Measuring the flow rate Q from an open pipe

Oct 02

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 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















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

Penstock & Hydropower Pipeline Analysis with PipeFlow®-Expert

Jul 16

Using PipeFlow®-Expert – For Penstocks & Hydro System Pipelines

SmallHydro’s Review of -

PipeFlow-Expert with a few Screenshots of Flow & Pressure Calculations


PipeFlow® Expert - Used to design and analyize Penstocks & Pipes In Hydro Systems.

A number of you readers have commented that you needed a better understanding of your Hydropower penstocks or hydro system pipelines. Some have remarked “that was fast” when a quick and accurate estimate was returned.

In this short review I’m letting you in on my secret, I use PipeFlow-Expert® by Daxesoft to make these initial penstock performance and energy loss design estimates.  It is a cycle of trial & re-trial to find the sweet spot for a hydro site. A design spot  between power ROI and the sites infrastructure costs. These hydro capital costs include; electric switchgear, turbine and penstock costs, etc.  Hint – the penstock and associated hardware are often one of the most expensive components in a hydro system. 

Penstocks also can pose a hidden system operational expense in lost hydro energy $ value.  When improperly designed, they can also have a long term risk to system integrity and safety issues as well.  You can see that it is vital to get the penstock design right as early as you can. Until you get the penstock and hydro turbine design nailed down the whole hydro project is at risk.

For a more detailed and accurate hydropower system penstock analysis I use the same software with a more detailed input model. The design results for this tool are amazing and the tool is quite simple to set up and operate too.  Maybe even fun, Ok I can hear you  – Now Jess don’t get carried away… ;-) 

I’ll just have to post you some videos showing how I use it.

This software has a great graphical interface with tabular data where it makes sense and a great fluid modeling and analysis engine to crank out your solutions on the double. Warnings and quick fix diagnostic pop-ups lead you to suspected areas within your proposed Penstock and pipeline designs that would cause either a very expensive failure or costly rework at a later time. PipeFlow Tools are worth gold to a hydropower system designer or site prospector.

But wait! Before you cart your Penstock over to their site, look at the special offer we have arranged for you first…

These screenshots are just a few of the many Hydro System relevant features of the PipeFlow Expert software. I chose them based on your Hydropower Penstock evaluation needs. For our current SmallHydro PipeFlow tools offer; See our Penstock Tools & Special Offers page. 

After getting our special code, visit the PipeFlow site for more details on the different Penstock & Pipeline evaluation tools.  I use these tools often while looking at overall Hydroelectric system efficiency and Hydro ROI $ optimization.  Note: Make sure you Get your SmallHydro Savings Discount coupon here first

PipeFlow® Expert is by far the Best tool I’ve used of for Penstock Pressure & Flow efficiency evaluations.   If your system is simple enough they even carry a less expensive ”Lite” version that will cover just a few pipes, valves and fittings for a real savings.

Of course everything PipeFlow makes has an instant Free download! So you should certainly try it before you buy it with this free limited capability trial version.  You’ll need to get our discount offer codes first though, to get your special SmallHydro discount savings applied later if and when you choose buy. Check with PipeFlow for all applicable terms and conditions.

Click on an image or link below to enlarge the details of the screenshot image. There are more od these features and worked examples described on PipeFlow’s site. Just get your SmallHydro Promo Code(s) first!


Pipe Flow Expert - Systems pipeline & penstock drawing tool details

Pipe Flow Expert - Systems pipeline & penstock drawing tool details


Pipe Flow System Drawing Area

Design, build, view and edit your system model with Pipe Flow Expert’s powerful and easy to use interface. The drawing area will let you define tanks, pipes, fittings, valves, pumps, components, flow controls and demands.


Hydro Turbine & Pump performace curve graph

Hydro Turbine & Pump performace curve graph


Pump Characteristics & Curve Data

Define a pump as a curve of flow versus head produced or as a fixed flow rate pump. Pumps can be saved and loaded independently to make it easy to copy and duplicate pumps as required.


