I built a closed loop system today. I used a mag 9 pump, 1'' pvc intake and 2 3/4'' returns. the problem I'm having is the pumps making a loud sucking sound right where the 1'' intake meets the pump. I used a 1'' to 3/4'' coupling to connect the pvc to the pump. What do you think I did wrong?
its kinda like giving birth, trying to push through a smaller diametre.
are u getting sparatic bubble mix in the return. because if its not cavitation, it could be drawing in a small amount of air at the reducer coupling
i dont know what the mag9 looks like, but if it has throttle control a tweak might fix it.
i think im going to let someone else step in that knows the pump that your talking about. ours differ in operation to yours
if there are no bubbles, then it will be pump cavitation (at least it sounds like this is what you are experiencing). The inlet pipework may be too small. Essentially what is happening is the pump is having a hard time getting sufficient water to the impeller. This causes small bubbles to form as the water is literally pulled apart by the pressure drop at the impeller. The bubbles then collapse as they leave the pump, causing the sound.
This is a damaging cycle. As the bubbles collapse, the water rushes in voilently. In industry you can chew through a steel impeller in days if the pump is cavitating.
Pumps have a characteristic called NPSHR (Net Positive Suction Head Required). Depending on your pipework, the NPSH Available is either higher or lower than the NPSHR. In your case, the pipework is restricting flow into the pump.
I have yet to see an aquarium pump with the NPSHR listed. My guess is that is you replace the inlet pipework with 3/4 inch ID, it should stop the suction sound. It will also dramatically increase the flow you're getting through the pump.
closed loop problems 
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