Here I go again, writing about net positive suction head (NPSH). I persistently harp on this subject because I frequently witness NPSH problems in the field. To paraphrase the words of the famous psychologist Carl Jung, “what you resist, will persist.”
You may believe that NPSH is something you can check and forget, but do you realize that both the system’s NPSH available (NPSHa) and the pump’s NPSH required (NPSHr) are continually changing over time? It’s like the air pressure in your car’s spare tire—forgotten in the trunk.
Background on NPSH: Energy & Pressure
Confused by the term NPSH? Think of it as a required energy level to prevent most cavitation. Pumps require a certain amount of NPSH (energy) to satisfactorily operate at a given point of head (pressure) and flow on the performance curve. These points are empirically determined by the manufacturer (sometimes calculated) and are noted as NPSHr.
The suction side of the pump system must, in return, provide a certain amount of NPSH (energy), and that is referred to as NPSHa. There must be more NPSHa than NPSHr for the pump to operate satisfactorily.
The difference between the two is referred to as the margin or ratio. Refer to Hydraulic Institute (HI) 9.6.1-2017 for more information on NPSH margins and ratios.
NPSH Tips to Keep in Mind
NPSH changes with a number of things, including:
Impeller diameter: For the same flow rate on a given system, an impeller of a smaller diameter will have a higher NPSHr. For proof, check it on a pump selection program. Use the same flow rate over a selection of four to five heads.
This phenomena occurs for several reasons and most are complex. The main reason has to do with a mismatch in the relative velocity of the fluid at the inlet of the impeller as compared to the outlet. Note that the higher the specific speed (NS) of the impeller design, the more the effect will occur.
Another often overlooked factor is a simple math axiom. NPSHr, also known as NPSH3, is normally defined as a 3 percent head drop at a fixed flow rate for the pump. A 3 percent head drop with a smaller impeller (implies lower head) is a smaller number than a 3 percent head drop of a larger impeller (implies higher head). In essence, what this result demonstrates is that, by default, pump manufacturers actually allow for less cavitation on smaller diameter impellers. Terry Henshaw, in one of his many outstanding articles, (Pumps & Systems May 2009) discusses this industry disjunct in more detail.
Clearance of impeller and wear rings: Over time as the clearances open up on the pump, the NPSHr will also increase. This effect is significantly worse for closed impellers than open or semi-open styles. A closed impeller that uses wear rings can have an increase in NPSHr by as much as 40 percent due to wear in the critical clearances. The main reason this increase in NPSHr occurs is recirculation flow at and near the impeller eye.