Viscosity correction should be completed before approving a centrifugal pump for a liquid other than water. A pump curve published for water does not guarantee the same flow, head, efficiency, input power, or required NPSH when the pumped liquid has higher viscosity. Selection should start with viscosity and density at the actual operating temperature, the applicable water curve, the operating point, and manufacturer evidence. ANSI/HI 9.6.7 provides guidance for evaluating this change; it does not turn an estimate into a universal guarantee.
Reviewed and updated: August 29, 2026. This article is an engineering guide for process-pump buyers, maintenance teams, and specifiers. Final selection should consider the approved fluid, manufacturer curve, suction conditions, motor, seal, and project safety controls.
Why a Water Curve Does Not Represent a Viscous Fluid
Many centrifugal-pump curves are developed with water or another low-viscosity reference liquid. When the pumped fluid is more viscous, friction losses increase inside the pump and throughout the system. As a result, the actual operating point may shift to lower flow and lower head, while efficiency decreases and absorbed power may increase.
The term viscous does not define an application by itself. Oil, a chemical solution, resin, or suspension can have very different density, temperature, solids content, and rheological behavior. Before requesting a replacement pump, confirm whether the liquid is Newtonian and whether the stated viscosity represents the normal, minimum, and maximum operating range.
What Changes: Flow, Head, Efficiency, and Power
Viscosity changes the relationship between velocity, internal slip, and hydraulic losses. At a minimum, a practical evaluation should review:
- Available flow and head at the actual operating point.
- Corrected efficiency, not only the efficiency shown for water.
- Input power, motor load, and thermal margin.
- Required NPSH and available margin at the most adverse condition.
- Starting torque, speed, control method, and behavior during temperature changes.
The effect is not a linear rule that can be applied with a fixed percentage. It depends on pump size, speed, impeller geometry, viscosity, density, and operating point. A correct calculation should preserve its assumptions and clearly separate measured values from estimated values.
What ANSI/HI 9.6.7 Covers
ANSI/HI 9.6.7 provides guidance for evaluating centrifugal-pump performance with viscous liquids using a water-performance reference. Its value is in structuring the correction of flow, head, and efficiency while reminding the engineer to review power and suction conditions as well. It is not automatic authorization to extrapolate any curve, fluid, or geometry.
Use the guideline together with the approved pump curve, manufacturer data, and project requirements. For non-Newtonian fluids, significant solids loading, or properties that change during the process, a Newtonian-fluid correction may not represent the complete behavior. In those cases, a supplier-specific review or another pumping technology may be required.
Data Required Before Correcting a Pump Curve
| Data | What to Record | Why It Matters |
|---|---|---|
| Viscosity | Normal, minimum, and maximum value; unit and test method | Defines internal losses and the applicability of the correction |
| Temperature | Startup, normal operation, maximum, and expected changes | Viscosity can change significantly with temperature |
| Density | Specific gravity or density at service temperature | Affects power, motor load, and pressure |
| Fluid | Newtonian or non-Newtonian; composition and solids | Determines whether the selected correlation is applicable |
| Operating Point | Flow, head, pressure, speed, and control range | The pump must be evaluated at the real operating point, not by model name alone |
| Installed Pump | Model, size, impeller, curve, seal, and motor | Supports the decision to correct, modify, or replace |
Viscous-Liquid Pump Correction Sequence
- Define the operating point and flow range, not only the average condition.
- Confirm viscosity, density, and temperature for each relevant operating condition.
- Select the correct water curve for the installed pump size, speed, and impeller diameter.
- Apply the correction method indicated by ANSI/HI 9.6.7 or the manufacturer and document the assumptions.
- Review the corrected flow-head-efficiency curve and absorbed power.
- Confirm available NPSH, required NPSH, starting torque, motor, seal, and speed limits.
- Validate the operating point against the system and define how the result will be verified in the field.
If the corrected curve does not cover the required duty or leaves insufficient margin, do not hide the problem by opening a valve further or simply oversizing the motor without reviewing pump selection. That can mask a hydraulically unsuitable pump.
