can I use a water pump curve for a viscous liquid?
Use it as a reference only. Apply a documented viscosity-correction method to the correct pump curve, then review the corrected duty, power, NPSH, startup and seal conditions.
Does ANSI/HI 9.6.7 provide one maximum viscosity for centrifugal pumps?
No single viscosity value determines suitability for every pump. Applicability and performance depend on the fluid, pump, operating point and the method’s limits.
Can the HI correction method be used for non-Newtonian fluids?
Do not assume so. The current standard scope describes Newtonian liquids. Non-Newtonian rheology, solids or changing fluid properties may require a supplier-specific method or another technology review.
Does lower efficiency always mean the next larger motor should be selected?
No. Calculate shaft power at consistent corrected duty points, review the full operating range and startup conditions, then confirm the motor and drive against their nameplates and project requirements.
Should a pump switch to positive displacement above a fixed cSt value?
Not from viscosity alone. Compare the corrected centrifugal-pump duty and operating constraints with the requirements for the alternative pump and the actual fluid.
Sources and Review Basis
- Hydraulic Institute, ANSI/HI 9.6.7-2021 — current edition listing and scope. Use the complete standard for correction equations, charts, limitations and worked calculations.
- Hydraulic Institute Data Tool: Viscosity — dynamic and kinematic viscosity terminology and unit conversion context.
- ANSI Pumps Pro Pump Power & Motor Sizing Calculator — shaft-power input workflow after the corrected duty values are established.
Final selection should use the current applicable standard, exact pump curve, fluid data and project requirements. This guide explains the review sequence; it does not certify a pump or motor selection.
