Question Set 42

Question No. 1

What is the purpose of a turning gear?


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 gearis engaged to the turbine to rotate the spindle and allow uniform cooling.

Question No. 2

What is the potential problem of shrunk-on-disc type rotor?


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.

1.     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. 3

What is the principle of a steam turbine?


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. 4

What is the function of a gland drain?


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. 5

What is the difference between partial and full arc admission?


In multi-valve turbine inlets, partial arc admission allows the steam to enter per valve openingin a sequential manner, so as load is increased, more valves open to admit steam. This cancause uneven heating on the high-pressure annulus as the valves are individually openedwith load increase. In full-arc admission, all regulating valves open but only at a percentage oftheir full opening. With load increase, they all open more fully. This provides more uniformheating around the high-pressure part of the turbine. Most modern controls start with full-arcand switch to partial arc to reduce throttling losses through the valves.

Question No. 6

What is the cause of circumferential cracking?


High cycle fatigue with or without corrosion.

Question No. 7

What is meant by the water rate of a turbine?


It is the amount of water (steam) used by the turbine in pounds per horsepower per hour orkilowatts per hour.

Question No. 8

What is gland-sealing steam?


Low-pressure steam is led to a sealing gland. The steam seals the gland, which may be acarbon ring or of the labyrinth type, against air at the vacuum end of the shaft.

Question No. 9

What is meant by critical speed?


It is the speed at, which the machine vibrates most violently. It is due to many causes, suchas imbalance or harmonic vibrations set up by the entire machine. To minimize damage, theturbine 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. 10

What is the essential distinguishing feature between a steam turbine and reciprocating steam engine?


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.

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