(Luca Brazi) wrote in message . com...
"Jon C Rupert" wrote in message ...
"Mort" wrote in message
. com...

"Luca Brazi" wrote in message
om...
Just switched out a Mag 3 for a Mag 7 in a closed loop SCWD
circulation system and the blasted thing is churning out microbubbles
and rattling unbearably (neither of which the Mag 3 did). It is
slightly better when I restrict flow a bit on the output side, but
that rather defeats the purpose of using the new pump now doesn't it?
There is a simple U tube connected to the pump inlet by about 2.5 feet
of 3/4" hose. There's got to be air getting in somewhere but I can't
find any leak. I didn't experience a similar "break in" with my Mag
3. Microbubbles means the leak, if any, has to be before the
impeller, right? Anyone with more experience want to tell me what's
happening here?


I could really be reaching here and I'll prolly get slapped for saying
this
but....


In a hydronic heating system (closed loop) there are occurances where if
the
pump is oversized or the line is undersized the pump will actually reduce
the pressure on its suction side to the point where the water will begin
to
turn to vapor. (ie=make bubbles) This could explain the rattling.

I seriously doubt this is the case. I'd be surprised if a Mag pump could
even do this but a mag 7 is considerably larger than a 3. Why the change?

What type of hose is it that you are using? Is there anything else
connected by a tee or otherwise near the outlet of the pump?

~Mort


You may have a good point. I think the technical term is cavitation. It
can destroy a pump and it typically happens when the pump is not properly
sized for the system.

The pump is on a 29gal approximately 2.5 to 3 feet below the tank's
top. There is a simple U tube connecting with 3/4" black vinyl tubing
running straight down to the inlet. From the outlet it runs up one
foot to the SCWD (ball valve in the middle) and the SCWD connects to U
shaped outlets than hang over the edge in each of the tank's back
corners. Given a 3' vertical run and the SCWD taking ~20% off the
flow rate, I estimated that the pump would be circulating around 300
to 350 gph. Since the connections on the pump are 1/2" MPT and FPT I
figured that 3/4" hose would provide sufficient water volume to the
pump. It certainly worked with the Mag 3, but I wasn't getting enough
circulation in the tank and wanted to up the water movement without
adding powerheads. Thanks for responding so quickly.

Still spewing microbubbles after running for 18 hours so I
disconnected it and put the Mag 3 back on. Circulation and no noise
or microbubbles. I remain confused but am willing to accept that 3/4
hose is too small a diameter for this span. So the question is: would
a system of sufficient diameter and vertical travel to avoid the
cavitation problem with a Mag 7 have any more GPH of flow than does
the Mag 3 with the existing dimensions?

Another option might be to split the hose with a "T" before the SCWD
and use a ball valve to divert a % of the outflow directly into the
tank through a third return. Principal cost of the experiment would
only be time and frustration but has anyone out there done anything
similar or know why it wouldn't work?