Measuring the flow rate Q from an open pipe

Oct 02
2009

How to Measure your Water Flow Rate Q from an Open Pipe:

There are times when it’s necessary to estimate the flow rate from a stream constrained to flow in a water jet flowing from an open pipe. This flow measurement method doesn’t require us to have precision fluid flow measurement instrumentation, other than a straight edge and plumb bob. Flow meters or weirs would likely be more accurate, but sometimes all you have is a filled pipe with a jet of water streaming out. If that’s the case, then try this method.

Pipe Flow Direct Q Measurement Diagram A

Pipe Flow Q Direct Measure Diagram A

There are two basic pipe flow measurement methods used to make this estimation. The first pipe flow measurement method involves pipes that are completely filled; that is they have no air in  the water pipe above the fluid exiting the pipe, the other method used is a partially filled pipe. The completely filled pipe flow case will be fairly easy to measure utilizing the table enclosed below. The other method we will cover in another SmallHydro.com blog post.

Meanwhile let’s examine how to measure the pipe flow rate Q from a water jet squirting from a full pipe:

What is needed to measure the completely filled pipe flow case is a simple straight edge ruler and a plumb bob marked off at an appropriate pre defined distance, in this case we use 13 inches. To utilize this method one simply measures out an appropriate extra distance parallel to the initial pipe exit flow direction up to an intercept point with the plumb bob line that is 13 inches below that point. See  flow measurement diagrams A, B & C.

You will move the ruler parallel to the pipeline until the plumb bob just touches the outer edge of the falling stream of water. By measuring this intercept distance X. for the given 13 inch plumb bob Y. value, you then simply use the table below with your pipe diameter and X value to look up the flow in gallons per minute or GPM.  You can convert these values to your desired standard units of measurement as well.

This water flow estimation technique is used in agriculture to measure irrigation flow from an open pipe. This method can also be used for smaller flows,  if a stream can be constrained temporarily to completely fill the inside of a pipe and flow out the end above the ground or stream tail water enough to not interfere with the plumb bob.

Keep in mind that if the pipe is small enough, direct measurement can be made by filling the bucket and timing the amount of time it takes to fill it, we’ll discuss this in another Small Hydro power stream flow measurement post and link to it later.

So, Let’s take a look at the measurement technique, see pipe flow measurement procedures, and diagrams A., B. and C. :

Tools needed to measure your hydropower system pipe flow rate Q:

  1. Straight edge ruler marked in inches
  2. Plumb bob or thinweight and string at least 2 feet long
  3. A pipe completely filled with the water flow jet about 18-24 inchs up in the air above any interference.

 

Procedure to measure potential hydropower system flow Q from a full flowing pipe:

  1. Tie plumb bob so it dangles 13 inches below the bottom of staight edge ruler
  2. Place straight edge ruler along top edge of pipe parallel to exit flow
  3. Shift ruler until plumb bob  just touches outer surface of the flow at 13 inches below the attachment to ruler.
  4. Measure X distance betwee pipe tip and bob string attachmet point.
  5. Measure inside diameter of pipe
  6. Use ‘X’ and Diameter with edges of table to read off flow rate in GallonsPer Minute (GPM)

 

Note: Keep ruler parallel to pipe at all times.

Pipe Flow Q Direct Measure Diagrams Full & Partial flow D-E

Pipe Flow Q Direct Measure Diagrams Full & Partial flow D-E

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Some more views of the same measurement, it works the same regardless of pipe angle. Of course bigger errors can happen at steeper angles.

Pipe-Flow-Q-Direct-Measure-Diagrams-B-C

Pipe Flow Q Direct Measure Diagrams B & C

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The following table can be used to look up the corresponding flow rate in gallons per minute or GPM when given your measured X. value.  See the reference equation given below for how this was computed. Also see the OSU irrigation and hydrology site for more information on these irrigation and other water flow measurement methods.  Click the water flow table picture below to get a closer view of the table or to print it  out.

 

Equation for computing Full Pipe Flow Rate in GPM

Equation for Full Pipe Flow Rate in GPM

Note  Disclaimer

- This is a pipe flow estimation table calculated for 13 inches of  ‘Y’ fall. For other ‘Y’ distances use the formula given.  Accuracy is not guaranteed, so check your work to make sure the results are adequate for your application.

 

 

Pipe Flow Direct Measure Flow - Q Chart

Pipe Flow Direct Measure Flow - Q Chart

9 Responses to “Measuring the flow rate Q from an open pipe”

  1. Open Pipe Flow Chart – Full Flow Calculation Table | Small Hydropower & Micro Hydropower says:

    [...] By popular demand, this a larger copy of the flow table mentioned in the Open Pipe Flow Measurement Method post. [...]

  2. peter says:

    Did you publish the method for partially full pipes? Where is it?
    Thanks,

  3. Jess says:

    Peter,

    Take a look at the “California Pipe” method post. Posted on 11/19/2009.

    http://smallhydro.com/200911/small-micro-hydro/measure-flow-rate-q-using-the-california-pipe-method/

  4. PKARTHIGAYAN says:

    THIS FLOW MEASUREMENT TECHNIQUE IS VERY MUCH USEFUL FOR HYDRO ENGINEERS LIKE ME AND AGRICULTURIST AS READY RECKONARE TYPE.SO KINDLY PUBLISHED THIS TYPE PRACTICAL ORIENTED SOLUTIONS IN THE NEAR FUTURE ALSO. AND GIVE ONE EXAMPLE TYPE SOLUTION ALSO LIKE TO CALCULATE SPILLWAY DISCHARGE IN ORIFICE TYPE SLUICE.

    REGARDS
    P.KARTHIGAYAN
    HYDRO ENGINEER,NEWDELHI,INDIA

  5. Ross G Jackson says:

    I have a quistion about vertical flow rate,

  6. differential pressures says:

    Above mentioned technique of water flow measurement is no doubt very useful but now its advancement in process and we are waiting for…… thanks to provide this great info that would be beneficial for everyone….again thanks

  7. Eve says:

    Hallo jesse,

    mine is not about measuring Q, however i need your comment. how to select a valve for the penstok. we had in mind using a burtterfly valve however what other options are there and there pros and cons.

  8. Roy Bird says:

    HI. I work in Papua Newv Guinea, and have a requirement for a small hydro plant that would be suitable for a smallvillage stiuation. The demarnd for this type of equipment is very high . Please advise me if you can help./

    ROY BIRD

  9. Barkat Nida says:

    Dear Roy Bird,

    I am consultant at Innovative Energy Foundation India.
    Please give following details about your situation:
    1-Flow velocity of Water
    2:-Quantity of flow
    3-Head
    If you face some problem in preparing the above information, please feel free to ask more questions till you solve the problem.
    Barkat Nida
    barkatnida@gmail.com

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