skimmer pump cavitating?

rwb500 · 02/18/2015, 04:40 PM

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 · 02/18/2015, 05:19 PM

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

rwb500 · 02/18/2015, 05:58 PM

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 · 02/18/2015, 06:01 PM

What model is it?

rwb500 · 02/18/2015, 06:12 PM

diablo dc3500s

Wazzel · 02/18/2015, 06:58 PM

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 · 02/18/2015, 07:01 PM

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 · 02/18/2015, 07:13 PM

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 · 02/18/2015, 07:15 PM

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

rwb500 · 02/18/2015, 07:58 PM

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 · 02/18/2015, 08:05 PM

here is an example of the intake design shown:

normal function:

when it gets messed up:

Kenmx10 · 02/18/2015, 09:04 PM

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

Wazzel · 02/18/2015, 09:08 PM

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 · 02/18/2015, 09:11 PM

Quote:

Originally Posted by Wazzel

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 · 02/18/2015, 09:17 PM

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 · 02/18/2015, 09:22 PM

Quote:

Originally Posted by Kenmx10

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 · 02/18/2015, 10:12 PM

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

rwb500 · 02/18/2015, 10:17 PM

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 · 02/18/2015, 10:58 PM

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 · 02/18/2015, 11:09 PM

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 · 02/18/2015, 11:12 PM

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

rwb500 · 02/18/2015, 11:19 PM

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 · 02/19/2015, 06:59 AM

Quote:

Originally Posted by 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.

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 · 02/19/2015, 07:06 AM

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 · 02/19/2015, 08:10 AM

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

02/18/2015, 04:40 PM	#1
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	skimmer pump cavitating? 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?

02/18/2015, 05:19 PM	#2
Wazzel Registered Member Join Date: Mar 2002 Location: Houma, LA Posts: 4,743	Are you running it at the recommended water depth? How long has it been since it has been cleaned? __________________ Mark Beware the light at the end of the tunnel. Sometimes it's a train. Current Tank Info: 120, LED, Bare Bottom, SPS/LPS

02/18/2015, 06:01 PM	#4
Wazzel Registered Member Join Date: Mar 2002 Location: Houma, LA Posts: 4,743	What model is it? __________________ Mark Beware the light at the end of the tunnel. Sometimes it's a train. Current Tank Info: 120, LED, Bare Bottom, SPS/LPS

02/18/2015, 06:58 PM	#6
Wazzel Registered Member Join Date: Mar 2002 Location: Houma, LA Posts: 4,743	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. __________________ Mark Beware the light at the end of the tunnel. Sometimes it's a train. Current Tank Info: 120, LED, Bare Bottom, SPS/LPS

02/18/2015, 07:13 PM	#8
Wazzel Registered Member Join Date: Mar 2002 Location: Houma, LA Posts: 4,743	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. __________________ Mark Beware the light at the end of the tunnel. Sometimes it's a train. Current Tank Info: 120, LED, Bare Bottom, SPS/LPS

02/18/2015, 05:58 PM	#3
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	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.

02/18/2015, 06:12 PM	#5
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	diablo dc3500s

02/18/2015, 07:01 PM	#7
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	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.

02/18/2015, 07:15 PM	#9
Fishmommy Registered Member Join Date: Feb 2013 Location: NH, USA Posts: 1,214	^this. Water depth must meet minimum spec and inlet needs to be completely unrestricted __________________ 135 DSA reef w/50 MRC sump. 10 + 5 QT, 180 planted, 75 planted

02/18/2015, 07:58 PM	#10
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	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?

02/18/2015, 08:05 PM	#11
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	here is an example of the intake design shown: normal function: when it gets messed up:

02/18/2015, 09:04 PM	#12
Kenmx10 Registered Member Join Date: Nov 2008 Location: Charleston S.C. Posts: 771	What does the motor sound like when it changes? Also does the water flow change or is it the air flow that changes?

02/18/2015, 09:08 PM	#13
Wazzel Registered Member Join Date: Mar 2002 Location: Houma, LA Posts: 4,743	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. __________________ Mark Beware the light at the end of the tunnel. Sometimes it's a train. Current Tank Info: 120, LED, Bare Bottom, SPS/LPS

02/18/2015, 09:17 PM	#15
viggen Registered Member Join Date: Sep 2004 Location: Dayton, Ohio Posts: 2,213	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. __________________ A tub of fish

02/18/2015, 10:12 PM	#17
Kenmx10 Registered Member Join Date: Nov 2008 Location: Charleston S.C. Posts: 771	Have you taken out the impeller and examined it. Sounds like a defect in the impeller or pump.

02/18/2015, 10:17 PM	#18
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	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.

02/18/2015, 10:58 PM	#19
Kenmx10 Registered Member Join Date: Nov 2008 Location: Charleston S.C. Posts: 771	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.

02/18/2015, 11:09 PM	#20
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	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.

02/18/2015, 11:12 PM	#21
Kenmx10 Registered Member Join Date: Nov 2008 Location: Charleston S.C. Posts: 771	Sorry, sounds like we are both guessing. Haha. Good luck

02/18/2015, 11:19 PM	#22
rwb500 Registered Member Join Date: Mar 2010 Location: Atlanta Posts: 1,694	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.

02/19/2015, 07:06 AM	#24
Wazzel Registered Member Join Date: Mar 2002 Location: Houma, LA Posts: 4,743	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. __________________ Mark Beware the light at the end of the tunnel. Sometimes it's a train. Current Tank Info: 120, LED, Bare Bottom, SPS/LPS Last edited by Wazzel; 02/19/2015 at 07:38 AM.

02/19/2015, 08:10 AM	#25
amutti Registered Member Join Date: Nov 2006 Location: MA Posts: 357	My Skimz DC does the same thing at 100% speed. It works perfectly at 80%. BRS said it was normal??