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      Viscosity Correction - Centrifugal Pumps

      Learn when and how to apply Hydraulic Institute (HI) viscosity correction for centrifugal pumps in FluidFlow, including setup steps, validity limits, best practices, FAQs, and a worked example.

       

       

      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


      1. In the Global Settings window, locate the Inclusions panel

      2. Enable the viscosity correction option

         


         

      3. Verify fluid properties are correctly defined (temperature and viscosity)

      4. 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.