Pump noise on lower PSI

[Grenademonkey543]

Hi all,

I have two 3/4 Northstar pumps and decided to swap them around since the falls pump is barely used and has less wear on it. The primary was starting to show signs of wear (lower PSI, higher noise). The swap went successful. The PSI went from 4 to 8 on non-solar loop and from 12 to ~ 15 on the solar. I couldn't be happier.

I did notice an interesting thing though. When running through the solar loop the pump runs super quiet. When closing the solar off, the pump significantly increases in noise (despite a lower PSI). Can anyone chime in as to what might be the cause?

Videos attached (redirecting to solar -> running on solar)

Closing solar:

Is it cavitation? Or just both pumps are worn enough to were changes in load trigger certain changes in characteristics.

 

[JamesW]

It might be cavitation. If more pressure reduces the sound, that could indicate cavitation.

 

[sktn77a]

2nd vote for cavitation. The pump impeller is trying to pump more water that it's receiving. That causes cavitation. When you add back pressure (and reduce flow) the water supply can now keep up with the pump - no more cavitation.

"The cause of cavitation in pumps is usually due to insufficient NPSH (Net Positive Suction Head) energy on the suction side of the pump."