The Hydraulic Institute (HI) has published ANSI/HI 9.6.1-2012 Rotodynamic Pumps Guideline for NPSH Margin. The new guideline serves as a replacement for the 1998 edition, which was later withdrawn completely in 2003.
This guideline covers rotodynamic, general purpose pumps with power levels as high as 5300 hp and impeller tip speeds less than 130 feet per second and describes the benefits to pump longevity of maintaining suitable NPSH Margin. Pump users should be aware that the full published pump head will not be achieved when the NPSH Available equals the NPSH3 of the pump. If the NPSH Available matches the NPSH3, the first-stage head will be 3% less than the fully developed value. Today's higher speed, higher energy density pumps might not achieve acceptable service life under suction conditions without an adequate NPSH Margin.
This document serves as a tutorial on the subject of NPSH as well as providing guidance for determining NPSH Margin. The following considerations are discussed relative to the sensitivity of a pump to NPSH-related problems:
Impeller inlet tip speed
Larger vs. smaller impeller eye diameter
Suction specific speed
Characteristics of the pumpage
Operating range of the pump and proximity to POR and AOR
Materials of construction and cavitation resistance
Impeller vane overlap
Pump size and relationship of size to cavitation damage
Duty cycle -time of operation under cavitating condition
Reduction of NPSHA with timeThis excellent NPSH reference helps pump users understand why it is essential to specify an appropriate NSPH Margin and learn about the negative aspects of specifying an excessive NPSH Margin.
The updated guideline is available for purchase at the HI eStore for $80.00 and is available in both hardcopy and pdf formats.
