Question Set 53

Question No. 1

What factors are responsible for turbine-blade failures?


In the high pressure cylinder, the turbine blades are mostly affected by:

1.     Solid-particle erosion (SPE)

2.     High cycle fatigue

Whereas, in the last few stages of the low-pressure cylinder, the blade damage is mainly afflicted by:

1.     Erosion

2.     Corrosion

3.     Stress/fatigue damage mechanism

4.     According to EPRI (Electric Power Research Institute, USA) data stress corrosion cracking and fatigue are the chief exponents for turbine-blade failures in utility industries.

Question No. 2

What factors cause excessive steam leakage under carbon rings?


1.     Dirt under rings: steam borne scale or dirt foul up the rings if steam is leaking under the carbon rings.

2.     Shaft scored.

3.     Worn or broken carbon rings.

These should be replaced with a new set of carbon rings. The complete ring is to be replaced.

Question No. 3

What factors contribute to excessive speed variation of the turbine?


1.     Improper governor droop adjustment.

2.     Improper governor lubrication.

3.     Throttle assembly friction.

4.     Friction in stuffing box.

5.     High inlet steam pressure and light load.

6.     Rapidly varying load.

Question No. 4

What is a balance piston?


Reaction turbines have axial thrust because pressure on the entering side is greater than pressure on the leaving side of each stage. To counteract this force, steam is admitted to a dummy (balance) piston chamber at the low-pressure end of the rotor. Some designers also use a balance piston on impulse turbines that have a high thrust. Instead of pistons, seal strips are also used to duplicate a piston’s counter force.

Question No. 5

What is a diaphragm (turbine)?


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 are fitted into the top of the casing, the other half into the bottom.

Question No. 6

What is a multiport 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 the valves in sequence. Such a device is a multiport valve. Using nozzles at full steam pressure is more efficient than throttling the steam.

Question No. 7

What is a radial-flow turbine?


In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades. They are used for special jobs and are more common to European manufacturers.

Question No. 8

What is a shrunk-on-disc rotor?


These are built by heat expanding the discs, so that upon cooling they shrink on the main rotor forging.

Question No. 9

What is a tapered-land thrust bearing?


The Babbitt face of a tapered-land thrust bearing has a series of fixed pads divided by radial slots. The leading edge of each sector is tapered, allowing an oil wedge to build up and carry the thrust between the collar and pad.

Question No. 10

What is an extraction turbine?


In an extraction turbine, steam is withdrawn from one or more stages, at one or morepressures, for heating, plant process, or feed water heater needs. They are often called”bleeder turbines.”

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