Q: What Causes Hydrostatic Head or Hydraulic Water Pressure?
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Q: How do you Measure Hydrostatic Head for
Small & Micro Hydropower System Site Evaluations?
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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
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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!
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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…
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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
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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
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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™
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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.
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