skimmer pump cavitating?

[rwb500]

my diablo DC skimmer pump will occasionally become louder and have a greatly reduced ouput. putting my finger over the air intake for a couple seconds and then releasing it fixes the problem and restores the pump to proper function.

it is more prone to this happening at high speed (6/6). At 5/6 it happens about once a day.

Is this some type of cavitation phenomenon? If i restrict the air intake a bit, will that prevent it?

[Wazzel]

Are you running it at the recommended water depth? How long has it been since it has been cleaned?

[rwb500]

the pump is clean, I just cleaned it before I set up the new skimmer. It is in pretty shallow water (~6"). The pump itself doesn't have a recommended depth but I can imagine that it is designed to function a little deeper, sucking less air.

[Wazzel]

What model is it?

[rwb500]

diablo dc3500s

[Wazzel]

The recommended water level for the 200-INT is 7-8.5 inches. It uses the same pump. It looks like you need to raise the water level in your sump 1-2 inches.

[rwb500]

the water depth recommendations for a skimmer have much more to do with the height of the skimmer outlet than anything to do with the pump air/water mix. have you ever heard of this happening with a skimmer pump? if excessive air intake is the problem I will not be re-doing my entire sump, I will just restrict the air intake.

[Wazzel]

A pump, any pump, needs a certain amount of inlet pressure to function properly. If it is cavatating it does not have enough inlet pressure. Restricting the air flow will not change that. I suspect the recommend water depth have lots to do with the operation of the pump and not so much with the skimmer body. I do not know the specifics on this pump, but I use to design and test pumps for a living. Centrifugal pumps all function similarily.

[Fishmommy]

^this. Water depth must meet minimum spec and inlet needs to be completely unrestricted

[rwb500]

well, it has now happened twice in the last hour. When it happens I can see a ring of bubbles just inside the opening of the intake volute. When I put my finger over the air intake, these bubbles disappear and the pump returns to functioning normally.

it is not cavitation within the pump caused by the impeller, and it doesn't seem to be excess air coming from the venturi, either. what gives?

[rwb500]

here is an example of the intake design shown:

https://lh4.googleusercontent.com/-TGfxRh_q1u8/VOVEUbcFWZI/AAAAAAAACDA/AN0T8JUBxuM/s640/IMG_2195.jpg

normal function:

https://lh4.googleusercontent.com/-LoBB_QPCMn8/VOVEUJL2RsI/AAAAAAAACC4/CtV8BFg0wqw/s640/IMG_2189.jpg

when it gets messed up:

https://lh5.googleusercontent.com/-YeZ9Da4ZnLw/VOVETtPJqjI/AAAAAAAACCw/4IK4S7Q_ze4/s640/IMG_2187.jpg

[Kenmx10]

What does the motor sound like when it changes? Also does the water flow change or is it the air flow that changes?

[Wazzel]

I really can not see anything in the pictures. Trying to think of what else it could be, but nothing else comes to mind. Is it pulling a surface vortex? If it is that is a water level issue too. That one can sometimes be fixed with extending the inlet. On vertical pumps or inlet pipes we would put rings to slow down the velocity around the inlet. You could try to rig a baffel over the inlet to see if that stops it.

[rwb500]

I really can not see anything in the pictures.

in the picture of when it is messed up - the intake is surrounded by a ring of air. it is a pretty dramatic contrast to the intake under normal operation.

[viggen]

I do not think the pump needs a certain amount of head pressure to work/not cavitate. It needs a certain amount of backpressure to ensure the proper amount of air/water inside the skimmer body to give optimal performance.

My bet is the pump itself is defective. I had one of the bubbleblaster 3000 pumps do the same thing. It would shut down/cavitate every once in a while... Called up coralvue and shipped the pump to them and they sent me a new one.

[rwb500]

What does the motor sound like when it changes? Also does the water flow change or is it the air flow that changes?

it sounds louder and higher pitched - as if the volute has too much air.

