Page 227 - Failure Analysis Case Studies II

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level by (a) ensuring blade frequencies within a narrow limit and thus avoiding resonant vibration
and (b) limiting steam bending stresses.
The total number of blades at a particular stage is divided into groups. Blades in each group are
tied by lacing rods passing through the outer and inner lacing holes. The rigidity of the group is
maintained by joining the lacing rods with blades by brazing at the hole edges. In respect of the
materials, the inclusions and surface defects are minimised to reduce the tendency for fatigue failure.
Regarding the environment, attention is given to steam quality so that corrosion fatigue does not
take place. A large amount of investment is made to achieve the best design of a turbine with a clean
steel having correct microstructure and mechanical properties. Failures that are still encountered are
mainly related to improper maintenance and operating conditions. Knowledge of failures due to
such reasons often go a long way in preventing failures and to greatly improve the economy of
power generation.
In this paper the results of the analysis of failed LP turbine blades of a 210 MW unit of one
thermal power plant are presented [4]. The unit was first overhauled after five years of commissioning.
Within nearly two years of operation after overhauling, the unit was forced to shut down because
of high level of noise and vibration at the LP zone during operation. On opening the turbine casing,
four turbine blades of the 29th stage were found fractured. In the present case, the stage where
failure took place, there are in total 120 blades and these were divided into 15 groups having eight
blades each. A stage is defined as the position of the wheel containing the blades, which is counted
from the position of the HP zone along the shaft towards the turbogenerator. In the present unit,
HP and IP zones contain 12 and 11 stages whereas in the LP zone, there are eight stages namely
stages 2431 among which the 25th and 29th stages (also known as Bauman stages) are very much
prone to vibration arising from steam flow during operation. The total number of start ups were
325 (Le., 46 times cold start and 279 times hot start). The turbine was operated mostly at 50 Hz;
however, for certain period of time, it was operated above/below this level. The durations for such
operations are 1850 h in the frequency range of 51-51.99 Hz and 200 h of 52-52.4 Hz. It was noted
that, due to some problems in grid frequency regulation, the unit was operated at 45.5 Hz for 13 s
over two days prior to failure.
The objective of the present paper is to analyze the causes of the failure Le., whether it is due to:
(a) material defects in the blade, lacing rod or brazed joints; (b) improper brazing process; (c)
improper plant operations in regard of deviations from the stipulated frequency criterion and/or
water chemistry.

2. EXPERIMENTAL DETAILS

A portion of the blade containing the fractured surface was collected from the assembly. The
virgin and service exposed brazing material and the inner and outer lacing rods were also obtained
from the plant. Specimens for microscopy were cut from blade and lacing rods and polished using
standard metallographic techniques and etched with nital. Optical microscopes and JEOL JSM
840A scanning electron microscope (SEM) were employed to observe the microstructure in order
to ascertain the quality of the material. Energy dispersive analysis by X-rays (EDX) attached to the
SEM was carried out to ascertain the composition of blade, lacing rod and braze joint. Fractography
was done in the SEM to analyze the fracture features. Portions of blades and rods were mounted
for hardness testing with IO kg loads in a Vickers indentation testing machine. Tensile tests were
performed in an INSTRON with a strain rate of 10-3/s. To simulate the effect of stress conditions
arising from the steam pressure on the blades, high cycle fatigue tests were carried out with single
edge notch (along with the width direction of the blade) test pieces (36 x 9.5 x 2.5 mm) deformed in
3-point bending on an AMSLER Vibrofore machine operated with a frequency of 50 Hz.

3. RESULTS
3.1 . Visual inspection
On opening the turbine casing, three blades were found fractured from the location of the inner
lacing hole and another one from the outer lacing hole. The assembly of the blades with inner and