4.4
"CAUSE OF PITTING
"An important step in any effort to minimize cavitation damage is to identify the
cause of the pitting. This requires careful examination of the extent and location
of the pitting as well as a review of the operating history of the unit including
operating heads and loading of the machine.
cause of the pitting. This requires careful examination of the extent and location
of the pitting as well as a review of the operating history of the unit including
operating heads and loading of the machine.
summarize typical locations of cavitation damage on propeller and Kaplan
turbines, Francis turbines, and pump-turbines, respectively, and identify possible
causes of the damage.
turbines, Francis turbines, and pump-turbines, respectively, and identify possible
causes of the damage.
"In analyzing cavitation damage, first check for local discontinuities in blade
shape or profile in the area immediately upstream of the damaged surface. Also
check whether or not the cavitation patterns are the same on each of the blades.
If damage varies from blade to blade and there are no apparent discontinuities
upstream, the problem may be on the overall blade profile or blade location. In
this case, a template should be made of a cavitation-free blade or the one with
the least pitting, and this template used to check the overall profile of the other
blades for possible modifications. If the pattern of damage is very similar from
blade to blade, and local profile discontinuities are not evident, the problem
becomes more difficult and other factors such as method of operation, operating
heads, etc., must be considered.
shape or profile in the area immediately upstream of the damaged surface. Also
check whether or not the cavitation patterns are the same on each of the blades.
If damage varies from blade to blade and there are no apparent discontinuities
upstream, the problem may be on the overall blade profile or blade location. In
this case, a template should be made of a cavitation-free blade or the one with
the least pitting, and this template used to check the overall profile of the other
blades for possible modifications. If the pattern of damage is very similar from
blade to blade, and local profile discontinuities are not evident, the problem
becomes more difficult and other factors such as method of operation, operating
heads, etc., must be considered.
"The possibility that damage is not the result of cavitation should be
investigated:
investigated:
Damage may be from corrosion, particularly if water has high oxygen
content or high dissolved solids.
content or high dissolved solids.
On carbon steel runners with stainless steel overlay, damage at the
interface of carbon steel and stainless steel is likely to be partially due to
galvanic action.
interface of carbon steel and stainless steel is likely to be partially due to
galvanic action.
Large voids beneath overlay are caused by galvanic corrosion which will
occur when there is a small hole in the overlay. The hole may be a defect
in the weld overlay or from cavitation pitting which has penetrated the
overlay.
occur when there is a small hole in the overlay. The hole may be a defect
in the weld overlay or from cavitation pitting which has penetrated the
overlay.
If water contains large amounts of entrained solids, the damage may be
caused by physical erosion rather than cavitation pitting.
caused by physical erosion rather than cavitation pitting.
"Input from the turbine manufacturer's hydraulic engineer in assessing the cause
of pitting is always valuable. This is one reason for inspection at an early stage
of operation. Even if damage is far less than the guaranteed amount, the
manufacturer should be asked to report on the cause of the damage.
of pitting is always valuable. This is one reason for inspection at an early stage
of operation. Even if damage is far less than the guaranteed amount, the
manufacturer should be asked to report on the cause of the damage.
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