In the world of psychology and specifically in the area of introspective issues, it is often declared that, “what you resist will persist.”
You’ve heard it before, and here we go again. Allegedly, the most misunderstood concept in the pump world is net positive suction head (NPSH). I have written several articles on the subject and so have many other pump technicians or engineers and so-called experts.
The NPSH name itself, an acronym, confuses most pump neophytes. The subject and required calculations confounds people who are new to the industry, those on the periphery (operators or administrators) and professionals who incorrectly believe they fully understand the subject even after 25 years in the business.
One of the fun parts of my position is teaching at several pump schools per year and devoting a major portion of the course to the subject of understanding the concept of NPSH and how to complete the calculations. In the teaching process, I cover the five main examples that you will likely encounter in normal industry applications. The examples are adapted from chapter 1 of the “Cameron Hydraulic Data Book.”
I will explain these five examples first in this column and over the next four months with the basic optimistic intent that once you learn these five examples and a few variations of each you will be able to handle the applications encountered in the real world. Between now and Thanksgiving, you may wish to read and file all five columns as a handy reference. As background, please re-examine two of my previous columns on the subject, one from Pumps & Systems August 2015 and the other from April 2018.
The net positive suction head is the total suction head in feet of liquid (or meters), less the vapor pressure (in feet or meters) of the liquid being pumped.
Think of head as an energy level and not as a force-like pressure. All values are absolute.
NPSHa is measured at the pump centerline or the impeller eye. These two things can be at different places or elevations. Think of NPSHa as the liquid’s available energy level at the inlet of the pump or the eye of the impeller. The liquid will flash to vapor if there is not enough NPSHa. Do not confuse NPSHa with suction pressure. While suction pressure is in some ways a component in the mix, there is something more complex to the story.
NPSHa is the amount of NPSH that the system has available at the eye of the pump impeller. This NPSHa value is entirely a function of the liquid, its properties, ambient conditions and the suction system design and geometry. Essentially, the calculation is about the suction system itself and has nothing to do with the pump. This calculation should be completed by the system owner, the end user and/or their engineer or consultant. For liability reasons, manufacturers are normally directed to not be involved in the customers’ calculations; however, as time marches on, the manufacturer is getting more involved mostly as a preservation issue.
NPSH required (NPSHr) is most commonly determined by the pump manufacturer by empirical methods and using standards and specifications from the Hydraulic Institute (HI). NPSHr values are normally reported on the performance curves for the pump.