Siphons for AP
General Siphon principles
Place a hose into a tank of water, remove all the air out of the hose, block off one end of the hose (with a thumb) and bring it out and below the water level in the tank and remove the thumb – water flows from the tank until either end of the hose is above the water level then air enters the hose and siphoning action ceases – this is a basic siphon.
Now change the 10mm hose for 150mm flexible hose and do the above test (use a bigger thumb) and what will happen is air will flow immediately up the pipe and no siphon action occurs, to overcome this put both ends under water and levels will move towards equilibrium; we are trying to incorporate siphon action with flood & drain to provide efficient methods of moving fish tank water cyclicly (the aquaponian way)
Flood & Drain
Siphons were introduced to assist in F&D of the grow beds, but as has been discovered, it has also branched out to header tanks and modified to work in fish tanks, so there is really no limitations.
Normal F&D involved water flowing into a grow bed (GB) at a greater rate than outflow (flooding), then stopping the inflow as the GB drained via a smaller drain pipe (10mm 1/2”) - to help fill the GB faster, a loop was put in the 10mm pipe so that it acted like a siphon and didn’t start draining water until the level in the GB rose above the height of the pipe loop – this type of F&D required the pump flow rate (inflow) to be higher than the outflow of the drain pipe and also required the pump to be cycled (switched on & off).
The next step in the evolution came about by trying to reverse the process, having a smaller pump running continuously and a drain pipe larger so that the outflow from the GB is greater than the inflow and somewhere a long the line it has become known as “auto-siphoning”
Continuous flow pumps and auto siphons
The problem with having a slower continuous inflow is that the siphon is not primed and a balance is met whereby the outflow equals the inflow and the GB remains in a flooded condition, to overcome this both ends of the pipe need to be submerged( as in the 150mm example).
A simple way is to put in a U-bend in the outlet pipe to trap water and not allow air to flow back in to the outlet pipe (see picture 2). How does this work - as the water level rises in the GB, the water tries to rise in the pipe but has resistance because of the trap, eventually air is forced out through the lower water trap – this will continue until the water level in the GB is above the top of the siphon pipe and then all the air will be forced out of the siphon pipes and the GB will be drained until one end of the siphon sucks in air (see picture 3)
Another challenge surfaced, although the GB drained down to the bottom of the pipe, the siphon and outlet pipes were still full of water so as water was added to the GB it was being siphoned straight out, the GB could not be then flooded again - air needed to be drawn into the siphon so that the water could all flow out and the F&D cycle could continue.
Types of siphons
There are 2 main types of siphons being used so far:
A basic siphon as described initially but the top of the siphon pipe needs to be below the highest level of the GB, the siphon can be mounted internally or externally with the outlet below directly through the base of the GB or out the side.
The water can begin flowing once the level is above the lower level of the horizontal part and to remove the air and start the siphon working effectively, the water level needs to rise to the top of the pipe, if the pipe is 40mm diameter then water level needs to increase by 40mm.
Decreasing the height diameter (40mm) can be achieved in 2 ways:
- putting a smaller sized horizontal pipe (20mm), but this reduces the outflow of the siphon
- by squashing the pipe and having an oval shaped pipe, this is the idea which led to the use of the “Bell & Siphon” method.
The siphon consists simply of a vertical standpipe in side the GB extending through the base of the GB, a larger pipe(siphon pipe) is placed over the standpipe, freestanding and taller than the standpipe, the siphon pipe is fitted with an end cap and the base has pieces cut out (see picture 1).
Water flows up between the walls of both pipes then down the inside of the stand pipe, calculations for sizes will be covered shortly.
