Backyard Aquaponics
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I think if you start thinking about the pump simply being on the end of a no holes overflow pipe (I think this is what werdna is suggesting) then it suggests u don't need the solenoid.

But why theorize when it can be tested by an easy experiment

Another way of thinking about it, take a hose or a u made out of PVC pipe and fully submerge it in a bucket till there are no air bubles in it. Turn the u upside down or grab the middle of the house and start lifting it out of the water while keeping both ends in the water the whole time.....does the PVC pipe or hose remain completely full of water or does it break suction and fill with air?

This simple experiment should confirm (or denie) the theory.

U could even lift he hose as high as your roof if you want to be totally convinced

I think 4xjbh is concerned with the water draining out of the pipework.

4xjbh, as long as both ends of the pipe are below the surface of the water, there is no way for it to suck air in.

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+1, and learning. Always approach life with your beginners mind.

That is not quite right. The "work" the pump has to do is reduced by the weight of the water on the downward side "sucking" water through the pipe. The circulation pump just has to overcome the friction in the pipe so it is essentially operating at zero head. THere are some limits to this though. Once the pipe is completely full of water the pressure in the pipe is going to decrease the higher you can. Roughly when you are 2m meters above the water level of the sump the pressure in the feed end of the pipe is going to =(0.8bar+friction loss in pipe work there after) absolute. For arguments sake this would be 0.81 Bar absolute (.1m of pressure loss due to friction). Or in other words -0.19 Bar gauge pressure or -0.19 negative pressure (you cant have negative pressure but pressures less than atmosphere are often called negative pressure). As the hegiht increases the pressure in the pipe will decrease and the ability of the water to hold gas will also decrease. If the flow through the pipe is at a velocity that is below flushing velocity or if the water flow is stopped (like overnight) then gas will collect at the top of the pipe and eventually cut of the flow.

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You learn something new every day. Thanks Stuart.

I was aware that eventually gas would break out when you went high enough, but i wouldnt have thought that 2m or so that 4xjbh was after would be high enough. Having solenoids on each end wont prevent this from happening though...

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10m is the absolute theoretical highest you can go because at that point the pressure in the top of the pipe equals 0. Or another way of looking at it is that the pressure in the pipe at the water level of the tank equals the air pressure. If the pump was contributing then you could go higher. For example if you had a tap on the exit and it was almost closed then the pump would have to work against that restriction and that would increase the pressure in the line meaning you could go higher but you wouldn't want to.

As for the gas coming out or in for that matter (relevant to aeration) any change in pressure changes the amount of gas that can be held in the water. So even a very small decrease in pressure could cause problems in a line that was turned off for a period of time.

If you really want to get picky your altitude is going to effect it as well around the caspian sea you could go higher where I am you would top out at around 9.8(?) something. Also a low pressure weather system could decrease the holding capacity of your water in the tank which would be further reduced by the low pressure in the top of the pipe. That could mean that your system could be working fine and then a storm could go through and turn off all your no holes overflows and such.

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Have inlet and outlet at same height, in sump.
Bleed all air out, by forcing water through with garden hose (has already been said).
If system is sealed (not leaks) system will stay full of water - always. As long as both inlet and outlet are covered with water.
Head pressure will be zero.
You don't need a large volume pump, as heater pipes are generally small bore.
No point in pumping heaps of water through, as it will not heat up.

Its simply atmospheric pressure at work.
Will love it!

That's why this works -
http://www.backyardaquaponics.com/forum/viewtopic.php?f=8&t=12498#p326822

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As I said it would be a problem if the flow was turned off. While the pipe is flowing the bubbles as they form will get carried through the pipe. However if the flow is stopped then depending one the size of the pipe it may only take a few ml of bubbles collecting at the top to block the flow or even just to reduce the effective size of the pipe so that the flow cant flush the air out of the pipe so that while it may not be blocked completely it soon will be.

I don't have a spread sheet setup for air but I do have one for oxygen. At sea level water at 18C can hold about [9.5mg/L O2] if the pressure is dropped to .6bar absolute, ie .4bar less than atmospheric or the pressure you would have in siphon that went up 4m before coming down, then the ability of the water to hold O2 would drop to [5.5mg/L O2]. This would be about 25ml of O2 gas produced for every L of water at that reduced pressure. If the pipe is flowing such a small amount of gas is going to be flushed through but if still the siphon could be broken.

When installing siphons comercially it is first recommended that you not do it because of problems with air accumulating in the siphon and cutting flow. Second if ou are going to do it you must have a suction port at the high point of the siphon so that it can be reprimed WHEN the siphon is broken for what ever reason. Three that if there is any equipment on the outflow side of the siphon that some sort of automatic shut off/alarm system is installed so that equipment does not run dry.

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With my NHO i had a small air collection point which was basically an upside down T that had a small siphon running out of it that sucked any air collecting out so it couldn't fill with air. I just used a length of flexible hose with an ID of around 3mm. When that small siphon wasn't running it would only take a matter of hours to kill the NHO. This could be another way you could overcome the problem of air collecting in a small pipe.

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There is no pressure difference in the pipe other than that created by moving water, and the weight of the water. If a system were fill at say 14.7 psi then the pressure at the top would be 14.7 psi no matter how high it went.

Actually no. The water column has mass and with gravity weight. If you take a cylinder place it under water so that it fills with water the pressure within the cylinder is the pressure caused by its depth in the water (gauge) plus the atmospheric pressure (guage + atmopheric = absolute pressure). If the cylinder is up ended under water and then lifted from the water by its base the pressure within the cylinder is going to be different depending on where you measure it. For the area of the cylinder under the water the pressure is going to be x mm of H2O (guage) + 10,000mm of H20 (atmospheric)= 10,000 + x mm H20 (absolute pressure). At the water level of the tank that the end of the cylinder is emersed in the pressure is going to be 0mm of H2O(gauge) +10,000mm H2O (atm) = 10,000mm of H2O (absolute). As you measure the pressure above the water line of the tank the pressure is going to decrease. At 100mm above the level of the water in the tank the pressure is going to be -100mm H2O (gauge) + 10,000mm H20 (atm)= 9,900mm H2O (absolute). The higher you go the less the pressure is going to be as long as the base of the cylinder does not go higher than the water in the tank. As soon as it does air will get in the pressures will equalise dumping the water in the cylinder back in to the tank. If you doubt this take a plastic milk bottle and fill it with water in a tank or sink lift it out of the water by its base. As you do so the pressure in the bottle will decrease and because the atmospheric pressure outside the bottle will then be greater the atmospheric pressure will push the sides of the bottle in. The more you lift the bottle out of the water the more the sides will get pushed in because the pressure in the bottle will is less than that outside. This will continue until you lift hte bottle out of the water at which point the water will empty from the bottle and the pressures will equalise.

In our pipe example the pump adds pressure to get the water moving and once the pipe is primed the water pressure will decrease as you go up the pipe. Once you get to the top and start coming back down all the pressure lossed going up is going to be regained.

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