skimmer pump cavitating?

[Wazzel]

Originally Posted by amutti

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]

Originally Posted by rwb500

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]

Originally Posted by Wazzel

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]

Originally Posted by viggen

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.