# Specific Speed of Hydro-turbines

• I_am_learning
In summary, specific speed of a turbine is defined as the speed at which a geometrically similar turbine would rotate when provided with unit head and unit discharge. The formula for specific speed is N = Ns * H^(5/4) / sqrt(P), where N is the actual speed, Ns is the specific speed, H is the head, and P is the discharge. This formula can be used to find the specific speed of a turbine when provided with a certain head and discharge. Textbooks often provide charts of specific speed versus head, as specific speed is dependent on both the turbine geometry and the head and discharge. This may cause confusion, but it is important to understand that specific speed is not a static value and will vary based
I_am_learning
I am again confused at this. As far as I know, specific speed of a turbine of given Shape and Size is defined as -"The speed at which geometrically similar (i.e. scaled down Version) turbine would rotate when provided with unit head and unit discharge"
The general text-book derivation for specific speed gives

But, I guess its written the other-way-round. I mean, most of the time, we have our turbine, and we have the specific speed for our turbine (owing to its shape & provided by the manufacturer), and we would like to find out at what speed it would rotate when we provide some head H and Discharge Q (P). So, I think, the useful way of writing the formula would be
N = Ns * H^(5/4) / sqrt(P) .
That is all for my understanding.
Now my confusion. :)
I also find that text-books gives charts for Ns Vs Head ? I thought, specific speed is dependent only on Turbine Geometry. Then how come, say a franchise turbine of given geometry have different specific speed for different heads? I feel like It kills the very own definition of specific speed.

I think your confusion arises due to a misunderstanding of the concept of specific speed. Specific speed is not a static value, it is dependent on the head and discharge of the turbine. So, when you provide a turbine with different heads and discharges, the specific speed will vary accordingly. The charts that you find in textbooks are actually plots of the specific speed for various heads and discharges. So, when you look at them, you can find the specific speed for a particular turbine based on the head and discharge you are providing it with. Hope this helps!

## What is the specific speed of a hydro-turbine?

The specific speed of a hydro-turbine is a measure of its efficiency and is represented by the letter Ns. It is calculated by dividing the turbine's rotational speed in revolutions per minute (RPM) by the square root of its flow rate in cubic feet per second (cfs).

## Why is the specific speed of a hydro-turbine important?

The specific speed of a hydro-turbine is important because it helps determine the type and size of turbine that is best suited for a specific application. It can also help predict the performance and efficiency of the turbine.

## What is a high specific speed in a hydro-turbine?

A high specific speed in a hydro-turbine indicates a turbine with a higher rotational speed and a lower flow rate. These types of turbines are typically used for high head, low flow applications, and are more efficient at producing electricity.

## What is a low specific speed in a hydro-turbine?

A low specific speed in a hydro-turbine indicates a turbine with a lower rotational speed and a higher flow rate. These types of turbines are typically used for low head, high flow applications, and are more efficient at pumping water for irrigation or other purposes.

## How does the specific speed of a hydro-turbine affect its design?

The specific speed of a hydro-turbine affects its design by determining the shape and size of the blades, the type of runner, and the overall efficiency of the turbine. Turbines with different specific speeds will have different designs to optimize their performance for their specific application.

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