5.5.2 Radial support pads
The provision of this system of kinematic support becomes progressively more difficult as the diameter of the diaphragm increases. As the size of turbine has increased, the basic concept of kinematic support has been modified to suit the circumstances.
One alternative has been to fit keys in both half-diaphragms so that each is supported independently at the horizontal joint. This arrangement requires that there is a gap between the half-diaphragms for expansion, resulting in a steam leakage across the half joint: it also requires more machining and fitting.
The difficulty in providing kinematic support for large diaphragms occurs mainly in LP turbines, where the outer ring diameter is large. In the LP turbine, the differential temperature between diaphragm and cylinder that has to be accommodated is much reduced. This, together with the need to provide some form of stop-plate for the top half-diaphragms, led to an alternative arrangement where, in the running condition, the top half-diaphragm is supported on the bottom half-diaphragm, which itself just rests in the recess in the cylinder. The clearance for differential expansion is achieved by exact machining of the outer diameter of the diaphragm.
A more practical arrangement is to machine a generous radial clearance between cylinder and diaphragm and to fit the half-diaphragm with a number of radial support pads which are then dressed to obtain the required location of the half-diaphragm.
Differential steam pressure forces the diaphragm outer rings against the downstream axial face of the circumferential recess in the inner cylinder. To prevent the diaphragms seizing in this recess, and to permit adjustment, small crushing pegs are fitted in the upstream face of the locating spigot.
Temporary support pads may be fitted and adjusted to achieve the correct positioning of the diaphragm in the cylinder. The packing bush radial clearances are bored to a charted size and checked with the rotor in position. If necessary, the temporary pads are adjusted to optimise radial clearances. When the desired alignment has been obtained, the thickness of the temporary support pads is gauged and the final support pads ground to size.