Tamar Designs
MEASURING YOUR POWER SOURCE
Before investing in any water power equipment, it is essential to measure your power source. You must know how much water you can deliver to the turbine and how high the water source is above the turbine. You must also know how long the penstock or feed-in pipe will be so you can calculate friction losses.
"Gross head" is the vertical height from the tail water level to the intake level when the turbine is stopped. "Net head" is the head available to drive the turbine when it is running at normal full load with all losses such as pipe friction, rise in tail water level, etc. taken into account. In the case of an impulse turbine, the height of the jet centre above the tail water level must also be deducted.
It is most important to distinguish between gross and nett head - with a long pipeline on a high head scheme or with a restricted tail race on a low head scheme, there may be as much as 20% difference between the gross and net heads.
When measuring head, impulse and reaction turbines have to be treated differently. On a Pelton or Turgo Impulse turbine (impulse turbines) you measure to the point of entrance (or nozzle), but with Francis and Kaplan turbines (reaction turbines) you measure to the surface of the discharged water. When measuring head elevation, you need not be concerned with horizontal distances and you do not necessarily have to travel in a straight line. For heads of 10 metres or less, you must be extremely accurate.
The gross head is usually measured by direct survey, and wherever possible we like to be provided with a sectional drawing showing the ground or proposed pipeline profile from head water level to tail water level. Very low head schemes of the type in which it is proposed to utilise an old water wheel pit to accomodate a turbine can frequently be surveyed by direct measurement or by use of a long straight edge and builders' level. Preliminary surveys of high head schemes can be made with an aneroid barometer or from direct map readings, Whilst this may be sufficiently accurate to enable an approximate quotation to be worked out, such figures should always be checked by direct survey before the scheme has been finally decided.
With existing schemes it is sometimes possible to obtain the net head from the pressure gauge readings, but it is most important that such readings be taken when the full flow of water available to drive the turbine is passing down the pipeline. The pressure gauge must be checked for accuracy and its elevation above the centre line of the pipe and above the tail water level must be given. The figure on the following page shows how the gross head is measured in impulse and reaction installations.
[ Gross Head Sketch ] [ Measuring Gross Head ] [ Measuring Flow Rate ] [ Water Flow Tables ]
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