Can Reducing Pipe Diameter Increase Water Flow Speed in a Gravity-Fed System?

[eeiko321]

Hi guys


this has probably been asked before but i can't seem to find something exactly the same.

pics will show you a filter and my small fish pond... water travels up in the blue drum, then up into the overflow which is 4" vertical pipe... then out the side and from 4" to 2" pvc... it doesn't got very far... arppox 17" away.

my question is... could the water be sped up that's fed by gravity? like, would reducing the 50mm to say... a 40mm or 32mm pipe?

its not coming through at a bad rate.. its just that I'm always thinking of ways to make improvements here.



any suggestions or more info... let me know

thanks heaps

 

[CWatters]

If I have understood correctly the answer is no. The outflow flow rate appears to be dictated by the rate coming into the blue tank - so speeding up the output by changing from 2" to say 4" won't help.

However if you were to increase the flow rate coming _into_ the blue tank a lot then there might come a point where the 2" outflow pipe can't cope. Then you would see the water level "backing up" in the 2" and then the 4" overflow pipe and eventually the blue tank would overflow. Only at that point would upgrading the output pipe help.

 

[eeiko321]

If I have understood correctly the answer is no. The outflow flow rate appears to be dictated by the rate coming into the blue tank - so speeding up the output by changing from 2" to say 4" won't help.

However if you were to increase the flow rate coming _into_ the blue tank a lot then there might come a point where the 2" outflow pipe can't cope. Then you would see the water level "backing up" in the 2" and then the 4" overflow pipe and eventually the blue tank would overflow. Only at that point would upgrading the output pipe help.

Hi watters

i appreciate your prompt reply.

yes, the blue drum is fed by Pond water to be recycled. it has an inlet about half way of the height, fed by a 4500L submersible pump.

i do understand the fact that, the bigger the pump ...pumping into the blue drum would increase the turnover.

but your saying that the 2" outlet, from the drum to the pond... like decreasing from 50mm (2") to say... a 32mm (1/1/4") flow rate won't be helped ?.

because the common myth surrounding that is what i hear people say about the native amazon BlowPipe theory... where a needle is inserted into a tube and acts as a dart when blown. now if it were a 2" pipe.. the needle will be going nowhere.


but yes, its the rate of water that is coming up from the blue drum into the overflow outlet in the centre...

hmm

 

[CWatters]

Correct. Making the outlet pipe bigger won't improve the flow rate.

At the moment the level in the blue tank stays roughly constant. This means the rate at which water enters and exits the blue tank must be the same. Therefore the limiting factor is the pump not the size of the outlet pipe.

Obviously if you increased the size of the pump it might be necessary to enlarge the outlet pipe as well but at the moment that's not the limiting factor. You can't get more water out then you put in.