Editing Water turbine (section)

== Design and application ==
[[File:Water Turbine Chart.png|right|350px]]

Turbine selection is based on the available [[water head]], and less so on the available flow rate.  In general, impulse turbines are used for high head sites, and reaction turbines are used for [[Low head hydro power|low head]] sites. [[Kaplan turbine]]s with adjustable blade pitch are well-adapted to wide ranges of flow or head conditions, since their peak efficiency can be achieved over a wide range of flows.

Small turbines (mostly under 10 MW) may have horizontal shafts, and even fairly large bulb-type turbines up to 100 MW or so may be horizontal.  Very large [[Francis turbine|Francis]] and Kaplan machines usually have vertical shafts because this  makes best use of the available head, and makes installation of a generator more economical.  [[Pelton wheel]]s may be either vertical or horizontal shaft machines because the size of the machine is so much less than the available head.   Some impulse turbines use multiple jets per runner to balance shaft thrust.  This also allows for the use of a smaller turbine runner, which can decrease costs and mechanical losses.

=== Typical range of heads ===
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• [[Water wheel]]<br/>
• [[Screw turbine]]<br/>
• [[VLH turbine]]<br/>
• [[Kaplan turbine]]<br/>
• [[Francis turbine]]<br/>
• [[Pelton wheel]]<br/>
• [[Turgo turbine]]<br/>
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0.2 < ''H'' < 4 &nbsp;&nbsp;(''H'' = head in m)<br/>
1 < ''H'' < 10<br/>
1.5 < ''H'' < 4.5<br/>
2 < ''H'' < 70<br/>
10 < ''H'' < 300<ref>{{cite web|url=http://www.alstom.com/power/renewables/hydro/hydro-turbines/francis-turbines/|title=Francis hydro turbines|website=alstom.com}}</ref><br/>
80 < ''H'' < 1600<br/>
50 < ''H'' < 250
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</div>

=== Specific speed ===
{{Main|Specific speed}}

The specific speed <math> n_s </math> of a turbine characterizes the turbine's shape in a way that is not related to its size. This allows a new turbine design to be scaled from an existing design of known performance. The specific speed is also the main criteria for matching a specific hydro site with the correct turbine type.
The specific speed is the speed with which the turbine turns for a particular discharge Q, with unit head and thereby is able to produce unit power.

=== Affinity laws ===
[[Affinity laws]] allow the output of a turbine to be predicted based on model tests.  A miniature replica of a proposed design, about one foot (0.3 m) in diameter, can be tested and the laboratory measurements applied to the final application with high confidence. Affinity laws are derived by requiring [[similitude (model)|similitude]] between the test model and the application.

Flow through the turbine is controlled either by a large valve or by wicket gates arranged around the outside of the turbine runner.  Differential head and flow can be plotted for a number of different values of gate opening, producing a hill diagram used to show the efficiency of the turbine at varying conditions.

=== Runaway speed ===
The '''runaway speed''' of a water turbine is its speed at full flow, and no shaft load. The turbine will be designed to survive the mechanical forces of this speed. The manufacturer will supply the runaway speed rating.