The two main data needed to design a
turbine are the maximum flow rate and the net head associated to this flow.
However, these two values do not allow to uniquely define the type of turbine
that must be designed; the designer is required to choose a third fundamental
parameter: the number of revolutions.
Then
it will be possible to define what we call the turbine specific speed.
But
what is the specific speed? Leaving aside the theory about mechanical similarities,
let's briefly explain what it is.
Where
Q maximum discharge [m^3/s]
H net head [m]
g gravity (normal 9.806[m/s^2])
angular velocity of
turbine [rad/s] calculated as: 
This parameter relates
to each other the characteristic data of the turbine, and has the peculiarity of
identifying families of geometrically similar machines (and then to scale them),
having very different powers and sizes, but with one fundamental common
characteristic: they are part of the only family of turbines that can process the
available head and the discharge with the highest performance possible.
With simple
calculations you can use the characteristic data of the turbine (H, Q and n) to
find a value of specific speed that
uniquely identifies the most efficient type of turbine and its main dimensions.
All this thanks to the experience gained in the design of more than a century
of hydropower construction and excellent theoretical studies that have enabled
the development of the theory of similarity at the basis of the specific speed
calculations.
This does not mean
that all turbines manufacturers build the same equipment once given the same
typical number. After choosing the type of turbine and the main dimensions
defined, each manufacturer has developed its own geometry and parameters that
differentiate the turbines in terms of operation, reliability, cost and
efficiency. Over the years, various design schools have proposed many
definitions of the specific speed. The UNI-ISO has tried to standardize them
into a single parameter, but in fact the definitions remain different.
In HPP-Design we use
the parameter k, seen above, and nq
In the picture, every
value of typical number of machine k (or nq) corresponds to a very specific
type of turbine, exception made for some overlapping areas in which there is no
univocal choice and the experience has shown that they can adapt well to different
types of turbines (Pelton / Francis and Francis / Kaplan).
The question that
arises is: how is this value really used?
Known the head and
discharge, a preliminary study is made to define the appropriate number of
revolution, taking into account some possible rotation speeds, and then calculating
the relative specific speed which corresponds to a well-defined design
solution. The possible solutions are then compared, in terms of performance,
cavitation behaviour, main dimensions, etc. and finally the choice is made for the
solution that best suits the specific project. Once the rotation speed is
fixed, starting from the specific speed it is possible to choose the type of
turbine and start the detailed design.
Hpp-design is the tool
that helps the designer to make a preliminary choice by comparing these
elements directly into the chosen page. Try it out!
