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
- Thrust bearing housing
- Ford Journal bearing
- Ahead casing proper
- Ahead exhasut belt
- (Astern casing and belt if fitted)
- Aft gland housing
- Aft Journal bearing
- Flexible coupling housing
- 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 EffectsThe turbine casing distorts due to the heat differential.
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
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
PipeworkLong 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.
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 arrangementsAllowance 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