Understanding the 2024 Updates to ANSI/HI 9.6.1 Rotodynamic Pumps Guideline for NPSH Margin
By Alex Moser, Senior Engineer of Standards and Training, Hydraulic Institute
The ANSI/HI 9.6.1 guideline establishes recommended net positive suction head (NPSH) margins for rotodynamic pumps of radial, mixed, and axial flow designs under varying operating conditions. These recommendations apply to a wide range of applications across ten distinct market segments, with some requiring additional considerations. Maintaining the recommended NPSH margin is crucial for reliable pump operation, as insufficient margins can lead to noise, vibration, loss of performance, and overall reliability issues. The revised guideline provides users with NPSH margin recommendations specific to each application.
Importance of NPSH in Pump Performance
NPSH is a critical factor in ensuring the reliable operation of pumps. Understanding the significance of the NPSH margin requires defining two key aspects:
- Net Positive Suction Head Available (NPSHA): A system characteristic determined by the system designer. It represents the total suction head of liquid (absolute) at the first-stage impeller datum, minus the absolute vapor pressure of the liquid.
- Net Positive Suction Head Required (NPSHR): A pump characteristic provided by the manufacturer, indicating the minimum NPSH value needed to achieve specified performance at a given flow rate, speed, and pumped liquid.
NPSHA should always exceed NPSHR and the difference between these values is known as the NPSH margin.
Cavitation occurs in rotodynamic pumps, typically at the inlet to the impeller, when the pressure at the impeller inlet drops below the pumped liquid’s vapor pressure. This results in the formation of a vapor bubble (cavity), which later implodes back to liquid when the bubble reaches a higher-pressure region in the impeller. To minimize cavitation and ensure the stated performance is achieved and reliability is not compromised, the NPSH margins recommended in ANSI/HI 9.6.1 should be applied.
Key Updates in the 2024 Edition of ANSI/HI 9.6.1
The 2024 edition of ANSI/HI 9.6.1 introduces significant revisions to better align industry best practices with real-world applications. This edition supersedes the 2017 version and includes key updates:
Transition from NPSH3 to NPSHR for NPSH Margin Determination
To simplify NPSH margin determination, the industry has transitioned from NPSH3 to NPSHR. Previously, NPSH3—the NPSH at which sufficient cavitation causes a 3% reduction in total head—was used to calculate the NPSH margin. However, not all manufacturers publish NPSH3 values, making it difficult for users to obtain this data.
To address this challenge, NPSHR now serves as the reference point, with the requirement that the manufacturer-supplied NPSHR must be equal to or greater than the tested NPSH3 value. This change streamlines the process while ensuring reliable pump operation.
Refinement of the NPSH Margin Considerations Section
Understanding how to evaluate the NPSH margin is essential for achieving optimal pump performance and reliability. The revised guideline introduces a structured approach to assessing the NPSH margin, considering the characteristics of the pumped liquid (pumpage), the pump’s physical effects, and the environment and site (system) conditions.
- Characteristics of the pumped liquid (pumpage): Factors such as temperature, abrasives, corrosives, entrained and dissolved gases, solids content, additives, and changes in fluid properties that influence cavitation potential should be considered.
- Characteristics of the pump’s physical effects: Elements including design features, pump rotational speed, wear, and aging can impact NPSHR over time.
- Characteristics of the environment and site conditions (system): Influences such as operating region, inlet conditions, transients, recirculation lines, parallel pumping, and biological or system aging effects must be factored into NPSH margin evaluations.
Since suction pressure is rarely measured continuously at the pump inlet, NPSHA values are typically estimated using standard industry methods. To address potential inaccuracies, the guideline provides recommended NPSH margins to compensate for variations in pump operation and deviations from assumed NPSHA values. By following these considerations, users can minimize the risk of cavitation and ensure reliable pump operation.
Simplification of the Guideline
The previous NPSH margin determination section has been eliminated, allowing better coordination between the applications section and NPSH margin considerations.
Additionally, the appendices have been replaced with new content focused on NPSH margin topics applicable across all market segments:
- Appendix A – NPSHA Determination and Considerations: When evaluating NPSH margin, understanding how NPSHA is determined is critical. This appendix provides an overview of the key factors influencing NPSHA, including the NPSHA equation, descriptions of essential parameters, and calculation examples. Since NPSHA varies depending on application and operating conditions, this section highlights best practices for evaluating liquid properties, system characteristics, and equipment variations. To ensure an adequate NPSH margin, users should always consider the lowest NPSHA value in system analysis while also reviewing how NPSHA and NPSHR fluctuate across the full operating range.
- Appendix B – Compensating for Insufficient NPSH Margin: When the recommended NPSH margin cannot be achieved, this appendix outlines strategies to mitigate cavitation-related risks. These strategies enhance the guideline’s applicability across industries by providing targeted guidance for addressing insufficient NPSH margin.
Industry Collaboration and Future Enhancements
By adopting the recommendations in this updated edition, users can improve process stability, reduce operational risks, and extend pump service life.
The Hydraulic Institute continues to seek field data to further enhance and expand the guideline in future editions. Industry professionals are encouraged to share field data, case studies, and research insights to refine best practices for NPSH margin determination.
The 2024 edition of ANSI/HI 9.6.1 offers a more precise and practical approach to NPSH margin considerations for rotodynamic pumps. By transitioning to NPSHR-based margin determination, refining guidance, and streamlining content, the new edition enhances usability across industries. Engineers, system designers, and pump manufacturers should familiarize themselves with these updates to ensure optimal pump selection and performance.
Next Steps
- Review the updated ANSI/HI 9.6.1 guideline to align with industry best practices.
- Evaluate your current pump selections to ensure adequate NPSH margin.
- Engage with the Hydraulic Institute to contribute field data and feedback.
For more information on ANSI/HI 9.6.1, as well as other Hydraulic Institute standards, guidelines, and publications, visit www.pumps.org.
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