Pipe Flow Expert - Unit selection window - allows custom Hydropower unit settings

Pipe Flow Expert - Unit selection window - allows custom Hydropower unit settings


Configure Pipe Flow Units

Choose the units that you want to work in for each field type. The units can all be set to metric or imperial on mass, or can be configured on an item by item basis as required.


PipeFlow Expert System Fittings database

PipeFlow Expert System Fittings database


Pipe Size & Materials Database

Choose from a database of standard pipe materials and standard pipe sizes for that material. New pipe material and size data can be added to the database as required.


PipeFlow Expert System Fittings database

PipeFlow Expert System Fittings database


Pipe Fittings Database

Choose from our database of fittings information to add fitting information onto pipes. New fittings data can be created by the user and stored in the fittings database.

Just add water!



How can we help you connect the drops?”

Your comments & questions are always welcome!

Measuring Hydrostatic Head (Pressure) – Step 2 of 12

Jun 24

Q: What Causes Hydrostatic Head or Hydraulic Water Pressure?

Q: How do you Measure Hydrostatic Head for
Small & Micro Hydropower System Site Evaluations?

A:  Hydrostatic Head – is the pressure rise caused by gravity acting on a column of water or fluid that is not in motion. In the case of a small or micro hydropower system it comes from the difference in elevation between the intake of the hydro system penstock and downhill to the input side of the water turbine.

Figure 1 Hydrostatic Head Measure using a Surveyors Transit or Straight Board & Levels – Method 1 of 3 see page link for more details on all 3 methods.

Link to Details on all 3 Methods for Measuring Hydrostatic Head for your site.

Hydraulic Head in Height Difference or Pressure

Head can be given as either the difference in elevation or vertical height (Head is a measured height in feet or meters) or it can be expressed as the pressure of the same water column at the turbine inlet (Pressure stated in PSI, KPA, Newtons/m^2, etc.) Either hydraulic head elevation difference or head pressure can be used as it is relatively simple to convert between them as we discuss later.

For Small & Micro Hydropower Systems, Measure Head Accurately!

The hydro system head measurements will need to be made very accurately since these head measurements play an important role in determination of other hydroelectric system parameters as well.

These head-pressure related system parameters include things such as; hydro turbine selection, system efficiency and fluid dynamic issues. For example; Kaplan, Cross-flow, Francis or Pelton turbines, hydrodynamics of turbine runner blade or bucket design, penstock material and strength, valve types, etc. are all impacted directly by the head measurement. All of these in turn impact the engineering and financial side of the hydro system design.

Because good hydro system design is so dependent on the accuracy of head measurement we will skip the use of mechanical altimeters and GPS units. Typically these have altitude errors on the order of tens of feet or more unless special survey quality GPS instruments are used. As you will see in method 3 below there are instruments that can do the job accurately, repeatedly, with ease and reasonably low cost.

Three accurate hydropower head measurement methods for measuring hydrostatic head are given in the links below, pick one that fits your need or budget and feel free to let us know about any new hydro head measurement methods you have:

The detailed article on the 3 Methods for Measuring Hydrostatic Head for your hydro site is on a different page.  Read the full article page on methods 1 to 3 for more…

Or you can visit the SmallHydro Info page if you wish to visit that article later.  We’ll be placing a few important reference article links there as we move through the 12 step Small & Micro Hydropower program. The summary follows…

Method 1 – Hydro Head Measure with Surveyors Transit, Levels & Pole

This method uses a Surveyor’s transit or contractor’s levels and a marked pole. You can use a 20 ft section of PVC pipe marked with a measuring tape attached for easier reading. The transit can be replaced by a straight board and level for economy, watch out for hydro head error buildup with short segments or warped boards or bad leveling though.   For more visit;  Method 1 to 3 details

Method 2 – Measure Head indirectly with Hose and Pressure gauge

If the distance is short enough, you can use one or more garden hoses to measure Head.  This method relies on the fact that each vertical foot of Head creates 0.433 psi of water pressure.  For example, 100 vertical feet will generate 43.3 psi in the lower end of your Penstock.  By measuring the pressure in the hose, you can use this conversion factor to calculate the elevation change of your system. We give metric conversions at the bottom of the detailed article too.

