Thursday, 24 May 2012

Damping wires, Lacing wires and shrouding are fitted to reduce stress due to vibrations in the blade excited by such as steam flow fluctuations as the blades pass the nozzles. This is referred to as the 'passing frequency'. This particularly occurs with partial admission To prevent spreading of the long thin blades found in the final stages of the LP turbine. Shrouding is not fitted to these blades to allow adequate drainage. Due to the high specific volume losses due to spillage is relatively small Steam changing direction as it passes over the blade tends to build up in the concave face. There is a tendency to flow to the tip where if unchecked it can spill over leading to considerable loss of efficiency. This is particularly important in parsons turbines expecially as the initial stages of the HP turbine where the steam has a low specific volume. The vibration associated with turbine blades is referred to as the 'clamp-pin' type and is determined by vieing the blades in their packets i.e. blade groups attached by their shroud. Frequency types The lowest frquency is of the whole packet vibrating. Higher frequency is where as equal number of blades bow in oposite directions Higher still frequencies occur where each blade vibrates Lacing, Damping and Binding wires There are four sources of vibration damping under normal operating conditions Internal damping of the blade material Inherent dry friction damping of the blade assembly at the root and tip Fluid damping or viscous damping due to the steam environment Mechanical damping through fitting of damping aids such as damping or lacing wires etc Lacing wires fitted at an anitnode provide a very effective form of dampening. However, the antinode may exist at different positions for the different types of vibration so a compromise on the position has to be reached. A Damping wire which is 'free fitting' is free to move within the holes. Centrifugal force throws the wire to the outside of the hile where frictional effects help dampen the vibration. The disadvantage of damping wires is that heavy fretting can eventually cause the holes to widen to an extent that the rotor has to be rebladed. Lacing wires are brazed in and are therefore strengthening and hence are not necessarily placed at an antinode but rather where the blade is thickest. Binding wire is used to strengthen the trailing edge of the blade. This is a very old fashioned technique and is little used. The use of round wire can lead to aerodynamic losses Snubber or bumbing blocks may be cast or forged into the blade. These have a highly aerodynamic form. The damping is then achieved by both the bumbing of the blades and the following resistance to breaking as a vacuum formed at the joined faces tries to hold them together. A certain amount of fluid damping also occurs. Shrouding May be fitted by brazing, welding or riveting. The shrouding is fitted over the blade, the tenon is then either riveted with 4 or 5 blows or welded. Care must be taken either method of fixing as it can lead to crack formation. Once the shrouding is fitted the surveyor may request a pull off test. The pull is determined by calculation and governed by the expected centrifugal stress on the shroud during normal operations. Centenary Shrouding For blade batches where the centrifugal stress on the shroud of very large LP blades is significant, then centenary shrouding is employed.

    The casing is made of four main parts
    • Bottom Half-If all the nozzles are contained in the top half, then the bottom half is subject to steam at wheel case pressure and temperature only and can therefore be made of cast iron.
      The bottom half in this case extends from end to end and contains the following listed from ford to aft
      1. Thrust bearing housing
      2. Ford Journal bearing
      3. Ahead casing proper
      4. Ahead exhasut belt
      5. (Astern casing and belt if fitted)
      6. Aft gland housing
      7. Aft Journal bearing
      8. Flexible coupling housing
      Ahead Nozzle box-Contains ahead nozzle, subjected to boiler pressure and temperature hence made from cast steel
    • Turbine casing cover- Subjected to reduced pressure and temperature and can therefore be made of cast iron
    • Astern Nozzle box- Seperate top covers may be supplied to allow ease of maintenance for thrust and journal bearing

LP Turbine Casing


To reduce windage losses the astern turbine exhausts in the same direction as the LP turbine. The Astern casing is located by crossed bars that are able to take the torque reaction from the fixed blading. The bar layout also allows for radial expansion as does the steam inlet which is fitted with a sliding coupling

Thermal Effects

 The turbine casing distorts due to the heat differential.
 exageration of distortion of casing
The pressure within the casing distorts casing halves shape to a more cylindrical one, with the high temperature creep results
Hence when the casing cools
exagerated distorted casing when cooled
The flanges become warped . This can be checked by laying a straight edge across the casing, measuring with a feeler gauge and keeping a log of the results.
No action should be taken unless absolutely necessary.
The casing may leak during warming through as the bolts fail to close the inner faces of the flange. If the leakage stops when the turbine is up to temperature then this is considered satisfactory.
However, if leakage still occurs the some machining must take place. If the leakage is allowed to remain then at high power output damage can ensue.
A temporary repair is with the use of Phurmanite, this is a goo which is pumped into the flange, under pressure through a tapped hole.

The use of shouldered bolts


Pipework

Plan of LP casing showing dummy bellows arrangement Long lengths of pipe work should be avoided, as should be tight bends as these can lead to fluid friction losses in the steam and pressure loss.
Hangers and sweeping curves before inlet to casing should be employed to ensure no weight on casing.
For the cross over pipes, to avoid large curves or frictional losses the following is now employed.
cross over pipe between HP and LP turbine casings
The pipes fitted to the casing should have large flexibly supported bends and/or bellows pieces. If not they can give side or top thrusts on the casing and lead to stressing and misalignement.
An alternative to sliding feet as shown is to use elongated holes. The holes being elongated in the direction of required expansion. The bolt is then of the loose fit design.
Care must be taken with all sliding arrangements to ensure freedom of movement. Surfaces should be kept clean, lubricated ( molybdenum disulphide ) and free of rust and paint.
Differing materials may be used for the varying components.

Expansion arrangements

Allowance for expansion over the temperature range in which the turbines operate is essential to reduce thermal stress, mechanical stress and maintain proper tooth contact and blade clearance. This is acheived by securing the turbine at one end and allowing to expand. The free end is normally the hotter end of the turbine where expansion is expected to be greatest.

The turbine is allowed to expand in the fore and aft direction by molybdenum disulphide lubricated sliding feet

An alternative mounting is by 'Panting plates'. This design is particularyly seen in HP turbines and in Turbo-alternators where there is less weight to support.

The turbine is rigidly attached to the gear casing or pedestal. The ford end is allowed to expand. The turbine movement is absorbed by the flexible coupling

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