How to Interpret the Corrected Curve
Compare the water curve with the corrected curve on the same axes and mark the expected operating point. A corrected curve will often show reduced hydraulic capacity and efficiency, but the shape and displacement should not be invented without data. Verify that power does not exceed the motor capability during startup, normal operation, and upset conditions.
| Review Result | Interpretation | Next Decision |
|---|---|---|
| Corrected curve covers the duty and the motor has margin | The pump may be a candidate under the documented conditions | Confirm seal, materials, control method, NPSH, and performance verification |
| Flow or head falls outside the required range | The water curve does not represent the application | Review impeller diameter, speed, another pump size, or another technology |
| Power or torque exceeds the allowable range | There is a risk of overload or incomplete startup | Review motor, speed, control, and hydraulic selection |
| Viscosity changes during the process | A single operating point does not represent the full cycle | Evaluate the worst case and the required control range |
NPSH, Starting Torque, and Sealing
Selection for a viscous liquid does not end with head. Temperature can change vapor-pressure conditions, while suction-line losses may increase. Review available NPSH at minimum liquid level and maximum temperature, and compare it with required NPSH corrected or confirmed by the manufacturer.
The motor and starter must also handle the starting torque of the fluid and pump. Review acceleration with a VFD, overload protection, and the possibility that the fluid cools during a shutdown. The mechanical seal, elastomers, wetted materials, and flushing plan must be compatible with the chemistry and temperature; a hydraulically acceptable pump can still be a poor selection if the sealing system is not suitable.
When a Centrifugal Pump May No Longer Be the Right Choice
There is no single viscosity value that requires a positive-displacement pump in every application. The decision depends on the corrected curve, flow range, shear sensitivity, control method, solids, startup, temperature, and the consequence of a performance deviation.
If the centrifugal pump loses too much margin, operates far from its intended region, overloads the motor, or cannot start repeatably, compare reselection with a positive-displacement pump or another technology. When the installed equipment is a Goulds 3196 or Durco Mark III, a replacement option should be evaluated only after correcting the curve and confirming that the hydraulic size, seal, and materials are suitable.
How to Prepare an RFQ for a Viscous Liquid
- Fluid, composition, viscosity, and density at normal and extreme conditions.
- Startup, operating, and maximum temperature; behavior during shutdowns.
- Minimum, normal, and maximum flow; head or pressure; and system curve.
- Suction level, vapor pressure, available NPSH, and suction-line conditions.
- Installed model, size, frame or group, speed, impeller, motor, and seal.
- Wet-end materials, shaft, sleeve, elastomers, and sealing plan.
- Required documents: corrected curve, power data, materials, inspection, and testing.
To organize the operating point, you can use the site’s pump calculators. If the review points to a Goulds 3196 or Durco Mark III configuration, send the nameplate, size, and fluid data before requesting a Goulds 3196 replacement or Durco Mark III replacement.
Request a Viscosity-Correction Review
Send flow, head, speed, viscosity, density, temperature, water curve, installed model, and motor data. We will review whether the application calls for correction, reselection, or a replacement-pump quotation.
Frequently Asked Questions
Can I use a water curve to select a pump for oil?
It can be used as the starting reference only when an appropriate correction is applied and power, NPSH, torque, temperature, and sealing are reviewed. It should not be treated as the final oil-performance curve.
Does ANSI/HI 9.6.7 define one fixed viscosity limit?
No. Applicability depends on the pump, fluid, operating condition, and correction method. A universal threshold can lead to incorrect selection.
Does viscosity affect NPSH?
It can affect losses and suction behavior; temperature and vapor pressure also change the available margin. NPSH should be reviewed for the actual condition, not only for water.
Should I request a replacement pump when viscosity changes?
Not necessarily. First correct the curve and confirm duty, power, NPSH, motor, seal, and materials. If the installed pump no longer covers the required range, then compare a verified replacement with another pumping technology.
