Handbook of Turbomachinery Second Edition Revised - Ventech!

Start–Stop Transients/Overspeeds
Large cyclic stresses on blades can occur during turbine run-ups and during
overspeeds. The effect is shown schematically in Fig. 45. Blades will
experience a half-stress cycle from zero centrifugal load to full centrifugal
load as the unit starts up. During operation, vibratory stresses are imposed
on the high mean centrifugal stresses as indicated schematically by the
numerous vibratory stress cycles shown in Fig. 45. With unit shutdown the
centrifugal stress cycle is completed.
These large cyclic stresses are most prominent on those blades subject
to the highest centrifugal stresses, e.g., the longest blades in the last few
stages of the LP turbine. Distribution of blade loads can occur that lead to
stresses that exceed local yield level and subsequently to low cycle fatigue
failures. This problem is particularly acute in locations of high stress
concentration such as blade root hooks and tiewire holes. It is less common
in the disk rim attachment hooks, where design stress levels are typically
lower.
Rotating blade failure can occur in a relatively few number of cycles.
For example, crack initiation has been experienced in L-0 blades in 200–
1,000 start–stop cycles depending on steam environmental conditions.
Clearly, for units cycled on a regular basis, a design low cycle fatigue life of
less than 500 start–stop cycles is marginal, particularly given the potential
for adverse environmental effects or the added stresses that result from
stress concentration in the gaps between the blade root and disk hooks.
Figure 45 Dynamic and static stresses during turbine start–stop operation.
Copyright © 2003 Marcel Dekker, Inc.