Mechanical Interview Questions 10
91. What is a stage in a steam turbine?
92. What is a diaphragm?
93. What are the two basic types of steam turbines?
94. What are topping and superposed turbines?
95. What is a combination thrust and radial bearing?
96. What is important to remember about radial bearings?
97. How many governors are needed for safe turbine
operation? Why?
98. How is a fly ball governor used with a hydraulic
control?
99. What is meant by critical speed?
100. How is oil pressure maintained when starting or
stopping a medium-sized turbine?
91. What is a stage in a steam turbine?
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.
92. What is a diaphragm?
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 diaphragm is fitted into the top of the casing, the other half into the bottom.
93. What are the two basic types of steam turbines?
o Impulse type.
o Reaction type.
94. What are topping and superposed turbines?
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.
95. What is a combination thrust and radial bearing?
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.
96. What is important to remember about radial bearings?
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.
97. How many governors are needed for safe turbine operation? Why?
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.
98. 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.
99. What is meant by critical speed?
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.
(Caution, be sure the vibration is caused by critical speed and not by some other trouble).
100. How is oil pressure maintained when starting or stopping a medium-sized turbine?
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.