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Old December 12th, 2017, 03:20 PM   #8
Tom Jenkins
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Join Date: Apr 2002
Location: Milwaukee, WI
Posts: 5,877
A couple of thoughts.

NPSH is typically absolute pressure (psia) not gauge pressure. Make sure you include approximate barometric pressure into your considerations.

NPSH is based on preventing cavitation, which is the formation of vapor bubbles at the impeller. It will depend on temperature, so make sure you have adequate safety margin for reasonable temperature variations.

Most pump curves show NPSHR for water. Gasoline and Diesel have different vapor pressures, and both will differ from water. Make sure your NPSHR is appropriate for the worst case fluid or somehow differentiate between them.

I wouldn't control the pump with a PID based on the suction side, particularly if this is a flooded suction pump. I would run based on another constraint, such as discharge pressure or flow rate if that matters, or else run flat out until I approach the NPSH limit. Then I would simply increment the speed down until NPSH is OK. I would use a simple deadband control like a home thermostat.
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