# Pipes & flow rate



## idris (28 May 2011)

Does anyone know how much pipe diameter and length effect flow rate?
I know it's not a simple relationship, and I don't want to get into complicated maths, but it would bs useful to know roughly.


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## Bobtastic (2 Jun 2011)

I would also like to know the answer to this.

I have recently created myself a spray bar out of clear acrylic tubing, and it occurred to me that the diameters that I have chosen may not have been ideal... I'm running the DIY bar on an Eheim Pro 3e 2078 which uses 22/16mm hosing. So my brain told me that I needed a length of acrylic tube with the same internal diameter as the hose. Is that the correct assumption?

I was thinking about this last night and it occurred to me that the supplied Eheim tubes do not have the same internal diameter as the hosing... So am I affecting, negatively, the flow rate of my filter?


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## George Farmer (2 Jun 2011)

idris said:
			
		

> Does anyone know how much pipe diameter and length effect flow rate?
> I know it's not a simple relationship, and I don't want to get into complicated maths, but it would bs useful to know roughly.


One factor remains constant; more hose length = less flow, as the pump has to work harder to shift a greater qty. of water.

Where it gets a little more complicated is hose diameter.

Ideally the hose diameter needs to appropriately match the filter pump rating.

Let's look at two scenarios.

1) You have a very powerful filter i.e. Fluval FX5, but with 12mm hose.  This will restrict flow considerably.  The ideal hose size for this filter is 1" (25mm).

2) You have a small filter i.e. Eden 501 nano external, but with a 25mm hose.  This will restrict flow because the mass of water in the hose is too large for the pump to shift effectively.

So from these two examples you can see that there is no simple formula.

As a very rough rule of thumb I would say this - 

Filters rated <600lph - 12/16mm hose
Filters rated  600>2000lph - 16/22mm hose
Filters rated >2000lph - 22mm+ hose

There are some tiny externals that come supplied with 9/12mm hose.

Most commonly available external canister filters on the market come supplied with 16/22mm hose (internal/external diameter).  

If you use a 12mm hose on these you can expect the flow to drop by up to 50% or more.  I have noticed this by using 12mm fitting on my Fluval G6, as it has an accurate built-in flow rate meter.

Another consideration is inline equipment i.e. external heaters and inline diffusers.  Even if they are supplied with the same size fitting as the hose, they will reduce flow because the diameter of the fittings are smaller to allow fitment.

Finally another consideration is how powerful the pump motors are, in terms of actual ability to pump under load, not just flow rate.  For instance a Koralia-type powerhead can shift several thousand litres per hour, but it's not designed to have any real load.  Higher quality pump motors should be able to pump water at the claimed rates, despite some clogging.

In summary, bigger filters are generally better, but by using smaller and longer hoses, you are losing a lot of flow.  I'm afraid there's no formula to determine actual qtys. as far as I know.  There maybe a hydrodynamics expert among us that may be able to help further...


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## Bobtastic (2 Jun 2011)

Hi George, 

Would having inlet/outlet tubing of the same internal diameter of the hosing be a positive or a negative?


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## Stu Worrall (2 Jun 2011)

good write up there george.  The other thing to take into consideration i guess is the speed of the flow.  I had 16mm in/out on my tetratec ex1200 which didnt seem to shift much around the tank.  I got hold of an ADA 13mm lily outlet which put more "sway" into the plants.  Without going into fluid dynamics and all that in theory i would have lost overall flow (lph) through the filter but the outlet speed into the tank has increased due to the smaller diameter.  This wont work on  all pumps though as the pump may just slow down and youll end up with the same velocity as before!

I had an interesting time with my nano last night going from the 16mm on a fluval 205 down to 10mm for the do!aqua lily!  I blew a quarter of the substrate away!


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## George Farmer (2 Jun 2011)

Bobtastic said:
			
		

> Hi George,
> 
> Would having inlet/outlet tubing of the same internal diameter of the hosing be a positive or a negative?


I would say positive.


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## Bobtastic (2 Jun 2011)

Goodo! I'll stop worrying about it then!


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## idris (2 Jun 2011)

George Farmer said:
			
		

> As a very rough rule of thumb I would say this -
> Filters rated <600lph - 12/16mm hose
> Filters rated  600>2000lph - 16/22mm hose
> Filters rated >2000lph - 22mm+ hose
> ...


Thanks George. Really useful.   Do you have any sense of how much difference a short section that is narrower would make? eg 16/22 pipe with a 50mm length at only 12mm?

