Page 143 - Steam Turbines Design, Applications, and Rerating

124 Chapter Six

selected for every blade size, the mechanical and aerodynamic charac-
teristics will also be the same for every blade size. Typical designs strive
to accomplish a number of objectives:

■ All stresses produced by centrifugal forces and steam bending mo-
ments in the blade and the root have the same magnitude for all
blade sizes.
■ The ratio of the blade natural frequency to the maximum rotational
frequency is identical for all blade sizes.
■ Steam conditions being equal before and after the blade annulus, the
flow velocities for every point of the flow area are the same for every
blade size. Hence, the Mach numbers are also the same.
■ If steam conditions upstream and downstream of the blade annulus
and peripheral speeds of various blade sizes are maintained con-
stant, the blade efficiency does not depend on the blade size. More-
over, the exit loss as a function of volume flow and peripheral speed
is the same for all blade sizes. Consequently, mechanical and aerody-
namic measurements obtained on a particular blade size can be
transferred to all the other sizes of a blade series. This also applies to
the operating experience as a whole, i.e., the experience gained with
certain blades of this series will apply to all blade sizes of this series.