5.1.2 Fixed blades — details and construction
There are two methods of constructing the stationary nozzle blading. Built-up blades, machined from the solid, are used for nozzle plates and steel diaphragms, whilst cast-in blades, formed from steel plates embedded into cast iron diaphragms, are used where temperatures are below 230°C.
On some of the latest machines, HP blades have been electrochemically machined. An impulse stage is frequently used for the first stage of the HP turbine. The fixed blades may then be incorporated in an integral nozzle box, thereby obviating the problem of pressure sealing at the inside of the blade ring. By absorbing a high heat drop, the nozzle box arrangement also reduces the pressure and temperature of the steam impinging on the HP rotor and inner casing.
Since impulse stage diaphragms have to withstand the bulk of the stage pressure drop, both diaphragm and blades need to be very robust. The diaphragm has the advantage of a relatively small diameter at the steam seal between diaphragm and rotor, but this labyrinth seal must be as good as possible to deal with the high pressure drop. The radial labyrinth seal ensures adequate insensitivity to axial displacement between the rotor and the casing. A typical impulse stage arrangement is shown in Fig 1.81.