Editing Pelton wheel (section)

== Design rules ==
[[File:S vs pelton schnitt 1 zoom.png|thumb|right|Sectional view of a Pelton turbine installation.]]
The [[specific speed]] <math>\eta_s</math> parameter is independent of a particular turbine's size.

Compared to other turbine designs, the relatively low [[specific speed]] of the Pelton wheel implies that the geometry is inherently a "[[low gear]]" design.  Thus it is most suitable to being fed by a hydro source with a low ratio of flow to pressure (meaning relatively low flow and/or relatively high pressure).

The specific speed is the main criterion for matching a specific hydro-electric site with the optimal turbine type.  It also allows a new turbine design to be scaled from an existing design of known performance.

<math>\eta_s=n\sqrt{P}/\sqrt{ \rho}(gH)^{5/4}</math> (dimensionless parameter),   <!-- !!formula missing g ?? --><ref name="Sayers1990">{{cite book|last=Sayers|first=A. T. |title=Hydraulic and Compressible Flow Turbomachines|url=https://books.google.com/books?id=YNpSAAAAMAAJ|year=1990|publisher=McGraw-Hill|isbn=978-0-07-707219-3}}</ref>

where:
* <math>n</math> = Frequency of rotation (rpm)
* <math>P</math> = Power (W)
* <math>H</math> = Water head (m)
* <math>\rho</math> = Density (kg/m<sup>3</sup>)

The formula implies that the Pelton turbine is ''geared'' most suitably for applications with relatively high hydraulic head ''H'', due to the 5/4 exponent being greater than unity, and given the characteristically low specific speed of the Pelton.<ref name=jcalvert>{{cite web|url=http://mysite.du.edu/~jcalvert/tech/fluids/turbine.htm#Impu|title= Technical derivation of basic impulse turbine physics|first= J.|last=Calvert|archive-url= https://web.archive.org/web/20210514022103/http://mysite.du.edu/~jcalvert/tech/fluids/turbine.htm#Impu|archive-date= 14 May 2021}}</ref>