"The most common, and usually the most successful, method for repair is to fill
the damaged area with weld material. The following items should be considered
when other repair methods are contemplated:
Experimental testing to date has indicated that non-fused materials have
less cavitation resistance than carbon steel. Therefore, except where a
profile adjustment is made, the use of these materials cannot be
considered a permanent repair.
Welding plate over a damaged area is only suitable in areas of low stress
and where there are no cyclic loading of vibration conditions.
Replacing damaged areas with new formed plates should only be used
in very severe and advanced cases of damage (i.e., holes through the
blade), as this method could be rather costly to implement "in site."
"Restoration of the runner to its original condition is the most straightforward
approach to repairs. By selecting a weld material of increased cavitation pitting
resistance, it will always be possible to decrease the rate of pitting to some
degree. In many cases, this is the only feasible method of repair, short of
redesigning and replacing the turbine runner. A disadvantage is that the
cavitation pitting problems may persist if the runner is restored to its original
profile, requiring repeated repairs which will build up residual stress in the
runner. Consequently, to reduce the incidence of repairs and prolong the
operating life of the runner, attempts at runner blade shape or profile correction
should be initiated when this is deemed feasible.
"Runner modifications require a thorough analyses and understanding of the
cause of the cavitation pitting. Basically, four types of modifications may be
necessary to reduce the damage:
Leading edge modification
Trailing edge modification
Addition of anti-cavitation fins (on propeller and Kaplan turbines)
"Some specific guidelines for runner modifications are presented later in this
section. The following general guidelines should be observed:
It is important that a well devised plan be made for any runner
modifications, involving if necessary, changes made in small steps with
more frequent inspection.