Ci= velocity of steam at inlet to blade, i.e. leaving nozzle( giving nozzle angle)
Ci rel= velocity of steam relative to the blade( giving blade inlet angle)
Co= Velocity of steam at outlet of blade
Parsons Impulse-Reaction
The original blade design was thin section with a convergent path. Blohm & voss designed blades similar to bull nose impulse blades which allowed for a convergent-divergent path. However due to the greater number of stages the system did not find favor over impulse systemsU/Ci = 0.9
If the heat drop across the fixed and moving blades are equal the design is known as half degree reaction.
Steam velocity was kept small on early designs, this allowed the turbine to be directly coupled to the prop shaft.
Increased boiler pressure and temperature meant that the expansion had to take place over multiple rotors and gearset.
As there is full admission over the initial stage, blade height is kept low. This feature alone causes a decrease in blade and nozzle efficiency at part loading. In addition, although clearances at the blade tips are kept as small as practical, steam leakage causes a proportionally higher loss of work extracted per unit steam
Blade tip clearances may be kept very tight so long as the rotor is kept at steady state.
Manoeuvring, however, introduces variable pressures and temperatures and hence an allowance must be made.
End tightening for blades is normally used. This refers to an axial extension of the blade shroud forming a labyrinth. When the rotor is warmed through a constant check is made on the axial position of the rotor. Only when the rotor has reached its normal working length may load be introduced. Alternatively tip tightening may be used referring to the use of the tips of the blade to form a labyrinth against the casing/rotor. This system is requires a greater allowance for loading and is not now generally used.
To keep annular leakage as small as possible these rotors tend to have a smaller diameter than impulse turbines.
To keep the mass flow the same with the increasing specific volume related to the drop in pressure requires an increase in axial velocity, blade height or both -see above. Altering the blade angle will also give the desired effect but if adopted would cause increased manufacturing cost as each stage would have to be individual. Generally the rotor and blading is stepped in batches with each batch identical.
The gland at the HP end is subjected to full boiler conditions and is susceptible to rub. The casing must be suitably designed and manufactured from relevant materials.
A velocity compounded wheel is often used as the first stage(s) giving a large drop in conditions allowing simpler construction of casing and rotor and reducing length. Special steels are limited to the nozzle box.