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pipe diameter, flow rate and velocity

Andrew Butler

Member
Joined
1 Feb 2016
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1,740
Location
Banbury, Oxfordshire
In need of someone who understands these things better than me.
I'm looking to see what size pipe is needed for a filter to operate without being restricted so in other words how big the diameter of a pipe needs to be to allow a particular amount of litres per hour.
Am I correct to think I need to know the velocity of the water to calculate this?
Hopefully someone out there understands these things and can give me some guidance.
I have used the calculator below but I think I need the velocity to get the correct answer.
http://www.pipeflowcalculations.net/pipediameter.xhtml

This is all to do with spraybar calculations; the general rule I was given was to match total area of holes in the spraybar to the diameter of the pipe BUT both the Fluval G3 and G6 have the same size pipes yet very different flow rates: 700 and 1000 litres per hour.
So this would mean if I followed the rule then my spraybar would have the same overall area of holes for both filters which I don't think is right and the overall area of holes I calculated is very different to the sprayar Fluval supply which is the same with both filters although it can restrict flow on the G6.

I look forward to someone with understanding to educate me and hopefully help me fill in the blanks.

Cheers
Andrew
 
Hi M8 :D

I use http://www.1728.org/flowrate.htm to get my flow rates when going from a 25mm pipe to many small holes in a spray bar. Using the cross sectional area of the 25mm and comparing to total cross sectional area of the holes in the spray bar doesnt work due to the 'Hagen–Poiseuille law' :bookworm:

I calculated the velocity from the pipe diameter and the pump/filter flow rate and say I got

a 28mm pipe and 3000l/h flow rate from pump/filter gives 135cm per sec Velocity

then I decide on the spraybar diameter holes say 4mm and use the 135cm per sec Velocity which yields 61 litres per hour

so to keep the same 3000l/h its 3000 /61 = 49 so 49 holes of 4mm diameter to yield 'about' the same flow rate

But if holes 5mm yeilds 95 l/h so 3000/95 = 31 holes.

well thats the method I used ;)
 
What I'm trying to do is take the flow rate and see how big the pipe would need to be for it to allow that amount of water free passage but no more; does that make sense?
As I said both the Fluval G3 and G6 have the same size hose but different flow rates so surely the one with less flow doesn't actually require the pipe to be that big to let that amount of water past?

'Hagen–Poiseuille law'
Hi Karl, First time I've ever heard of that one :pompus:

I just run how my maths compared to the results using the above and the results aren't far off but I take that on board for the future.
area of 28mm pipe = 615.75
area of 4mm pipe = 12.57 (615.75/12.57=48.98568) Hagen–Poiseuille law = 49.18033
area of 5mm pipe = 19.63 (615.75/19.63=31.3678) Hagen–Poiseuille law = 31.57895
 
Fluval G3 and G6 have the same size hose but different flow rates so surely the one with less flow doesn't actually require the pipe to be that big to let that amount of water past?

Yes/No/maybe- all depends on why they used the same size pipe. The one with the lower flow rate with a smaller pipe may take a big hit due to the resistance in the pipe and it may need that size pipe to maintain that flow rate. So its pump dependant also, plus I always find out what the smallest diameter is within the pump/filter assembly and that will be the point of maximum resistance and than make sure all piping is bigger than that and swept bends where possible or extra diameter bends.
 
It's to calculate the area of holes needed in the spraybars for each one; they can't both be the same surely? If you want to get 7000 LPH through a hole you wouldn't physically need the same size you would to get 10000 LPH through would you?
I'm leaving the pipe the same size just want to be able to calculate the area of holes needed to match the flow. How do I find out the velocity by only knowing the flow rate?
 
You could just try out different hole sizes and spacing useing sacrificial pipe.
You might find your local B&Q sells 20mm black pipe used by electricians or my favourite.. 21mm overflow pipe.
Armed with some pvc tape, battery drill and a few drill bits around 3-6mm, you could experiment and find the best format for your individual pump power.
I think that maybe the efficiency of the pump, it’s wattege and impella design would make equations differcult?
 
