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boiler plant operation > Turbines > Flashcards

Turbines Flashcards

1
Q

How do we determine turbine hp, state method

A 
BASED ON EXTERNAL DIAMETER OF STEAM SUPPLY PIPE
LESS THAN 1 ¾ INCHES = < 9 HP
> 1 ¾ - 3 ½ INCHES = 50 HP
> 3 ½ - 5 INCHES = 150 HP
 > 5 INCHES = UNLIMITED HP
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2
Q

How is oil fed to steam turbines

A

Gravity fed system (smaller turbines)
Lube oil pumps for larger turbines (forced fed turbine)
- AC powered auxiliary pump pump
- DC powered auxiliary pump (battery power)
from supply tank (reservoir), oil is pumped to bearings and return to tank
vapor extractor removes vapors from lube oil reservoir and creates a slightly negative pressure ( oil returns freely to the reservoir and removes moisture)
lube oil cooler used to control temperature between 130 - 140 deg F

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3
Q

What do thrust bearings do.

A

Prevents the shaft from moving too much in an axial direction

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4
Q

What is axial clearance and where in the turbine is axial clearance most important

A

Axial clearance is how much the shaft can move endwise without coming into contact with the stationary components.
Axial clearance is most important in the impulse section of the turbine to reduce free expansion

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5
Q

What is radial clearance and where in the turbine is radial clearance most important

A

Radial clearance is the distance from the blade tips to the casing
Radial clearance is most important in the reaction section of the turbine to prevent tip leakage. The lube oil maintains radial clearance

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6
Q

Describe how a Kingsbury Thrust bearing works. What are the benefits of using a kingsbury thrust bearing

A

A kingsbury or tilting pad thrust bearing consists of a collar to which is attached some shoes with a babbited face. The shoes ride on either side of the thrust collar limiting how much the rotor can move endwise.
Benefits are if there is a rub you may rub out only a couple of shoes (easily replaced)

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7
Q

Describe how a tapered land thrust bearing works

A

A tapered land thrust bearing consists of a solid face with grooves (lands). Solid face forces oil into grooves (lands) MAKE OIL FLOW INTO AND OUT OF THE BEARINGS
Downside vs kingsbury is if there is a rub you have to replace the entire bearing

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8
Q

Name the components of a gland steam sealing system

A

Steam supply valve
leakoff or control valve
gland steam condenser
gland steam exhauster

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9
Q

What are governors

A

governors control the speed of the turbine

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10
Q

How does a flyball governor work

A

A flyball governor (direct acting governor) is the simplest type of governor. It consists if 2 weighted balls attached by a worm gear to the main turbine shaft. At 0 speed the weights are all the way in and the control valve which controls the steam is wide open. As shaft speed increases centrifugal force will cause the flyballs to move toward the outer position compressing the spring causing the valve to move toward the closed position

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11
Q

What are some problems with flyball governors

A

Problems are hunting or speed droop, (doesn’t give exact speed all the time), usually these types of governors are found in auxiliary turbines.
Hunting or ‘speed droop’ in these types of governors can range from 6 - 10%

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12
Q

How does a MHC governor (mechanical hydraulic control) work

A

MHC governors have a servomotor attached to reduce hunting and a speed changer that allows you to dial in a specific speed

  • With the turbine at rest the flyballs are at thier innermost position causing the pilot valve to move and allow high pressure oil under the servomotor opening the valve wide.
  • As the turbine speed increase the flyballs move outward compressing the spring and positioning the pilot valve to allow oil to drain from the servomotor closing the valve.

Advantage is that speed droop can be fine tuned with the speed changer. Speed drop 3 - 5%

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13
Q

What are the 2 types of governors

A
  • Centrifugal Mechanical Governors, direct acting governors, flyball governors
    • Centrifugal Hydraulic (MHC)
  • Oil operated - oil relay governors
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14
Q

What is a meant by a simple cycle plant in relation to gas turbines

A

A simple cycle plant consists of a gas turbine connected to a generator. The exhaust steam exhausts to atmosphere and the heat from the exhaust steam is not recovered

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15
Q

What is a meant by a combined cycle plant in relation to gas turbines

A

A combined cycle plant consists of a gas turbine and its generator exhausting into a heat recovery steam generator (HRSG) to make steam to run a steam turbine and its own generator

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16
Q

What is a Brayton Cycle and what is a Rankine cycle

A

A brayton cycle is a gas cycle and a rankine cycle is a steam cycle. A combined cycle plant uses both the brayton and rankine cycles and is more efficient than a simple cycle plant

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17
Q

what is a steam turbine

A

Steam turbines are heat engines that convert heat energy to mechanical power. Steam turbines are used to drive fans, pumps, generators, chillers and other machinery to perform work.

