Monday, 12 May 2014

A rainwater system with syphons and leaks

I've been trying to work out a system of cascading water barrels. Ideally each barrel would contain enough water for one watering. When it emptied it would fill from the next one up, and then when it was full it would empty and water the garden. This would get regular irrigation for the garden. Better still it would only work in the early morning or in the evening, and would not work when it was raining, but those are separate issues.

Right now I just want to think of a system that will run only on gravity and rainwater.

Each water tank needs up to three pipes connecting to it: water in, water out via a tap at the bottom, and an overflow. Generally I want to get water out of the tank at the bottom, so the tanks further up the system don't necessarily need a water-out tap, or if they do it will normally be closed.

The water-in can go on the lid of the tank, which should be straightforward. The overflow needs to go high up on the side, and usually will be dry, so the seal around the hole is not critical. The water-out needs to be near the bottom, and will be wet and under pressure, so the seal is critical. It would be a good thing if I could get out of having to make these. 

If I use a syphon overflow, so a pipe goes into the higher tank high up on the side, and the pipe reaches all the way to the bottom of the higher tank, then feeds into a lower tank so the other end of the pipe is at or below this end, then it will work as a syphon. When the water goes above the overflow hole and the high point of the overflow pipe, the pipe will fill with water, then water will head down the other side of the pipe, and suck water from the bottom of the upper tank. And because it will work as a syphon, it will continue to suck water out either until all the water has gone into the lower tank, or if the lower tank is only a little lower, then it will empty until it reaches an equilibrium and the water will go to the same level in the two tanks. 

Once it's in this situation, adding more water to the top tank will send some water down to the lower tank, so that it will maintain this equilibrium, even if the level of water in the top tank is below the overflow level. 

I was thinking that I could use the same kind of syphon overflow between each tank, from the top to the bottom. However, because the syphons are going to keep tanks in equilibrium after they have filled up, they are going to try to get to the same level, and all the water will syphon itself out. 

So a syphon overflow may work for two tanks that are on more or less the same level, but will not work so well for the whole array. 

Maybe what I need instead is a leaky system. Rather than trying to make leak-free taps coming out of the tanks, perhaps I should be letting water gradually leak down the cascade.

Or perhaps I need to think about leaking air. The syphon won't carry on trying to keep the two tanks level if air is leaking into it. If I can get a little air hole at the top of the syphon tube, when the water goes above that level, the syphon tube will fill with water and start syphoning out. It will carry on syphoning out while there is still water at the top of the syphon tube, but as more and more air gets in, the flow will decrease and then stop. The bigger the air hole, the faster air will leak in and the less the water will syphon out. This could limit the amount of water going to the tank below each time.

Of course another priority is a low-maintenance, self-cleaning system, and the chances are that if the system relied on leaks to work properly, then rather than leaks getting worse as they do when you don't want them, they would fill themselves in.