Steam Turbine Interview Questions - Part 01

Steam Turbine Important Interview Questions with Answers - Part 01

Question No. 01
What are the two basic types of steam turbines?
Answer:
1. Impulse type.
2. Reaction type.

Question No. 02
How can steam turbines be classified?
Answer:
The classification of steam turbine are as follows:

By the action of steam:
  1. Impulse.
  2. Reaction.
  3. Impulse and reaction combined.
The number of step reductions involved:
  1. Single stage.
  2. Multi-stage.
  3. Whether there is one or more revolving vanes separated by stationary reversing vanes.
The direction of steam flow:
  1. Axial.
  2. Radial.
  3. Mixed.
  4. Tangential.
  5. Helical.
  6. Re-entry.
The inlet steam pressure:
  1. High pressure.
  2. Medium pressure.
  3. Low pressure.
The final pressure:
  1. Condensing.
  2. Non-condensing.
The source of steam:
  1. Extraction.
  2. Accumulator.
Question No. 03
Why should a steam or moisture separator be installed in the steam line next to a steam turbine?
Answer:
All multistage turbines, low-pressure turbines, and turbines operating at high pressure with saturated steam should have a moisture separator in order to prevent rapid blade wear from water erosion.

Question No. 04
Besides lubrication, which are two functions of lubricating oil in some turbines?
Answer:
In large units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.

Question No. 05
Do the radial axial-bore cracks occur in the LP rotor/shaft alone?
Answer:
These are also known to occur in the HP as well as HP rotors.

Question No. 06
Do you think that turbine blade failure is the only cause of unreliability of steam turbines? Does upgrading of turbine means replacement of blades and/or improvement of blade design?
Answer:
  1. Like the blades, the steam-turbine rotors are highly stressed components. They are subject to cracking by a variety of failure mechanisms. Rotor failures do occur. And when they occur the result is catastrophic with the complete destruction of the unit and the total loss of generating capacity.
  2. Therefore, special attention should be given to rotor upgrading and repairing techniques.
BLADE FAILURES
  • Unknown 26%
  • Stress-Corrosion Cracking 22%
  • High-Cycle Fatigue 20%
  • Corrosion-Fatigue Cracking 7%
  • Temperature Creep Rupture 6%
  • Low-Cycle Fatigue 5%
  • Corrosion 4%
  • Other causes 10%
Besides, many damage mechanisms operate in combination of, poor steam/water chemistry, certain blade design factors that vary from one turbine manufacture to other, system operating parameters,

Question No. 07
How do the problems of vibration and fatigue arise with steam turbine blades?
Answer:
  1. These arise due to flow irregularities introduced because of manufacturing defects, e.g. lack of control over tolerances.
  2. System operating parameter, e.g. low flow may excite various modes of vibration in the blades.
Question No. 08
How does the dirty safety trip valve trip the safety trip at normal speed? What is the remedy to it?
Answer:
Dirt may find its way to the safety trip valve and get deposited around the spring end cap end. This will block the clearance between the safety trip valve and the spring end cap. As a result the steam pressure in the spring cap gets lowered allowing the valve to close.
The remedy is,
The spring end cap as well as safety trip valve should be cleaned.

Question No. 09
What maybe the possible causes for the safety trip tripping during load variation?
Answer:
  1. Light load and high inlet steam pressure.
  2. Safety trip set very close to the operating speed of turbine.
Question No. 10
What is the safe maximum tripping speed of a turbine operating at 2500 rpm?
Answer:
The rule is to trip at 10 percent over speed. Therefore, 2500 x 1.10 = 2750 rpm

Question No. 11
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.

Question No. 12
What is the potential problem of shrunk-on-disc type rotor?
Answer:
  1. 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.
  2. As a result corrodents’ impurities like chlorides concentrate at key ways. This factor coupled with high stress concentration lead to SCC attack on key-way areas.
Question No. 13
What is the principle of a steam turbine?
Answer:
  1. 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.
  2. As a result of its change in direction across the blade, the steam will impart a force to the blade.
  3. 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.
Question No. 14
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 feed water heaters. Often, gland drains are led to a low-pressure stage of the turbine to extract more work from the gland-sealing steam.

