Pipe Calculation for Water Flow

I know only the flow rate (40 m3/hr) of water but velocity and pipe diameter are unknown.

How can I calculate the unknown diameter and velocity?

Please help.

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• Utility Pipe Network
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• Pipes and Pipelines

17 Answers

1. in case of flow measurement you can get velocity as well at the time of installation of fixixng portable water flow meter you should measure the diameter of pipe before installation. 

2. Keep in mind that all of the responses talking about what the velocity should be, typical values, etc. have nothing to do with answering your basic question.

   If you know flow then you have to have either the inside pipe diameter or the velocity. Don't cloud the issue about other things that have nothing to do with your question. Either somehow measure the diameter of the pipe is typically the only thing you can find out where you know the flow. Simply you could wrap a flexible tape measure around the pipe and calculate the diameter from the circumference.

3. Hi Arshad, you can do these following approximations:

    

   1. Can you measure pressure differences and flowrate between 2 points?

   If yes you can use simplified Bernoulli Equation to find the Area of the pipe with the following calculations (assume that there is no changes in area and no changes in height between those 2 points of measurements):

   A (area of pipe) = sq root of (rho*g*(Q2 squared - Q1 squared) / 2*(P1-P2))

   You can then easily find the diameter of “A” using area of circle formula.

    

   2. You can use an approach by looking for flowmeters specs. Most flowmeters have a link between their average flowrate capacity and pipe diameter. As you mentioned flowrate of 40 m3/h, if this is the average/permanent/most dominant flowrate, then the closest diameter of flowmeter that will match this requirement is diameter (DN) between 50-65 mm.

    

   Hope this help.

   1 Comment

   1. You can later calculate the velocity based on Flowrate (Q) and diameter of the pipe both for option 1 or 2.

4. You have to know the pipe diameter. Can you find the pipe exposed where you can either measure the circumference or diameter to find the outside diameter? Assuming it is standard commercial pipe then look up the wall thickness in the pipe table and calculate the actual inside diameter.

   v=21.22 Q/d^2 where

   v = velocity is meters/sec

   Q=Flow in Liters/Minute (convert your flow required)

   d=pipe inside diameter in mm (Note: d^2 is inside diameter squared)

5. You will have to assume velocity of flow in the pipe. Secondly you must know the levels of the junctions/starting point/delivery point. Then only you will be able to calculate head available and required.  Design of gravity distribution is very critical particularly when the slopes are less. You can take help of EPANET software

6. Hi Mr. Arshad,

   The desired velocity for long pipelines is nearly nearly  1.5 m/s, We calculate the diameter of pipe assuming the velocity at 1.25 m/s (Q=AV) and adopt the next available diameter of the pipe in the market. This works well for longer pipe lines up to 5 km

7. Hi Mr. Arshad,

   The desired velocity for long pipelines is nearly nearly  1.5 m/s, We calculate the diameter of pipe assuming the velocity at 1.25 m/s (Q=AV) and adopt the next available diameter of the pipe in the market. This works well for longer pipe lines up to 5 km

8. Dear Mohd Arshad，

   In my understanding， your question is below：

   You have a big customer with water demand 40m3/hr（average water flow）, and you need to confirm pipe DN /velocity during design phase.

   On this condition, you need to know this bid customer's consumption pattern and internal tank volume. And then, please check the public water network, including water pressure, pump operation,...

   Finally, you will address the fitful pipe DN and velocity.

   BR,

9. Dear Mohd,

   You have received very good comments of this professionals!
   
   I would like to add that the velocity is a crucial parameter to determine the internal diameter or viceversa, on the other hand the material and pressures are other important variables to solve your system.
   
   The instalation and use of the water are other paramters to have into account.
   As an example, if you have short distances, and must pump between high differential pressure (example: inlet pump and outlet pump pressure), as  example boilers, you could use 4,5 m/s, and maybe more... but if you must pump water in a zero slope line for several kilometers, you will want the less possible velocity that gives you less lost in the water system, then you could think in 0,6 to 1 m/s...
   
   Regards,
   
   Orlando D. Gutiérrez Coronado

    

    

    

    

    

    

    

    

    

    

   1 Comment

   1. hi, Mr ornaldo,

      thank you very much,

       

10. I assume this is a design question? If this is an existing system you can't solve it - as others have indicated, you need two terms to solve Q=VA for the third term. And that assumes the pipe is filled.

