Propulsion

Turbine Construction

Fabrication

Rough Forging.

The forging is heavily worked to hammer weld  internal holes and defects. 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


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

Homogenous rotor with even grain flow

Even expansion

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 )



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

Blade material



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.



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 important and must take into account;

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


Single Cylinder Design

These are usually found on short run ships such as passenger ferries although there present day use is very restricted .The Main advantage is very short warm up times. For this design criteria the Single Cylinder Double Casing turbine was developed


Advantages of using A single cylinder

Advantages of using A double casing


Warming Through


The main object of warming through is to ensure straightness of the rotor.To do this a negligible temperature gradient must exist throughout the rotor.


There is a tendency for the rotor to hog where the steam is introduced( that is to say the rotor bends due to temperature gradient rather than sagging under gravitational forces) with the rotor steam is introduced. Hence the rotor must be rotated.


The graph below indicates the importance of this

The line is the out of balance force due to centrifugal force equal to the mass of the rotor.


Hence, the offset at 3000rpm to cause an out of balance equivalent to the mass of the rotor is 0.102 mm


testing of the engines after shut down ahead and astern should be taken as part of the warming through process. Close watch of the relevant nozzle box temperatures is a good indication of the condition of the turbine.

Second object of warming through is to prevent distortion of the casing.


Rotation of the rotor churns up the steam and provides adequate mixing. With underslung condensers the temperature gradient is virtually unavoidable, hence separate condensers are better.


The third objective is to prevent thermal stresses caused by the temperature gradient in thick materials such as at the bolt flanges. Vertical slots are often provided to help alleviate this problem, this distortion can also lead to non concentricity of the casing


This is particularly prevalent in open cylinder designs such as axial plane or double casings.


Heat transfer rate is at its greatest where the steam is condensing on the surface of the casing. This in turn is governed by the inlet pressure of the warming through steam. Hence, warming through in steps providing adequate period to stabilise the temperature at each step.


Complete warming through cannot occur until nearly at full power , hence, warming through much above atmospheric saturation temperature is pointless.


Also as part of the LP turbine runs at lower temperature, warming above 100
oC is unnecessary. Protracted warming through periods are unnecessary. A temperature of 82oC at the LP inlet belt in 30 mins is acceptable

Vibration caused by an out of balance of the rotor may be alleviated by running for a short period at reduced engine speed followed by a slow increase in speed