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Fourth Year Millwright > Steam Turbines > Flashcards

Steam Turbines Flashcards

1
Q

Why is steam superheated?

A

Because it has more potential energy, and when superheated, it is very dry

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2
Q

What is another name for the insulation that covers the turbine to retain heat and protect personnel?

A

Lagging

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3
Q

What is the purpose of the sentinel valve, and where would it be found?

A

It is simply a warning device it is not designed to relieve or reduce pressure. It is found in smaller turbines

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4
Q

Is the governor and emergency trip mechanism the same?

A

No they are independent. The governor tries to maintain a speed, and the emergency trip immediately shuts off the steam to The turbine in the event of overspeed

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5
Q

What is the principle of operation of a steam turbine?

A

A steam turbine is a type of external combustion engine

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6
Q

What are the two ways that steam turbines can be divided into?

A

Impulse, and reaction operation types

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7
Q

Describe reaction blades and impulse blades

A

Reaction blades are not symmetrical and look like an airfoil, impulse blades are symmetrical

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8
Q

What is the purpose of impulse nozzles?

A

The nozzles increase the velocity of the steam, and give up pressure. It directs the steam to the proper angle for the moving blades

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9
Q

Describe impulse blades

A

There symmetrical and parallel to the shaft axis. They act like a bucket in the steam hits the middle of it imparting energy. There is no change in steam pressure across impulse blades because there is no change in size of the opening from one side of the blade to the other

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10
Q

Why is no sealing required on either side of the rotating blades on impulse blading?

A

Because there is no change in size in the space between the entry and exit there is very little leakage

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11
Q

How does the moving blade on the rotor on impulse blading move

A

The rotor, which is sometimes called a bucket, converts the kinetic energy of the moving steam into mechanical energy

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12
Q

When does steam impart its energy on reaction blading?

A

It pushes on the blades as it leaves. There is an increase in velocity, and a resulting drop in pressure across these blade types

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13
Q

How do reaction turbines frequently orient the blades?

A

They put a velocity compounded impulse blades at the inlet to reduce the number of rows needed

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14
Q

What is the purpose of putting velocity compounded impulse section at the inlet of a reaction turbine

A

The turbine machine is much shorter, and there is no pressure drop across the impulse blading therefore at the high-pressure end of the turbine the sealing between the rows of impulse wheels is not necessary

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15
Q

Where is most damage incurred on a turbine?

A

At the low pressure end, 5th location

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16
Q

What are two problems that develop from the pressure drop across reaction blades?

A

The steam leaks around the periphery of the blades, so clearances must be kept to a minimum.
The pressure drop creates an unbalanced axial thrust.

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17
Q

Describe a low pressure blade with a twisted design

A

They have partial impulse profile near the base and a reaction profile near the tip

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18
Q

How is a shroud attached?

A

It is riveted in sections over a tenon

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19
Q

What is the purpose of the shroud?

A

Maintains blade spacing
Reduces leakage
Strengthens the whole blade
Reduces blade vibration

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20
Q

What is used to further strengthen the blades?

A

Wire lacing adds strength and stiffness. It also dampens the blades and reduces the tendency to vibrate

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21
Q

What is the critical speed of a rotor?

A

It is determined by its natural frequency. At this speed, the motor will shake violently. If the turbine must run at a speed above its critical speed, it must accelerate quickly through the critical speed

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22
Q

Describe the Babbitt used for bearings in a turbine

A

Due to the high heat, Babbitt is normally tin based on a turbine

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23
Q

What happens with pressure and velocity when steam passes through a reaction type nozzle?

A

Steam pressure decreases, steam velocity increases, and steam volume increases

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24
Q

Describe a reaction turbine

A

There is often more than one casing, cylinder, or section. They are joined together, and are a HP, IP, and LP section

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25
Q

What are things used to handle the axial thrust?

A

Dummy piston, thrust bearings, dual flow rotors(enters from the middle), and opposed flow designs

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26
Q

Why is a reaction turbine inefficient at higher pressures?

A

Because of the leakage past the blade tips. This is why they will use impulse blading in the first HP section, making it a combination impulse/reaction turbine

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27
Q

What is a tandem compound turbine?

A

Consists of several sections(or cylinder, or casing) connected to each other at their shafts in a line with one single output

28
Q

What is a cross compound configuration?

A

The HP section will not be in line, it is piped to the next section (IP/LP) each section will have its own output

29
Q

Describe extraction turbines

A

Steam is extracted from the turbine at different stages, depending on requirements. It is used for feed water heating, and plant process heating. It is also extracted to avoid wet steam at the later stages of the turbine.

30
Q

What is done to help avoid wet steam in the later stages?

A

Reheat steam is introduced to help

31
Q

What is a condensing turbine?

