Question

Pitting occurs downstream of the spillway due to cavity. Describe the formation of pitting and show two examples of pitting. This question is open-ended. Therefore it must be supported by relevant literature.

Answer 1

Pitting due to Cavitation damage is often one of the more serious problem in operating a
hydraulic structure. In a matter of days
it can damage a structure to the point of failure. Millions of dollars have been spent to repair structures that were damaged from pitting erosion.

The mechanism of pitting formation due to cavitation and damage:
Cavitation is a hydraulic phenomenon
similar to the boiling of a liquid, except
it is based on a pressure change instead
of a temperature change. It results from
the expansion and collapse of small bub-
bles, or nuclei, of vapor and gas in a
liquid. The nuclei expand in a low-
pressure region and collapse rapidly in a
high-pressure region. The collapse pro-
duces noise and can cause vibration and
damage to nearby materials.
Cavitation Nuclei: The nucleus is the
basic element involved in cavitation.
Nuclei may be present in a liquid as the
result of air entrainment or organic reac-
tions. They can result from small vol-
umes of gas trapped in surface irregular-
ities of boundaries. Contaminantsus-
pended in the liquid can supply nuclei
from gas trapped in their surfaces. It can
be assumed that a liquid, unless specially
treated, will contain sufficient nuclei for
cavitation to occur.
Nuclei growth. The growth of a nucleus
in a low-pressure region is dependent on
local pressure and on the size of the
nucleus.3 When the pressure is below a
critical level and the nucleus is larger
than a critical size, vaporous growth will
take place. The mechanism of growth is
based on a pressure drop, which can
result from acceleration of a liquid or be
produced in vortexes present in zones of
separation. The pressure in a vortex can
be much lower than the pressure of the
surrounding liquid. Cavitation occurs at
mean liquid pressures higher than the
vapor pressure of the liquid. A nucleus
begins to grow when its internal pressure
overcomes surface tension. The growth
of a bubble in cavitation can involve a
volume change of 50 000 times.

Nuclei collapse. The collapse of an
expanded nucleus or vapor cavity is the
next step in cavitation. Once a growing
nucleus reaches a higher local pressure,
it becomes unstable and collapses.5 The
high-pressure region can be caused by
the deceleration of the liquid by expan-
sion in area or by dissipation of vortexes.
Both of these types of pressure recovery
occur downstream from an offset or
valve. The collapse occurs almost instan-
taneously, and pressures exerted on
nearby liquid or solids can be of an
extreme order of magnitude.6
Much of the energy produced during
the collapse of a cavitation nucleus is
stored as potential energy in the sur-
rounding liquid and remaining cavity.
This energy can cause a rebound or
regrowth and collapse of the remaining
cavity. It has been observed1 7 that one
bubble may have several rebounds, with
each collapse causing cavitation.

Examples of damage
Practically every hydraulic structure
connected with high-velocity flows has
been subject to cavitation damage of one
form or another. From the days of the
first steamship propellers to today's large
flow structures, cavitation damage has
been a major problem.
There are examples of damage to
almost every component of a hydraulic
structure. Damage to entrances of struc-
tures has occurred in the outlet conduits
of Tygart Dam in Panama, Madden Dam
on the Chagres River, and Norris Dam in
Tennessee. The crests of dams such as
the Bonneville Dam on the " Columbia
River have been damaged. Lucky Peak
Dam has been subjected to severe dam-
age from surface roughness. The Grand
Coulee Dam has suffered damage from a
bulge that formed in the spillway. Davis
Dam, Pine Flat Dam, and Detroit Dam
have undergone damage from voids left
in their finish surfaces. Cavitation dam-
age downstream of gate slots has been a
serious problem for the Tygart, Bonne-
ville, Parker, Norris, Libby, Fort Peck,
Dennison, Norfolk, and Tarbela dams.
Steps and protrusions of joints have
caused damage in the Bull Shoals, Sho-
shone, Palisades, and Dworshak dams.
Damage to exits has occurred at the
Folsom Dam in California, and damage
to baffle piers has taken place at the
Bluestone and Clayton dams.