# pump cavitation



## gsubioguy (Aug 2, 2019)

I have a brand new mag 18 on a 120RR w/ 40g sump--has only been used during the initial fill of the tank for leak check and evaporation measurements.

I'm not seeing any "large" micro bubbles in the tank nor am I seeing bubbles exiting the returns. However, I clearly hear the pump cavitating and the water in the DT has an ever so faint milky hue. The water in the return chamber isn't low enough for a vortex to form and I am not getting any visible bubbles past the second baffle (see image for baffles). About the only potentially useful info I've come across thus far is a pump of this size should utilize 1.5" return lines (I plumbed 1" from sump to top of tank to a tee to dual 1" runs leading to quad 3/4" loc line return outlets). Basic fluid dynamics says the 3/4" is the ultimate effector on the pump, not the 1" but (and correct me if I am wrong) wouldn't restricting the flow of the pump have the opposite effect on pump? Or the same as adding head height? The only way I've been able to stop the "noise" is by closing the swing valve nearly completely on the return line (thus creating even more back pressure on the pump).

Short of remaking the whole return line setup (and I'm not sold that is the cause) my last thought is to remove the face plate from the pump while submerged and perhaps free any air pockets that might be in it.

Anyone have any further suggestions?


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## Deeda (Oct 12, 2012)

Off the top of my head and never having run a sump, try putting a sponge along the bottom length of the chamber right before the pump chamber to catch any potential bubbles. This is often done when using a sump with baffles.

How high is the water in the pump chamber above the pump all the time that the pump is operating?

Do you have a sponge filter or strainer on the pump intake port?


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## gsubioguy (Aug 2, 2019)

Unfortunately it's not pulling air through the sump (three sets of course foam in place the whole two days its run). And it's not pulling air from the surface of the return chamber.


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## Ronzo (Sep 8, 2013)

gsu;

I'm no expert in this field, so typically just try to revert back to basic science when I need to figger something like this out...a positive displacement pump cavitates as a result of the input flowrate not keeping up with the impeller, so the impeller cavitates on the "pulling" side, and by reducing the output flowrate by constricting the return pipe, you are limiting the flowrate which the unconstricted pump ideally wants to move, and this in-turn reflects back to the input side and stops cavitation...sooo, I think you should look at the input side of pump!...increasing the pump input pressure (can input port be lowered?) or flowrate (although I can't quite see how)...but maybe I've given you some ideas to think about...maybe I've just stated the obvious....let us know how you make out and what your solution turns out to be.

Cheers from Connecticut!


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## Deeda (Oct 12, 2012)

Thanks for the additional pics! Any chance you can post a video with sound of the sump running and the pump chamber?


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## gsubioguy (Aug 2, 2019)

Yesterday I removed the face plate and cleaned the seal, impeller and housing then reassembled submerged to further help eliminate internal air. Initially appeared to remedy but after a few minutes it was cavitating.

Deeda: 
I'll video & upload that this evening. It isn't horrendous but noticeable and like someone popping gum or crinkling a can, once you notice it you become fixated. Not to mention it potentially chewing up an impeller.

Ronzo: 
Since the pump isn't ingesting air, I assume it is due to low inflow. Short of putting an elbow on the input (& an exacerbating the problem), it cannot go any lower. I've even removed the prefilter and guard/grate to eliminate restrictions. Perhaps taller return chamber to increase the water pressure on the inlet would 
help but that would require a whole new sump--initial baffle height is as high as I'm comfortable going and the downstream baffle heights are each only 1" lower than the previous height setting baffle. The stand is 36" tall with tall doors so I can easily fit a taller sump if that ends up being the course of action.

Currently, the return chamber has ~6" of water column (~3.6gal) between min & max (pump forming a vortex to back siphon overflowing baffles). I can add probably another 2-3 gallons to the system and still not flood with pump off. Might try this approach since it won't have any negative effect since no skimmer.

Thank yall for the advice! Again, will get a video up this evening.


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## Ronzo (Sep 8, 2013)

gsu;

Cavitation by definition is not due to moving bubbles in the waterstream (and located where the pump is, its certainly not ingesting any!), so therefore it must a result of air being generated on the impeller, due to the vacuum occurring on the pulling side, actually momentarily pulling the dissolved air in the water out of suspension...so much for the science lesson, sorry...

...so here another idea...locate the return pump outside the sump, and more importantly, LOWER and connected to bottom of last chamber with a hose...yes it would make noise while pumping away air, but once this was gone, the increase of the height of water column at input of return pump (and therefore pressure of course) if enough, would very well prevent the cavitation and associated noise. I don't know if your setup allows this, but if Sump is not on the floor, and there's room, you could just try it as an experiment.

BTW...is there any way to (electrically) reduce speed or power of pump (and propensity of it wanting to cavitate at the current water column height)?

Good Hunting!


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## gsubioguy (Aug 2, 2019)

Ronzo said:


> gsu;
> so much for the science lesson, sorry...


Degree is in biology so no need fretting science lessons plus I am a bit of submarine history buff so know that cavitation is dO2 coming out of solution due to low pressure then the bubbles "popping" as they collapse under pressure.

It would be possible to plumb the pump externally but it'd take a bit of work. As for the pump itself, it's an AC so no throttling. Might end up putting it on fb/craigslist and getting a DC pump.


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## gsubioguy (Aug 2, 2019)

Video is mp4 so cannot upload so I'll add the link to it on youtube. The video makes it appear a good bit louder than it actually is & ignore the cloudiness.


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## Ronzo (Sep 8, 2013)

gsu;

Before moving that pump on, be advised that an AC powered pump _can_ be throttled back by an inexpensive speed controller (degree and experience is in electronics!).

...but I'm interested to hear vid also...

Cheers

Edit: After hearing vid...its tough for me to make any calls from that...


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## gsubioguy (Aug 2, 2019)

Ronzo said:


> gsu;
> 
> Before moving that pump on, be advised that an AC powered pump _can_ be throttled back by an inexpensive speed controller (degree and experience is in electronics!).


Nice! I have a basic understanding of coding for Arduino (C++, I believe?). Could a potentiometer be used?


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## Ronzo (Sep 8, 2013)

gsu;

...not a potentiometer alone, but a pot will be the control input to TRIAC or SCR chopper circuit which controls the motor current. Search AC motor speed control. A control rated for a few amps, like that of a range hood, or similar, will be modestly priced and in the power range you need.

Cheers


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## gsubioguy (Aug 2, 2019)

Additional volume (what I can safely add in my sump design) didn't remedy the cavitation. :?


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