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18
Q

What are some advantages of steam turbines

A 
Ability to use high pressure/ high temperature steam
High efficiency
High rotational speed
High capacity to weight ratio
No internal lubrication
Oil free exhaust steam
Can be built very small to very large
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19
Q

What are some disadvantages of steam turbines

A

For slow speed applications reduction gears are required
Steam turbines can not be made reversible
The efficiency of small simple turbines is poor

20
Q

What’s the formula for determining turbine horsepower, steam flow formula

A

HP = Ws (H1-H2)/2545

Ws = weight of the steam LBS/HR
H1 = enthalpy at throttle
H2 = enthalpy of the exhaust
2545 = number of btu's in a mechanical horsepower hour

Weight of the steam multiplied by the enthalpy at the throttle minus the enthalpy at the exhaust divided by 2545 ( BTU’S in a mechanical HP hour)

21
Q

What are the types of turbines

A

Backpressure turbines (non- condensing turbines, auxiliary turbines) used to drive auxiliary equipment, always exhaust at pressures higher than atmosphere

Condensing turbines, always exhaust to less than atmospheric conditions (partial vacuum)

Bleeder turbines, have pipes coming off the turbine at various pressure ranges that are used to heat feedwater, there are no control valves on these pipes, the more steam put into the turbine the more steam used to heat feedwater.
Definition of a bleeder turbine It is an uncontrolled extraction used to heat feedwater.

Extraction turbine, uses grid valves to control the downstream process pressure

Mixed pressure turbines, used in combined cycle plants that generate steam in a heat recovery steam generator at multiple pressures

22
Q

What are the advantages of a back pressure (non condensing) steam turbine

A 
Simple configuration
Low capitol cost
low need of cooling water
High total efficiency
Acts as a pressure reducing valve using exhaust steam to do more work
23
Q

What are the three operating principals of steam turbines

A

Impulse, reaction, and composite

24
Q

Describe how the impulse principle of a turbine works

A

Steam is directed through the 1st stage nozzle experiencing a pressure drop as the steam passes through the nozzle (whenever there is a drop in pressure volume and velocity increases) the velocity of the steam is increased and the high velocity steam is directed at the moving blades giving up some of that velocity to get that work done (turn the rotor), it then passes through the redirecting blade (stationary blade) which sends the steam through a second row of moving blades.

The moving blades in an impulse turbine are half mooned shaped) to accept the high velocity steam.

Steam velocity is always twice the blade speed

25
Q

What happens to pressure, velocity and volume of the steam in the impulse stage of a steam turbine

A

Steam pressure decreases (pressure drop) as it passes through the nozzle increasing both the volume and velocity of the steam, as the steam strikes the first set of moving blades the velocity of the steam decreases while pressure and volume remains constant, Pressure, volume, and velocity remain constant as the steam passes through the (redirecting blade) into the second set of moving blades where the velocity of the steam decreases while pressure and volume remain constant

26
Q

Describe the rateau stage of a steam turbine

A

Differs from the curtis stage in that the stationary blades act as a second set of nozzles, as the steam experiences a pressure drop again through the stationary blades the volume and velocity of the steam increases before bieng directed to the second set of moving blades

27
Q

What are the advantages of the curtis stage

A

You can start out with large pressures and take a large pressure drop in a small space

You can also use high temperature steam and take a large temperature drop through the curtis stage with no ill effects

Allows for reduced length of rotor

Less expansion and contraction problems

Suitable for high pressures/temperatures

Disadvantage is poor efficiency

28
Q

Describe how a helical flow turbine (terry turbine) works

A

A helical flow turbine has one rotating wheel, as steam passes through a nozzle high velocity steam hits the blades causing them to rotate, the steam gets redirected and the remaining velocity once again passes through the same set of moving blades.

29
Q

Name 3 stages of an impulse turbine

A

Curtis stage
Rateau stage
Helical flow

30
Q

Explain the how the reaction principle in steam turbines works

A

The leaving velocity of the steam from the blades causes the rotation, leaving velocity from the blades is higher than the entering velocity, steam velocity is equal to blade speed.

