I use tube diffusers on the bottom of my pond. They are about 24 inches long, black rubber tubes with an air connector at one end. When new, they provide lots of tiny champagne bubbles that make their way to the surface. Surface disruption is not too visible.

After a few months, they get a leak in them and begin to produce larger bubbles that bubble up at the surface like boiling water. It is obvious when they are leaking and big pain to pull up the (heavy) unit and replace the tube.

My aerator maker tells me it's muskrats biting holes in the tubes, but I don't think so. I think its air pressure that busts a small slit leak in the tube.

Anyone had experience with this? Replacement tubes are not cheap, although I did find a guy on Alibaba that sells them for a fraction of what they cost here.

Thanks

Do you have your diffusers sized properly to the CFM the pump puts out? If you are pushing too much air through the membranes it could cause them to fail prematurely.

Sounds like pressure to me too. Especially, when you say a new diffuser emits tiny bubbles and the broken one is a rolling boil. Is the diffuser tube rated to accept the CFM coming from your compressor? One thing to look at would be the operating pressure of the system when you have a new diffuser installed. Should be a gage at the compressor. I am not an aeration expert but, if a new diffuser is hardly disturbing the surface then you are probably not generating much of a vertical water column either. Everything sounds to me, with the info provided, that your diffuser is undersized.

EDit:

Snrub,

Just saw your post. Ha Ha Great minds think a like!!

I do have a pressure gauge in my compressor cabinet. It is connected to the manifold that feeds the air to the diffusers.

I have three diffusers running off the compressor - one close, one in the middle and one on the far side of the pond. I give the far one a bit more air than the middle one, and the close one gets the least. They seem to be balanced.

What is the typical pressure reading I should be looking for on the gauge?

thanks

If the type of diffuser you use has no internal restrictor, then the pressure will equal roughly a half psi for each foot of depth water head pressure. So if you diffuser sits in 10' of water, the pressure is going to run about 5 psi.

The diffuser membrane itself will have very little back pressure resistance. Probably less than a half psi. If the membrane itself is causing a pressure rise it would mean you are likely pushing way too much air through it. Even in open air with no water back pressure, the membrane will not build up hardly any pressure unless you are putting too much air through it.

When my 9" round membrane diffusers are operating under water at 9' and about 4.5 psi, they bulge up about two inches in the middle. They only bulge up slightly more than that out of water. As the membrane baloons up, the holes or slits expand allowing more room for air to escape. So within the design limits of the membrane, it will only bulge up a little more at maximum recommended flow compared to minimum recomended flow.

You really need to know your pump cfm output rating at the depth you are running your diffusers. Divide that number by 3 (because you have 3 diffusers) and that will give the cfm per diffuser as long as you have them approximately balanced where each is putting out about the same air. Then see what your diffusers are rated at in cfm. They will have a range of acceptable flow rates. For example, a diffuser might be rated at 1.2-2.8 cfm. Anything within that range is ok. If a person is pushing for example 4 cfm through that diffuser that is rated maximum of 2.9, the holes or slits (depending on the design) will be forced open too wide and will stretch or rupture the membrane. Basically the same as blowing up a balloon till it pops.

The pressure is really irrelevant. The pressure is what it takes to push the air against the head pressure of the water just enough to overcome the water pressure and force bubbles out. This pressure is dependent on the depth of water (water head pressure), not the back pressure provided by the membrane.

An exception to the above............. some diffusers may have an internal orifice to resist air flow. This is used to help balance flow between multiple membranes or perhaps prevent damage from excessive flow. If a diffuser has an internal orifice, it may show back pressure above what the water head pressure is if air flow in cfm is above what the diffuser needs to operate properly.

In addition to what Snrub said, here are a couple of excerpts from tech papers you might find helpful


From a paper by M. K. Stenstrom and D. Rosso at UCLA

Both coarse and fine bubble diffusers present a pressure drop. The operating pressure of a diffused air system must include pressure drop in pipelines, the hydrostatic pressure of the water at the diffuser submergence, and the pressure drop of the diffuser.
• A diffuser pressure drop is called the “dynamic wet pressure” and includes both the pressure loss through the diffuser but also the surface tension of the fluid being aerated.
• Coarse bubble diffusers have very low DWP, generally only 5 to 10 mbar. Fine pore diffusers always have more, with ceramic devices having DWPs from 15 to 30 mbar. Membrane devices have higher DWPs, as much as 45 mbar. Consultant manufacturers data and verify DWP when clean water testing

And from Hazen and Sawyer

Regardless of type, model, or manufacturer, the fine-pore diffusers show decreased efficiency and increased pressure drop with increasing air flow rate.

Hope this helps. Good luck. Please let us know what you figure out.

Bill D

Edit: If you are not familiar with mbar pressure units, 45 mbar is about 0.6 psi. The reason I provided these excerpts is to show that aeration is not just buying a compressor somewhere, some hose somewhere and a diffuser somewhere. You need to consider all factors together and the system needs to be designed to match all components to the application.

If animals are biting the membrane to cause larger holes you should see probably multiple tooth marks in the membrane. I have seen membranes on tube diffusers with larger holes or tears but these were created by mechanical punctures. Consider buying a different type of membrane rather than regular EDPM membranes.

Hobart,

I am curious. You say you are running 3 diffusers and you balance them by watching the bubbles on the surface? Are they all pretty much at the same depth or do you have one in a lot deeper water than the others? If yes, does it seem the shallow ones bust more? If yes, then you might want to think about restrictors on the shallow water diffusers as Snrub mentioned. I am not a pro, IMHO you should present your problem to someone that is.