Question No. 15
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.

Question No. 16
What is the cause of circumferential cracking?
Answer:
High cycle fatigue with or without corrosion.

Question No. 17
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.

Question No. 18
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.

Question No. 19
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. Be sure the vibration is caused by critical speed and not by some other trouble.

Question No. 20
What is the essential distinguishing feature between a steam turbine and reciprocating steam engine?
Answer:
  1. 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.
  2. 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.
Question No. 21
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.

Question No. 22
What is the remedy for rotor-surface cracking?
Answer:
Current rotor/shaft should be machined off (skin-peeling).

Question No. 23
What is to be done in case of cracks originating at the lacing-wire holes?
Answer:
These are to be weld-repaired. However the following factors must be considered:
  1. The length of the crack that appears on the pressure and/or suction face.
  2. Whether the cracks propagate towards inlet end, discharge end and or both.
Question No. 24
How does the damage to turbine-blades tell upon the efficiency of the unit?
Answer:
The damage to blade profiles changes the geometry of steam flow path and thereby reducing the efficiency of the unit.

Question No. 25
How can the problem of excessive speed variation due to throttle assembly friction be overcome?
Answer:
The throttle should be dismantled. Moving parts should be checked for free and smooth movement. Using very fine-grained emery paper, the throttle valve seats and valve steam should be polished.

Question No. 26
How can damaged tenons be repaired?
Answer:
By adopting modern welding techniques, tenons can be rebuilt This in some cases results in extended blade life.

Question No. 27
How can the fatigue damage on high-pressure blades be corrected?
Answer:
Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc admission. This can be corrected by switching over to full arc admission technique.

Question No. 28
How does pressure monitoring ensure detection of turbine deposits?
Answer:
  1. Pressure of steam expanding in the turbine is measured at characteristic points, i.e., at the wheel chamber, points of pass-out, inlet/outlet of HP, IP and LP stages of the turbine.
  2. The turbine manufacturer provides the pressure characteristics in the form of graphs.
  3. At 1st commissioning, the user supplements these theoretical curves with those derived from actual measurements. These are actual pressure characteristics for a clean turbine. Now these pressure characteristics are compared with those obtained during operation in the later period.
  4. Under identical conditions, an increase in pressure shows the formation of deposits.
  5. For a steam throughput in the range 70-100%, an increase in wheel chamber pressure of more than 10% indicates severe blade depositions.
Question No. 29
How is oil pressure maintained when starting or stopping a medium-sized turbine?
Answer:
An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by a hand crank; others are motor-driven. This pump is used when the integral pump is running too slowly to provide pressure, as when starting or securing a medium-sized turbine.

Question No. 30
In which cases does moisture-impingement and washing erosion occur?
Answer:
1. These are encountered in the wet sections of the steam turbine.
2. For nuclear power plants, these wet sections can involve parts of high pressure cylinder.

Question No. 31
In which section of the steam-turbine rotors is the problem of rotor failure mostly prevalent?
Answer:
Rotor failures occur mostly on the large low-pressure rotors.

Question No. 32
In which turbines, is this pressure-velocity compounding principle employed?
Answer:
In the Curtis turbine.

