Page 57 - Steam Turbines Design, Applications, and Rerating

38 Chapter Two

With few exceptions, modern, state-of-the-art diaphragms are typi-
cally manufactured in four styles or type configurations: convention-
ally cast, investment cast, milled and welded, and spoke type.
Conventionally cast diaphragms generally consist of preformed noz-
zle blade sections of cold rolled 405 stainless steel material cut to
length corresponding to proper nozzle height. The individual nozzle
sections are positioned in proper location and ductile cast iron material
(ASTM A-536, Gr. 80-55-06) poured to form the diaphragm; the resul-
tant steam path is not entirely of stainless steel construction.
The cast conventional diaphragm is limited to low-temperature
applications (500°F and less) and stage pressure drops of approxi-
mately 25 psig. Its frequency of use is decreasing, being gradually
phased out by the other basic diaphragm types especially in the small
nozzle height range. While the basic diaphragm is less expensive than
other types, some problem areas are associated with cast conventional
diaphragms:

■ The actual nozzle area is difficult to control. In many cases expensive
hand filing is required to remove core sand from the nozzle passages
and to obtain the proper areas.
■ Scrap rates are high. The quality of the casting obtained from this
method is difficult to control, in particular at the bore.

Investment cast diaphragms consist of individual nozzle blocks of 17
percent chrome material (17-4 PH) (Fig. 2.11), assembled to inner and
outer steel rings (ASTM A-283 Gr. D) and welded together by the sub-
merged arc welding process (Fig. 2.12). Note then this nozzle block has
sidewalls included resulting in a total stainless steel steam path with
the investment cast process. The nozzle blocks can be made to a much
closer tolerance and with a smoother finish than with conventional
casting methods.
With overall steel construction, the allowable stage temperature is
750°F (400°C), and the allowable pressure drop across the diaphragms
is not a severe limitation because the actual thickness can be increased
to that required.
The inside diameter of the nozzles of a family of diaphragms with
approximately the same pitch diameter remains constant, so that the
wheel outside diameter is also constant. This, many times, results in
the possible use of a standard turbine wheel.
Recent uses of investment cast diaphragms include a 12.5-MW con-
densing turbine with 13.0-in (330-mm) nozzle height and with a
53.25-in (1353-mm) pitch diameter. The investment cast diaphragms
have proven superior to the conventional cast type (closer area con-
trol, smoother finish, sidewalls) and, with mechanical design modifi-