Unfortunately it confirms my fears:
My Rena xP2 is rated at just over 1000lph. So I was expecting about 500-600lph with filter media in the canister. 
It comes with aprox 14mm (ID) inlet & outlet pipes and 16/22 hose. I'm using the stnadard hoses, but have used 16/12 acrylic pipes for the inlet & outlet.
A rough measurement has given me 350-400lph, which is disappointing. So I do wonder whether I need to remake my tubes out of something larger. 
Now can I get 16/22 hose over 16/22 pipe?  :?


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## Bobtastic (3 Jun 2011)

Hey Idris, 

I purchased some acrylic tubing from zee ebay that has an ID of 16mm but has an OD of 20mm and I've been able to bend the tube with some success and attached 22/16mm hosing over it. I simply started the kettle boiling, opened the lid so that it didn't stop boiling and plunged the end of the tube in. It get malleable enough to slide on with no problems.


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## idris (3 Jun 2011)

Thanks Bob. I hoped that was the case but wasn't 100% confident it would stretch that far.


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## Bobtastic (3 Jun 2011)

I guess different hoses have slightly different properties so you may or may not have any problems. I also got a 15mm rod to seal the end and once superglued, heated and the outer tube squeezed around it the seal it water tight.


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## Alastair (3 Jun 2011)

That's a big drop in flow with your xp2 idris. If you think your going to suffer flow wise, give me a shout and for a donation to ukaps, as I've done with another member on here I'll gladly pop you out an ex1200 withall bits etc. I can't be bothered waiting to sell it anymore.


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## idris (3 Jun 2011)

Alastair said:
			
		

> That's a big drop in flow with your xp2 idris. If you think your going to suffer flow wise, give me a shout and for a donation to ukaps, as I've done with another member on here I'll gladly pop you out an ex1200 withall bits etc. I can't be bothered waiting to sell it anymore.


*Dear all, 
Alastair is officially the nicest man on the planet today!*
Alastair, you will have a PM shortly!

Bob, which supplier did you get your pipe and rod from? The one I got my smaller diameter stuff from does 15mm rod but not 16mm. Just because I can get quite anal about details (you may have noticed that by now  ) I'd ike to find 16mm rod and 16mm (ID) tubes if I can.


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## Bobtastic (3 Jun 2011)

Alastair does seem to be the supplier of free filters to the UKAPS massive! I'll just have to wait and see if anything I need comes up for grabs! 

I got my tubing from clearplasticsupplies. I wanted to get a 16mm rod too, but couldn't seem to find one... Like I said I just used super glue to fix the rod in place, then I cut off the excess. I then heated up the end of the spray bar with the plugging rod and squished it until it was sealed.


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## Alastair (3 Jun 2011)

idris said:
			
		

> Alastair said:
> 
> 
> 
> ...



Aww gee thanks idris ha ha. It's not a problem glad I can Help. It's a one off though I'm normally very grumpy ha ha. 

Bob, if I have anything spare you might be looking for let me know ;0)


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## Charlieh (3 Jun 2011)

Back to the original question !  You can't really predict flow directly from pipe diameter alone.

In a nutshell fluid flowing through a pipe is subject to resistance from various sources one of which is friction caused by rubbing against the sides of the pipe. The amount of friction is related to the pipe material, length and bore and also to the amount of fluid (flow) and speed - thus to maintain the same flow (litres or gallons per hour) through a narrower pipe the fluid has to move faster (metres or feet per second) but in so doing the friction increases which in turn decreases the flow unless a stronger pump is employed.  As a general rule of thumb, for a given flow, doubling the pipe diameter will decrease fluid speed by 4.

These are links to good articles on fluid dynamics as related to aquariums

http://www.advancedaquarist.com/2003/1/aafeature2

http://www.advancedaquarist.com/2009/8/aafeature


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## idris (3 Jun 2011)

Thanks Charlieh. That's gonna take some reading, and seveal cups of tea, and some more reading, and some more tea, but I shall try. 
(To think I did some fluid mechanics 23yrs ago - really didn't think some pretty fish in the corner of the kitchen would re-open those old wounds  )


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## Charlieh (5 Jun 2011)

Morning Idris - I agree. Fluid dynamics are not nice. You're probably not bothered now but I just had a thought. I remembered seeing a table of flows under gravity (drains essentially) for different size pipes so you don't have to factor in pumps, velocities, lengths etc. I've had a search and it's Table 1 at this link

http://www.advancedaquarist.com/2009/11/diy

So a 3/4 inch pipe (18mm id) carries approx 1.6 times more flow than a 1/2 inch pipe (13mm id) and approx 0.7 times less than a 1 inch pipe (24mm id). Don't forget the pipe sizes are imperial (I've used pressure pipe for the ids) but they're close enough to our metric size tubing to be representative.

You could always test it by siphoning water into a bucket through different size tubing and measuring how long it takes to fill.


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