Method 3 – Measure Hydro Head with Precision Zip-Level Pro 2000™

Zip Level Pro 2000 Measure Head

Zip Level Pro 2000 Measure Head

This method is unique to DoradoVista’s approach to hydropower head measurement. We will also show how to use it for accurate flow measurement in a later post about flow measurement methods. It is a very accurate differential pressure based geotechnical instrument. It is extremely accurate when used according to the instructions given by Technidea the manufacturer. This instrument saves time, labor (one person operation is easy) and is the most accurate method we know of for a decent price. 

Read SmallHydro’s Review of the ZipLevel Pro 2000

Small Hydro Penstock & Turbine Foundation Issues

Apr 27

Hi there,

I have only subscribed yesterday to the Small & Micro Hydropower Newsletter and I really need your help,we are based in South Africa and we started refurbishing a Francis turbine and will commission it early may 2009.  Our hydropower company is in Africa.

Q: Currently we are busy proposing for a new site with a 80m head and would like to install a Pelton turbine I need some info regarding the hydropower foundation setup.
Please can you give me some guidelines I will be very thankful.

Hope to hear from you soon



Dear Mark,

We’re glad to have you join us!

A: May I suggest the first thing you need to establish for Pelton wheel hydropower design infrastructure is the same as your need for good turbine and penstock choice. Basically you need to know your static head pressure (80m) AND flow rate (m^3/s) The penstock and turbine are chosen to gain the best fit for efficiency of the available flow regime, but there is a dynamic force caveat.

Penstock length and diameter and geometry (turns & angles of turns) will determine the maximum dynamic flow forces at your thrust blocks or pipe anchor points. Every change in angle and every flow rate change will generate thrust forces, From Newton’s first law of motion. Your infrastructure & power-house design must exceed these forces by a wide margin or there will be BIG problems.

One more non-linear issue is that you must keep the linear flow rate below 5ft/s or 1.5m/s to avoid the effect of catastrophic water hammer. Remember, you are moving a freight train’s weight of water in the penstock there’s a lot of energy stored in the water’s momentum (Momentum=M x V.) The kinetic energy is (E = 1/2 m x v^2.)

If the jet(s) of a Pelton turbine gets plugged by debris the flow can stop abruptly causing a huge energy or pressure pulse to form(a.k.a. water hammer.)  This in turn will lead to huge forces on the thrust blocks and penstock walls (including the turbine mount & thrust structure) but every bit of the penstock structure will be exposed to huge pressure waves traveling back and forth at the speed of sound in water (pressure reflections).

It is not unusual for steel pipe to rupture or collapse depending on the magnitude and sign of this pressure/vacuum wave. All of these design issues are pretty much the same as large scale water conveyance pipelines in any system. For example eg. sewer or domestic supply.

So, start with your flow needs and penstock design to determine each thrust block’s forces and the terminal powerhouse (turbine foundation) forces. Make sure you consider maximum flow & catastrophic pulse generation. Then engineer each concrete and steel anchor to exceed these by a good margin. A good pipeline designer should be able to do these calculations and design this aspect of your system.

Note: All DoradoVista Small& Micro Hydropower site recommendations are not meant to replace a good design or design engineer, they are for informational and technical seeding of ideas only. You must get a locally qualified engineer to evaluate all aspects of your hydro system design including all of our recommendations for your project. Hydro system safety and economics design issues are both involved here even at 80 m of head.

That discussion on good penstock and turbine foundation design is just for starters –

Here’s a link to some more Hydropower books from that we’re collecting on various Hydro topics. It’s from our Small & Micro Hydro Reader’s corner.

The inexpensive CD’s on that web page (from the DOE on hydro) may likely give more detail on your topic question.

DoradoVista, Inc.

Hydro Prospector Jess

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