Thursday, 24 May 2012

Turbine Construction

Vertical Casting

Only the bottom part of the ingot is used.

Rough Forging

It is a requirements that forgings are heavily worked. Any small holes or defects canbecome hammer welded together. No forging is carried out below the plastic flow temperature as this can lead to work hardening. Forging will allow continuous grain flow
ultimate tensile stress and elongation checked. This must be near enough equal in all 3 directions.
After rough machining it is put in for a thermal stability test. For this final machining is given to the areas indicated. The end flange is marked at 90' intervals. Then the rotor is encased in a furnace. Pokers are placed onto the machined areas and accurate micrometer readings taken. The rotor is rotated though 4 positions marked on the flange.
The rotor is then heated to 28'C above normal operating temperature and slowly rotated.
Measurement is then taken at hourly intervals until 3 consistant readings are taken ( hence the rotor has stopped warping). The rotor is then allowed to cool and a set disparity allowed.
For turbine sets operated at greater than 28'C above their designed superheat then run the risk of heavy warping as well as high temperature corrosion and creep.
Final machining is now given. The rotor is statically balanced and then dynamically balanced and check to ensure homogenity.The rotor is bladed then again dynamically balanced.

HP rotor

Most modern HP rotors are made of a single gashed forging of high quality steel.A hole of 50mm is bored axially through the rotor to allow for internal ispection and to remove impurities and internal flaws which can cause premature failure. In addition to the blade wheels also found on the rotor are; Thrust collar, Journal bearing surfaces, Oil thrower, Gland, Conical seat, thread or flange to attach flexible coupling
Simple HP rotor
Most modern HP turbine rotors are of the Rateau or pressure compounded design.
Reduced number of stages ( 8 to 10 ) give a shorter rotor and provides savings in weight and length. Also provides for better critical vibration characteristics.

    Rotors are solid forged providing
    1. Homogenous rotor with even grain flow
    2. Even expansion
    3. Good thermal stability with less likelihood of distortion under high temperatures
After forging the rotor is machined, wheels may be parallel or slightly thickened at the base . The methods is also used for the LP turbine which has 7 to 9 stages plus 2 to 3 astern.
After rough machining rotor is given a thermal stability test, after further machining and fitting of blades the rotor is given a static and dynamic balance.
This design is known as the Gashed disc rotor and gives a minimum shaft thickness and hence a minimum area for gland sealing to prevent steam leakage.

    Material ( up to 566oC )
    • 0.27 - 0.37% Carbon
    • 1.0% Manganese
    • 0.04% Sulphur
    • 0.2% Silicon
    • 1.0% Chromium
    • 0.5% Nickel
    • 1.5% Molybdenum
    • 0.3% Vanadium

LP Rotor

LP rotor The loss of efficiency due to the two stage velocity compounding of the astern turbine is more than made up by the reduction in windage whilst running ahead ( the design must still be able to supply 70% of the ahead revs which approximates to 40% of the ahead power) The impulse blading may have up to 20% reaction effect at the mean blade height.
The astern stage consists of one single wheel two stage velocity compounded followed by a single stage wheel.


    Material
    • 0.45% Carbon
    • 0.15% Silicon
    • Trace amounts of phosphorus and sulphur

    Blade material
    • 11.5 - 13.5% Chromium
    • 1% Nickel,manganese and silicon
    • 0.12% Carbon
    • Trace amounts of sulphur and Phosphorus

Built Up design

The Stal-Laval LP turbine is designed not to be flexible.This is possible as the problems of gland leakage is not so great as on the HP turbine, the HP turbine has reduced diameter rotor so reducing the gland sealing area but allowing flexibility. Having a stiff rotor allows the Astern turbine to be built up and hence allows the bulk of the LP rotor to be forged from a low grade steel whilst only the Astern parts are made from the material necessary to withstand the superheated steam.
If the rotor was flexible and a built up astern turbine wheels fitted then a possibility of fretting exists.
stal laval lep turbine astern end
The use of separately machined astern wheels allows the original forging to be more simplistic.
The forging of the higher grade steels required for use in superheat conditions require an increased amount of energy, and hence expense, in the original forging and subsequent machining process.
Another big advantage is that the astern wheels being of smaller mass and free to expand means that they can take more rough treatment then if they formed part of a single mass. The discs are forged from solid ingots and then machined so as to produce a force.shrink fit when heated and hydraulically pressed onto shaft.

    The fit is all importnat and must take into account;
    • Stretch under centrifugal force ( particularly under overspeed)
    • Discs reach working temperature before shaft when warming through
The disadvantage of force fit is that under high temperature condition, the metal being subject to stresses is susceptible to creep.
The result of thisis that due to the radial and tangential stress the wheel tends to grow in size. The wheel tends to loosen and fretting corrosion can take place
For HP rotors, generally, one wheel per step is allowed with a small clearance between each wheel. The whole is secured by a locknut and each wheel keyed to ensure positive transmission of torque. These keys are displaced by 180' for each step.
For LP turbines 3 wheels per step can be accommodated.
As the combined rotor shaft/wheel hub diameter is about twice that of the gashed rotor the sealing surface is greatly increased

Relative volumes of steam in HP and LP turbines


It can be seen that whereas the increasing volume of the steam in the HP turbine is moderate, The increase in the LP turbine is significant requiring blade height to be increased in successive stages. In the final stages both the height and the angle of the blades have to be altered. See notes on taper/twisting of blades) e

1 comment:

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