Steam Turbine Interview Questions - Part 02

Steam Turbine Important Interview Questions with Answers - Part 02

Question No. 51
In which turbine tip leakage is a problem?
Tip leakage is a problem in reaction turbines. Here, each vane forms a nozzle; steam must flow through the moving nozzle to the fixed nozzle. Steam escaping across the tips of the blades represents a loss of work. Therefore, tip seals are used to prevent this.

Question No. 52
What are four types of thrust hearings?
  1. Babbitt-faced collar bearings.
  2. Tilting pivotal pads.
  3. Tapered land bearings.
  4. Rolling-contact (roller or ball) bearings.
Question No. 53
What are some conditions that may prevent a turbine from developing full power?
  1. The machine is overloaded.
  2. The initial steam pressure and temperature are not up to design conditions.
  3. The exhaust pressure is too high.
  4. The governor is set too low.
  5. The steam strainer is clogged.
  6. Turbine nozzles are clogged with deposits.
  7. Internal wear on nozzles and blades.
Question No. 54
Why is it necessary to open casing drains and drains on the steam line going to the turbine when a turbine is to be started?
To avoid slugging nozzles and blades inside the turbine with condensate on start-up; this can break these components from impact. The blades were designed to handle steam, not water.

Question No. 55
What steam rate is as applied to turbo-generators?
The steam rate is the pounds of steam that must be supplied per kilowatt-hour of generator output at the steam turbine inlet.

Question No. 56
What is the operating principle of a reaction turbine?
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.

Question No. 57
What is a multi-port governor valve? Why is it used?
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 these valves in sequence. Such a device is a multi-port valve. Using nozzles at full steam pressure is more efficient than throttling the steam.

Question No. 58
Besides lubrication, what are two functions of lubricating oil in some turbines?
In larger 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. 59
By monitoring the exhaust steam temperature, how can the blade deposition be predicted?
  1. Immediately after the 1st commissioning, the different values of exhaust temperature for different steam flow rates are precisely determined and plotted against steam flow. This will produce the first actual graph. This is for a clean turbine.
  2. Similar graphs are to be drawn at later periods for comparing with the initial graph.
  3. A rise in exhaust steam temperature under the same conditions refers to deposit formation.
  4. An increase of exhaust steam temperature by more than 10% in the range of 70 to l00% steam flow indicates inadmissible blade depositions. Shutdown is to be taken and blades are to be washed off deposits.
Question No. 60
Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?
You should keep the cooling water circulating for about 15 mills or more so that the condenser has a chance to cool down gradually and evenly. Be sure to have cooling water flowing through the condenser before starting up in order to prevent live steam from entering the condenser unless it is cooled. Overheating can cause severe leaks and other headaches.

Question No. 61
How can problems of "excessive vibration or noise" due to piping strain be avoided on steam turbines?
  1. The inlet as well as exhaust steam lines should be firmly supported to avoid strains from being imposed on the turbine.
  2. Adequate allowance should be made for expansion of steam pipes due to heat.
Question No. 62
How can the detection of deposits in a turbine be made during operation?
  1. Pressure monitoring.
  2. Internal efficiency monitoring.
  3. Monitoring exhaust steam temperature.
  4. Monitoring specific steam consumption.
Question No. 63
How can the disadvantages of the impulse turbine be overcome?
  1. By Velocity compounding
  2. By Pressure compounding
  3. By Pressure-Velocity compounding.
Question No. 64
How can the misalignment be rectified?
The bolts holding the flanges together are to be tightened. The coupling is to be checked for square between the bore and the face. At the same time axial clearance is to be checked. Using gauge block and feeler gauges, the gap between coupling faces 1800 apart is to be measured. After rotating the coupling-half 1800, the gap at the same points is to be measured. After this, the other coupling is to be rotated 1800 and the gap at the same points is to be re-measured. These measures should come within a few thousands of an inch. Dividing the coupling faces into four intervals, the distance between the coupling faces at this intervals is to be measured with the aid of a gauge block and feeler gauges. These gaps measurements should come within 0.005 inch for proper angular shaft alignment. After proper alignment at room temperature, the two halves of the coupling are to be connected.

Question No. 65
How can the speed variation be reduced by making a governor droop adjustment?
If the internal droop setting is increased, the speed variation will reduce.

Question No. 66
How does improper governor lubrication affect? What is the remedy to it?
In the event of low governor oil level or if the oil is dirty or foamy, it will cause improper governor lubrication.

The remedy is:
  1. The dirty or foamy lube oil should be drained off; governor should be flushed and refilled with a fresh charge of proper oil.
  2. In the event of low level, the level should be built up by make- up lube oil.
Question No. 67
How does solid-particle erosion occur?
Solid-particle erosion, i.e. SPE occurs in the high-pressure blades. And it takes place when hard particles of iron exfoliated by steam from superheater tubes, reheater tubes, steam headers and steam leads strike on the surface of turbine blades.

