Thread: 2” returs from BD and skimmer

Hi Guys, First post here even though I have had my pond for 10 years now. I already know that I should have gone with 4” returns but the pond was built as a pump fed system and somebody convinced me that 2” returns would be fine, which to be fair they have been, BUT I am now thinking of changing to gravity fed and not sure that I can achieve enough flow. The pond is 13,000 litres and 1.3m deep at BD. I understand that if I lower the outlets of the returns into the filter I will get increased flow, I Ideally would like to to achieve 13,000 litres an hour. My pump is variable so have no problem pulling that volume but my question is, will there be enough flow from the returns to manage that flow rate? I have seen a chart which shows that a 2” pipe with 1” drop will give 1080 gallons per hour but at 3” drop it gives 1883 gallons per hour, so 2 x2” pipes with a 3” drop should give me over 3600 gallons per hour which would be plenty. Before I start digging, to lower my filter, I just wanted to check if my assumptions are correct? Finally, the other bit that confuses me is that if I produce a 3” drop into the filter I presume that I must still have the top edge of the filter above the pond water level i.e. if the inlets are 3” below pond water level the top of the filter must be say 4” above inlets, so that it doesn’t overflow if the pump switches off? Oh incase you are wondering why I want to convert to gravity is that I want to create a pumped return to create more movement in the pond as I am getting some sludge build up which the BD isn’t collecting. Obviously, other advantages are that the pump is running with clean water and the filter system will be more efficient as the pump won’t be mulching the crud before it enters the filter. Sorry it’s a long post. Thanks.

Thanks. Just to clarify, It’s a 2” bottom drain and a 2” skimmer which are pumped out through a dry mounted pump at pond water level (So not gravity fed into filter) which pumps into an OASE Screenmatic 36 filter (Above grond level) which in turn gravity feeds back into pond via waterfall. My aim was to hopefully replace the Screenmatic (which will only operate with a pumped system) with a Gravity fed filter which then pumps back into pond. My question is, will I get sufficient flow from the two x 2” drains to fill the gravity fed filter quickly enough for me to be able to pump back into the pond at a rate of about 10,000 litres per hour, which my existing pump will deliver comfortably even running at about 50% capacity.

The water level in the filter will find its balance point according to the pump out rate and the maximum input flow of the pipes.

If you are pulling water out of the filter and retuning it to the pond at 10,000 gallons an hour, then the water level in the pond will rise and the level in the filter will fall. Water will flow through the pipes at the rate you have described. If it is a little slower than your calculation, then the level in the filter will be a bit more than the 1" below the pond level that you suggest. If it's faster then the difference will be less.

Depending on the flow capacity of the filter, that may add a little more head and increase the difference in levels.
I have a gravity fed nexus (with dracodrum2) and there is never less than 1 inch difference between the input water level and the output water level. Obviously that increases as the drum blocks, and then returns to 1" after the cleaning cycle.

Make sure that when all pumps are off and the levels equalise, that the water level in the filter does not overflow.

You could add air into the main pond if your trying to push more sediment towards bottom drains.

If you don't like air in the pond, you could use positioned submersible water pumps. They are also making large aquarium powerheads that sweep nowadays I believe.

Thanks everyone. My initial thought was to add jets/powerheads to create a circular movement, as Noo suggested, but just wanted to investigate the gravity filter option. I understand about the water levels dropping in the filter but I suppose another way of putting my question is will the filter ever empty because the 2 X 2” drains can’t drain enough water out of the pond quickly enough for the flow that I want i.e. about 10,000 litres per hour. I will probably just try it out with a drum barrel, before investing into a new filter, to see if the 2” drains will give me enough flow. If not, I will go with the jets.

I have a vari pump so will set it up and gradually increase flow rate to see what the maximum I can get before the filter starts to empty. Was trying to avoid retro fitting drains for aesthetic reasons but will have a look at what’s available. Any suggestions?

I tried, but nowhere near enough flow into the filter to produce even 6,000 litres/hr. So will either try just moving the pond water with air stones and some jets or consider adding a mid level drain (which i could do quite easily as it’s 24” above ground).

If I fitted a mid level drain. What size would you suggest? I currently have 2” from my BD and skimmer. Would I then run all 3 drains into the sieve? And then pump after the sieve into my filter?

I'm puzzled that you're only getting 6000lph from the 2 2" pipes.
How far below the pond surface level do they enter your filter?
They need to be well below the waterline when all is static to get maximum flow. If they are higher in the filter and become exposed when the pump is running, then you won't get any more flow from them.
A picture or two would help

Thanks GDL, I have attached a drawing which might help. The 2 X 2” pipes don't actually enter the filter but the pump which is at pond water level. My original post was asking if lowering the 2” pipes would produce a greater flow, so that I could fit a sieve or settling chamber before the pump to get rid of larger particles before they enter the pump. I assumed that lowering the pipes would increase the flow rate which is what you are now saying. However an earlier response from another member suggested that lowering the pipes would make no difference! So if I was to put some sort of sieve or settling chamber BEFORE the pump, which had the inlet’s at about 1m below the pond water level do you think my flow would be enough so that I could pump 6000 litres per hour without it draining dry?

Originally Posted by ColinH27

So if I was to put some sort of sieve or settling chamber BEFORE the pump, which had the inlet’s at about 1m below the pond water level do you think my flow would be enough so that I could pump 6000 litres per hour without it draining dry?

Exactly so!
It doesn't matter how far they are below the surface in the chamber, just that they stay below the surface while the pump is running.

The water level in the chamber will drop as you pump water out of it, as the difference between the water surface of the pond and the water surface in the chamber increases, then the flow through the pipes will increase.
If you look at THIS CALCULATOR, you will see the required drop that you need to get the flow. It is dependent on the pipe material (the smoother it is, the less friction), the length of pipe (more length = more friction losses) and the drop.

How many bends it has in it also has an impact. Each bend adds affectively to the length of the pipe. A swept 90 degree bend adds less than a tight 90 degree one. Unless you have built a real spaghetti of bends, I wouldn't worry about them, just accept that the difference in height will be slightly more than the calculator gives.

Taking a 4.8cm ID plastic pipe that is 5m long and a head of 10 cm, gives a flow of 1.718 l/s, or 6185 lph.
Suck the water out faster to increase the head to 20cm gives you 2.5 l/s, or 9000 lph.

You have 2 pipes, probably with unequal lengths. so the flow through the shorter one will be greater than the longer one.