I've got some sacrificial pipe if I need it but I thought it would be possible to determine how big a pipe would need to be and fit X amount of water through per hour.
 
If you want to get 7000 LPH through a hole you wouldn't physically need the same size you would to get 10000 LPH through would you?

No depends on the head of pressure the pump can generate, higher the head of water the pump can generate the higher the pressure, higher the flow rate, plus higher the flow rate the more kinetic energy the water has to create FLOW/turnover in the tank

How do I find out the velocity by only knowing the flow rate?

http://www.1728.org/flowrate.htm you also know the diameter of the pipe so just enter the flow and you get the velocity.

The calculations just give an idea of the flow OFC then its trail and error, why do you need to know the flow anyway ? Think it because you thinking of the x10 guideline for output, if it is this is a guide to help you get enough tank turnover/flow in the tank. If your filter gave an output of a litre an hour but your powerhead was creating lots of flow in the tank the output of the filter/spraybar is irrelevant, as long as theirs enough filtration. So many holes in a spraybar of small diameter create many 'jets' with high energy water which push a higher volume of water around but also reduces the filters output. But a few big holes or many big holes push little water about the tank as the water has less energy but there will be a higher. So filter output isnt everthing.

As our Clive says 'FLOW' is king, not filter output, filter output x10 is an estimate of manufacturers stated outputs that you are advised to match the tanks size, But the trick is getting a good turnover by the use of well placed spraybars lily pipes etc

You could just try out different hole sizes and spacing useing sacrificial pipe.

:thumbup:, I did the maths then just used smaller holes and checked the tank flow, then increased the holes 0.5mm and a few more to try and generate a Gyre in my tank, one spray bar has never been changed as its fine the other I have modded here and there and have a spare also with differant size holes, using one ATM with tape over some holes ;)
 
The calculations just give an idea of the flow OFC then its trail and error
So what you're saying is ignore most of the things I have read about calculating spraybar holes and spacings? - Namely match overall area of holes in spraybar to filter pipe area.

So if we look at the 2 filters I'm talking about; Fluval G3 and G6
Fluval G3 - 700lph flow rate - 1285lph pump performance - head height 180cm
Fluval G6 - 1000lph flow rate - 2460lph pump performance- head height 210cm
Both of these use 16mm internal diameter pipe.

There are so many conflicting things online about spraybars but the above calculation seemed one that most people suggested, including the experienced ones.
It seems pointless doing the maths if after that it's just trial and error.
How will you ever truly know that your filter is pushing the correct amount of water through? Even if you set it up and time how long it takes it to fill a bucket that doesn't account for the resistance the water would normally give your filter in the aquarium.
 
i get what you are saying Andrew as water can only flow at a certain maximum rate through a hole before it cannot flow any faster. it will be a trial and error exercise the main problem will be excess back pressure and pump premiture failure if it is worked too hard.
 
i get what you are saying Andrew as water can only flow at a certain maximum rate through a hole before it cannot flow any faster. it will be a trial and error exercise the main problem will be excess back pressure and pump premiture failure if it is worked too hard.
I also get what is meant about velocity and that being a big factor.
If 100 people run in single file then they will get past a point faster than 100 people walk past it - unsure if this is a good analogy or not!
 
Fluval actually send a spraybar with the G3 and G6 but it is the same one for both.
I have used it before BUT not only is it a bit fiddly and not fit my tanks but I found it to restrict flow once the slightest bit of detritus had built up.
I thought I would have a look at it and see how many holes and what size they were:
There are 14 x 3mm holes on each spraybar which equates to an area of 98.96.
The 16mm pipe has an area of 201.06 so the supplied spraybar doesn't even cover half the area of the 16mm pipe BUT remember it only takes the slightest bit of build up to restrict the flow.
 
The 16mm pipe has an area of 201.06 so the supplied spraybar doesn't even cover half the area of the 16mm pipe BUT remember it only takes the slightest bit of build up to restrict the flow.

