Closed loop

[Guest reeffoto]

I am finishing up installing my new 150 gallon tank and am having a problem with the closed loop.  I am getting constant air blowing out.  The pump is level with the top of the tank so that I could reduce the head on the return and get more flow.  I am starting to think this might be the cause since the pump is having to do more suction that just a syphon bringing the water to the pump.  I have checked for leaks in the intake side of the pvc to the pump and have found none.  I primered and glued all fittings and even put aquarium silicone on the joints.  Am I correct in what I think the problem is or does someone have another idea of what it could be.  The end of the intake goes into the tank about 10 inches so it's not getting the air from there.  The pump is a mag 12 and I did prime it and some water is flowing along with a lot of air.

[krish]

Refetto,

 I have a mag7 closed loop on my 120.

 I have it such that the intake plumbing is oversized than

 the intake of the pump. The pump is on the stand behind

the tank. The intake is like an inverted "J". I have a union

that can be removed to pour water to maintain/start

the siphon. I have the return also submerged in water.

 

The only time i get air bubbles is until the air gets purged out.

This happens if the main pump is off and the water level drops

exposing the sea swirl nozzle.

 You probably will not lose much head if the pump is lower.

Keeping it lower will help the siphon. FWIW, i removed the

SCWD and replaced it with a sea swirl. The tank looks a

lot lot better. The corals are happy.

From my experience with the closed loop, have a ball valve

to drain the loop if you need. Have another ball valve on

the return to stop the water and open the union to let

air in and break the siphon. This is invaluable, considering

that it took me 15 minutes to replace the SCWD with the

sea swirl and have the tank up and running.

 

To digress, i would like  some of the the single stranded

copper wire that you were planning to get rid off. But

will pick it up around Jan 10 or so,if that is available and fine

with you.

 

Good luck

 

thanks

krish

[iceman]

Do you have any union-type valves on the loop?  If so, do they have the rubber o-ring installed?  When I set mine up I couldn't figure where the air was coming from until I checked my union valve and found the o-ring on the floor.

[Guest snapper]

Technically speaking, no one has a "closed loop" in this hobby unless the top of the tank is completely sealed and no free surface is in contact with the atmosphere.

 

I know, yadda yadda, quack quack.

[michaelg]

Semantics my friend- it is all in how you define a "closed" thing- whether it be a loop or a mind.....

 

Bumping the intake side might just be the trick.  A long time ago, there was discussions on a process known as catavation- where by the suction would take water and put it into a gas....  I never really thought this was possible though, and suspect there is a leak somewhere.  Did you tape the threads with teflon tape?

[Armson]

Hey Krish,

 

If your not using your scwd any more I will take it off your hands.

 

 

 

 

B :D

[GaryL]

here is something that discusses the process of cavitation it happens more in automobiles (water pump). i also agree that it is an air leak somewere in the system. hope this helps, G

 

Suction Cavitation

 

Suction Cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.

 

Discharge Cavitation

 

Discharge Cavitation occurs when the pump discharge is extremely high. It normally occurs in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater similar to what occurs in a venturi and turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition due to the high pressure condition premature failure of the pump mechanical seal and bearings can be expected and under extreme conditions will break the impeller shaft

[Guest reeffoto]

There was a air leak in the line.  Was just a pain to find.  Yes cavitaion is a bad thing.  One place I really hated cavitaion was when I was on a sub.  Noise on a subamarine is a very bad thing.  Thanks for the input.

[Sph2sail]

From Cruising World: Hands On Sailor column on noise abatement, their is a nice definition of cavitation:

 

The creation of cavities or bubbles in water by a swiftly moving object [sph:like a prop or impeller].  Cavitation bubbles result from localized low pressure that occurs around a propeller blade, primarily at the tip.  When the pressure in the water drops, the boiling point drops as well, which causes the water to boil in this region, forming bubbles of water vapor.  When a bubble moves out of the low-pressure region, its existence is no longer sustainable, and it implodes with a sharp crack, like a tiny thunderclap.

 

Would seem to me that cavitation can not cause lots of bubbles in a pump's effluent, as the low pressure point/lowered boiling point would not last too long.

 

Glad you found the air leak.

 

s