Turbine parts(c.87-89)

Diaphragms are used between successive pressure stages in impulse turbines to act as nozzle support. They prevent the passage of steam from stage to stage except through these nozzles.

They are usually made in halves, split along the casing centre line, with each half in recesses machined in the cas­ing. Diaphragms are secured so that the upper halves may be readily lifted with the upper casing.

They may be of cast iron for temperatures up to 450°F, in which case the corrosion-resisting steel vanes are cast integ­ral with the diaphragm; they may be of plate or cast steel, in which case the nozzle vanes are welded to the diaphragm.

A nozzle is an orifice with rounded inlet and outlet edges, through which steam expands, converting a portion of the available heat in the steam to kinetic energy or velocity. How­ever, all the available heat is not converted to velocity due to friction eddies or other losses in the nozzle. The efficiency of the nozzle is also affected by the conditions of the steam, form of inlet and outlet, length of nozzle and smoothness of finish.

Nozzles for present day practice are usually made of cor­rosion-resisting steel or other alloys.

They may be of square, rectangular or round cross-sec­tions, and either machined in solid sections or built up by using vanes or partitions.

Nozzles are either of the convergent or convergent-diver­gent type, depending upon the critical pressure ratio.

Convergent nozzles are commonly called “non-expanding nozzles” and convergent-divergent nozzles, “expanding nozzles”.

Rotors are made of steel forging, either machined from one solid forging or are of built-up construction.

Solid forged rotors may be used for both impulse and impulse-reaction turbines. This type of construction should always be used for large high-speed units, generally 10,000 r.p.m. or over.

Built-up rotors are also used for impulse and reaction turbines. This type is used for small or low-speed impulse turbines.

When the rotor is finish machined, journals are ground and grooves cut for gland strips and blading.

There are many variations in the design of blading. The change in inlet and outlet angles is necessary for each steam condition. The method of fastening the blade to the rotor dif­fers. In Fig. 10 are shown some common types, with special reference to types of roots: T-root, inverted T-root, dove-tail and serrated root. It should be noted that contact between the blade root and groove is not made at all points, but gen­erally what is known as “3-point contact”.

Words to be Learnt

successivestageпоследовательная ступень exceptза исключением to split (split) разнимать recess канавка, паз machined обработанный orifice отверстие edge кромка friction eddy завихрение трения smoothness шлифовка rectangular прямоугольный solid section сплошная секция partition перегородка convergent суживающийся divergent расширяющийся

steel forging стальная поковка solid forging цельнокованый built-up rotor составной ротор journal шейка вала to ground укладывать groove зд. паз gland strip уплотнительное кольцо fastening крепление reference ссылка root ножка, основание лопатки inverted перевернутый dove-tail root ласточкин хвост serrated root пилообразная ножка