Introduction
Most centrifugal pump curves are generated using clean water. When your process fluid is more viscous than water, the pump performance will deviate from these curves. Increased viscosity negatively affects pump performance—more power is required to pump the same volume of liquid. This translates to decreased head and efficiency curves.
In FluidFlow, performance curves are corrected according to methods from Hydraulic Institute (HI) 9.6.7, an American National Standard Guideline for the Effects of Liquid Viscosity on Rotodynamic (Centrifugal and Vertical) Pump Performance.
When Viscosity Correction Is Required
Threshold: Kinematic viscosity ≥ 4.3 cSt at the pump inlet
Validity range of HI method: 4.3–3300 cSt
Fluids: Newtonian only
Pump types: Centrifugal (rotodynamic) only
Expected Performance Effects
Head and Flow: typically reduced compared to water curves
Efficiency: most significantly impacted
Power: often increases due to lower efficiency
Note: 1 cSt = 1 mm²/s. Confirm viscosity at actual operating temperature.
How to Apply Viscosity Correction in FluidFlow
Viscosity correction is automatically applied to all applicable centrifugal pumps when enabled
Activation Requirements
Enable "Include Viscosity Correction Factor for Centrifugal Pumps" in Global Settings:
Options → Calculation → Global Settings or press F2
Pumped fluid kinematic viscosity must be within 4.3–3300 cSt
Fluid must be Newtonian
Implementation Steps
In the Global Settings window, locate the Inclusions panel
Enable the viscosity correction option
Verify fluid properties are correctly defined (temperature and viscosity)
Run calculation and confirm success with "Pump is Viscosity Law Corrected" message
Interpreting Results and Design Implications
Head and Flow: Expect modest reductions compared to water curve duty points
Efficiency: Anticipate significant decreases; review motor sizing and electrical components accordingly
Operating point: Prepare for position shifts on the corrected performance curves
Documentation: Record viscosity correction settings for reference
Worked Example
Shown is a sample system where we are pumping the fluid Fuel Oil No. 4. The option to apply viscosity correction has been switched OFF in the top model and ON in the bottom model. We can see below the difference in pump power requirement, efficiency and duty pressure rise.
The viscosity-corrected model (bottom) shows reduced efficiency compared to the uncorrected model (top)
Power requirements increase when viscosity correction is applied
Pressure rise capability decreases with viscosity correction enabled
Note the "Pump is Viscosity Law Corrected" message in the bottom model
This demonstrates the importance of applying viscosity correction for accurate sizing
Without correction, pump and motor selection would be undersized for the actual application
Best Practices
Always verify fluid viscosity at actual operating temperature
Enable viscosity correction at the beginning of your design process to avoid late-stage rework
Distinguish between dynamic viscosity (μ, cP or Pa·s) and kinematic viscosity (ν, cSt or mm²/s) in calculations
Account for viscosity variations across the operating temperature range in your design safety margins
Document all viscosity correction assumptions in project specifications
Consult pump manufacturers when operating near the viscosity correction limit boundaries
Note: Disable the viscosity correction feature when working with non-Newtonian liquids.
FAQs
Q: Does this apply to all pump types?
A: No. It applies to centrifugal (rotodynamic) pumps only.
Q: What fluid types are supported?
A: Newtonian liquids with kinematic viscosity between 4.3–3300 cSt.
Q: How do I know if the viscosity correction is active?
A: Check the Messages panel after solving—it will display "Pump is Viscosity Law Corrected" when correction is applied.
Q: Can I enable viscosity correction for individual pumps?
A: No, the setting is global and applies to all centrifugal pumps in your model.
Q: What if viscosity exceeds 3300 cSt?
A: The HI method is out of range; obtain manufacturer data or consider alternate approaches.
Q: Does the correction change NPSH?
A: The HI viscosity correction focuses on head, flow, and efficiency. Evaluate NPSH and cavitation margins separately using pump data and operating conditions.
Conclusion
Applying the Hydraulic Institute's viscosity correction in FluidFlow generates realistic pump performance predictions for viscous, Newtonian fluids. To implement this effectively: enable the correction setting, verify fluid viscosity at actual operating temperature, and carefully evaluate the duty point, efficiency, and motor sizing requirements. These steps help prevent underperformance and eliminate costly redesign later.
By enabling viscosity correction, your model accurately reflects real operating conditions—protecting performance integrity, maintaining efficiency, and ensuring proper equipment sizing to avoid expensive design and commissioning problems.