I think the others have explained how to calculate the pressure drop
in pipes with different flow rates.
One thing you maybe did not understand is that the pipe has to be
sized much bigger than would be needed just to carry the water. If
you work out how much water the pipe can carry when it uses up all
the pressure just to carry it then this will not be so useful. You
want most of the pressure to be still there when you get to the
turbine. (The remaining pressure known as net head is what produces
The flow is controlled by the turbine (by the nozzle size usually)
and not the penstock pipe. The pipe needs to be big enough that it
can deliver that flow without losing too much pressure. Obviously
the nozzle affects the water flow in the pipe quite dramatically but
without that you will get no power. You will get a lot more water
flowing through the pipe if it is just wide open at the end, but all
the pressure is wasted in pushing the water through the pipe and it
emerges with no useful power.
For a given pipe, and if you have plenty of water, then the most
power you can get is by limiting the flow so that 1/3 of the pressure
is dropped in the pipe and 2/3 in the turbine. If you use less water
you will get more pressure but less power. If you use more water you
will get less pressure and less power.
I hope this helps.
>I want to build a microhydropower system in the mountains, the aim
>would be to generate 4kW, but alos 2kW is acceptable.
>I have read and undertood the theories abot head and flow, but I don't
>know how to calculate the effective water flow taht would pass through
>the penstock/nozzle/turbine; the idea is that the waterflow of the
>creek is big enough, and I want to install just a sort of collection
>system that would pour water into the penstock, as much as the
>penstock/nozzle would run.
>How do I calculate, at least approximatively, the waterflow through a
>vertical plastic pipe of, let's say, 2" diameter and a head of X
>And more, if the pipi would finish with a 1/2" nozzle, how this would
>affect the waterflow ?
Scoraig Wind Electric