Smaller holes restricts the output of the filter but the water is coming out with more energy which can result in better tank turnover with good placement 'trail and error', and 'Tank turnover is the FLOW which is King' in Clives posts
 
So to surmise:
The only way to work out the number of holes needed in a spraybar is 'trial and error' - Curse all those people that published that formula and made me think it was the way to go!

Smaller holes restricts the output of the filter
Just remember this does damage the pump in your filter over time.
 
The only way to work out the number of holes needed in a spraybar is 'trial and error'

No, the calculations get you close to the number and size of holes required, just start off with smaller holes IMO and check tank turnover

Just remember this does damage the pump in your filter over time

So does putting filter media in the filter as it increases the load on the pump, esp if its crammed with fine media, doubled the life of my pump by getting a spare pump and swapping it over when it needs a good clean ;)
 
Hi everyone,

I'm going to jump in here as I am in the middle of my spray bar adventure! I have constructed a spray bar for my FX4 with the excellent guidance of Andrew which uses the theory of matching pipe size to the number of holes. This led to the current 1m spray bar that is sitting in my Juwel Vision 260.

Whilst doing my water change this morning I thought I would play around with the bar a little as I believe the pressure isn't sufficient for the size of the tank (this might also be contributing to my CO2 issues). This first video is the bar with no tweaks and therefore at my 'normal' flow and pressure:



Then I decided to start blocking holes. I didn't do any calculations as to which ones to block. I just started in the middle and tried to miss an equal number each time. This is the bar with 10 holes blocked.



And then I built up to this which is 17 holes blocked.



From what I have read and seen, an ideal pressure would get the water to hit the front glass. Whilst there is a clear progression from the first video to the last, do you think that I have reached an adequate pressure to help the flow and distribution in the tank? On the flip side by covering 17 of the 40 holes, is this likely to lead to issues with the filter/pump. I have 2 more sections of piping and will be producing a new bar when I know what to do with it.

Other than playing around with the hole configuration, is there anything else that could be done? The hosing pipe that leads to the bar is quite long. I made it this way when installing the FX4 so I knew I had enough hose - would reducing this help? Would removing some of the media from the FX4 be beneficial? I have more in there than it came with as I still have media from my Juwel internal in there when I was trying to start the FX4 up.

As always, any ideas and suggestions welcome!
 
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Hi Tom,
As you know it was me that told you to match total area of holes in spraybar to pump hose so I take the blame for that here.
the calculations get you close to the number and size of holes required
In this case Karl you can see it is nothing like close!
do you think that I have reached an adequate pressure to help the flow and distribution in the tank?
In my opinion the spraybar still looks down on power, I would be tempted to angle it up just a little - not saying that will solve this but I would have my spraybar angled slightly upwards anyway.
I would be tempted to try covering a few more holes too and see what it's like then.
is this likely to lead to issues with the filter/pump
I can't answer that but you could try and see if you can map the flow rate of your filter with/without the spraybar - the FX4 is meant to have a flow rate of 1700 LPH
Would removing some of the media from the FX4 be beneficial?
If you check out Karls posts I'm sure he has a picture of the media he runs in his FX6 which might give you an idea.

I think this is going to be a bit trial and error unless someone else can tell you better.
 

I use the same amount of sponge but in the centre red baskets have fully packed the media which came with the FX4 (this fills the baskets) and on top of that I have bags of cerax from the Juwel. Perhaps I have gone a little OTT!

As a side note @Zeus. that tank is fantastic - what an focal point for the room. Looking at mine I should be covering it in a black sheet and pretending its not there. Maybe I could just stick a picture of yours on the front and pretend! :lol:
 
I use the same amount of sponge but in the centre red baskets have fully packed the media which came with the FX4 (this fills the baskets) and on top of that I have bags of cerax from the Juwel. Perhaps I have gone a little OTT!
why not try emptying all the baskets and just see what impact that has on the spraybar before you block anymore holes up?
 
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