Calculate Backyard Aquaponic and Aquaculture Pipe Flows

calculate pipe flows for gravity

Following the short article When Your Flow Rate Goes Wrong some members and readers have asked for more detail on how to work out which pipe size is suitable for them to use in their backyard aquaculture an aquaponic systems.  There is just a little math involved but it is not too complicated and this article will help you work it out, even another table!  I have to stop doing this…

The simplest way to work your gravity flow rates out is to use the pipe manufacturers website where most will have some tools for plumbers to work with when doing drainage pipes in buildings.  But if you want to take the next not so easy way, but certainly the most interesting, is to work it out for yourself, you will need some help from your good mate Hazen-Williams and lots of coffee!

The formulas are quite simple, but the way they all go together can be a little mind bending and I simply have no room for writing it out so here is a table to play with.  I can not say if this table is of any use and is only a guide and no guarantees it is correct!  My apologies to our imperial users, this one is in metric.  If time permits or there is enough requests I will convert the formulas to your weird way of measuring things!

eGravity Pipe Size and Flow RateAs with any math table we have to make a few assumptions about what you are doing.  The table assumes a PVC friction coefficient of 130 and all the flow rates (FR) are in litres per hour to maintain a 1m/sec pipe velocity which is certainly good enough for small home systems.  The velocity of the water in the pipe is important to carry the solids to their destination, your grow beds or preferably filters.

The pipe sizes will need to be rounded up to the next actual pipe size available and the flow rates are the minimum to maintain that 1m/sec velocity.  When you upgrade the size of the pipe you will lose some velocity in the pipe but there is plenty there.  Keep in mind, combining multiple formulas in one table can only do so much so it is a general guide which you should check with some of the math below if you are not sure.

In general sizes and flows with single pipe sizes in a system will work well with the table and give you a rough idea where to start.  No allowance for friction loss for bends, so keep your pipe lines as straight as you can.

My Fish Farm - Earthan Group

Let’s take my fish farm for an example to help you use this table.

Starting from the fish tank under gravity flow to the manifold and onto the filters.  My Fish Farm is a little interesting because it uses two outlets (top and bottom) from each tank.

The two sets of pipe are as follows:

  • Bottom drain is 25mm x 1500mm long on each tank, which connects to a 40mm x 3600 manifold.
  • Top skimming outlet is 40mm x 2000mm long connecting each tank to a 40mm x 4000mm manifold.
  • Drop in the entire system is only 50mm.
  • Overall flow rate is 4500 litres per hour and each tank runs about 1500 litres per hour.  Roughly
    70% running out of the top and 30% out of the bottom.

The top drain manifold is 40mm x 4000mm long with a 50mm drop using the table above we need 4494 litres per hour to maintain the pipe velocity of 1m/sec and it tells us the pipe should be about 50mm which it is not but knowing the formula helps:

Flow rate = 0.285*130*Pipe Diameter in meters^2.63*Drop/Length^0.54*60*1000*60

Flow rate = 0.285*130*0.04^2.63*50/4000^0.54*60*1000*60

Flow rate for a 40mm pipe with that drop is, you guessed it 2635 litres per hour.

As I am not concerned with velocity in each of the short 40mm x 2000mm outlets on the tank, they are sized the same for convenience but if you want to work it out by the table a 28mm pipe would be very close. (upgraded to 32mm a 25mm pipe would be just a little to small).

The bottom drain has a similar set up with 40mm x 3600mm manifold.  The table tells me I would need a 44mm pipe to get 3639 litres per hour flow but I only need 1800 LPH or there about. So let’s work this one out.

Pipe diameter = Flow rate m3/s/0.285*130*drop/length^0.54^1/2.63

Pipe diameter = 0.0005/0.285*130*50/3600^0.54^1/2.63

Pipe diameter for 1800 litre per hour flow rate with 50mm drop is 34mm.  So I upgraded to 40mm.  On top of that each tank has a 25mm x 1500mm long running 900 litres per hour each which the table gives us 22mm.  Again upgraded.  I measured the water flows for all the bottom drains and the current flow rate is 1440 litres per hour total.

Hot Tip: In short, if you are not sure, upgrade your pipe size and allow access to pull a brush through the pipework to keep them clean.  This is very handy even if you have the right velocity to clean out the bio film build up.

If you managed to read all of that, good on you!  You will no doubt need a break, I know I do.

Perhaps if the need arises I will do a pumping one.

Until then, happy growing.
Regards
Paul

Following the short article When Your Flow Rate Goes Wrong some members and readers have asked for more detail on how to work out which pipe size is suitable for them to use in their backyard aquaculture an aquaponic systems.  There is just a little math involved but it is not too complicated and this article will help you work it out, even another table!  I have to stop doing this…

The simplest way to work your gravity flow rates out is to use the pipe manufacturers website where most will have some tools for plumbers to work with when doing drainage pipes in buildings.  But if you want to take the next not so easy way, but certainly the most interesting, is to work it out for yourself, you will need some help from your good mate Hazen-Williams and lots of coffee!

