Pump sucking in Air

[defigart]

I have a pump that is sucking in air and sending out water with micro bubbles.

 

My thoughts. Since there are LOTs of tiny bubbles coming out, then the pump must be sucking in air prior to going through the impeller, and not in the impeller housing.

 

I can seal all the PVC connections with solvent, but was wondering if anyone else had any ideas.

[siskiou]

How far under water is the pump located?

Is it sucking in air from the surface, or does it seemingly come out of nowhere?

And what type of pump is it? Return, powerhead for circulation?

[andy]

If it is on a closed loop, it might be cavitating.

[defigart]

Its an inline pump. I suspect cavitation might be an issue. How do I prevent it from happening.

[andy]

Cavitation happens when the pump isn't able to draw enough water into it's intake; solutions will vary depending on exactly why the pump is getting starved. Possibilities include larger diameter drain (plumbing, bulkhead, strainer, etc) or restricting the return (valve, usually).

 

Is this an over-the-edge (as in, not-drilled) closed-loop?

[defigart]

Yes here are some pics of it. It came with 3/4" PVC threaded attachments, so I used 3/4" PVC into, and out of the pump. I also ran an airline up into the part of the line going to into the pump to a small power head to suck the air out of that section of the line.

 

Pump has 12ft of head, and I am using less than 4ft to go back up to the tank.

[andy]

I had a feeling... I tried something very similar with an Eheim 1262 driving a SCWD. I wasn't able to get free enough flow (using 1.5" PVC) over the top of the siphon to avoid cavitating the pump. It drove me nuts, eventually becoming more bother than it was worth.

 

You could try restricting the pressure side, but that kind'a kills the whole point of the closed loop, right? Be sure to post if you find a solution.

[defigart]

What is I temperariily restricted the pressure side, then when the pump was pumping well then unrestricted it?

 

Or do you think the issue is the distance down to the pump? I could raise it up and closer to the tank, if you think that will make the difference.

[andy]

I don't think the problem is the distance to the pump. On a true closed loop, the pressure is constant throughout the loop so there is no head pressure -- the pressure on the drain side and the return side are equal so no head. Anyways, when I tried it my pump was maybe 8" down from the top of the tank.

 

I'm beginning to think it's that siphon messing up the system -- somehow it's reducing the pressure on the drain side of the loop, which leads to the cavitation. Maybe someone with a stronger background in hydraulics will chime in.

[defigart]

For now I put the pump back into is submersible form, and placed it in the tank. ( I don't like it but at least its not cavitating anymore.) If I have to I can hide the pump behind a large piece of rock later.