Chapter 1 - Criteria for turbine application Flashcards
(43 cards)
1
Q
What are the main types of steam turbines?
A
- condensing turbines
- noncondensing turbines
2
Q
What do steam turbines do?
A
Steam turbines are used in powerplant cycles to convert heat energy from steam generating equipment into mechanical work.
3
Q
What are the industrial applications of a small single stage turbine?
A
- pump drivers (up to 50MW)
- Large compressors
- Generation range (7.5-900KW)
- 1000 to 6500 rev/min
4
Q
What is regenerative cycle?
A
When a steam turbine bleeds (extracts) steam to heat feedwater
5
Q
What is another term for regenerative cycle?
A
bleed turbine
6
Q
Explain automatic extraction turbine.
A
When steam is extracted or bled from several points usually from one to eight points
7
Q
Turbine cases are designed to handle to high pressures and temperature, while maintaining what?
A
- resisting distortion
- maintaining all clearances
- correct alignment on rotating blading
8
Q
Describe split casings
A
- good for inspection upon removal, but no ideal for external flanges due to lack of temperature transfer and strength
- higher chromium alloys are used in higher temp. applications (creep resistance)
- surfaces steam tight, dowel pins and grooved slots
- higher pressure/temp application require flange heating to evenly distribute heat in case of expansion
- centerline support be designed to expand and contract evenly with the unit to maintain alignment
9
Q
Describe double casing
A
- used for very high pressure
- highest pressure is injected to the inner casing, the inner casing exhausts out the end of the turbine into the outer casing
- Exhausting the inner casing into the outer is done to reduce stress and promote even heating
10
Q
Describe cylinder casing drains
A
- used to remove remove water from turbine
- water is bad for blades promotes corrosion and material loss
- LP section designed usually to exhaust max at 14% wetness
- drains are placed on the casing in critical places to remove condensate especially on LP
11
Q
Name the 3 types of turbine rotors
A
- solid forged rotor
- disc rotor
- welded rotor
12
Q
Describe a solid forged rotor
A
- Forged rotors of this type have wheels and shaft, which has been machined from one solid forging (one piece).
- used in HP and IP sections
- used based on your start-up procedure because it requires a long warm up
13
Q
Describe a disc rotor
A
- Also known as: “build-up rotors”
- constructed of many separately forged discs or wheels
- discs are slotted into the machined grooves along the motor
- the outer rims of the wheels have machined grooves for blading
- Used for LP cylinders
- wheels or discs can come loose if improper warm-up/expansion occurs
14
Q
Describe a weldor rotor
A
- built up from discts and two shaft ends which are joined together by the welding in the inner circumference
- discs must be very strong and small holes must be drilled in the discs to allow steam in for heating (expansion)
- Grooves are machined on the outer edge of the discs to allow for blading
- induction must be applied before welding on rotor (automatic argon arc welding)
15
Q
Name the 3 types of blading
A
- Reaction blading
- impulse blading
- combination of impulse and reaction
16
Q
Describe reaction blading
A
- pressure drops across both fixed and moving blades
- tight clearances are required with this method to ensure no steam leaks by both rotating & stationary blading.
- any steam that leaks by sections performs no work and causes loss of efficiency
17
Q
Describe Impulse blading
A
- machined from solid bars and are slotted into the discs and tangs are left at the tips for schrouding attachment
- there is no pressure drop across moving blades in an impulse turbine
- sealing is not as important as a reaction turbine, although proper sealing is required with this design between diaphragms to prevent leakage between stages
- shrouding helps guide steam to incr. efficiency
18
Q
What are the two main forces on blading?
A
- bending force
- throwing out force
19
Q
Where are the greatest stresses in blading?
