NPSHR and Impeller Diameter
Okay, this may take longer than 60 seconds.
Prior to computerized software for pump selection, manufacturers simply drew one NPSHR curve for the pump on the head versus capacity curve. It was used regardless of the impeller diameter chosen. This is not a good idea for the smaller diameter selections.
NPSHR for a fixed flow rate is less for the larger diameter impeller than the smaller diameter impeller. Or stated in another way, at a given flow rate the NPSHR increases as the diameter of the impeller decreases. (Note the head changes but the flow rate remains at some fixed value.)
As an alternative method to thinking about the science behind this, open your Summit Select program and select a Summit Pump model as below:
- 2196 in 60 hertz
- Perform a Design Point Search for ambient temperature water
- 250 GPM at 65 feet TDH
- Select the pump size 2 x 3 – 6 at 3500 RPM
- Open the curve
You will note that for 250 GPM the 6.00 inch impeller will require about 11 feet NPSHR, the 4.63 inch impeller will require 13.3 feet NPSHR and the minimum 4.00 inch impeller will be 18 feet NPSHR.
The 3 condition points for the 2 x 3 – 6 (3500 RPM).
- 250 GPM at 150 feet 6.00 inch and NPSHR 11.2 feet.
- 250 GPM at 65 feet 4.625 inch and NPSHR 13.4 feet.
- 250 GPM at 30 feet 4.00 inch and NPSHR 18 feet
Note your results may vary by a small fraction. Also be aware if you adjust the impeller trim from the original pump search, the flow and head will change due to the Summit Select’s predicted system curve at that trim.
– The Summit Pump Team
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