Page 226 - Failure Analysis Case Studies II

Failure Analysis Case Studies N
D.R.H. Jones (Editor)
0 2001 Elsevier Science Ltd. All rights reserved 21 1

AN INVESTIGATION OF THE FAILURE OF LOW
PRESSURE STEAM TURBINE BLADES

N. K. MUKHOPADHYAY, S. GHOSH CHOWDHURY,* G. DAS,
I. CHATTORAJ, S. K. DAS and D. K. BHATTACHARYA
National Metallurgical Laboratory, Jamshedpur 831007, India
(Receiued 16 March 1998)

Abstract-An analysis of the failure of LP turbine blades of a 210 MW thermal power plant has been presented
in this paper. The blade material is of 12% Cr steel with tempered martensitic microstructure. Microstructural
analysis as well as hardness and tensile tests did not indicate any degradation in terms of microstructure and
mechanical properties. Physical discontinuities were observed in the braze joint which might have been formed
due to improper brazing operation. Failure of the brazed joints between the blade and lacing rod was found
to be due to improper brazing operations and corrosion effects during service. Fractographic evidence showed
that the cracks were initiated from various points on the blade surface, which were at the interface with the
lacing rod. Striations and beach marks were also observed which indicated the occurrence of high cyclic
loading on the blades. Frequency data obtained from plant indicated the possibility of excessive Vibration
generated due to fluctuation in grid frequency during operation. Thus, the situation was aggravated due to a
resonant condition of vibration, facilitating the propagation of cracks which were initiated earlier. Q 1998
Elsevier Science Ltd. All rights reserved.
Keywords: Thermal power plant, turbine blade failure, vibration. fatigue.

1. INTRODUCTION

Steam turbine blades are critical components in power plants which convert the linear motion of
high temperature and high pressure steam flowing down a pressure gradient into a rotary motion
of the turbine shaft. As the steam enters turbine from the boiler, it passes through different stages
such as high pressure (HP), intermediate pressure (IP) and low pressure (LP) zones. Statistics shows
that LP turbine blades are generally more susceptible to failure compared to those of the HP and
IP. There are various mechanisms by which LP blades fail [I-31. Almost 50% of the failures are
related to fatigue, stress corrosion cracking and corrosion fatigue. The fatigue failure takes place as
a result of vibration arising from the fluctuation of bending stress due to the asymmetric flow of
steam. Once a crack is initiated, the component is assumed to have failed since crack growth takes
place rapidly. Even this fatigue failure can be accentuated by corrosion. Creep damage is not
important for the LP blades. It is reported that failure initiates from various locations of the blade
and these are 26% from shroud, 20% from lacing hole, 40% in the aerofoil region and 14% in the
blade attachment [l]. Therefore, the mechanism of the failure varies along the length of the blades.
In general, LP blades in a steam turbine assembly are designed to run for 30 years, but many
cases of premature failure of blades are encountered in practice. A recent survey indicates that
causes for about 40% of the failures could not be pinpointed [l]. To reduce the incidence of failure,
it is necessary to take into account all the aspects important for the performance of a blade. Thus,
it is necessary to understand the metallurgy of the blade material, operating stresses and the
operating environment. As the blade’s design is complex, the actual state of stress is highly compli-
cated. However, if the design conditions do not deviate in service, the state of the stresses in a blade
should not cause any concern. The stresses acting on the blades mainly originate from centrifugal
loading and vibratory response of the blades. Vibratory stresses are normally maintained at low

*Author lo whom correspondence should be addressed.
Reprinted from Engineering Failure Analysis 5 (3), 18 1-1 93 (1 998)