then changes direction and increases its speed relative to the speed of the blades. A pressure drop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating through the rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature, reflecting the work performed in the driving of the rotor. Blade efficiency Energy input to the blades in a stage: is equal to the kinetic energy supplied to the fixed blades (f) + the kinetic energy supplied to the moving blades (m). Or, = enthalpy drop over the fixed blades, + enthalpy drop over the moving blades, . The effect of expansion of steam over the moving blades is to increase the relative velocity at the exit. Therefore, the relative velocity at the exit is always greater than the relative velocity at the inlet . In terms of velocities, the enthalpy drop over the moving blades is given by: (it contributes to a change in static pressure) The enthalpy drop in the fixed blades, with the assumption that the velocity of steam entering the fixed blades is equal to the velocity of steam leaving the previously moving blades is given by: where V0 is the inlet velocity of steam in the nozzle is very small and hence can be neglected. Therefore, A very widely used design has half degree of reaction or 50% reaction and this is known as Parson's turbine. This consists of symmetrical rotor and stator blades. For this turbine the velocity triangle is similar and we have: , , Assuming Parson's turbine and obtaining all the expressions we get From the inlet velocity triangle we have Work done (for unit mass flow per second): Therefore, the blade efficiency is given by Condition of maximum blade efficiency If , then For maximum efficiency , we get and this finally gives Therefore, is found by putting the value of in the expression of blade efficiency Operation and maintenance Because of the high pressures used in the steam circuits and the materials used, steam turbines and their casings have high thermal inertia. When warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allow superheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steam