STEAM TURBINES QUESTION ANSWERS CLASS NOTES

STEAM TURBINES QUESTION

ANSWERS CLASS NOTES  

Question answers are very helpful in

understanding and clarity about the

topic. Fundamentals become well

understood. Apply these in real

life with ease.

1. What is a steam turbine?


Steam turbine is a high speed rotating machine. It converts the heat energy of steam into work/motion/ electrical energy.


2. Why is steam turbine widely used?


Steam turbines are widely used to develop power as their efficiency is highest as compared to other turbines.


3. Name the various types of steam turbines.


There are two types of steam turbines.
(i) Impulse turbine
(ii) Reaction turbine

4. What is an impulse steam turbine?


Total expansion (total pressure drop) is in fixed nozzles located outside the turbine casing. In this case, velocity of steam from the nozzle is very high. The jet of steam coming out of the nozzle strikes the blades mounted on the rotor. The rotor  connected to generator produces power. It has been observed that the velocity of steam jet is almost two times the velocity of the moving blades.


5. What is a reaction steam turbine?


In a reaction turbine, nozzles (called as fixed blades) as well as moving blades are mounted on the rotor. There is a pressure drop in the nozzles as well as in moving blades. Pressure drop takes place in stages i.e. total pressure drop does not take place in one set of nozzles. Therefore, velocity of the steam jet is less than that in case of an impulse turbine.

one stage = set of fixed nozzles + set of moving blades

  Number of stages covers the total pressure drop.

6. What is a stage in a turbine?


In an impulse turbine, stage means set of nozzles outside the turbine + moving blades on the rotor.
In a reaction turbine, stage means one set of fixed blades + one set of moving blades.


7. What is degree of reaction?


DEGREE OF REACTION OR REACTION RATIO (ρ)
It is the ratio of static pressure drop in the rotor to the static pressure drop in the stage
ρ = (Static pressure drop in rotor/Total static pressure drop in one stage)
OR
ρ= (adiabatic enthalpy drop in rotor/ Total adiabatic enthalpy drop in one stage)
Degree of reaction has the symbol ρ (row). It has no units.
Degree of reaction in an impulse turbine will be zero.
Hence degree of reaction is applicable to the reaction turbine.
If degree of reaction is greater than 50%, only then it is a reaction turbine.
Larger the degree of reaction, larger will be the efficiency of the turbine. Therefore it helps in the selection of a turbine.
50 % degree of reaction means pressure drop in the fixed blade = pressure drop in the moving blade.

8. What can be the possible reasons for a turbine not developing the designed full power?


The following can be the possible reasons for a turbine not developing the full power:
(i) The supply steam pressure as well as temperature is not as per the design conditions.
(ii) Pressure at exit of the turbine is too high. It should be under vacuum condition.
(iii) Partially worn out nozzles and blades
(iv) Overloading of the turbine
(v) Blockage of steam strainer
(vi) Incorrect governor setting

9. What is governing a steam turbine?


Steam turbine governing controls the rate of flow of steam into a steam turbine. It maintains a constant speed under all loads. The variation in speed can have a significant impact on its performance.

10. What are the different methods of steam governing in a steam turbine?


There are four different methods of governing the turbines.
(i) Throttle governing
(ii) Nozzle governing
(iii) By pass governing
(iv) Combination of (i), (ii) and (iii) methods

Q 11. Why use super-heated steam in a steam turbine?

All steam turbines use super-heated steam to avoid condensation. Due to condensation, velocity decreases. There is  large volume reduction in changing steam into water. The ratio of volume reduction is of the order of 700 times. It will cause erosion (rapid wear of turbine blade or erosion causes pits on the turbine blade). It is only due to abrupt decrease in speed.

Q 12. What is steam rate?

 Amount of steam (kg) at the inlet of the turbine to generate 1 kWh electric energy. 1 kWh of electric energy is 1 unit of electric energy

Q 13. What is remedy for eroded part of a turbine/compressor/pump?

Compensate erosion by any one of the following techniques:
(i) Deposit of metal by welding
(ii) By spray of molten metal over the eroded part in a most systematic manner

(iii) Insertion of filler strips or patch plates

Q 14. What is a bleeder turbine or an extraction turbine?

It is in a multi-stage steam turbine.  There is extraction of steam from stages for heating the feed water. Thus, heat input to boiler reduces. The thermal efficiency increases.

Q 15. Why is steam trap? 

Steam trap separates water condensed in the steam pipe line leading to steam turbine. If water condenses on the turbine, then Cavitation (damage) occurs to turbine. The life of the turbine greatly reduces.

Q 16. What prevents the turbine to produce full rated power?

(i) The inlet pressure and temperature of the steam are much lower as per design conditions.

(ii) Exhaust pressures are high.

(iii) Some impurities block nozzles.

(iv) Nozzles and turbine blades worn.

Q 17. How many governors does the safe operation of a steam turbine?


Two governors operate safely the steam turbine. One of these is used in an emergency. It stops supply of steam if the speed of the turbine exceeds 10 % above the maximum rated speed. The second governor is the main governor which makes the turbine to run at constant speed.

Q18. What are the main causes of vibration in a steam turbine?


(i) Loose parts
(ii) Unbalanced parts
(iii) Misalignment of parts
(iv) Loose foundations


Q 19. What is the purpose of an air ejector in a steam condenser?

An air ejector is a high velocity jet which acts like a vacuum pump. It removes non-condensing gases from the condenser to maintain required vacuum in the condenser. Maintaining vacuum in the condenser reduces heat lost to the cooling water in the condenser. Proper vacuum in the condenser increases the power output of the turbine. It will thus increase the thermal efficiency of the cycle.

Q20. What would happen on vacuum loss in the condenser of a steam turbine?

 Loss of vacuum in the condenser decreases the work obtained from the turbine. Heat lost to condenser cooling water increases. The thermal efficiency will decrease.

 Q21. Discuss a Regenerative Rankine cycle used in a steam turbine.

Multi-stage turbine uses the Regenerative Rankine Cycle. Steam from within a stage heats feed water. There is a series of feed water heaters. It preheats water on its way from the condenser to the boiler. In such a turbine, feed water temperature becomes temperature of the boiler. This preheating increases the thermal efficiency of the thermal cycle of the turbine. In these heaters, steam do not come in physical contact with the water.  These heaters function as an ordinary tubular heat exchanger. Regeneration increases the feed water temperature and reduces the heat input in the boiler. This improves the efficiency of the cycle.

 Q22. Why is reheating used in Rankine cycle?

Use reheat when two or three turbines used in series for power generation. Steam coming out of the high pressure turbine goes to the boiler for reheating. Reheated steam passes through a second turbine. The reheat temperature is the same as inlet temperature to the first high pressure turbine. However the reheat pressure needed is only one fourth of the original boiler pressure. This reheat prevents the vapor from condensing during its expansion. Thus this does not damage the turbine blades. This reheat increases the work output and thus increases the thermal efficiency too. It has been found that maximum two stages of reheat are useful. For more than two stages, efficiency starts decreasing.

These days, two stages of reheat are quite common in steam power plants using three turbines in series. In each case, steam temperature is the temperature of the steam entering the high pressure turbine. This steam is now fed to the medium pressure turbine. Reheat the steam from medium pressure turbine in the boiler.  Steam is then fed to the low pressure turbine. This double reheating significantly increases the thermal efficiency of the cycle. It is important to remember that the temperature at the inlet of each turbine is same.

Q23. Why is steam expanded on turbine below atmospheric pressure?

To get more work and high efficiency

 

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