The fixed blades in the stationary section are tear dropped shaped (restricted area) as steam passes through the restricted area there is a 2nd pressure drop which increases both the volume and velocity of the steam which is directed to the 2nd set of moving blades

Thrust is an issue in turbine using the reaction principle and is not an issue with a turbine using the impulse principal

31
Q

Name some components of a steam turbine

A 
Casing
Governor
Valves (main stop/reheat stop)
Rotor (stiff/flexible)
Diaphragms
Bearings (journal/thrust)
Blades (shroud band)
32
Q

What is expansion in regards to steam turbines

A

2 types of expansion

  • Absolute expansion (shell expansion) is the measurement of the shell expansion with respect to a stationary point on the turbine (how much the turbine grows from its cold point` until fully warmed up measured by a reference point on the foundation)
  • Differential expansion, how much the rotor grows relative to the casing, the rotor will always react more quickly to temperature changes than the casing which has more surface area than the rotor
33
Q

Explain the terms rotor long and rotor short

A

Rotor long means the rotor has grown or is growing more quickly than the casing, only occurs during start up. to correct rotor long you would cool the rotor by increasing steam flow (lower back pressure, reduce steam temperature, go to partial ARC admission)

Rotor short means the rotor is cooling more quickly, this only happens when the unit has tripped and is restarted. To correct rotor short you would decrease steam flow (raise back pressure, increase steam temperature, Full ARC admission)

Never trip unit during rotor long condition, you could lose control

34
Q

How are turbine rotors classifies

A

Stiff rotors operate below the first critical speed (used for low speed applications)
Flexible rotors operate above the first critical speed (used for high speed applications)

35
Q

What are critical speeds in relation to steam turbines

A

Critical speeds are when a harmonic imbalance (vibration) in the rotor occur when changing speeds from high speed to low speed or low speed to high speeds because the rotor is not balanced for certain speeds causing excessive vibration of the rotor. Pass through critical speeds quickly by increasing the ramp rate

36
Q

How are rotors constructed

A
  • forged from 1 solid block of metal
  • welded construction.
    Rotors are constructed of carbon steel for temperatures below 650 deg f and alloys for temps above 650 deg f.
37
Q

Explain what is meant by gravity bow in turbine rotors

A

The rotor if sitting and not rotating is supported by 2 journal bearings at each end. The weight of the rotor and the effect of gravity will cause the rotor to bow (gravity bow). To correct this you put the rotor on turning gear (rotate very slowly 2-3 rpms) takes bowing out of the shaft

38
Q

What is thermal bow

A
  • Thermal bow occurs when the turbine trips and the rotor goes from operating speed to 0 speed and turning gear doesn’t engage
  • the upper half of the casing gets hotter causing the rotor to expand upwards

extremely dangerous condition due to tight clearances

39
Q

What is a turning gear in a steam turbine

A

It is a small electric motor with a geared coupling to keep the shaft rotating to cool down properly to correct / prevent bowing
- Rotate rotor to prevent/remove bowing of the rotor
- Gives inertia to rotor during roll - up to ensure full control
- It may (will) be required to rotate the rotor manually if turning gear fails to engage
Always follow manufacturers guidelines

40
Q

What is another name for the stationary blades in the turbine

A

Diaphragms

41
Q

Where is the crushing pin used in a stem turbine

A

Pin located on the outside of diaphragms, It is used to keep the diaphragms snug within the casing

42
Q

What are shroud bands

A
  • Shroud bands are thin strips of metal that goes over the blade ends on the tenons at the ends of the blades
  • used to stiffen the blades against vibration
  • contain the path of the steam so the steam gets directed through the blades and doesn’t go over the blade tips
43
Q

What are sealing strips

A

Sealing strips are used in reaction steam turbines

- reduces steam loss around blades and warn of possible damage from improper clearance

44
Q

Why is there a pressure drop in a Curtis turbine?

A

There is a large pressure drop in the first set of stationary blades called the nozzle block. This large pressure drop provides a large velocity increase. This velocity is used to rotate the moving blades. It also allows for construction of a smaller turbine. (Then explain how a Curtis turbine works and draw the pressure/velocity diagram…

45
Q

Conduct an overspeed trip on a steam turbine

A

REMOVE LOAD FROM TURBINE (UNCOUPLE DRIVEN EQUIPMENT)
ROLL TURBINE TO 200 – 500 RPM AND TRIP.
RELATCH AND ROLL TO SOUND OUT TURBINE.
PROCEED TO OPERATING SPEED (GOVERNOR TAKES OVER)
INCREASE SPEED TO TRIP POINT
HAVE 2 – 3 PEOPLE (DEPENDS ON SIZE OF TURBINE)
- 1 person with hand held tach
- 1 at the steam stop valve
- 1 at trip
CONDUCT TEST 2 – 3 TIMES
IT MAY REQUIRE ADJUSTMENT
TRIP SHOULD OCCUR 110% OF RATED SPEED (+/- 5%)

boiler plant operation (12 decks)

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