Depending on the capacity of a given coal fired boiler, the steam turbine rotor train consists of the HP, IP, up to three LP rotors, and the generator rotor. The rotor shafts are coupled together to form a single shaft which rotates at 3000 rpm to generate electricity at 50 Hz (UK) or at 3600 rpm to generate electricity at 60 Hz (North America). Each rotor consists of multiple wheels of radial-mounted aerofoil blades (known as stages), which become incrementally larger from the first stage at the inlet to the last stage at the outlet. Between each rotating stage of blades, a static wheel of blades known as the diaphragm, acts
1. Rail Unloading House 8. Primary Air Fan 15. Main Chimney 22. Stator 29. High Pressure Feed Heaters
2. Junction House 9. Boiler Burner 16. Super Heater 23. Generator Transformer 30. Economiser
3. Coal Conveyor 10. Boiler 17. High Pressure Turbine 24. Condenser 31. Steam Drum
4. Boiler Coal Bunker 11. Forced Draught Fan 18. Boiler Reheater 25. Condensate Extraction Pump 32. Cooling Tower
5. Bucket Wheel Machine 12. Air Heater 19. Intermediate Pressure Turbine 26. Low Pressure Feed Heaters 33. Circulating Water Pumps
6. Coal Feeder 13. Electrostatic Precipitator 20. Low Pressure Turbine 27. Deaerator 34. Circulating Water Make-Up Pumps
7. Pulverising Mill 14. Induced Draught Fan 21. Rotor 28. Boiler Feed Pump 35. FGD Absorber Tower
11
Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011
upon the steam exiting the previous stage to optimise the flow of steam onto the subsequent stage. Most turbine designs incorporate a dual flow construction where steam is fed into the centre of the rotor and expands outward through incrementally larger stages to the outer ends (outlets). However many turbines can incorporate a single flow design where steam enters at the inlet end through subsequently larger stages to the outlet end.
The typical HP turbine, pictured in Figure 2-2, is subject to steam with the highest pressures of 156 bar at 565 ºC, and is therefore constructed with the smallest turbine blades (buckets) which increase in size through subsequent stages as the steam gives up its energy. This type of turbine acts predominantly as an impulse turbine where the fixed veins
(diaphragm) act as nozzles to direct high velocity steam, which has significant kinetic
energy, onto the buckets to create turning force in the rotor16. The blade design of these
turbines tend to be of short aerofoils with fixed cross section along their entire length and shroud rings tying the full wheel of blades together at their tips; the shroud rings also
incorporate sealing rings to prevent steam leakage around the outside of each stage17.
Figure 2-2 Typical dual flow high pressure (HP) turbine17
The typical IP rotor, pictured in Figure 2-3, is subject to steam at 565 ºC but at a relatively lower pressure of 40.2 bar; this means that it is of similar but scaled up design to the HP
12
Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011
rotor. The dual flow configuration has steam supplied to the centre of the rotor and expands through subsequently larger stages to the exhaust at the outer ends. As in the HP turbine, the IP turbine acts predominantly as an impulse turbine where the diaphragm creates nozzles to direct steam onto the rotating buckets of the rotor, imparting the kinetic energy of the steam to rotation of the rotor. However, in the latter stages of more modern rotors, where the kinetic energy of the steam reduces as the pressure reduces, the aerofoils are designed such that they act as reaction turbines. Reaction turbine blades of these latter stages employ advanced aerodynamic features so that they react with the flow of steam over their profile; this creates a pressure differential which produces rotational torque similar to the lift created by an airplane wing. These stages of blades therefore act as impulse turbines at their base
and have some features of reaction turbines at their tips18.
Figure 2-3 Photograph of a dual flow intermediate pressure turbine
The typical LP rotor, pictured in Figure 2-4, is powered by the exhaust steam at 306 ºC and 6.32 bar, from the IP rotor. The rotor train might consist of up to 3 LP turbines which
STEAM FLOW ~2m
13
Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011
are critical to the efficiency of power production and can typically produce up to 40 % of the
total power15. As the pressure of the steam has reduced significantly the stages of blades are
markedly larger in both length and width, creating a much larger surface area. The LP rotor has a significantly different design where the first four stages act mainly as impulse turbines having some aerodynamic features to produce reaction forces toward their tips (similar to the latter stages of the IP rotor). However the last two or three stages of blades consist of free standing aerofoils designed to work together as a system; acting predominantly as reaction turbines over the length of the blades. This design employs advanced aerodynamic features including meridional flow path contouring, axial and tangential compound lean of the L-0 nozzle, and tailored exit profiles from the L-1 stage to allow optimum radius ratio in
the L-0 blade1819.
Figure 2-4 Photograph of dual flow low pressure turbine
During service, the last stage blades of an LP rotor, which are typically greater than 1 m long, and can weigh around 40 kg each, will rotate at 3000 rpm (50 Hz); the enormous centrifugal forces exerted upon these blades mean that they are among the most highly stressed components in a power station. Damage such as cracking in the root fixings of turbine blades can be caused by the start-up loading stresses, thermal stresses, residual manufacturing stresses, and normal loading stresses due to the centrifugal forces acting on
STEAM FLOW Last stage blades L-0 Stage 5 blades L-1 Coupling ~3.5m
14
Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011
the blades. The root areas of the blades are subjected to the highest stresses and can be damaged over time during normal running conditions, but more susceptible to damage due to non-ideal conditions occurring during running; such events as loss of vacuum or over speed are factors leading to the initiation of cracking. Any such cracking will propagate under normal running loads, but under transient loading, such as start-up and shut-down
cycles, cracks propagate readily leading to subsequent failure20. Failure of even a minor
stage blade of a steam turbine rotor can lead to significant damage to the rotor as it is ejected through subsequent stages. The loss of a last stage blade of an LP rotor can lead to the total destruction of the LP, and generator rotors, due to the chain of events which occur after the sudden loss of balance in the rotor train. Figure 2-5 illustrates the catastrophic damage which occurred after a last stage blade was ejected through the 400 mm thick steel casing of the LP turbine, crashing through the roof of the turbine hall, and subsequently landing in a lay-down yard 100 metres away. The sudden unbalance of the LP turbine led to the rotor train coming to a full stop from 3000 rpm in several seconds. The force broke the 500 mm diameter main rotor shaft, damaging the generator turbine casing, which exploded,
destroying both the LP and generator rotors. The cost to the utility company ran into millions of pounds for the replacement of the rotors and loss of generation. No lives were lost due, in main, to it occurring in the early hours of the morning.
15
Thesis – Ultrasonic phased array testing in the power generation industry – Novel wedge development November 2011