4.7
"RUNNER MODIFICATIONS
"Runner Blade Leading Edge Cavitation
"Cavitation at the leading edge of the runner blade as a result of water not
approaching the blade at the proper angle.
approaching the blade at the proper angle.
"The existence of one or more good blades with zero or minimal cavitation pitting is
indicative of variation in blade angle or profile at the leading edge. In this case, the
entrance angle and profile of the good blade should be checked relative to others. A
template made from the good blade can be used for adjusting the leading edge of the
other blades.
indicative of variation in blade angle or profile at the leading edge. In this case, the
entrance angle and profile of the good blade should be checked relative to others. A
template made from the good blade can be used for adjusting the leading edge of the
other blades.
"If the location and extent of damage is about equal on all blades, then the problem
is likely due to design or to operating conditions (i.e., incorrect head, excessive head
variation, or operation at low load). Assuming the problem is related to design or due
to incorrect head, the cavitation can be mitigated by gradual reshaping of the blades.
is likely due to design or to operating conditions (i.e., incorrect head, excessive head
variation, or operation at low load). Assuming the problem is related to design or due
to incorrect head, the cavitation can be mitigated by gradual reshaping of the blades.
cavitation. In some cases, it may be necessary to add material to the opposite side
of the blade to retain the structural integrity of the blade. Any adjustment to blade
shape should be small and, at any one attempt, not more than 1/8 inch (3 mm) of
blade thickness should be removed.
of the blade to retain the structural integrity of the blade. Any adjustment to blade
shape should be small and, at any one attempt, not more than 1/8 inch (3 mm) of
blade thickness should be removed.
"Turbine Discharge Edge Cavitation
"Cavitation pitting at the discharge edge is usually a result of the edge thickness.
Grinding the edge as shown on
Grinding the edge as shown on
serve to mitigate this problem.
"Installation of Anti-Cavitation Fins
"On propeller or Kaplan turbines where there is pitting on the runner periphery, or on
the suction side of the blade immediately adjacent to the periphery, the problem may
be mitigated by addition of anti-cavitation fins to the blade periphery. The fins are
easiest to install with the runner removed; however, they may be added with the
runner in place. Typical dimensions for anti-cavitation fin installation are shown on
the suction side of the blade immediately adjacent to the periphery, the problem may
be mitigated by addition of anti-cavitation fins to the blade periphery. The fins are
easiest to install with the runner removed; however, they may be added with the
runner in place. Typical dimensions for anti-cavitation fin installation are shown on
acceptable by the turbine manufacturer.
"On Kaplan turbines, where one-half or all of the discharge ring is cylindrical in shape
rather than spherical, there will be a discontinuity (large gap) between the blade
rather than spherical, there will be a discontinuity (large gap) between the blade
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