Pump Selection and Operating Point: Evaluating Viscosity Effects on Installed Centrifugal Pump Curves

Centrifugal pump curves are typically developed using water as the standard test liquid. When the installed liquid is more viscous than that baseline, the pump will not deliver the same head, efficiency, or power performance shown on the curve. Pump selection and operating point review must therefore distinguish the manufacturer’s water-based performance from the corrected installed performance before the motor, duty, or operating range is accepted.

The pump curve and system curve are both affected by viscosity, but in different ways.

Higher liquid viscosity increases internal losses within a centrifugal pump. The supported correction reduces head and efficiency while increasing the shaft power required for the duty. At the same time, higher viscosity increases piping friction, which steepens the system resistance curve.

Figure 1. Effect of increased fluid viscosity on centrifugal pump performance: both head (H) and efficiency (η) shift downward from the water curves (blue) to the viscosity-corrected curves (orange).

The installed operating point is the intersection of these corrected relationships. Correcting only the piping pressure drop while leaving the pump curve at its water-test condition is insufficient. Likewise, correcting the pump curve while modeling the network with the wrong fluid viscosity is incomplete.

The calculation must use one consistent fluid property basis across the pump and the system.

FluidFlow applies viscosity correction in accordance with HI 9.6.7, the Hydraulic Institute's American National Standard for the Effects of Liquid Viscosity on Rotodynamic Pump Performance.

The documented application range is:

Where possible, the most reliable approach is to obtain pump curves (capacity, efficiency, and NPSH) from the manufacturer for the actual fluid at operating conditions—especially for highly viscous fluids. In that case, viscosity correction in FluidFlow would not be required.

Begin with the manufacturer’s pump data. The capacity curve is mandatory; efficiency and NPSHR curves are recommended because viscosity assessment must include more than head and flow.
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Verify the following before calculating:

Turn automatic sizing off and select the manufacturer pump entry. Rating mode allows FluidFlow to calculate where that pump operates against the system resistance. The engineer remains responsible for matching the database entry to the installed machine and current impeller.

The viscosity correction is enabled by default and can be reviewed in the Calculation Options window under Global Settings, accessed through F2 or Options → Calculation.

Figure 2. In FluidFlow, viscosity correction is applied by default and can be disabled in the Calculation Options.

Before applying the correction, confirm that the curve you entered is the manufacturer’s water-based curve at the same speed and impeller trim. If the curve is already supplied/derated for the viscous operating fluid (or has already been corrected by the manufacturer), the viscosity correction does not need to be applied—otherwise, you risk over-correction and misreading the operating point.

Figure 3. Simplified workflow for FluidFlow centrifugal pump curve viscosity correction.

FluidFlow performs the correction; the engineer must document the calculation setting and the fluid property source so the result can be reproduced.

With the correction enabled for an eligible viscous liquid, the expected direction is unfavorable: corrected head decreases, efficiency decreases, and required shaft power increases compared with the water-based curve.

The new operating point also reflects the steeper system curve caused by higher pipe friction. Depending on the connected system, the flow can move left of the water-case duty. Review the pump and system curves together rather than comparing one corrected head value at a fixed flow.

Power deserves a separate check. An uncorrected model can understate shaft power requirements and overstate efficiency. Compare the calculated requirement with the motor basis and manufacturer limits, but leave final motor thermal and mechanical acceptance to the responsible electrical, mechanical, and vendor review.

NPSH must also be reviewed at the corrected operating point. FluidFlow can report NPSHa from the suction-side hydraulics and NPSHR from the installed curve where supplied. The required margin remains a project and vendor criterion.

Viscosity often changes with temperature, so the normal operating case may not be the limiting case. The source guidance specifically calls for hot and cold pump duty checks.

A colder liquid can increase viscosity, raising pipe resistance and increasing the pump correction penalty. A hotter liquid can reduce viscosity but change vapor pressure and therefore affect NPSHa. The governing case for power is not necessarily the governing case for suction margin.

Consider alternative low-temperature cases in which the liquid may be significantly colder than during normal operation, such as initial system fill or pump startup following exposure to very cold ambient conditions. Model each credible temperature scenario with a consistent property basis. Do not mix cold viscosity with hot vapor pressure or use one temperature to represent initial fill, startup, and normal operation without justification.

FluidFlow applies the documented correction and computes the steady-state operating point. It does not validate the laboratory property data, guarantee that the manufacturer’s water curve is representative of the installed pump, or approve operation outside the vendor’s published range.

The correction does not cover non-Newtonian behavior or solid-induced pump losses. Those cases require a different route.

Final acceptance must reconcile corrected duty, motor capability, NPSH margin, minimum flow, runout, and the manufacturer’s allowable operating envelope.

It is enabled by default, but the engineer should verify the setting in Calculation Options for each file. A model can inherit project settings, and an eligible Newtonian case should not be accepted on the assumption that the default remained unchanged.

No. The documented method is for Newtonian liquids. Non-Newtonian viscosity changes with shear rate, and the shear conditions inside the pump are not represented by one routine viscosity value. Disable the standard correction and use test or manufacturer data.

Confirm whether the curve data you entered is the manufacturer’s water-based curve at the same speed and impeller trim, or whether it has already been supplied/derated for the viscous fluid and temperature. HI 9.6.7 is intended to correct a water-based curve; applying it to an already corrected curve will over-correct and can mislead the operating point interpretation.

The same viscosity used to correct pump performance affects pipe Reynolds number, friction factor, and pressure drop. The installed duty is set by the corrected pump curve and the system resistance calculated with the same fluid properties.

There is no universal answer. A cold, high-viscosity case can govern power and flow, while a hotter case can reduce NPSHa through higher vapor pressure. Calculate both and route the acceptance criteria to the relevant project and vendor requirements.

Viscosity assessment is an installed-system calculation, not a curve-only adjustment. Verify HI 9.6.7 applicability, apply the actual Newtonian fluid properties, calculate the corrected pump and system curves, and repeat for credible temperatures. FluidFlow computes the shift; the engineer decides whether the corrected operating envelope is acceptable.