i'm using a centrifugal pump to pump RO water.  The RO water originally has some fine air bubbles.  But they should leave over time.I have a flooded suction with a 2.5" suction, and about 1ft of straight pipe before the suction.I'm recircing the water right back into the tank, and it just continues to create bubbles and sometimes airy-foam.  The recirc back into the tank is under the liquid level and you can just see all the air bubbles discharging.I dont have any leaks in the system, pump doesnt vortex or appear to suck air in any of the fittings before the inlet.  Any ideas? 
 

I was wondering about that.It's a Self-priming pump direct coupled pump, and there isn't a gearbox etc.  It has a mechanical seal.None of the pump leaks, and I've had it up to 100 psi on the discharge.  I would think that if someone were lose/worn and it was sucking air, that when the pump was off, or under high pressure, it would leak water out..  
 

1gibson, I tightened the bolts on the housing.  8 bolts.  eahc turned about 1/8th of a turn.  And the bubbles went away.I guess i don't understand why it didn't leak under high pressure.  But it's fine now.I spent all morning trying different combinations with the pump and the system to see if i could isolate where the air was coming from, finally decided it was the pump and tightened the bolts.. viola.Seals may not leak under pressure but will under vacuum.  Kind of unidirectional that way.  High pressure may be enough to force the seal tight, low pressure below atmospheric may not.  At rest the seal has enough self-pressure, squeeze, to not leak.  It's also not unusual for some air to come out of solution in the impeller inlet eye and not fully re-dissolve into the liquid. This can happen well before the onset of cavitation due to small disturbances in the flow path.

2011-09-01

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