Power head blowing bubbles

[showey]

Maybe a newbie question but I have been trying to find out why a 402 powerhead 5" below the water suface occasinally blows out bubbles? I cant find any signs of wear.
Can someone help please?

 

[jdieck]

When the pressure drops in a fluid with dissolved gases, the gases tend to vent out of the liquid (in this case water and CO2 Nitrogen and Oxygen). This effect is much like opening a bottle of soda pop. In any case similar thing happens in a powerhead.
Pressure in the water drops as it is pulled trough the pump inlet so some of the dissolved gases in the water will get released specially if the inlet is somehow restricted (Say in the 402 you if you have the inlet partially closed to regulate flow).
This bits of gas get to accumulate inside the rotor housing and eventually comes out in the form of a bubble that the impeller catches and churns into thousands of microbubbles.
A potential solution is to insure there is no restriction in the inlet or set the powerhead deeper so the added on pressure from the water column help prevent or reduce the effect.
If it is for water circulation the use a propeller type powerhead has a lot less tendency to the effect.

 

[showey]

BAM!
Now thats an answer.
Thank you!

 

[kidako]

I would have just said turn it over to let any trapped air out.
what jdiek said is my final answer ?

 

[jdieck]

Yes you can try it upside down, that helps some times. If your 402 has the otion of flow control and reverse flow try have it upside down when you start it up to insure that any air trapped in the reversal chamber has come out.

 

[showey]

Been there, done that. I DO have the flow restricted and I AM going to get one of the Prop types. In the mean time, I little more flow and a little deeper in the tank has made a big difference. Thanks again!!

 

[Logzor]

I had the same problem. They were creating a small whirlpool and siphoning down air from the surface.