Steam Turbine and Steam Nozzle | Interview Question and Answers

Steam Turbine and Steam Nozzle | Interview , viva , oral Question and Answers

1. What is meant by super saturation in steam nozzles?

Owing to the high velocity, the residence time of steam in the steam nozzle is small, and there may not be sufficient time for necessary heat transfer and the formation of liquid droplet. Consequently the condensation of the steam may be delayed for a little while. This phenomenon is known as super saturation.

2. Define stagnation enthalpy

The stagnation enthalpy represents the enthalpy of fluid when it is brought rest adiabatically.

3. What are the different forms of steam nozzles?

Convergent nozzles
Divergent nozzles
Convergent divergent nozzles

4. Define coefficient of nozzle or nozzle efficiency

Nozzle efficiency = (actual enthalpy drop) / (isentropic enthalpy drop)

5. What is the effect of friction on the flow through a steam nozzle?

The expansion will not be isentropic and enthalpy drop is reduced
The dryness fraction of the steam is increased
The specific volume of steam is increased

6. What are the differences between super saturated flow and isentropic flow in steam nozzles? Super saturated Flow Isentropic Flow

Entropy is not constant Entropy is constant
Reduction in enthalpy drop No reduction in enthalpy drop
Mollier diagram could not be used to Mollier diagram can be used to solve the problem

7. What are the reasons for the drop in velocity of the steam for a given pressure drop in steam nozzle?

Friction between the surface of the nozzle and steam
Due to internal fluid friction in the steam
Due to shock losses

8. What are the effects of super saturation in nozzles?

The dryness fraction of the steam is increased
Entropy and specific volume of the steam are increased
Exit velocity of the steam is reduced
Mass of the steam discharged is increased.

9. What are the limits for super saturation in steam nozzles? Why?

The super saturation occurs upto above 0.94 dryness fraction and beyond that the condensation of steam occurs suddenly and irreversibly at constant enthalpy and then remain in stable condition.

10. What are the main functions of steam nozzles?

To supply high velocity jet of steam in steam turbine
To inject feed water in to the boiler in a steam injector.

11. Define indicated pressure ration in steam nozzles

There is only one value of the ratio (P2/P1), which produces maximum discharge from the nozzle. That ratio is called Critical Pressure Ratio.

12. What are the factors those change the fluid properties while a fluid flows through a nozzle with no work or heat transfer?

Change in flow area
Frictional forces

13. Explain super saturated flow (or) metastable flow in steam nozzle.

When super heated steam is expanded isentropically, it starts condensing at its meet with dry saturated line. But in nozzles, the velocity of steam is high and hence the time available is very less (about 0.001 sec). So, the condensation phenomenon does not start at point ‘2’ for a flow of point 1 to point 5. As a result of this, the steam continues to expand in dry condition up to point 3. The steam between state 2 and 3 is said to be supersaturated or metastable state and the flow of such super saturated steam is known as supersaturated or metastable flow. A limit to the super heated state was observed by Wilson and a line drawn on the chart through the observed points is known as Wilson line. This line becomes the saturation line for all practical purposes. Beyond this Wilson line, the steam suddenly starts to condense and restores its normal equilibrium state.

14. What are the effects of super saturation?

The super saturation increases the specific volume and entropy of the steam.
Super saturation reduces the heat drop. Thus exit velocity of the steam is reduced.
Super saturation increases the dryness fraction of the steam. The temperature at which super saturation occurs will be less than the saturation temperature corresponding to the pressure. Therefore the density of the super saturated steam will be more than for the equilibrium condition, which gives the increase in mass of steam discharged.

15. Define nozzle efficiency.

It is the ratio of actual enthalpy drop to isentropic enthalpy drop.

16. Differentiate super saturated flow and isentropic flow.

Super saturated flow Isentropic flow

Entropy is not constant Entropy remains constant Super saturation reduces the heat drop.

therefore exit velocity is reduced No reduction in enthalpy drop. Molliear diagrams cannot be used Moiller diagrams can be used

17. Mention the applications of nozzle.

To inject feed water into the boiler in steam injectors.
To maintain, high vacuum in power plant condensers.
To supply, high velocity jet of steam jet in steam turbines.
To remove, air in condenser.

18. What are the advantages of convergent divergent nozzle?

The steam enters the nozzle at high pressure with negligible velocity and leaves at high velocity with low pressure.
Convergent-divergent nozzles are used in back pressure turbine.

19. What is the purpose of divergent portion after the throat section of nozzle?

It accelerates the steam leaving the nozzle.
It does not affect the discharge of steam passing through the nozzle.

20. Define degree of super saturation.

The ratio of super saturation pressures corresponding to the temperature between super saturated region is known as the degree of super saturation.

21. Define blade efficiency or utilization factor.

It is the ratio of rotor blade work to energy supplied to the rotor.

22. Define degree of reaction.

It is defined as the ratio of the actual isentropic heat drop to the total heat drop in the entire stage.

23. What is compounding and explain the purpose of compounding?

Compounding is the method in which multiple system or rotors are keyed to common shaft in series and the steam pressure or jet velocity is absorbed in stages as it flows over the rotor blades.

Purpose of compounding: Reduction of pressure (from boiler pressure to condenser pressure) in single results in the very high velocity entering the turbine blades. Therefore, the turbine rotor will run at a high speed about 30,000 rpm which is not useful for practical purpose. In order to reduce the rotor speed up to about 400 m/sec, compounding of steam turbine is necessary.

24. What are the types of compounding in steam turbines?

Velocity compounding, Pressure compounding and Pressure velocity compounding.

25. What are the advantages and disadvantages of velocity compounded turbines?

Advantages:

The cost of turbine is less because less number of stages.
It occupies less area.
The system is reliable and easy to operate.
Turbine casing is very simple and need not be very strong.

Disadvantages:

The friction losses are large due to very high steam velocity in the nozzle.
Low efficiency because blade speed ratio is less than the optimum value.
The power developed in the later rows is only a fraction of power developed in the first row.

26. What is governing of steam turbine and state the various methods of governing?

Governing is the method of maintaining the constant speed of the turbine irrespective of load variation by varying the flow rate. The various methods of governing in steam turbines are Throttle governing, Nozzle control governing, By pass governing, Combinations of throttle and nozzle governing and Combinations of throttle and by pass governing.

27. What is the fundamental difference between the operation of impulse and reaction steam turbines?

In impulse turbine, the steam completely expands in the nozzle and its pressure remains constant during its flow through the rotor blades. In reaction turbine, the steam expands partially in the nozzle and remaining in rotor blades.