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Friday, 16 August 2019

SPECIFIC SPEED OF TURBINE AND ITS SIGNIFICANCE

We were discussing a new topic, in the subject of fluid mechanics and hydraulics machine, i.e. an introduction to hydraulic machinevarious types of hydraulic turbines and some important terminologies associated with a hydraulic turbine such as Gross head, Net head and efficiencies of a hydraulic turbine


Now we will focus here to understand the term specific speed in turbines with the help of this post. We will also find out here the significance of specific speed in turbine. 

Specific speed

Specific speed is basically defined as the speed of a turbine which is identical in shape, geometrical dimensions, blade angles, gate opening etc., with the actual turbine but of such a size that it will produce unit power when working under unit head. 

Specific speed will be denoted by symbol Ns. 

Different types of turbines are compared by using the value of specific speed as every type of turbine will have different specific speed. 

Where,
N = Speed of actual turbine
Ns = Specific speed of turbine
P = Power developed or shaft power
H = Head under which the turbine is working 

Significance of specific speed 

There is very important role that specific speed plays during the selection of type of turbine. Specific speed will predict the information about the performance of a turbine. 

According to the specific speed, turbines will be classified as mentioned here. 

High specific speed turbine

Specific speed of such turbines will be in the range of 255 to 860 and hence such turbines will be termed as high specific speed turbines. 

Kaplan and propeller turbine are the best examples of high specific speed turbines. 

Medium specific speed turbine 

Specific speed of such turbines will be in the range of 50 to 250 and hence such turbines will be termed as medium specific speed turbines. 

Francis turbine is the best example of medium specific speed turbine. 

Low specific speed turbine 

Specific speed of such turbines will be in the range of 8 to 30 with single nozzle and up to 50 with multiple nozzles. Hence such turbines will be termed as low specific speed turbines. 

Pelton turbine is the best example of low specific speed turbine. 

Specific speed for various types of turbines 

Specific speed for various types of turbines are mentioned in following table as displayed here. 


Do you have any suggestions? Please write in comment box.  Further we will find out, in our next post, Governing of turbine.

Reference: 

Fluid mechanics, By R. K. Bansal 
Image courtesy: Google 

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