Question No. 68
How does the foreign-particle damage of turbine blades arise?
It occurs due to impact on blades by foreign particles (debris) left in the system following outages and become steam-borne later.

Question No. 69
How does this modification reduce the vibration fatigue damage?
  1. Joining the blade segments together at the shroud band increases the length of the arc-to a maximum of 360° that alters the natural frequency of the blade grouping from the operating vibration mode.
  2. This design has gained considerable success in commercial service.
Question No. 70
How is a fly-ball governor used with a hydraulic control?
As the turbine speeds up, the weights are moved outward by centrifugal force, causing linkage to open a pilot valve that admits and releases oil on either side of a piston or on one side of a spring-loaded piston. The movement of the piston controls the steam valves.

Question No. 71
How is pressure compounding accomplished?
  1. This is accomplished by an arrangement with alternate rows of nozzles and moving blades.
  2. Steam enters the 1st row of nozzles where it suffers a partial drop of pressure and in lieu of that its velocity gets increased. The high velocity steam passes on to the 1st row of moving blades where its velocity is reduced.
  3. The steam then passes into the 2nd row of nozzles where its pressure is again partially reduced and velocity is again increased. This high velocity steam passes from the nozzles to the 2nd row of blades where its velocity is again reduced.
  4. Thus pressure drop takes place in successive stages. Since a partial pressure drop takes place in each stage, the steam velocities will not be so high with the effect that the turbine will run slower.
Question No. 72
How is pressure-velocity compounding accomplished?
  1. It is a combination of pressure compounding and velocity compounding.
  2. Steam is expanded partially in a row of nozzles whereupon its velocity gets increased. This high velocity steam then enters a few rows of velocity compounding whereupon its velocity gets successively reduced.
  3. The velocity of the steam is again increased in the subsequent row of nozzles and then again it is allowed to pass onto another set of velocity compounding that brings about a stage-wise reduction of velocity of the steam.
  4. This system is continued.
Question No. 73
How is the washing of turbine blades carried out with the condensate?
  1. The washing is carried out with the condensate at 100°C.
  2. The turbine is cooled or heated up to 100°C and filled with the condensate via a turbine drain.
  3. The rotor is turned or barred by hand and the condensate is drained after 2 to 4 hours.
  4. It is then again filled with the condensate at 100°C (but up to the rotor centre level), the rotor is rotated and the condensate is drained after sometime. This process is repeated several times.
Question No. 74
How is turbine blade washing with wet steam carried out?
  1. Wet steam produced usually by injecting cold condensate into the superheated steam, is introduced to the turbine which is kept on running at about 20% of nominal speed.
  2. For back-pressure turbine the exhaust steam is let out into the open air through a gate valve. For a condensing turbine, the vacuum pump is kept out of service while cooling water is running, with the effect that the entering cooling steam is condensed. The condensate is drained off.
  3. The washing steam condition is gradually adjusted to a final wetness of 0.9 to 0.95.
Note, it is important
  • Not to change washing steam temperature by 10°C/min,
  • To keep all turbine cylinder drains open.
Question No. 75
How is velocity compounding accomplished?
  1. This is accomplished by an arrangement with alternate rows of fixed blades and moving blades. They mounted on the casing while the moving blades are keyed in series on a common shaft. The function of the fixed blades is to correct the direction of entry of steam to the next row of moving blades.
  2. The high velocity steam leaving the nozzles passes on to the 1st row of moving blades where it suffers a partial velocity drop.
  3. Its direction is then corrected by the next row of fixed blades and then it enters the 2nd row of moving blades. Here the steam velocity is again partially reduced. Since only part of the velocity of the steam is used up in each row of the moving blades, a slower turbine results. This is how velocity compounding works.
Question No. 76
How many governors are needed for safe turbine operation? Why?
Two independent governors are needed for safe turbine operation:
  1. One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
  2. The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
Question No. 77
How many types of particle-impact damage occur in turbine blades?
  1. Erosion/corrosion.
  2. Foreign-particle impacts.
  3. Solid-particle erosion.
  4. Water damage.
Question No. 78
How to prevent turbine deposition?
Answer: By upgrading the quality of steam and by ensuring proper quality of the following.
  1. Boiler feed water quality.
  2. Steam boiler model.
  3. Boiler design.
  4. Boiler operation.
Question No. 79
How will you detect that misalignment is the probable cause of excessive vibration?
  1. Coupling to the driven machine is to be disconnected.
  2. The turbine is to be run alone.
  3. If the turbine runs smoothly, misalignment, worn coupling or the driven equipment is the cause of the trouble.
Question No. 80
How would you slop a leaky tube in a condenser that was contaminating the feed water?
To stop a leaky tube from contaminating the feed water, shut down, remove the water-box covers, and fill the steam space with water. By observing the tube ends you can find the leaky tube. An alternate method is to put a few pounds of air pressure in the steam space, flood the water boxes to the top inspection plate, and observe any air bubbles. Once you have found the leaky tube, drive a tapered bronze plug (coated with white lead) into each end of the tube to cut it out of service. This allows you to use the condenser since the tubes need not be renewed until about 10 percent of the tubes are plugged.

