Closed loop Cavitation?

[bluewatercandy]

I built a closed loop system consisting of two 1.25" pipes over the side of the tank. They have what must be 500 1/4" holes drilled in each one. These pipes then tie into a 2" pipe outside the aquarium. A Gen X PCX70 pump (1500gph) picks it up from there and pumps through a 1" ball valve and then into 1" pvc pipe that makes a complete loop around the top of the tank. There are 5- 1/2" flexible ball socket returns off this loop.

My problem is that I can only run the pump valved back to about 3/4" flow. If I open the valve any more than that I hear cavitation or something in the pump impeller housing and get microbubbles.

Is there not enough water being supplied to the pump? Or could it be that this pump must have a certain amount of backpressure? I have been toying with buying a sequence dart, but would like to just swap pumps and not redo the entire CL.

What is going on with this thing?

[asm481]

Are you sure it isn't just still got some air in the uptake pipe. How long has it been running. I have a blueline pump pushing 1100 gph with an over the back supply that is only 1" pvc with the same 500 1/4 inch holes and no cavitation. If you have unions and valves on both sides of the pump try and flush it. remove the return side and rig up a tube to let you run the pump with no restriction to pull any air out. Your setup of 2 pipes tee'd into one leaves a lot of place for air to get stuck on original startup.

[bluewatercandy]

I will try that. The system has been running for about a year and it seems that every once in a while I can get away with opening the valve slightly. Do you think that a sequence dart would work with this type of setup, not having the option to drill the tank?

[ctripi]

I have a similiar closed loop set-up with a dart. You might have air trapped in the portion of your intake as it goes over the top of the tank. For my over-the-top 1.5" intakes which feed into the 2" dart intake, I used the following parts from intake to pump.....threaded overflow strainers....up to slip-slip 90 degree elbow (now going over the top of tank)....to slip-slip tee (continuing over the top and comprising the back). The top part of the tee has a slip-fpt bushing and threaded plug while the bottom part drains down to the pump. The reason why I mention the slip-fpt bushing set-up is that I feel it's needed to prime the loop without any air. I take a threaded plug out of the top of the tee, replace the threaded overflow strainer with another threaded plug, fill the loop until it almost spills out of the top, close the loop and replace the strainers. No air, no cavitation. PS...very pleased with my dart loop.

[bluewatercandy]

Thanks everyone! Sounds like I just need to do a little fine tuning.

[dsanfilippo]

Its possible too that you have a small pinhole air leak on the suction side that pulls air in when the pump is moving a lot of water...just a thought