Question No. 33
What are the disadvantages of velocity compounding?
Answer:
  1. Steam velocity is too high and that is responsible for appreciable friction losses.
  2. Blade efficiency decreases with the increase of the number of stages.
  3. With the increase of the number of rows, the power developed in successive rows of blade decreases. For as much as, the same space and material are required for each stage, it means, therefore, that all stages are not economically efficient.
Question No. 34
What are the main causes of turbine vibration?
Answer:
  1. Unbalanced parts
  2. Poor alignment of parts
  3. Loose parts
  4. Rubbing parts
  5. Lubrication troubles
  6. Steam troubles
  7. Foundation troubles
  8. Cracked or excessively worn parts
Question No. 35
What are the possible causes of a governor not operating?
Answer:
  1. Restriction of throttle valve reflex.
  2. Failure of governor control on start-up.
If it is found that after start-up, the speed increases continuously and the governor is not closing the throttle valve, it may be that the governor pump has been installed in the wrong direction.

Question No. 36
What are two types of clearance in a turbine?
Answer:
  1. Radial - the clearance at the tips of the rotor and casing
  2. Axial - the fore-and-aft clearance, at the sides of the rotor and the casing
Question No. 37
What is a combination thrust and radial bearing?
Answer:
This unit has the ends of the Babbitt bearing extended radially over the end of the shell. Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.

Question No. 38
What is an air ejector?
Answer:
An air ejector is a steam siphon that removes non-condensable gases from the condenser.

Question No. 39
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).

Question No. 40
What is to be done for erosion-induced damage on high-and low-pressure stage blading?
Answer:
  1. 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, Satellites is generally deposited onto the damaged site. Following this, the weld is subjected to stress-relieving and re-profiling.
  2. 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.
  3. 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.
Question No. 41
What is the function of a thrust bearing?
Answer:
Thrust bearings keep the rotor in its correct axial position.

Question No. 42
What is done when cracks due to SCC or corrosion-fatigue are found?
Answer:
The damaged blade is usually replaced, as repairing is difficult.

Question No. 43
What is a stage in a steam turbine?
Answer:
In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a "stage." A single Curtis stage may consist of two or more rows of moving blades.

Question No. 44
How does deposit formation on turbine blades affect turbine efficiency?
Answer:
About 500 g of deposits distributed more or less evenly all over the blading section can bring down turbine efficiency by 1%.

Question No. 45
How does the internal efficiency monitoring lead to the detection of turbine deposits?
Answer:
  1. Process heat drop.
  2. Adiabatic heat drop.
  3. The process heat drop and adiabatic heat drop are obtained from a Mollier- Chart for the corresponding values of steam parameters - pressure and temperature - at initial and final conditions.
Question No. 46
What are the possible causes of the speed of the turbine rotor increasing excessively as the load is decreased?
Answer:
  1. Throttle valve not closing fully.
  2. Wearing of throttle valve seats.
Question No. 47
What are the points of SCC attack? What are these points in particular?
Answer:
SCC attack predominates where corrodents deposit and build up i.e. in those blading areas where flowing steam cannot provide a washing effect.
These points are:
  1. Tie wires.
  2. Tie wire holes.
  3. Brazing.
  4. Blade covers.
  5. Tenons holes.
Question No. 48
What are the causes of radial axial-bore cracks on HP/IP rotors/shafts?
Answer:
  1. The predominant cause is creep, which may act with or without low cycle fatigue.
  2. Also the cracks result due to poor creep ductility due to faulty heat treatment process.
Question No. 49
What is the operating principle of an impulse turbine?
Answer:
The basic idea of an impulse turbine is that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward. The velocity of the steam is about twice as fast as the velocity of the blades. Only turbines utilizing fixed nozzles are classified as impulse turbines.

Question No. 50
Despite preventive measures, damage due to moisture impingement has been found, in certain cases, in the shield and beyond. Why?
Answer:
  1. Shields are designed and fabricated on the basis of predicted range of steam/water quantities impacting the blades at specific angles.
  2. Now if the operating conditions deviate significantly from design parameters then the erosion damage will occur. And in some cases it may go beyond nominal erosion wear and warrant repair.
  3. Also the corrosion of casing can occur due to blockage/clogging of water drains or extraction thereby forcing the water back into the casing. If this condensate water is carried over to steam path and impacts the blade, thermal-fatigue failure can occur within a short period.

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