Question Set 48
Question No. 1
How is pressure compounding accomplished?
Answer:
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. 2
How is pressure-velocity compounding accomplished?
Answer:
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. 3
How is the washing of turbine blades carried out with the condensate?
Answer:
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. 4
How is turbine blade washing with wet steam carried out?
Answer:
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. 5
How is velocity compounding accomplished?
Answer:
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. 6
How many governors are needed for safe turbine operation? Why?
Answer:
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. 7
How many types of particle-impact damage occur in turbine blades?
Answer:
1. Erosion/corrosion.
2. Foreign-particle impacts.
3. Solid-particle erosion.
4. Water damage.
Question No. 8
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. 9
How will you detect that misalignment is the probable cause of excessive vibration?
Answer:
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. 10
How would you slop a leaky tube in a condenser that was contaminating the feed water?
Answer:
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.