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Question & Answers on Steam Turbines 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. What is an air ejector? Answer: An air ejector is a steam siphon that removes non-condensable gases from the condenser. 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 Answer: In an extraction turbine, steam is withdrawn from one or more stages, at one or more pressures, for heating, plant process, or feedwater heater needs. They are often called "bleeder turbines." What is combined-cycle cogeneration? A nswer: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. What is done when cracks due to SCC or corrosion-fatigue are found? Answer: The damaged blade is usually replaced, as repairing is difficult. What is gland-sealing steam? Answer: Low-pressure steam is led to a sealing gland. The steam seals the gland, which may be a carbon ring or of the labyrinth type, against air at the vacuum end of the shaft.
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. What is meant by critical speed? Answer: It is the speed at, which the machine vibrates most violently. It is due to many causes, such as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine should be hurried through the known critical speed as rapidly as possible. Caution: Be sure the vibration is caused by critical speed and not by some other trouble. What is meant by the water rate of a turbine? Answer: It is the amount of water (steam) used by the turbine in pounds per horsepower per hour or kilowatts per hour. What is the cause of axial-bore cracks? Answer: Inadequate toughness of rotor steel and transient thermal stresses. What is the cause of circumferential cracking? Answer: High cycle fatigue with or without corrosion. What is the cause of turbine deposits? Answers: 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. What is the definition of a steam turbine? Answers: 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. - Neilson defines it
: The turbine is a machine in which a rotary motion is obtained by the gradual change of the momentum of the fluid.
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.What is the difference between partial and full arc admission? Answer: In multi-valve turbine inlets, partial arc admission allows the steam to enter per valve opening in a sequential manner, so as load is increased, more valves open to admit steam. This can cause uneven heating on the high-pressure annulus as the valves are individually opened with load increase. In full-arc admission, all regulating valves open but only at a percentage of their full opening. With load increase, they all open more fully. This provides more uniform heating around the high-pressure part of the turbine. Most modern controls start with full-arc and switch to partial arc to reduce throttling losses through the valves. What is the essential distinguishing feature between a steam turbine and reciprocating steam engine? Answers:
In a steam turbine, the heat energy of steam is converted into kinetic energy by allowing it to expand through a series of nozzles and this kinetic energy of steam is then imparted to the turbine blades mounted on a shaft free to rotate to drive this prime mover. In a reciprocating steam engine, the pressure energy of steam is directly utilized to overcome the external resistance. Here, the utilization of the KE of input steam is negligibly small. What is the function of a gland drain? Answer: The function of a gland drain is to draw off water from sealing-gland cavities created by the condensation of the sealing steam. Drains are led to either the condenser air-ejector tube nest or the feedwater heaters. Often, gland drains are led to a low-pressure stage of the turbine to extract more work from the gland-sealing steam. What is the function of a thrust bearing? Answer: Thrust bearings keep the rotor in its correct axial position.
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 tenon-rivet head damage occurs. What is the nature of circumferential cracking in shrunk-on-disc rotors in steam turbines? Answer: Regions of high stress concentration give birth to this type of cracking. It begins in corrosion pits and propagates towards the bore by high-cycle fatigue. It may culminate in a catastrophe, if it penetrates the bore (happily this usually does not occur). 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. What is the operating principle of a reaction turbine? Answer: A reaction turbine utilizes a jet of steam that flows from a nozzle on the rotor. Actually, the steam is directed into the moving blades by fixed blades designed to expand the steam. The result is a small increase in velocity over that of the moving blades. These blades form a wall of moving nozzles that further expand the steam. The steam flow is partially reversed by the moving blades, producing a reaction on the blades. Since the pressure drop is small across each row of nozzles (blades), the speed is comparatively low. Therefore, more rows of moving blades are needed than in an impulse turbine. What is the possible cause of slow start up of a steam turbine? Answer: This may be due to high starting torque required by the driven equipment. What is the potential problem of shrunk-on-disc type rotor? Answers:
It is the failure due to circumferential cracks, which are not limited to old rotors of early models (1960), but they also take place on present-day rotors. As a result corrodent impurities like chlorides concentrate at key ways. This factor coupled with high stress concentration lead to SCC attack on keyway areas. What is the principle of a steam turbine? Answers:
If high-velocity steam is allowed to blow on to a curved blade, the steam will suffer a change in direction as it passes across the blade, and leaves it as shown. As a result of its change in direction across the blade, the steam will impart a force to the blade. This force will act in the direction shown. Now if the blade were free in the direction of force as depicted. if, therefore, a number of blades were fixed on the circumference of a disc which is free to rotate on a shaft, then steam blown across the blades in the way described, would cause the disc to rotate. This is the working principle of a steam turbine. What is the purpose of a turning gear? Answer: Heat must be prevented from warping the rotors of large turbines or high-temperature turbines of 400°C or more. When the turbine is being shut down, a motor-driven turning gear is engaged to the turbine to rotate the spindle and allow uniform cooling. What is the remedy for a bent steam turbine shaft causing excessive vibration? Answers:
The run-out of the shaft near the center as well as the shaft extension should be checked. If the run-out is excessive, the shaft is to be replaced. What is the remedy for rotor-surface cracking? Answer: Current rotor/shaft should be machined off (skin-peeling). What is the remedy of the damage to blade profiles?
Answers:
Upgrading the turbine and depending on the extent of damage, upgrading may involve: - weld repair of affected zones of the blade,
- replacement of damaged blades by new ones and of new design,
- replacement of base material,
- application of protective coatings to guard against corrosion and erosion damage.
- What are the remedies to this failure?
Answers: For existing rotor, weld repair may be a choice; otherwise retire it. For new rotors, materials with improved pitting resistance should be used. What is the safe maximum tripping speed of a turbine operating at 2500 rpm? Answer: The rule is to trip at 10 percent overspeed. Therefore, 2500 x 1.10 = 2750 rpm. 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. What is to be done for erosion-induced damage on high-and low-pressure stage blading? Answers:
In such cases welding repair can be a good solution and this can be carried out during a normal maintenance outage without removing the blade. Using oxyacetylene torch, Stellite is generally deposited onto the damaged site. Following this, the weld is subjected to stress-relieving and re-profiling. In case of erosion penetrating the erosion shield and extending to the base material, a filler material of consistent or identical composition of blade material is used. In some cases use is made of Inconel alloy to build up the metal base. Therefore, using welding or brazing technique, a new shield can be attached to the blade. If brazing technique is followed, the rebuilt section is stress-relieved prior to the attachment of shield to it. If, on the other hand, the shield is attached by welding, then they are stress-relieved together. What is to be done in case of cracks originating at the lacing-wire holes? Answers:
These are to be weld-repaired. However the following factors must be considered: - The length of the crack that appears on the pressure and/or suction face.
- Whether the cracks propagate towards inlet end, discharge end and or both.
What maybe the possible causes for the safety trip to trip at normal speed? Answers:
Excessive vibration. Leakage in the pilot valve. Deposition of dirt in the safety trip valve. What maybe the possible causes for the safety trip tripping during load variation? Answers:
Light load and high inlet steam pressure. Safety trip set very close to the operating speed of turbine. What other parts of the steam turbine blades suffer from damage? Answers:
Blade roots. Shroud band.
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