I can't tell what changes - but the skimmer body does not fill up as high. It could be less water and less air, or less water and more air.

there is no surface vortex. I have taken photos of the only visible changes, revealing a ring of air bubbles forming around the inside of the intake volute.

[Kenmx10]

Have you taken out the impeller and examined it. Sounds like a defect in the impeller or pump.

[rwb500]

the pump is in perfect shape. did you look at the pictures? there is clearly a strange phenomenon occurring that allows a pocket of air to exist in the pump intake. I think I will just design a different style of venturi adapter. That should fix it.

[Kenmx10]

The picture is showing the water intake, So if air is coming out around the water intake, It would lead me to believe there is a problem with the impeller , or the impeller is running reverse. But without inspecting the impeller, we're just guessing.

[rwb500]

air is not coming out, it is simply sitting there. it looks like bubbles are being created at the sharp lip of the intake, and being sucked into the pump, creating too much air in the volute. you can see all of this in the picture. Is it possible that an area of sufficiently low pressure (sharp edge of narrow pump volute) would cause gases to come out of the water?

perhaps I will simply round that edge with a file.

I have just assembled the pump, it is in perfect shape. It works perfectly until it is cranked up to 5/6 or 6/6, at which point this happens. Additionally, the impeller running in reverse is both impossible on a DC pump and the water would still flow in the correct direction, as it is a centrifugal pump. Also the skimmer is still filling with water and air, just not quite as much.

I was hoping someone would have had a similar experience. I can tell that you are just guessing.

[Kenmx10]

Sorry, sounds like we are both guessing. Haha. Good luck

[rwb500]

yeah i have no clue. and its a very difficult situation to describe. oh well i'm filing it round right now, i'll post back.

[Wazzel]

I do not think the pump needs a certain amount of head pressure to work/not cavitate. It needs a certain amount of backpressure to ensure the proper amount of air/water inside the skimmer body to give optimal performance.

My bet is the pump itself is defective. I had one of the bubbleblaster 3000 pumps do the same thing. It would shut down/cavitate every once in a while... Called up coralvue and shipped the pump to them and they sent me a new one.

That is incorrect. The term is NPSH, net positive suction head. It is the amount of pressure required at the inlet of the impeller to keep it from cavitating. All centrifical pumps have some sort of NPSH requirement. Our tiny aquarium pumps are not exempt.

Back pressure in the skimmer is regulated by the outlet valve. Skimmers do not neen to sit in any water to operate properly.

[Wazzel]

Now that I can see the pic on a larger monitor, you are indeed cavitating. You can try to round the corners of the inlet to soften the effect. It may or may not work. Two thing that would work is to slow the flow or raise the water level to 7-8.5 inches as recommended by the skimmer manufacture.

There is more than one type of cavitation. You can get it at reductions in flow area/increase in flow velocity. Venturis and orifice plate are know to have cavitation issues. Sometimes it is used on purpose. What is happening with you is Hydrodynamic cavitation. It is different that what we generally thing of when we talk about cavitation associated with pumps, which is at the impeller inlet.

[amutti]

My Skimz DC does the same thing at 100% speed. It works perfectly at 80%. BRS said it was normal??

[Wazzel]

My Skimz DC does the same thing at 100% speed. It works perfectly at 80%. BRS said it was normal??

It should not be happening. The venture inlet is not properly sized for the flow. Rounding off the inlet corner might help.

[rwb500]

thanks Wazzel! I had never heard of cavitation not involving impeller blades, but I'm glad you know what is happening. I filed that sharp edge down, I can't imagine that the problem will happen any more. The plastic was pretty thick so I was able to round it down a good amount.

amutti - i think these pumps come with venturi adapters that are a bit too narrow for the pumps to run at full speed. a pump with adjustable speed really needs different sized venturi adapters to maintain low pressure, but avoid cavitation, at the venturi.

[Wazzel]

I am not 100% sure it will remove the problem. Sharp transitions are a problem with flow disruptions. Anything that you can do to smooth the transition will help, the unknown is if it will be enough. It might push the problem further down. Keep an eye on it for a while.

