Page 248 - Steam Turbines Design, Applications, and Rerating

Reaction vs. Impulse Type Steam Turbines 227

Full-admission stages. The dynamic stress on the remaining stages,
with full admission (Fig. 12.6), is estimated with the aid of the follow-
ing equation:

(12.1)
σdyn =± S × V × H ×α×σ b
where S = stimulus
V = resonance rise
H = vibration form factor
α= bonding factor
σ b = static bending stress due to flow forces

Here it is assumed that the dynamic loading on a blade, i.e., the cyclic
stress, is proportional to the static loading due to the flow forces
exerted by the steam. This is a perfectly logical assumption when one
considers merely the wake behind the trailing edges of the blades, e.g.,
or the pressure differences in exhaust casings, which can lead to fluc-
tuations in flow-induced forces.

Figure 12.6 Reaction stages with full admission. (Asea
Brown-Boveri, Baden, Switzerland)