Steam Turbine Interview Questions- Part 03

Steam Turbine Important Interview Questions with Answers - Part 03

Question No. 101

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. 102

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. 103

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. 104

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. 105

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. 106

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. 107

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. 108

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. 109

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. 110

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.

Question No. 111

What are the losses in steam turbines?

Answer:

1. Residual Velocity Loss - This is equal to the absolute velocity of the steam at the blade exit.
2. Loss due to Friction - Friction loss occurs in the nozzles, turbine blades and between the steam and rotating discs. This loss is about 10%.
3. Leakage Loss.
4. Loss due to Mechanical Friction - Accounts for the loss due to friction between the shaft and bearing.
5. Radiation Loss - Though this loss is negligible, as turbine casings are insulated, it occurs due to heat leakage from turbine to ambient air which is at a much lower temperature than the turbine.
6. Loss due to Moisture - In the lower stages of the turbine, the steam may become wet as the velocity of water particles is lower than that of steam. So a part of the kinetic energy of steam is lost to drag the water particles along with it.

Question No. 112

At what points does corrosion fatigue does show up?

Answer:
It attacks trailing edges, near the base of the foil and also the blade-root serration’s.

Question No. 113

What are the possible causes for the turbine not running at rated speed?

Answer:
The possible causes are:

1. Too many hand valves closed,
2. Oil relay governor set too low,
3. Inlet steam pressure too low or exhaust pressure too high,
4. Load higher than turbine rating,
5. Throttle valve not opening fully,
6. Safety trip valve not opening properly,
7. Nozzles plugged,
8. Steam strainer choked.

Question No. 114

What are the possible causes of excessive vibration or noise in a steam turbine?

Answer:

1. Misalignment.
2. Worn bearings.
3. Worn coupling to driven machine.
4. Unbalanced coupling to driven machine.
5. Unbalanced wheel.
6. Piping strain.
7. Bent shaft.

Question No. 115

What are the stresses to which a steam turbine rotor is subjected during its service life?

Answer:

1. Mechanical stress - The factors that contribute to mechanical stress in the shaft are the centrifugal forces and torque’s generated due to revolving motion of the shaft as well as bending arising during steady-state operation.
2. Thermal stress - Transient operating phases i.e. start-up and shutdown the genesis of thermal stress induced to the turbine shaft.
3. Electrically induced stress - They originate due to short circuits and faulty synchronization.

Question No. 116

What are three types of condensers?

Answer:

1. Surface (shell-and-tube)
2. Jet
3. Barometric.

Question No. 117

What are topping and superposed turbines?

Answer:

Topping and superposed turbines are high-pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine is at the same pressure as the old boilers and is used to supply the old turbines.

Question No. 118

What design modification is adopted to reduce susceptibility of last low pressure stages to fatigue failure?

Answer:
One modification is to join the blade segments together at the shroud band.

Question No. 119

What does "upgrading" generally means in the context of steam turbines?

Answer:
Upgrading is a most widely used tern. It encompasses a variety of meanings verses life extension, modernization and up-rating of steam turbines.

Question No. 120

What does the term "ramp rat" mean?

Answer:

Ramp rate is used in bringing a turbine up to operating temperature and is the degrees Fahrenheit rise per hour that metal surfaces are exposed to when bringing a machine to rated conditions. Manufactures specify ramp rates for their machines in order to avoid thermal stresses. Thermocouples are used in measuring metal temperatures.

Question No. 121

What factors are responsible for turbine-blade failures?

Answer:

In the high pressure cylinder, the turbine blades are mostly affected by:

1. Solid-particle erosion (SPE)
2. High cycle fatigue

Whereas, in the last few stages of the low-pressure cylinder, the blade damage is mainly afflicted by:

1. Erosion
2. Corrosion
3. Stress/fatigue damage mechanism
4. According to EPRI (Electric Power Research Institute, USA) data stress corrosion cracking and fatigue are the chief exponents for turbine-blade failures in utility industries.

Question No. 122

What factors cause excessive steam leakage under carbon rings?

Answer:

1. Dirt under rings: steam borne scale or dirt foul up the rings if steam is leaking under the carbon rings.
2. Shaft scored.
3. Worn or broken carbon rings.

These should be replaced with a new set of carbon rings. The complete ring is to be replaced.

Question No. 123

What factors contribute to excessive speed variation of the turbine?

Answer:

1. Improper governor droop adjustment.
2. Improper governor lubrication.
3. Throttle assembly friction.
4. Friction in stuffing box.
5. High inlet steam pressure and light load.
6. Rapidly varying load.

Question No. 124

What is a balance piston?

Answer:

Reaction turbines have axial thrust because pressure on the entering side is greater than pressure on the leaving side of each stage. To counteract this force, steam is admitted to a dummy (balance) piston chamber at the low-pressure end of the rotor. Some designers also use a balance piston on impulse turbines that have a high thrust. Instead of pistons, seal strips are also used to duplicate a piston's counter force.

