Fluid Engineer Explanation Please-inverted P-trap

[sometimewoodworker]

more about siphons 

[Sophon]

47 minutes ago, sometimewoodworker said:

Fancy overflow pipes, that if fed with too much flow can auto-siphon and so empty the tank. ???? 

I don't think that is correct. Even an auto-siphon requires the level where the water exits to be lower than the top of the reservoir the water is drawn from. Otherwise you could empty the water back in the original reservoir and create a perpetual motion machine. 

 

So even if an automatic siphon is created, the tank shouldn't empty to below the (approximately) 700 liter level.

[VocalNeal]

I think the P-trap things is confusing the issue? The purpose of a P-trap is exactly that to trap a column of water to stop foul vapours from coming back up the pipe. In this case there are no foul vapours. 

As far as I can see the purpose of the open top tanks is to act as aeration basins?  Using two doubles the aeration potential. So we are asking here how to connect the two tanks to prevent overflow?  If the system is in balance there should be no overflow?  But life is not prefect. 

If each tank has a separate overflow it is not possible for the tanks to be emptied by siphon effect as the overflow stops when the water level drops below the overflow?

 

Or we are trying to somehow stop the pumps from emptying the aeration tanks if the inflow stops?

[sometimewoodworker]

18 minutes ago, Sophon said:

I don't think that is correct. Even an auto-siphon requires the level where the water exits to be lower than the top of the reservoir the water is drawn from. Otherwise you could empty the water back in the original reservoir and create a perpetual motion machine. 

 

So even if an automatic siphon is created, the tank shouldn't empty to below the (approximately) 700 liter level.

If you go through the thread the picture you show is not going to be final.

 

there will be the current up pipe, U bend, and a down pipe that will connect into the return. That means that the auto-siphon can be created. 
 

Of course if the picture was final an auto-siphon could not happen and the U bend would be pointless.

Edited October 25, 2020 by sometimewoodworker

[carlyai]

1 hour ago, sometimewoodworker said:

You have the physics correct. 
 

Too much flow is the amount that will cause the air in the down pipe to be sucked into the outflow. This will change an overflow system to become an auto-siphon so emptying the tank. Changing the clean out screwed fitting at the very top to a mesh filter that will allow air in but keep crunches out will make the auto-siphon possibility much less and would probably eliminate much more than a 10% loss of volume 

But but, probably would also depend on the size of the P- trap pipe. Just thinking about this. If the pipe is too small then there won't be the flow. So maybe a 3" pipe would work better than a 40mm drain pipe.

If the physics is correct then that's an answer to my question. Thanks.

[sometimewoodworker]

1 hour ago, carlyai said:

But but, probably would also depend on the size of the P- trap pipe. Just thinking about this. If the pipe is too small then there won't be the flow. So maybe a 3" pipe would work better than a 40mm drain pipe.

If the physics is correct then that's an answer to my question. Thanks.

Sorry that’s exactly the wrong answer. The smaller the pipe and trap the greater the chance of the auto-siphon happening. The greater the pipe diameter the the smaller the change of an auto-siphon happening.

[sometimewoodworker]

It’s trivial to create an auto-siphon in a small diameter system, as you can see in the videos posted. The larger the pipe the greater the volume of water (litres per minute/litres per second) that has to be pushed through to allow the air column in the down pipe to be sucked through the output side. 
 

So allowing air to enter, through a filter, at the top and the pipes to be greater (maybe double) the incoming supply should reduce the danger to near zero.

[carlyai]

2 hours ago, sometimewoodworker said:

It’s trivial to create an auto-siphon in a small diameter system, as you can see in the videos posted. The larger the pipe the greater the volume of water (litres per minute/litres per second) that has to be pushed through to allow the air column in the down pipe to be sucked through the output side. 
 

So allowing air to enter, through a filter, at the top and the pipes to be greater (maybe double) the incoming supply should reduce the danger to near zero.

Yes that's what I meant.

I have a 3" drain, but because of the filter  fittings size is only 1.5" I reduced the 3" pipe drain outlets to 1.5" pipe, and use 1.5" pipe.

So would it be better to have 3" pipe from the drains, inverted  U bend and P-trap and then reduce to 1.5" after the U bend? Or it shouldn't matter as I have to reduce to 1.5" somewhere?

[carlyai]

7 hours ago, VocalNeal said:

I think the P-trap things is confusing the issue? The purpose of a P-trap is exactly that to trap a column of water to stop foul vapours from coming back up the pipe. In this case there are no foul vapours. 

As far as I can see the purpose of the open top tanks is to act as aeration basins?  Using two doubles the aeration potential. So we are asking here how to connect the two tanks to prevent overflow?  If the system is in balance there should be no overflow?  But life is not prefect. 

If each tank has a separate overflow it is not possible for the tanks to be emptied by siphon effect as the overflow stops when the water level drops below the overflow?

 

Or we are trying to somehow stop the pumps from emptying the aeration tanks if the inflow stops?

The idea of the inverted P-traps is to stop the tanks overflowing. If the water level in the tank will stay at the same level as the bottom of the P-trap then the tank won't overflow.

I don't think there will be a siphon effect as the pump (4000 L/H) is managing just over 2000 L/H delivered to the 2   1000 L tanks. That's what is required, to turn over the tank water once per hour.