FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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CHECKING FOR CRACKS IN GAS TURBINE ROTOR DISCS
J. ran den Aude I and 4.8. Niebciq
d'eifiufflicuic Canada Inc., Hamilton, Cnta'iic
1. INTRODUCTION
A gas turbine consists of a compressor and a turbine. The turbine drives the
compressor and a mechanical output shaft. The compressor becomes warm (from
compressing air), the turbine becomes hot from the flame which burns in the
compressed air in the combustion chamber. The higher the flame temperature,
the higher the turbine's efficiency, but the material is also used closer to
its limitations. The turbine discs (Figure 1) appear to have a finite life
and although most may last seemingly forever, there is still a fair percentage
that does not. This means that discs are considered to fail eventually
and that a close watch is to be kept on the disc quality at overhauls, usually
in the field. The type of failure is most often a crack in the tooth-serrations
(Figure 2) and so far, only two nondestructive tests appear useful
for the detection of root cracks, i.e., Pénétrants and Eddy currents. The
former is tedious, messy and difficult to evaluate in tight locations, the
latter is quicker, more definite and very sensitive, but is affected by material
edges and can therefore not test the entire length of the serration.
2. PENETRANT TESTING
The third party is now faced with a possible residue of two previous penetrant
applications in a crack and has to go through an extensive cleaning process
which might or might not remove all the old penetrant residue.
The cleaning process involves glass beading to remove scale, vapour degreasing
to dissolve trapped penetrant and perhaps a wash with acetone to enhance
a difficult cleaning process. At this point, it should be realized that we
are dealing with an entire turbine rotor from which only the turbine blades
were removed. Turbines come in different sizes, but a length of 7 m and a
mass of 12 tonnes, is not uncommon.
The pénétrants currently used are biodegradable and water washable. It was
found that the post emulsifiable pénétrants although superior when properly
used, lose all of their advantages when the emulsif1er is left on too long.
Apart from having no control over an operator in the field, the operator has
a huge component on his hands that has to be treated in its entirety, left to
soak a precise time and then wash. Such a procedure is impossible in the time
slots available. Water washable pénétrants are more tolerant and appeared to
be much better suited for the inspection of large components.