A
- LP section because blades are the largest
- double flow LP sections are designed and employed to cut forces in half versus a longer single flow design (you can get away with smaller blades this way)
20
Q
Describe blade shrouding
A
- it is employed on the end of the blades and provides a right clearance to the casing
- promotes steam through the blading to ensure efficiency and prevents steam leakage through stages
- constructed of stainless steel which is strong and corrosion resistant
- machined and welded if needed
21
Q
Describe diaphragms
A
Fixed blading of impulse turbine consists of nozzles mounted in diaphragms. constructed of two solid halve which fit into slotted grooves in the turbine casing
22
Q
Name 4 shaft sealing designs
A
- stuffing box
- carbon rings
- labyrinth seals
- water seals
23
Q
Describe carbon rings
A
spring backed carbon rings made of graphite and are self-lubricating
24
Q
Describe labyrinth seals
A
- usually employed in large machines, where carbon rings cannot be used (exceeds carbon ring limits)
- causes eddy currents in the steam passing through the seal reducing steam velocity and causing small pressure drops
- steam leak off points are remove condensate and steam from the seals
25
Describe water seals
- used on the turbine ends for sealing from steam leakag
- high purity water or condensate is injected into the seal
- the seal centrifugal pump runner inside is fixed to the turbine shaft which makes the seal go positive and ensure steam sealing
- no leakage
- Not effective until running speed is achieved
- quenching water fed to seal could cause shaft to crack
- requires more adjusting than steam glands
26
Name two types of bearings
- ring oiled bearings
| - pressure fed bearings
27
Describe ring oiled bearings
- ride freely on the journals revolving with them dipping into oil contained in the bearing housing
- they carry oil to the top of the journal from the resevoir
- rotation of the shaft forces the oil around the journal
- this type may be fitted with a cooling jacket of auxillary cooling
28
Describe pressure fed bearings
- large turbines employ bambbitt-lined sleeve bearing
- they are highly reliable and require minimal maintenance
- have a very low coefficient of friction
large steam turbine with these types of bearings are usually supported with two main bearings at each end
- oil is pumped into the bearing housing by a circulating pump through oil groove. This forms an oil wedge in the journal, bearing and shaft float on the oil.
- Cooling via a jacket or auxillary cooler usually required for this type of system
29
Describe axial expansion
the expanding steam flow is parallel to the line of the shafts
30
Describe radial expansion
(traverse) expansion at right angles to the shaft
31
Describe Impulse turbines
have a small amount of thrust most still require a thrust bearing or thrust counteracting method
32
Describe reaction turbines
- have more thrust than impulse turbines
- requires thrust bearings designing the turbines with double flow acting flow and other methods to help offset thrust by balancing forces through the turbine
33
Name the 4 types of thrust bearings
- ball bearings
- tappered land bearings
- Michelle bearings
- kingsburry bearings
34
Describe ball bearings
small turbines (impulse turbines) employ ball bearings "double row" or "deep groove" type bearings
35
Describe tapered land bearings
- large turbines
- they build up an oil wedge from oil being forced through grooves in the bearing on the journal creating a separating film
36
Describe kingsburry bearings
- thrust bearings
- consists of tilting pads which are free to rock
- the oil wedge forms within the pockets of the bearing
- require large amounts of oil and lot of heat to be carried away from the oil
37
Describe dummy pistons
- steam is fed from upstream in the turbine down a balance pipe and into a clearance pocket at the end of the rotor counteracting turbine thrust
- calculation is done to find the amount of force you have to counteract the units of thrust
38
Describe thrust adjusting gear
- used to project axial seals and bearings it fits on like a piston cylinder which can be adjust on start-up to be pushed in the direction of the exhaust for max clearance between moving / stationary blades to avoid rubbing
- in op. and temp. reached the thrust block is adjusted to tighten the tolerances (incr. efficiency)
39
Name two types of nozzles
- convergent nozzle
| - convergent divergent nozzle
40
Describe convergent nozzle
- used for small pressure drops
- as the pressure drop across the nozzle the velocity increases
- Any extra NRG in this type of nozzle is given up in the form of eddy currents
41
Describe convergent divergent nozzles
- used for large pressure drops
- the pressure at the narrowest part of the nozzle is at critical pressure
- in the divergent section is where the pressure drop occurs and steam velocity incr, with next to no eddy currents
42
What is Newtons second law and equation?
Change in momentum F=ma
43
How do steam turbine produce mechanical energy?
steam turbine blades are used to direct steam onto turbine blades at the correct angle resulting in the efficient energy conversion
