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Consider the following statements regarding a turbine: 1. Specific speed plays an important role in the selection of the type of turbine. 2. An increase in specific speed of the turbine is accompanied by higher maximum efficiency. 3. The runner of too high specific speed with high available head increases the cost of the turbine on account of the high mechanical strength required. Which of the above statements are correct?

Consider the following statements regarding a turbine: 1. Specific speed plays an important role in the selection of the type of turbine. 2. An increase in specific speed of the turbine is accompanied by higher maximum efficiency. 3. The runner of too high specific speed with high available head increases the cost of the turbine on account of the high mechanical strength required. Which of the above statements are correct? Correct Answer 1 and 3 only

Concept:

Specific speed plays an important role in selecting the type of turbine. The performance of the turbine can be predicted by knowing the specific speed of the turbine.

To compare the characteristics of machines of different types, it is necessary to know the characteristics of an imaginary machine identical in shape. The imaginary turbine is called a specific turbine. The specific speed provides a means of comparing the speed of all types of hydraulic turbines on the basis of head and horsepower capacity.

If a runner of high specific speed is used for a given head horsepower output, the overall cost of installation is lower. The selection of a high specific speed reaction runner would reduce the size of the runner to such an extent that the discharge velocity of water into the throat of a draft tube would be excessive. This is objectionable because a vacuum may be created in the extreme case.

The runner of too high specific speed with available head increases the cost of the turbine on account of high mechanical strength required. The runner of too low specific speed with low available head increases the cost of the generator due to the low turbine speed.

An increase in the specific speed of the turbine is accompanied by lower maximum efficiency and greater depth of excavation of the draft tube. In choosing a high specific speed turbine, an increase in the cost of excavation of foundation and draft tube should be considered in addition to this efficiency.

The weighted efficiency over the operating range of the turbine is more important in the selection of a turbine instead of maximum efficiency.

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