Question Set 17

Question No. 1

Why an increase in area is required to produce an increase of velocity in case of supersonic flow?

Answer:

Increase in area for increase in velocity for supersonic flow is required because the density decreases faster than velocity increases at supersonic speeds and to maintain continuity of mass, area must increase.

Question No. 2

Under what circumstances would there be an increase in pressure in a divergent nozzle?

Answer:

For subsonic flow at inlet section of a diffuser a lower velocity and higher pressure will exist at the exit section. For supersonic isentropic flow at the inlet section a higher velocity and lower pressure will exist at the exit but if a shock wave occurs in the diffuser then a higher pressure will exist at the exit.

Question No. 3

What are some conditions that may prevent a turbine from developing full power?

Answer:

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. 4

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. 5

What is a diaphragm?

Answer:

Partitions between pressure stages in a turbine’s casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragms is fitted into the top of the casing, the other half into the bottom.

Question No. 6

What are the two basic types of steam turbines?

Answer:

1.     Impulse type.

2.     Reaction type.

Question No. 7

What are topping and superposed turbines?

Answer:

Topping and superposed turbines arc high pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.

Question No. 8

What is a combination thrust and radial bearing?

Answer:

This unit has the ends of the Babbitt bearing extended radically 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. 9

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 clearance between the shaft and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor, great harm can be done to the turbine.

Question No. 10

How many governors are needed for safe turbine operation? Why?

Answer:

Two independent governors are needed for safe turbine operation. One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed). The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.