Air is required to break the siphon seal and allow the water trapped inside the siphon pipes to fall back into the GB or down the outlet pipe, to get air into the siphon is quite simple and there are many places that have proved to work:
1) put a hole and tube on the upside of the siphon as shown in pic 1
2) run the tube up to the top of the upper horizontal part of the siphon, a hole is in the end cap where the air tube could be fitted (pic 1)
3) it can be placed on the GB wall (below the high water mark) and joined to the outlet pipe outside of the GB
The main point is that when the water falls below the air tube, air is then sucked in – this air will flow with the water until the GB water level falls below the siphon end pipe. Water flow in the siphon then ceases but air is still sucked through the tube filling the siphon with air and breaking the seal, it is important to have the end of the air tube at least 10mm above the lower height of the siphon pipe. In pic1 the air tube is 25mm above the base, the support legs are 15mm deep.
The air tube need not be there at all, the hole would have been sufficient but with air flowing with the water, water velocity decreases, test results:
40mm pipe will displace 6,000lph, by adding air into the flow the output decreased to 5,000lph.
Pipe sizes and calculations
Tests were carried out using 40mm standpipe, with a gap height of 12.5mm and GB area of 350cm2, these figures will be used and calculated for different size pipe configurations in the following table:
Pipe | Area | Circ | min Ht | max Flow | min Inflow | Outer pipe | Cut outs
40 .. 1,256 .. 125 ... 10 ..... 6,000 ..... 920 ........ 65 ....... 20/55
32 .... 805 .. 100 .... 8 ..... 3,850 ..... 590 ........ 50 ....... 15/35
25 .... 490 ... 80 .... 6 ..... 2,340 ..... 360 ........ 40 ....... 10/15
20 .... 314 ... 62 .... 5 ..... 1,500 ..... 230 ........ 32 ....... 10/10
Pipe – standard pipe size in metric (mm)
Area – cross sectional area of stand pipe
Circ – circumference of pipe
Height – minimum clearance needed for sufficient volume of water to flow into standpipe unimpeded (higher means more air, means more water flow (lph))
Outer pipe – minimum size of outer siphon pipe needed for sufficient volume of water to flow between standpipe and siphon walls unimpeded
Max flow – maximum flow of pipe, for continuous pump flow, the maximum input should not be greater than half this figure.
Min inflow – minimum flow rate into the GB - on tests, the flow rate worked down to 360lph before false starts were occurring, 10% was then added to this figure. - minimum inflow is calculated on lph required to flow into a GB area of 1 cubic metre at minimum height clearance
Outer pipe – the siphon pipe size should be a minimum of 50% greater then the stand pipe size so as not to impede water flow
Cut outs – these are the pieces cut out of the bottom of the siphon pipe to have leg supports and to allow sufficient water flow –3 legs are used for supports at all time and the depth is set for 15mm on each leg
(20/55) – legs are 20mm wide, distance between each leg is 55mm (these figures are approximate)
**** to work out how long to make cutouts:
1) wrap a piece of string around the pipe
2) lay the string down flat and measure the distance
3) deduct the width of 3 legs (width depends on pipe size but not critical)
4) remainder divide by 3, this is the distance between legs
5) mark on pipe with pencil to make sure before cutting
PLC’s, Smart Relays and Solenoid valves
This is still in the theory and testing stage and is being discussed here, but basically if one GB which holds 300litres is connected to a 1,000l fish tank then at some time there is going to be 30% of the water not available to fish to swim in – now add 2 more 300l GB and at some stage there may be only 100l for the fish. Two ways of overcoming this problem is:
1) have a reservoir tank to hold sufficient water to flood the GB (sump and header tanks are being utilised)
2) fill GBs in series rather than parallel.
Put a sol valve on the input of each GB and open/close in sequence to direct the pump flow, this can be done in by timing the inflow or using a sensor to switch when at a pre-determined height.
The advantage of sol valves/PLC devices is that the GB F&D can be done sequentially and thus reduce the amount of water outside the fish tank to 1 GB size at a time.
Solenoid valves come in standard sizes and are sold at irrigation places or “B”, sizes for our use would be 20 or 25mm
Programmable Logic Controllers (PLC), Smart Relays
Programming devices used to open/close the sol valves
Links to Auto siphon explanations and pictures
Stepped Autosiphon system
Pipe in pipe concept
http://backyardaquaponics.com/forum/alb ... pic_id=191