Question No. 81
How would you stop air from leaking into a condenser?
First, find the leak by passing a flame over the suspected part while the condenser is under vacuum. Leaks in the flange joints or porous castings can be stopped with asphalt paint or shellac. Tallow or heavy grease will stop leaks around the valve stems. Small leaks around the porous castings, flange nuts, or valve stems can always be found by the flame test. So, you might have to put the condenser under a few pounds of air pressure and apply soapsuds to the suspected trouble parts.

Question No. 82
In how many patterns are tie wires used?
  1. In one design, tie wire is passed through the blade vane.
  2. In another design, an integral stub is joined by welding/brazing.
Question No. 83
In steam turbines, is there any alternative to the shrunk-on-disc design?
Two designs are available at present:
  1. Welded rotor in which each individual disc is welded, instead of shrunk, onto the main shaft.
  2. Mono-bloc rotor in which the entire shaft and blade assembly is manufactured from a single forging.
Question No. 84
In which case does upgrading imply life extension of steam turbines?
For a capital-short electric utility plant, upgrading comes to mean extending the life of that plant scheduled for retirement.

Question No. 85
In which cases does erosion corrosion damage appear?
Answer: It is commonly encountered in nuclear steam turbines and old fossil-fuel-fired units that employ lower steam temperatures and pressures.

Question No. 86
In which cases does upgrading mean up-rating the turbine capacity?
For an electric utility system facing uncertain load growth, upgrading is chiefly up rating.
It is an inexpensive way to add capacity in small increments.

Question No. 87
In which part of the steam turbine does corrosion fatigue occur?
In the wet stages of the LP cylinder.

Question No. 88
In which part of the steam turbine does stress corrosion cracking (SCC) occur?
In the wet stages of the low-pressure turbine.

Question No. 89
What are the basic causes of the problems are?
  1. Normal wear.
  2. Fatigue failure due to high stress.
  3. Design deficiency.
  4. Aggressive operating environment
Question No. 90
In which turbine is this pressure compounding used?
In the Rateau turbine.

Question No. 91
In which turbine is velocity compounding utilized?
In the Curtis turbine.

Question No. 92
In which zone of steam turbines has temperature-creep rupture been observed?
Damage due to creep is encountered in high temperature (exceeding 455°C) zones. That is, it has been found to occur in the control stages of the high-pressure and intermediate-pressure turbines where steam temperature sometimes exceed 540°C. In the reheat stage, it has been observed that creep has caused complete lifting of the blade shroud bands.

Question No. 93
Is there any adverse effect off full-arc admission operation?
At low loads, this results in a heat-rate penalty, due to throttling over the admission valves.

Question No. 94
Is there any other type of racking occurring in HP/IP rotors and causing rotor failures?
  1. Blade-groove-wall cracking.
  2. Rotor-surface cracking.
Question No. 95
Of all the factors that contribute to the unreliability of steam turbines, which one is the most prominent?
It is the problem of turbine blade failures that chiefly contribute to the unreliability of steam turbines.

Question No. 96
Rim cracking continues to be a problem of shrunk-on-disc type rotors in utility steam turbines. Where does it occur?
Cracking has been located at the outer corners of tile grooves where the blade root attaches to the rotor.

Question No. 97
So can you recommend this technique as a permanent measure?
No, this can be recommended in extreme cases or at best temporarily.

Question No. 98
What should be the more sound approach in the case of steam turbine blade failure?
The more reasonable and better approach is to replace the damaged blades with new ones that are stiffened by:
  1. Secreting the interface surface of individual blades so they interlock, or
  2. Welding the blades together.
  3. In some cases, a single monolithic block is machined out to manufacture the blades in a group.
  4. In some other cases, blades themselves are directly welded into the rotor.
Question No. 99
Steam blowing from a turbine gland is wasteful. Why else should it be avoided?
It should be avoided because the steam usually blows into the bearing, destroying the lube oil in the main bearing. Steam blowing from a turbine gland also creates condensate, causing undue moisture in plant equipment.

Question No. 100
What are the consequences of turbine depositions?
The consequences of turbine depositions have three effects.

A. Economic Effect:
  1. Reduction in turbine output
  2. Decrease in efficiency requiring higher steam consumption.
B. Effect of Overloading and Decreasing Reliability in Operation:
  1. Pressure characteristic in the turbine gets disturbed with the effect that thrust and overloading of thrust bearing increase.
  2. Blades are subjected to higher bending stresses.
  3. Natural vibrations of the blading are affected.
  4. Vibration due to uneven deposition on turbine blading.
  5. Valve jamming due to deposits on valve stems.
C. Corrosion Effect:
  1. Fatigue corrosion
  2. Pitting corrosion.
  3. Stress corrosion.

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