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.
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:
- Straight edge ruler marked in inches
- Plumb bob or thinweight and string at least 2 feet long
- 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:
- Tie plumb bob so it dangles 13 inches below the bottom of staight edge ruler
- Place straight edge ruler along top edge of pipe parallel to exit flow
- Shift ruler until plumb bob just touches outer surface of the flow at 13 inches below the attachment to ruler.
- Measure X distance betwee pipe tip and bob string attachmet point.
- Measure inside diameter of pipe
- 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.
Some more views of the same measurement, it works the same regardless of pipe angle. Of course bigger errors can happen at steeper angles.
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.
- 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.