Question Set 45

Question No. 1

What is the function of a thrust bearing?


Thrust bearings keep the rotor in its correct axial position.

Question No. 2

What is done when cracks due to SCC or corrosion-fatigue are found?


The damaged blade is usually replaced, as repairing is difficult.

Question No. 3

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.

Question No. 4

How does deposit formation on turbine blades affect turbine efficiency?


About 500 g of deposits distributed more or less evenly all over the blading section can bring down turbine efficiency by 1%.

Question No. 5

How does the internal efficiency monitoring lead to the detection of turbine deposits?


1.     Process heat drop.

2.     Adiabatic heat drop.

3.     The process heat drop and adiabatic heat drop are obtained from a Mollier- Chart for the corresponding values of steam parameters – pressure and temperature – at initial and final conditions.

Question No. 6

What are the possible causes of the speed of the turbine rotor increasing excessively as the load is decreased?


1.     Throttle valve not closing fully.

2.     Wearing of throttle valve seats.

Question No. 7

What are the points of SCC attack? What are these points in particular?


SCC attack predominates where corrodents deposit and build up i.e. in those blading areas where flowing steam cannot provide a washing effect.

These points are:

1.     Tie wires.

2.     Tie wire holes.

3.     Brazing.

4.     Blade covers.

5.     Tenons holes.

Question No. 8

What are the causes of radial axial-bore cracks on HP/IP rotors/shafts?


1.     The predominant cause is creep, which may act with or without low cycle fatigue.

2.     Also the cracks result due to poor creep ductility due to faulty heat treatment process.

Question No. 9

What is the operating principle of an impulse turbine?


The basic idea of an impulse turbine is that a jet of steam from a fixed nozzle pushes against the rotor blades and impels them forward. The velocity of the steam is about twice as fast as the velocity of the blades. Only turbines utilizing fixed nozzles are classified as impulse turbines.

Question No. 10

Despite preventive measures, damage due to moisture impingement has been found, in certain cases, in the shield and beyond. Why?


1.     Shields are designed and fabricated on the basis of predicted range of steam/water quantities impacting the blades at specific angles.

2.     Now if the operating conditions deviate significantly from design parameters then the erosion damage will occur. And in some cases it may go beyond nominal erosion wear and warrant repair.

3.     Also the corrosion of casing can occur due to blockage/clogging of water drains or extraction thereby forcing the water back into the casing. If this condensate water is carried over to steam path and impacts the blade, thermal-fatigue failure can occur within a short period.

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