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Help me understand airline check valves

PostPosted: Sat Jun 01, 2019 1:37 pm
by Samadhikash
I think I'm missing something about check valves. I understand how the valve itself works, but I'm not understanding why or when they are required. If I put one end of a tube in the tank (let's say it's connected to a sponge filter) and the other end on the floor, I could wait 10 years and it will never start a siphon. If I put an air pump on the floor end and let it run till it bubbles in the tank, I can take the hose off and put it back on endlessly without ever starting a siphon. So what I'm trying to figure out is what conditions are necessary to allow an air line on an air pump to start siphoning water from the tank.

I know it sounds silly, but I'm asking honestly. I feel like I'm back in high school and struggling to get a passing grade in physics. Under what "real world" conditions does an air line slipping off a pump (or the pump failing) will it start to siphon water from the tank? I feel like I could set up an experiment in a lab using altered pressure gradients to make it happen, but those conditions would never be naturally created in the living room. Short of sucking on the floor end of the airline to intentionally fill it with water to start a siphon, I can't make it work.

FWIW, I use check valves. But I really don't understand what has to happen for there to be a problem they can prevent.

Re: Help me understand airline check valves

PostPosted: Sat Jun 01, 2019 1:42 pm
by noddy
No idea. I just added one to my fry tank after not using one for 25 years for no reason whatsoever.

Re: Help me understand airline check valves

PostPosted: Tue Jun 04, 2019 5:42 pm
by AV8TOR
The check valve is a simple device that one way air can flow through but nothing can flow back the other direction. Try it by blowing on each end. Where it comes in is when the power goes out. It will stop your little air line from being a siphon and draining water from your tank

Re: Help me understand airline check valves

PostPosted: Mon Jun 10, 2019 6:40 am
by nodima
I understand your question, and I think Yogi Berra sums it up with this (paraphrased) In theory, there is no difference between theory and reality, but in reality there is a difference.

I always imagine a large fish like an oscar to mouth the airstone and push water back through it.

Re: Help me understand airline check valves

PostPosted: Mon Jun 10, 2019 8:11 am
by Ichthys
If the power goes out and the airpump stops suddenly, water will push the air back up the tubing to an inch or so above the water level, which in some cases can mean going over the back... and of course if it goes too far it will empty the tank.

Check valves prevent this. Another way to prevent it is to have the airline loop more than an inch or so above the water level. Given that check valves also severely reduce the airflow, I know my preferred method.

Re: Help me understand airline check valves

PostPosted: Thu Jun 13, 2019 2:56 pm
by Ronzo
Sam; They are a secondary safety valve (and cheap insurance!) to prevent the airline (which is normally flowing air in one direction) from possibly flowing water in the other direction after extraordinary events take place...

Ich; It is quite unlikely, but I agree that with the possible inertia of a column of water rushing up the airhose after a pump or hose failure, if the inertia is sufficient to make it over the hump and to the downhill side, a siphon might be started with disastrous results...

I have to disagree that checkvalves "severely reduce the airflow"...only in the reverse direction!

Cheers

Re: Help me understand airline check valves

PostPosted: Mon Jun 17, 2019 8:06 am
by Narwhal72
What ichthys said is correct. In the event of a sudden shutoff there is an inertia generated bounce back that can cause a siphon to form if the aquarium is full and the water is able to make it over the hump. I have experienced it myself.

IME it matters on the design and age of the check valve and air pump on how much air flow is reduced. Most check valves use a silicone rubber valve that is pushed open by the air flow on one side and squeezed shut when the air flow is not present or from the reverse side. If the silicone is soft and pliable and the air pump has good pressure there is little resistance to airflow. But if the silicone has aged and become stiff it takes more pressure to keep it open and many air pumps are low pressure which can severely reduce the air flow. That is why the check valves should be replaced regularly. Otherwise you risk blowing out the diaphragms of the air pumps from the increased back pressure.

Andy

Re: Help me understand airline check valves

PostPosted: Mon Jun 17, 2019 9:32 am
by Samadhikash
Thanks for the replies. I can visualize the "bounce back" when the air supply suddenly stops. I have a better understanding of the perfect storm factors that enable the siphon. Appreciate the input.

Re: Help me understand airline check valves

PostPosted: Mon Jun 17, 2019 2:14 pm
by Ichthys
A lot of (brand new) check valves severely reduce air throughput, ime. Maybe I was using cheap ones. You can hardly even blow through some of them...

Re: Help me understand airline check valves

PostPosted: Mon Jun 17, 2019 4:01 pm
by Narwhal72
The ones that have a metal spring inside are the worst. I can barely blow through them myself. The old Whisper check valves were always the best for me. But if you didn't replace them regularly they would fail on you (usually when you needed them).

I don't need them anymore since my air now comes from a pvc manifold and runs around the top of my fish room thank goodness.