Page 250 - Steam Turbines--Design, Applications, and Rerating by Heinz-Bloch, Murari-Singh

Reaction vs. Impulse Type Steam Turbines 229

Figure 12.8 Stimulus S in relation to reference axial gap c:1 =
impulse turbine; 2 = reaction turbine. (Asea Brown-Boveri, Baden,
Switzerland)

prevent this resonance effect. The damping wire is inserted through,
but not attached to, the blades, while the lashing wire is firmly fixed to
them. The moving blades of impulse stages are joined by shroud bands,
as illustrated earlier. All in all, the reaction type of construction claims
to have certain advantages of dynamic stressing of the blades.

Blade damage. It has been suggested that the moving blades of an
impulse turbine are subject to a greater inducement to vibrate than the
blades of a reaction machine. This contention has occasionally surfaced
in statistics on blade damage. One of these lists one case of damage to
impulse blades for every million component running hours, while the
rate for reaction turbines was only once per 3.8 million component
hours. If the number of components (number of stages) is also taken
into consideration, the average for an impulse turbine would appear to
be one case of blade damage in 25,000 h of operation, as against one in
56,000 h for the reaction turbine. Whether these statistics compare only
reputable, experienced manufacturers of the two turbine types or are
perhaps biased in a number of ways is a matter of conjecture.

Blade clearances. Axial clearance is an important factor in the effi-
ciency of an impulse stage; however, in reaction machines the radial
clearance must be as small as possible. To avoid blade damage if the