Mag Drive Pumps producing air

[dr_bull]

I have 2 mag drive pumps one is a 1800 and the other is a 950. They are used as the return pumps on two of my tanks. For some reason no matter what I do I always here a slight air noise inside the pump near the impeller area which produces some microbubbles in the display. I have all my plumbing sealed off really well and the pumps are ran externally. I use Teflon tape on the threaded connections to the pump, and hose clamps wherever the tubing meets a connection. There are no leaks anywhere. I am wondering if the pumps are some how sucking in air near the inlet somewhere or if this air noise/bubbles are being produced inside the pumps somehow? Both pumps draw their water from a sump/fuge that is well baffled with no visible microbubbles entering the return area.

Anyone have any idea what my problem could be here? I would love to get rid of the microbubbles in my displays.

 

[AZDesertRat]

Are the suction side lines undersized or restricted in any way? How are they plumbed, through submerged bulkheads with very short large diameter lines hopefully, or up and over the sump side, or....? Are there any sponges, micron filters, elbows, valves, vertical lifts etc on the suction side? Is the discharge side smaller diameter piping than the suction side and does it have a valve that can be used to restrict the discharge to see if cavitation is a problem?

 

[dr_bull]

Here are some pictures for you:

Both pumps are fed the same way, I do have a ball valve on the output of each pump so I can try restricting the flow a bit.

 

[jdieck]

The inlet over the side is restricting the flow causing cavitation. You can try increasing the inlet diameter but needs to be 11/2" which may cause all kind of issues for priming.
When ran external pumps it is always advisable to do it via a bulkhead in the bottom. In your case if you do not want to drill the tank it seems you have enough room to run the pumps submerged so no inlet piping is required. That will solve your problem.
By the way, cavitation may wear out the pump very quickly due to vibration.

 

[dr_bull]

Actually the sump is running fresh water at the moment for testing. But that whole area will be dedicated to a fuge so running the pump submerged is not an option. I thought about drilling the tank before but don't have the proper equipment to do so. The 950 on my other tank is drawing the same way from a 10 gallon sump which already has an established fuge and no room for the pump to sit inside the sump. Would reducing the flow via the ball valve stop the cavitation from occurring?

 

[jdieck]

Reducing the flow may work but Mag Drives may overheat running externally as they are cooled by the water flow.
To confirm cavitation you can restric the output, if the noise and/or bubbles stop then you have cavitation.
Also try running it inside just for a test (Easy to do when you have flex hoses and se how it runs. If so you may need to reduce the size of the sump and dedicate a section for the pump via an added on baffle.

 

[dr_bull]

Ok thanks for the input I will try the things you suggested and see what happens.

 

[jdieck]

Good Luck!

 

[AZDesertRat]

The pumps are way underfed for one and are not designed to function well in an up and over configuration. They are designed to have a completely unrestricted flooded suction as in submersible applications. If you want to run it externally sucessfully you really need to drill the sump, install a 1.5" or 2" bulkhead and run as short a piece of pipe as possible and as large as possible. Your discharge flows much smoother than your suction which also leads to cavitation. Corrigated of flex type tubing like you have used has very turbulent flows and a high C factor or resistance to flow.
I would rethink your pump situation and either drill the sump or go submersible with nothing attached to the suction side. If you absolutely must do it external like you have it use 1.5" or 2" for the up and over piece and a bushing right at the pump to reduce to the inlet thread size so you have as little restriction as possible but you may still have problems.

 

[dr_bull]

Thanks for all the input, I have solved my issue by putting the pumps inside the sumps. The microbubbles are gone and my flow has greatly increased in the tanks. I had a couple threaded strainers that fit in the ends of the mag drives to help keep the larger debris out of the pumps in the sump. It was well worth the space sacrifice to get rid of them microbubbles!

strainer link

 

[AZDesertRat]

Make sure to keep the strainer clean or better yet install a 3/4" tee with two strainers for lower entrance velocity and less clogging.

 

[dr_bull]

<a href=showthread.php?s=&postid=9007894#post9007894 target=_blank>Originally posted</a> by AZDesertRat
Make sure to keep the strainer clean or better yet install a 3/4" tee with two strainers for lower entrance velocity and less clogging.

Yup will keep it clean, interesting idea about the T I may give that a shot.

 

[AZDesertRat]

The lower the entrance velocity the less stuff will collect on the screen surface and the less you restrict the flow.