    If this is a design problem, you need to determine if this is a pipe flowing full, such as a force main, or a partially full pipe, such as a gravity sewer. There are different procedures for each.

11. Good morning,, you can adopt a velocity in a range of 1,5-2 m/s (1.8 m/s for instance,). This velocity will give you the internal diameter of the pipe: Section=Flow/Velocity. Then you can select your pipe based on a supplier catalog and the nearest available section ..;

    1 Comment

    1. Hi Philippe

       thank you 

12. Normally 40 m3/h flow you should select approx.. 100 mm ND pipe. The internal diameter depends on the material and class of the pipe you selected. For example, PVC pipe class 12, will have velocity of 1.37 m/s; if select a HDPE pipe will be 110 mm ND and for class 16, will have velocity 1.79 m/s.; if select steel pipe, if it is steel pipe with class 4, the velocity will be 1.35 m/s.

13. Hi.

    You could try Pipe Spec V3 from PAM - Saint Gobain.

    Download link:

    http://www.saint-gobain-pam.co.uk/pipespec

     

    It is a really simple / basic tool for what you need!

    Dr. Toth Szilard

    +40 (0) 726 166 802

14. Hi Mohd,

    In addition to Paul's message,  pipe selection may also depend on the nature of water (water supply, stormwater or industrial effluent) to be conveyed within that pipe. 

    Based on the pipe material selected along with the type of water, friction loss could also be a parameter influencing the velocity within the pipe.

     

    Hope it helps!

    regards Thomas 

     

15. Hi Mohd,

    Normally we could use the simple Q = VxA, however for this to be used we need 2 known parameters.

    Just a couple of questions for you:

    1. What type of pipe material it it?

    2. What is the distance from the source to the outlet (where you have 40m3/hr)

    3. What is the static head (fall or lift) between the source and outlet; or, what is the source pressure if pumped?

    4. What is the residual pressure?

     

    We may be able to estimate the pipe diameter and velocity if we have a little more information?

     

    We can work out what pressure drop/increase there is between the source and outlet, and divide this by the distance, it would at least provide some indication of pipe size and also velocity (hopefully you know the pipe material, which will help further).

     

    Hope you can provide some further info and we can look into it.

     

    Paul.

    1 Comment

    1. Hi paul,

       Thanks for your help

       There is lots of parameters required to design a pipe, I hadn't thought about that.

       I can give you only some parameters.

       1.Material:mild steel

       2.Water is being pumped up to to 6 m hieght through a 7  meter long pipe.

       3.Pump type( filter feed pump). Centrifugal.

       4.4.It a industrial wastwater treatment plant.

       5.Rest of the parameters you can consider.

       And please share with me the procedure of doing all calculation.

       Thanks again

       Regards

       MOHD ARSHAD

       1 Comment reply

       1. Hi Mohd, so sorry about the late reply, i didn't realise you had replied to my answer.

           

          I can tell you that in irrigation (PVC) mainlines we use a maximum velocity of 1.5m/s. This is for two main reasons: 1. To reduce the risk of water hammer, and 2. To reduce any excessive friction loss in the pipe (because of increased pumping cost).

          In steel pipework however, I  go up to a maximum of 2.2m/s because the pipe is generally going to handle any water hammer issues better than PVC.

          Unfortunately, with your system we don't have any pressures (pump/pipe discharge) to work with so I can't really give you an estimate of size. If we knew the pressure differences between the pump and the discharge then we could calculate the headloss through the length of pipe and find a size.

          Obviously there would be a drop of 60kPa (if a static lift of 6.0m) because of the height, but this isn't anything to do with friction. I'm sorry that I can't be of any more help to you Mohd.

          Paul.

16. Good Afternoon Ian;

    For your consideration 

    http://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html 

    Warmest regards,

     

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17. Hi Mohd, we generally use the Darcy Wiesbach formula for pipe flow where your inputs are pipe diameter, flow rate, pipe roughness (depending on type of material) .  You choose different pipe diameters and generate different flow velocities and friction losses.  You then select the optimum pipe size that will give you a lowish friction loss and a flow velocity that is acceptable (say between 1 and 4 m/s max).  Attached is a simple nomogram for HDPE pipeflow based on this equation. 

    1 Comment

    1. If it's a discharge pipe, i.e. outlet of a pumping station, trunk main, etc., it's a matter of optimum: take a range of 2-3 diameters and use Q=V x A. Minimum velocity should not be lower than 2m/s.