Question Set 51
Question No. 1
Usually it has been found that SCC attack takes place particularly at key-ways of shrunk-on-disc rotors of low-pressure turbines. Why are key-ways prone to SCC attack?
Answer:
1. Key-ways shrunk-fit each disc onto tile rotor shaft. They improve the rigidity of the connection between the disc and the central shaft However, key ways are subjected to abnormal centrifugal forces due to high over speed, that reduce the amount of shrink. Tangential stresses tend to gravitate at the key-way connection and steam tends to condense.
2. It is a one-piece-construction, and thus has inherent rigidity.
3. Advanced steel making techniques enable building of mono block rotors almost free from non-metallic inclusions and gas bubbles. Even large mono block rotors of clean steel are being manufactured today.
4. It exhibits lower inherent stresses.
5. The chance of disc loosening during operation is eliminated.
6. Highly stressed key-way is eliminated.
Question No. 2
What are four types of thrust bearings?
Answer:
1. Babbitt-faced collar bearings
2. Tilting pivotal pads
3. Tapered land bearings
4. Rolling-contact (roller or ball) bearings
Question No. 3
What are four types of turbine seals?
Answer:
1. Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
2. Labyrinths mated with shaft serrations or shaft seal strips.
3. Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
4. Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.
Question No. 4
What are some common troubles in surface-condenser operation?
Answer:
The greatest headache to the operator is loss of vacuum caused by air leaking into the surface condenser through the joints or packing glands. Another trouble spot is cooling water leaking into the steam space through the ends of the tubes or through tiny holes in the tubes. The tubes may also become plugged with mud, shells, debris, slime, or algae, thus cutting down on the cooling-water supply or the tubes may get coated with lube oil from the reciprocating machinery. Corrosion and dezincification of the tube metal are common surface condenser troubles. Corrosion may be uniform, or it may occur in small holes or pits. Dezincification changes the nature of the metal and causes it to become brittle and weak.
Question No. 5
What are the advantages of steam turbines over reciprocating steam engines?
Answer:
1. Steam turbine has higher thermal efficiency than reciprocating steam engines.
2. The brake horsepower of steam turbines can range from a few HP to several hundred thousand HP in single units. Hence they are quite suitable for large thermal power stations.
3. Unlike reciprocating engines, the turbines do not need any flywheel, as the power delivered by the turbine is uniform.
4. Steam turbines are perfectly balanced and hence present minimum vibrational problem.
5. High rpm 18000 - 24000 can be developed in steam turbines but such a high speed generation is not possible in the case of reciprocating steam engines.
6. Some amount of input energy of steam is lost as the reciprocating motion of the piston is converted to circular motion.
7. Unlike reciprocating steam engines, no internal lubrication is required for steam turbines due to the absence of rubbing parts.
8. Steam turbines, if well designed and properly maintained, are more reliable and durable prime movers than steam engines.
Question No. 6
What are the advantages of velocity compounding?
Answer:
1. The velocity compounding system is easy to operate and operation is more reliable.
2. Only two or three stages are required. Therefore, first cost is less.
3. Since the total pressure drop takes place only in nozzles and not in the blades, the turbine casing need not be heavily built. Hence, the economy in material and money.
4. Less floor space is required.
Question No. 7
What are the advantages of welded rotors?
Answer:
1. Welded rotor is a composed body built up by welding the individual segments. So the limitations on forgings capacity do not apply.
2. Welding discs together results in a lower stress level. Therefore, more ductile materials can be chosen to resist SCC attack.
3. There are no key-ways. So regions of high stress concentrations are eliminated.
Question No. 8
What are the basic causes of the problem of rotor failure?
Answer:
1. Normal wear.
2. Fatigue failure due to high stress.
3. Design deficiency.
4. Aggressive operating environment
Question No. 9
What are the differences between impulse and reaction turbines?
Answer:
1. The impulse turbine is characterized by the fact that it requires nozzles and that the pressure drops of steam takes place in the nozzles.
2. The reaction turbine, unlike the impulse turbines has no nozzles, as such. It consists of a row of blades mounted on a drum. The drum blades are separated by rows of fixed blades mounted in the turbine casing. These fixed blades serve as nozzles as well as the means of correcting the direction of steam onto the moving blades.
3. In the case of reaction turbines, the pressure drop of steam takes place over the blades. This pressure drop produces a reaction and hence cause the motion of the rotor.
Question No. 10
What are the factors that contribute to bearing failure in a steam turbine?
Answer:
1. Improper lubrication.
Only the recommended lubricant should be used.
2. Inadequate water-cooling.
a. The jacket temperature should be maintained in the range of 37-60°C
b. The flow of cooling water should be adjusted accordingly.
3. Misalignment.
It is desirable that ball bearings should fit on the turbine shaft with a light press fit. If the fitting is too tight, it will cause cramping. On the other hand, if the fitting is too loose it will cause the inner race to turn on the shaft. Both conditions are undesirable. They result in wear, excessive vibration and overheating. And bearing failure becomes the ultimate result.
4. Bearing fit.
5. Excessive thrust.
6. Unbalance.
7. Rusting of bearing.