The formulas are quite simple, but the way they all go together can be a little mind bending and I simply have no room for writing it out so here is a table to play with.  I can not say if this table is of any use and is only a guide and no guarantees it is correct!  My apologies to our imperial users, this one is in metric.  If time permits or there is enough requests I will convert the formulas to your weird way of measuring things!

eGravity Pipe Size and Flow RateAs with any math table we have to make a few assumptions about what you are doing.  The table assumes a PVC friction coefficient of 130 and all the flow rates (FR) are in litres per hour to maintain a 1m/sec pipe velocity which is certainly good enough for small home systems.  The velocity of the water in the pipe is important to carry the solids to their destination, your grow beds or preferably filters.

The pipe sizes will need to be rounded up to the next actual pipe size available and the flow rates are the minimum to maintain that 1m/sec velocity.  When you upgrade the size of the pipe you will lose some velocity in the pipe but there is plenty there.  Keep in mind, combining multiple formulas in one table can only do so much so it is a general guide which you should check with some of the math below if you are not sure.

In general sizes and flows with single pipe sizes in a system will work well with the table and give you a rough idea where to start.  No allowance for friction loss for bends, so keep your pipe lines as straight as you can.

My Fish Farm - Earthan Group

Let’s take my fish farm for an example to help you use this table.

Starting from the fish tank under gravity flow to the manifold and onto the filters.  My Fish Farm is a little interesting because it uses two outlets (top and bottom) from each tank.

The two sets of pipe are as follows:

  • Bottom drain is 25mm x 1500mm long on each tank, which connects to a 40mm x 3600 manifold.
  • Top skimming outlet is 40mm x 2000mm long connecting each tank to a 40mm x 4000mm manifold.
  • Drop in the entire system is only 50mm.
  • Overall flow rate is 4500 litres per hour and each tank runs about 1500 litres per hour.  Roughly
    70% running out of the top and 30% out of the bottom.

The top drain manifold is 40mm x 4000mm long with a 50mm drop using the table above we need 4494 litres per hour to maintain the pipe velocity of 1m/sec and it tells us the pipe should be about 50mm which it is not but knowing the formula helps:

Flow rate = 0.285*130*Pipe Diameter in meters^2.63*Drop/Length^0.54*60*1000*60

Flow rate = 0.285*130*0.04^2.63*50/4000^0.54*60*1000*60

Flow rate for a 40mm pipe with that drop is, you guessed it 2635 litres per hour.

As I am not concerned with velocity in each of the short 40mm x 2000mm outlets on the tank, they are sized the same for convenience but if you want to work it out by the table a 28mm pipe would be very close. (upgraded to 32mm a 25mm pipe would be just a little to small).

The bottom drain has a similar set up with 40mm x 3600mm manifold.  The table tells me I would need a 44mm pipe to get 3639 litres per hour flow but I only need 1800 LPH or there about. So let’s work this one out.

Pipe diameter = Flow rate m3/s/0.285*130*drop/length^0.54^1/2.63

Pipe diameter = 0.0005/0.285*130*50/3600^0.54^1/2.63

Pipe diameter for 1800 litre per hour flow rate with 50mm drop is 34mm.  So I upgraded to 40mm.  On top of that each tank has a 25mm x 1500mm long running 900 litres per hour each which the table gives us 22mm.  Again upgraded.  I measured the water flows for all the bottom drains and the current flow rate is 1440 litres per hour total.

Hot Tip: In short, if you are not sure, upgrade your pipe size and allow access to pull a brush through the pipework to keep them clean.  This is very handy even if you have the right velocity to clean out the bio film build up.

If you managed to read all of that, good on you!  You will no doubt need a break, I know I do.

Perhaps if the need arises I will do a pumping one.

Until then, happy growing.
Regards
Paul

About the author

Paul Van der Werf

Paul is the Operations Manager for a 4400m2 integrated aquaculture pilot project in the United Arab Emirates desert he designed and built. This is a commercial aquaponics pilot to evaluate integrated farming in arid climates.

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7 Comments on “Calculate Backyard Aquaponic and Aquaculture Pipe Flows

  1. Caca

    Great calculation on flow rate Paul.
    Thank you.

    P.S. Psst, Don’t tell anyone, I don’t get it at all. ;-)

    1. No worries I will keep it to myself lol

  2. gulmarg

    Hi
    Curious to know what ‘Drop’ in the table above refers to

    CHeers

    1. Hi gulmarg,

      The drop means the fall from the inlet of the pipe to the outlet measured vertically.

      For example if the water is entering the pipe at 1000mm and exiting at 800mm the fall or drop will be 200mm

      Regards
      Paul

  3. gulmarg

    Hi Paul,
    Thanks for the response. I’m designing a backyard aquaponics setup and trying to figure out pipe diameter sizing for my system. I’ve plugged the pipe diameter equation you have used above (Pipe diameter = 0.0005/0.285*130*50/3600^0.54^1/2.63) into excel and I’m not getting the value you got (34mm). I’m getting 0.05208. Curious to know if any brackets are needed for the equation.

    Cheers

  4. Pingback: Get your flow rates right and your fish will thrive | Earthan Group Pty Ltd

  5. Pingback: Design Flow Rates for Commercial Aquaponics - Earthan Group Pty Ltd

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