Question No. 125

What is a diaphragm (turbine)?

Answer:
Partitions between pressure stages in a turbine's casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used.

One-half of the diaphragms are fitted into the top of the casing, the other half into the bottom.

Question No. 126

What is a multiport governor valve? Why is it used?

Answer:

In large turbines, a valve controls steam flow to groups of nozzles. The number of open valves controls the number of nozzles in use according to the load. A bar-lift or cam arrangement operated by the governor, opens and closes the valves in sequence. Such a device is a multiport valve. Using nozzles at full steam pressure is more efficient than throttling the steam.

Question No. 127

What is a radial-flow turbine?

Answer:

In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades. They are used for special jobs and are more common to European manufacturers.

Question No. 128

What is a shrunk-on-disc rotor?

Answer:

These are built by heat expanding the discs, so that upon cooling they shrink on the main rotor forging.

Question No. 129

What is a tapered-land thrust bearing?

Answer:

The Babbitt face of a tapered-land thrust bearing has a series of fixed pads divided by radial slots. The leading edge of each sector is tapered, allowing an oil wedge to build up and carry the thrust between the collar and pad.

Question No. 130

What is an extraction turbine?

Answer:
In an extraction turbine, steam is withdrawn from one or more stages, at one or more pressures, for heating, plant process, or feed water heater needs. They are often called "bleeder turbines."

Question No. 131

What is combined-cycle cogeneration?

Answer:
A combined cycle using a gas turbine or diesel, usually driving a generator in which the exhaust gases are directed to a waste heat-recovery boiler or heat-recovery steam generator (HRSG). The steam from the HRSG is then directed to a steam turbo-generator for additional electric power production. The use of the exhaust heat from a gas turbine improves the overall thermal efficiency. In cogeneration, electric power is produced, but part of the steam from the HRSG or from extraction from the steam turbine is used for process heat, hence the term cogeneration-the simultaneous production of electric power and process heat steam.

Question No. 132

What is important to remember about radial bearings?

Answer:
A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder.

These bearings must be accurately aligned to maintain the close clearances between the shaft and the shaft seals, and between the rotor and the casing. lf excessive bearing wear lowers the rotor, great harm can be done to the turbine.

Question No. 133

What is the cause of axial-bore cracks?

Answer:
Inadequate toughness of rotor steel and transient thermal stresses.

Question No. 134

What is the cause of turbine deposits?

Answer:
The turbine deposits are steam-born foreign matters settled on turbine blades. Substances dissolved in the BFW transfer partly from the water to steam, during the process of evaporation. They get dissolved in the steam and are carried into the steam turbine.

Question No. 135

What is the definition of a steam turbine?

Answer:
A steam turbine is a prime mover that derives its energy of rotation due to conversion of the heat energy of steam into kinetic energy as it expands through a series of nozzles mounted on the casing or produced by the fixed blades.

1. Neilson definition: The turbine is a machine in which a rotary motion is obtained by the gradual change of the momentum of the fluid.
2. Graham's definition: The turbine is a prime mover in which a rotary motion is obtained by the centrifugal force brought into action by changing the direction of a jet of a fluid (steam) escaping from the nozzle at high velocity.

Question No. 136

What is the harm if the rotor is over speed?

Answer:
Over speed rotor grows radially causing heavy rub in the casing and the seal system. As a result, considerable amount of shroud-band and tenons-rivet head damage occurs.

Question No. 137

What is the nature of rotor surface cracks in steam turbines?

Answer:
They are shallow in depth and have been located in heat grooves and other small radii at labyrinth-seal areas along the rotor.

Question No. 138

What is the remedy for a bent steam turbine shaft causing excessive vibration?

Answer:

1. The run-out of the shaft near the centre as well as the shaft extension should be checked.
2. If the run-out is excessive, the shaft is to be replaced.

Question No. 139

What is the remedy of the damage to blade profiles?

Answer:
Upgrading the turbine and depending on the extent of damage, upgrading may involve:

1. Weld repair of affected zones of the blade,
2. Replacement of damaged blades by new ones and of new design,
3. Replacement of base material,
4. Application of protective coatings to guard against corrosion and erosion damage.

Question No. 140

What is the solution to the problem of SCC/corrosion fatigue of steam turbine blades?

Answer:
It involves changing the blade material as well as minimizing the presence of corrodents in steam to a permissible level.

Question No. 141

What maybe the possible causes for the safety trip to trip at normal speed?

Answer:

1. Excessive vibration.
2. Leakage in the pilot valve.
3. Deposition of dirt in the safety trip valve.

Question No. 142

What other parts of the steam turbine blades suffer from damage?

Answer:

1. Blade roots.
2. Shroud band.