A

It is a turbine that exhaust steam to a condenser (heat exchanger) and turns it back to water then on the boiler feed pump

32
Q

Where is the highest pressure on a turbine?

A

In the steam chest

33
Q

Describe nozzles

A

Stationary blades, or orifices that direct the steam, and exchange high pressure for high velocity

34
Q

What do larger turbines have, to help with sag in the shaft?

A

Self aligning spherical seat for the bearings

35
Q

What type of bearings are used on larger turbines, especially ones with reaction blading?

A

A thrust collar, and a set of tilting pad bearings

36
Q

What type of seals are used on turbines?

A

Small turbines have carbon seals, and larger ones use labyrinth seals

37
Q

How are carbon rings seals held in place?

A

Anti rotation tabs

38
Q

What is an annulus seal? When is it used?

A

They consist of a set of knife edges fitting close to a journal like section of the shaft. They are used when thermal growth exceeds the capability of a labyrinth seal because axial movement is not a factor

39
Q

How are coupling bolts reinstalled?

A

They are typically marked for a specific location due to fit and balance

40
Q

Why do most failures occur? Catastrophic failures?

A

Regular failures are due to some type of lubrication issue catastrophic issues are usually because of an over speed condition

41
Q

What is an acceleration governor?

A

Used on power generation stations, it closes quickly the steam admission valve in case of a sudden removal of load

42
Q

Describe the over speed trip

A

It is a mechanical or mechanical/hydraulic activated system. Most use a spring loaded tripping bolt or weight located on the turbine shaft

43
Q

When will an over speed trip be activated?

A

They are usually set to 110-115% of the operating speed

44
Q

What happens after an over speed has been tripped?

A

The main stop valve is closed, and a blowdown valve opens to relieve internal pressure back into the condenser

45
Q

When should an over speed trip be tested?

A

After doing any maintenance on the trip system, or after any major overhaul. Always test it twice to be sure.

46
Q

When are case drains opened?

A

They are opened and checked for water before start up, and after shutdown

47
Q

How does the turbine keep contaminants out?

A

The steam chest has a witches hat strainer

48
Q

For smaller impulse type turbines, what is the length?

A

Approx 6 feet

49
Q

What are two ways steam admission valves can be opened?

A

Either a cam lift, or a bar lift. The lift set up will be arranged so steam can be admitted progressively

50
Q

What are condensers usually made up of?

A

Shell and tube heat exchangers.

A vacuum is created in the condenser, reducing much of the back pressure in the final stages

51
Q

Where would you find a relief valve or rupture disc on a non condensing turbine?

A

Between the turbine and the turbine discharge isolation shut off. Used to prevent explosion if case is over pressurized, or if the isolation shut off is closed

52
Q

How are generators cooled?

A

Larger generators are cooled with hydrogen. They are very effective.

53
Q

What is a turning gear or barring gear used for?

A

It enables the slow rotation of the rotor while there is no steam in the turbine. Speed 1-60 rpm. Used when turbine is invariably heated for even hearing. Used to prevent permanent bowing. Usually mounted so the drive is in the bottom of the pedestal

54
Q

What is the main Oil pump run off of?

A

It is driven off the rotor

55
Q

How long is the jacking oil left on

A

Until approximately 50 to 60 rpm

56
Q

Why are large bolts hollowed on the flanges and frame?

A

So they can be heated internally with electric heating elements for proper torquing

57
Q

What is the clearance on the Babbitt in journal bearing?

A

.001 of an inch per inch of shaft diameter

58
Q

How is rotors axial position set?

A

It is set with shims at the thrust bearing location in the turbine housing

59
Q

What are the two functions of the thrust bearings?

A

To absorb any axial thrust on the rotor, to keep the rotor in the specified position in the casing

60
Q

When do you require multiple thrust bearings?

A

When the shafts are joined with flexible coupling’s then each rotor assembly needs a thrust bearing if solid couplings are used only one is needed

61
Q

What is one method for checking rotor to labyrinth seal clearances?

A

Using a lead wire

62
Q

What is used to help guide the top casing on the bottom and locate it properly

A

Guide pins are used

63
Q

What are the steps when tensioning bolts with the both stretch method?

A

Measure the length of the bolts were started while it is loose and check torque sequence

Tighten the stud to the specified starting to work with the wrench

Check the charts that indicate the amount of stretch that is achieved by a specified amount of nut rotation in degrees

Heat the stud and turn the nut the predetermined distance

After the stud has cooled measure the stud length to ensure that it is within the specified limits

64
Q

How big should the foundation of the turbine be?

A

It should be at least 2 1/2 times the mass of all the turbine assembly. It should also be isolated from the other building floors

65
Q

What stops the bearings from rotating?

A

Dowels for anti-rotation tabs

66
Q

Where’s the greatest amount of thermal stress created?

A

In the heavy steel sections of the casing during start up

Fourth Year Millwright (8 decks)

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