5.2.1 Fixed and moving blades — details and construction
This type of blading was designed and developed by Sir Charles Parsons and lends itself to economical production of blades from standard rolled sections. Only a small heat drop can be accommodated per blade row, due to the comparatively high velocity ratio required for good efficiency; this means that a large number of expansions are required. Steam approaches the moving blades with a velocity that is low and substantially axial in direction.
Consequently, the driving force applied to the moving blades arises almost entirely from the reaction force of the steam as it accelerates through the moving blades. The force applied to the moving blades is thus fairly steady, with very little disturbance arising from the nozzle wakes of the fixed blades, so fairly high bending stresses can be employed in the moving blades without risk of fatigue failure due to vibration.
Since the pressure drop across the fixed blades is small, diaphragms are not required, but small tip clearances are needed throughout the turbine to prevent excessive leakage losses. This was achieved in earlier turbines by axial sealing and end-tightening.
With solidly-coupled shafts which expand either side of a single thrust bearing and with multi-casing arrangements, end-tightening is not feasible and a compromise has to be reached between fine clearances to reduce leakage and large clearances to accommodate differential expansion when starting.
Modern reaction turbines generally employ a combination of axial and radial sealing at the stators and rotors, and for this purpose, the rotor blades are provided at their outer edge with shrouds formed of sections integral with the blade (see Fig 1.83). These mate with replaceable finned sealing segments in the casing. Fins formed on the inner diameter of the fixed blades provide sealing at the rotor.
Small reaction-type blades can be manufactured by cutting from rolled bar of the requisite profile or machining from bar, while larger blades may be separately cold-rolled after which the root portion is heated and forged. Blades may also be machined from envelope forgings or produced by precision forging, so that the profile requires no further machining; this is valuable where tough materials make machining difficult. Final machining can also be awkward where integral shrouds and root fixings interfere with the machine tool. To facilitate manufacture, modern practice is to braze together short groups of blades before machining the circumferential serrations on the sides of the roots, see Fig 1.84. The brazing also permits easier assembly. Groups of blades are fitted in the rotor or casing in circumferential grooves which have corresponding circumferential serrations machined in their walls. The blades arc secured in the grooves by similarly serrated side-locking pieces cut from rolled strip. A suitable stop plate at the half joint locates the fixed blades in the circumferential groove and prevents the blade segments from rotating due to torque reaction.