[rwb500]

thanks, i sure will. would a less restrictive venturi adapter be another possible solution?

[Wazzel]

thanks, i sure will. would a less restrictive venturi adapter be another possible solution?

Yes, reducing the velocity of the flow by running the pump slower or a larger opening would address the problem.

[viggen]

That is incorrect. The term is NPSH, net positive suction head. It is the amount of pressure required at the inlet of the impeller to keep it from cavitating. All centrifical pumps have some sort of NPSH requirement. Our tiny aquarium pumps are not exempt.

Back pressure in the skimmer is regulated by the outlet valve. Skimmers do not neen to sit in any water to operate properly.

Correct.... however my uneducated opinion is that if a skimmer has a ideal depth of say 8 inches.... and the skimmer is running in say 4 inches.... and experiences cavitating. I would highly doubt that these manufacturers would make such a product that is that finicky on the pump working or not. It obviously would affect bubble/water mixture but at least to me it shouldn't cause cavitation. I know I can run my BB5000 w/o and plumbing on either end of the pump and it doesn't cavitate.... that's in about 4.5in of water which is my sumps water depth. Possibly cavitation isn't the issue but the pumps computer chip is faulty causing it to shut off briefly

So the heigth of the skimmer has nothing to do with backpressure that the pump sees? If the skimmers reaction chanmber is 4ft tall it still only sees the head pressure as a 1ft tall skimmer assuming the exhaust water exits at the same level? I always assumed a certain skimmer manufacturer mounts their pumps higher in the body to reduce backpressure....

I also thought some skimmers are designed to sit in water while others are designed for external. As with the OP's pump, it does not seem to be one setup for running externalls due to looking like the same venture/intake that my BB3000 (that did exactly what he is experiencing) & BB5000 use. I do agree that the pump could sit in the sump and the skimmer itself could sit outside.... which then depends on the quality of the craftmanship.... will it leak

[Wazzel]

Correct.... however my uneducated opinion is that if a skimmer has a ideal depth of say 8 inches.... and the skimmer is running in say 4 inches.... and experiences cavitating. I would highly doubt that these manufacturers would make such a product that is that finicky on the pump working or not. It obviously would affect bubble/water mixture but at least to me it shouldn't cause cavitation. I know I can run my BB5000 w/o and plumbing on either end of the pump and it doesn't cavitate.... that's in about 4.5in of water which is my sumps water depth. Possibly cavitation isn't the issue but the pumps computer chip is faulty causing it to shut off briefly

So the heigth of the skimmer has nothing to do with backpressure that the pump sees? If the skimmers reaction chanmber is 4ft tall it still only sees the head pressure as a 1ft tall skimmer assuming the exhaust water exits at the same level? I always assumed a certain skimmer manufacturer mounts their pumps higher in the body to reduce backpressure....

I also thought some skimmers are designed to sit in water while others are designed for external. As with the OP's pump, it does not seem to be one setup for running externalls due to looking like the same venture/intake that my BB3000 (that did exactly what he is experiencing) & BB5000 use. I do agree that the pump could sit in the sump and the skimmer itself could sit outside.... which then depends on the quality of the craftmanship.... will it leak

The bubbles being produced at the inlet are vapor pockets being produced, cavitation. Pumps have parameters they need to operate properly. Minimum water level is one of them. It varies between pump models. For internal skimmers the minimum water level is the water level in the sump, not the water level in the body of the skimmer.

Pumps on skimmers only see the pressure difference between the water level in the sump and the level in the skimmer body if internal or the distance between the nozzle and the level in the skimmer body if external (assuming a recirculating external with a feed pump). That height is regulated by the outlet valve. Closing the valve creates a loss and that loss is reflected in the height of water in the skimmer body. A tall skimmer will see more pressure at the pump than a short skimmer because of the operation height of the water in the body.

Not much difference in internal vs external, mostly plumbing and pump choices as far as I can tell. The basic operating principal is the same.