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vanes or turbine buckets. Cavitation can not only severely damage the pump or turbine, but it can also
substantially reduce the capacity and therefore lessen the efficiency.
substantially reduce the capacity and therefore lessen the efficiency.
Abrasive erosion is the mechanical removal of metal by suspended solids, such as sand, in the liquid
flowing through an impeller or turbine. The rate of wear is directly related to the velocity of the liquid, so
wear will be more pronounced at the discharge of the nozzle of impulse turbines, near the exit vanes and
shrouds of pump impellers, and near the leading edge of reaction turbines buckets where the liquid velocity
is highest.
flowing through an impeller or turbine. The rate of wear is directly related to the velocity of the liquid, so
wear will be more pronounced at the discharge of the nozzle of impulse turbines, near the exit vanes and
shrouds of pump impellers, and near the leading edge of reaction turbines buckets where the liquid velocity
is highest.
Corrosion damage to submerged or wet metal is the result of an electrochemical reaction. The
electrochemical reaction occurs when a galvanic cell is created by immersing two different elements in an
electrolyte, causing an electric current to flow between the two elements. The anode, or the positive
electrode of the cell, gradually dissolves as a result of the reaction. With the water acting as an electrolyte,
irregularities, such as variation in surface finish or imperfections in the metal's composition, create small
galvanic cells over the entire surface of the metal. Corrosion damage occurs as the anodes of these cells
dissolve. Corrosion, unlike abrasive erosion, is generally independent of the liquid velocity. Pitting caused
strictly by corrosion will be uniform over the entire surface.
electrochemical reaction occurs when a galvanic cell is created by immersing two different elements in an
electrolyte, causing an electric current to flow between the two elements. The anode, or the positive
electrode of the cell, gradually dissolves as a result of the reaction. With the water acting as an electrolyte,
irregularities, such as variation in surface finish or imperfections in the metal's composition, create small
galvanic cells over the entire surface of the metal. Corrosion damage occurs as the anodes of these cells
dissolve. Corrosion, unlike abrasive erosion, is generally independent of the liquid velocity. Pitting caused
strictly by corrosion will be uniform over the entire surface.
Diagnosis of the problem can be difficult as the damage may be caused by more than one action. As a
metal corrodes, the products of corrosion form a protective film on the metal surface. This film protects
the base metal from further corrosive attack. An erosive environment will tend to remove this film leaving
the metal susceptible to corrosion damage. Similarly, where cavitation erosion is occurring, the metal will
be prone to further damage from corrosion.
metal corrodes, the products of corrosion form a protective film on the metal surface. This film protects
the base metal from further corrosive attack. An erosive environment will tend to remove this film leaving
the metal susceptible to corrosion damage. Similarly, where cavitation erosion is occurring, the metal will
be prone to further damage from corrosion.
Severe erosion or corrosion damage may warrant the replacement of the damaged parts with parts
constructed of a material that is more erosion or corrosion resistant. If severe cavitation erosion
occurs during normal operation, a new impeller or runner or other design changes may be
required. Obviously, replacing an impeller or other major components can be a very expensive
endeavor and should only be done after careful economic analysis. Some factors to take into
consideration when making an analysis are the cost and effectiveness of past repairs and any gain
in efficiency or output that may be obtained by replacement.
constructed of a material that is more erosion or corrosion resistant. If severe cavitation erosion
occurs during normal operation, a new impeller or runner or other design changes may be
required. Obviously, replacing an impeller or other major components can be a very expensive
endeavor and should only be done after careful economic analysis. Some factors to take into
consideration when making an analysis are the cost and effectiveness of past repairs and any gain
in efficiency or output that may be obtained by replacement.
Except for severe cases, repair instead of replacement is the most economical solution. The repair
procedure will depend on the cause of the damage. Welding is the most successful method of repair for
cavitation damage. Repair with non-fusing materials, such as epoxies and ceramics, is generally not
successful because the low bond strength of these materials, usually less than 3,000 pounds per square
inch, is not capable of withstanding the high shock pressures encountered during cavitation. Prior to any
weld repair, a detailed welding procedure should be developed. Welding performed incorrectly can cause
more damage by distortion and cracking than the cavitation did originally. Cavitation repair is discussed in
more detail in Facilities Instructions, Standards, and Techniques (FIST),
procedure will depend on the cause of the damage. Welding is the most successful method of repair for
cavitation damage. Repair with non-fusing materials, such as epoxies and ceramics, is generally not
successful because the low bond strength of these materials, usually less than 3,000 pounds per square
inch, is not capable of withstanding the high shock pressures encountered during cavitation. Prior to any
weld repair, a detailed welding procedure should be developed. Welding performed incorrectly can cause
more damage by distortion and cracking than the cavitation did originally. Cavitation repair is discussed in
more detail in Facilities Instructions, Standards, and Techniques (FIST),
Volume 2-5, Turbine Repair
.
Corrosion or erosion damage, if the pitting is deep enough, can also be repaired by welding. If the pitting is
definitely not caused by cavitation, other coatings or fillings may be acceptable. The epoxies and ceramics
discussed earlier, if properly applied, can be helpful. in filling pitting damage caused by corrosion or
erosion. In a corrosive environment, a coating of paint, after the original contour has been restored, can
definitely not caused by cavitation, other coatings or fillings may be acceptable. The epoxies and ceramics
discussed earlier, if properly applied, can be helpful. in filling pitting damage caused by corrosion or
erosion. In a corrosive environment, a coating of paint, after the original contour has been restored, can